Cognitive dysfunction is a common feature of autoimmune encephalitis. Pathogenic neuronal surface antibodies are thought to mediate distinct profiles of cognitive impairment in both the acute and chronic phases of encephalitis. In this review, we describe the cognitive impairment associated with each antibody-mediated syndrome and, using evidence from imaging and animal studies, examine how the nature of the impairment relates to the underlying neuroimmunological and receptor-based mechanisms. Neuronal surface antibodies, particularly serum NMDA receptor antibodies, are also found outside of encephalitis although the clinical significance of this has yet to be fully determined. We discuss evidence highlighting their prevalence, and association with cognitive outcomes, in a number of common disorders including cancer and schizophrenia. We consider mechanisms, including blood-brain barrier dysfunction, which could determine the impact of these antibodies outside encephalitis and account for much of the clinical heterogeneity observed. A receptor) and metabotropic (GABAB receptor) postsynaptic receptors that are bound by GABA, the major inhibitory neurotransmitter in the central nervous system (CNS). While long-range GABAergic neurons do exist, the majority of research attention relevant to cognition has focused on GABAergic interneurons, which appear to have a central role in the synchronisation of network activity and the generation of oscillations in different frequency bands, processes felt to facilitate the efficacy of information processing1.An expanding array of pathogenic neuronal autoantibodies are being identified, each targeting different neuronal surface antigens and thought to cause distinct and clinically recognisable encephalitic syndromes. These antigenic targets have wide-ranging properties and distributions in the central nervous system but common to almost all autoimmune encephalitides is cognitive dysfunction. The nature of cognitive impairment and associated neuroimaging findings varies between syndromes and gives insight into the antibody-mediated mechanism of action. Neuronal autoantibodies have also been reported—so far mainly in the peripheral blood—in individuals without frank encephalitis. Although their significance outside the encephalitic context is not yet clear, there is a growing body of evidence to suggest these autoantibodies have pathogenic potential even in the absence of the encephalitic syndrome. In this review, we outline the cognitive profile of each of the commonest autoantibody-mediated encephalitides and consider the role of neuronal antibodies outside encephalitis. While a treatment of the neurotransmitter basis of cognition is beyond the scope of this review, it is important to note that these autoantibodies largely serve to disrupt the signalling transmission of neurotransmitters such as glutamate and γ-aminobutyric acid (GABA) which are integral to cognition. Glutamate is a ubiquitously distributed excitatory neurotransmitter that also acts as an intermediary in cerebral metabolism; the ionotropic glutamate-specific N-Methyl-d-aspartate receptor (NMDAR) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are vital components of long-term potentiation (LTP) and long-term depression, processes understood to be the major synaptic substrates of learning and memory; changes in the neuronal surface density of these receptors therefore have direct effects on neuronal signalling with downstream impact on brain connectivity and cognitive processes. GABA receptors are present as ionotropic  epitopes, or even entirely different proteins, may contribute to the clinical expression of disease in some disorders.Detailed neuropsychological characterisation is often challenging in the acute phase of autoimmune encephalitis due to the severity of clinical symptoms. Accordingly, in the acute phase clinical descriptions tend to be qualitative and the more extensive cognitive testing possible in the post-acute and chronic phases is frequently authored from a neurorehabilitation perspective, potentially introducing a selection bias towards cases with more severe dysfunction. While we describe the acute and chronic cognitive deficits separately, in practice such distinctions are not so easily delineated and there is often significant overlap. Table 6. While most frequently described in women of child-bearing age, it is increasingly recognised in children and older adults of both sexes6.NMDAR encephalitis is both the most common and best-defined cause of autoimmune encephalitis. Its onset is often heralded by an influenza-like prodrome followed by a characteristic progression from psychotic symptoms and cognitive impairment to seizures, movement disorder, autonomic instability and loss of consciousness8. Atypical, unusual presentations of cognitive impairment are occasionally described in NMDAR encephalitis; case reports illustrate disruption to temporal orientation with a loss of age awareness and also transient epileptic amnesia, characterised by repeated, brief episodes of anterograde and retrograde amnesia10. Another recent case report documented a patient who, due to the nature of their presentation with memory loss, cognitive fluctuations, visual hallucinations and sleep disorder, was initially misdiagnosed with Lewy Body Dementia before NMDAR antibodies were identified leading to effective immunotherapy treatment11. In older adults with NMDAR encephalitis cognitive impairment is often more prominent which may account for some of the variability seen12.Cognitive dysfunction is often profound and in the acute phase typically extends across all domains. Deficits in executive function and memory are most marked but attention, language, visuospatial processing and social cognition are also affected to varying degrees13. As in the acute phase, episodic memory and executive function are most consistently affected 1 year after initial presentation14. A recent systematic review found chronic cognitive impairment in up to three quarters of patients, with timely immunotherapy the most important factor determining positive outcomes8. This emphasises the persistent and major morbidity of cognitive impairment in anti-NMDAR encephalitis for patients, in addition to the importance of early diagnosis and appropriate treatment13.While cognitive dysfunction is extensive in the acute phase it can also persist for years after the initial insult16. The antibody-mediated disruption of the interaction between the NMDA and ephrin-B2 receptor is central to this, causing the NMDAR to become displaced which allows subsequent internalisation17. Indeed, animal studies suggest that if ephrin-B2 is co-administered, the pathogenic effects of the antibodies are blocked and no downstream effects are seen18.The NMDA receptor is a tetrameric ligand-gated ion channel which mediates excitatory transmission in the CNS and is crucial for LTP, the neural substrate for learning and memory. In NMDAR encephalitis, there is substantial intrathecal production of IgG NMDAR antibodies which target the NR1 subunit of the receptor, causing a reversible and titre-dependent internalisation of the NMDA receptor with subsequent reduced receptor density and reduction in NMDAR-mediated currents19. Furthermore, in vivo infusion of NMDAR antibodies reduces excitatory postsynaptic currents in rat hippocampal neurons and simultaneously impairs learning and spatial working memory20. Infusion of inflammatory cytokines also reduced excitatory postsynaptic currents and led to further impairment in learning performance suggesting there may be other additive factors influencing cognitive dysfunction15. More recently, object recognition has been shown to be impaired following injection of NMDAR antibodies to rat hippocampi, widening the application of NMDAR antibody-mediated cognitive dysfunction21. NMDAR antibodies cause a dose-dependent increase in extracellular glutamate, akin to the elevated level of glutamate following ketamine administration which is associated with cognitive effects22.Accordingly, NMDAR-dependent LTP is depressed in mouse hippocampal slices with prolonged exposure to CSF from patients with NMDAR encephalitis23. Indeed, reduced functional connectivity between the hippocampus and the medial prefrontal cortex and impaired connectivity within the medial temporal lobe (MTL) network was observed in patients with NMDAR encephalitis and shown to predict the severity of memory impairment  and for this reason VGKC antibodies should not be routinely tested in the initial investigation of a patient with a suspected autoimmune CNS disorder32.The first potentially reversible, immunosuppression-responsive form of limbic encephalitis was described nearly 20 years ago33. Unlike other autoimmune encephalitides (and most autoimmune conditions) LGI1 encephalitis is most common in middle-aged males. Although LGI1 encephalitis is phenotypically similar to other paraneoplastic and non-paraneoplastic limbic encephalitides, faciobrachial dystonic seizures (FBDS) are unique to LGI1 encephalitis and are a useful clinical differentiator34.Typical presenting symptoms of LGI1 encephalitis are those of a limbic encephalitis; symptoms include cognitive impairment, behavioural changes and focal seizures. Seizures typically precede the onset of cognitive impairment, with a progressive amnesia usually developing at their crescendo36. Isolated amnestic syndrome can occur in up to 10% of cases of LGI1 encephalitis35 and, in the absence of seizures and where the onset is insidious, LGI1 encephalitis can mimic other syndromes of cognitive impairment such as neurodegenerative dementias37. Indeed, numerous case reports have documented LGI1 encephalitis misdiagnosed as Alzheimer’s disease, Creutzfeld-Jacob Disease and Dementia with Lewy Bodies before further investigation elucidated the true cause and led to reversal of the cognitive impairment with immunotherapy38.Prominent amnesia is a hallmark of limbic encephalitis associated with LGI1-antibodies; autobiographical memory is particularly impaired, often with significant confusion and disorientation39. In LGI1 encephalitis, most patients have a chronic cognitive impairment, with memory predominantly affected but deficits of attention and executive function also reported42. Greater disease severity, delays to immunotherapy or longer courses of immunotherapy (likely mandated by greater disease severity) are all associated with more profound cognitive dysfunction in LGI1 encephalitis40. Indeed, early treatment of isolated FBDS significantly reduces the risk of developing cognitive impairment, highlighting the importance of recognising FBDS early44.Looking at long term outcomes for patients with previously defined VGKC antibodies collectively, without differentiating LGI1 or CASPR2-positive patients, cognitive deficits correlate with antibody titre and are most marked for verbal memory, while processing speed and executive function are relatively spared45. However, given the marked differences in clinical presentation with AMPAR encephalitis, it is likely that LGI1 antibodies have other downstream effects in addition to modulation of AMPARs. In one study of 103 patients, IgG1 antibodies occurred more frequently in patients with cognitive impairment, suggesting a role for complement-mediated pathology in this symptom domain43. Serum IgG VGKC antibodies from patients with limbic encephalitis led to cell excitability with an increased tonic rate of firing and strengthened mossy fibre-evoked synaptic responses in CA3 rat hippocampal slices—the sera used in this study was later found to have LGI1 rather than CASPR2 antibodies46. A selective VGKC antagonist mimicked these effects suggesting that the antibody-mediated increase in cell excitability is directly related to reduction in VGKC function. Petit-Pedrol et al. later demonstrated LGI1 antibody-mediated reductions in synaptic density of both Kv1.1 VGKC and AMPAR receptors with simultaneous, reversible memory deficits47. They observed hyperexcitability, increased glutamatergic transmission and reduced synaptic failure rate with a severe impairment to neuronal plasticity and LTP in CA1 and the dentate gyrus. A recent study further explored the pathogenic mechanisms of LGI1-antibodies and suggested the possibility of synergistic contributions. Monoclonal antibodies derived from patients with LGI1-mediated disease were found to target multiple epitopes, with distinct specificities to the protein’s LRR and EPTP-domains. In this study, LRR and EPTP-binding LGI1 antibodies seemed to mediate distinct functional effects. LRR-directed antibodies bound and internalised the LGI1-ADAM22/23 complexes. Surprisingly, this mechanism was independent of IgG bivalency, as both IgG4 antibodies and Fab fragments retained this capacity. The dominant effect of EPTP-binding antibodies, on the other hand, appeared to be disruption of the interaction between LGI1 and its receptors. Both LRR- and EPTP-specific antibodies completely abrogated LTP induction at CA3-CA1 hippocampal synapses, which translated to impairment of recognition memory in mice exposed to LRRP-antibodies48. These findings likely explain the amnestic syndrome experienced by patients with LGI1 encephalitis.Unlike NMDAR antibodies, LGI1 antibodies are predominantly of the IgG4 subclass, although the IgG1 subclass may also contribute to pathology. LGI1 is a trans-synaptic protein which complexes presynaptic ADAM23 with postsynaptic ADAM22. LGI1 antibodies act directly to disrupt this binding which subsequently reduces synaptic AMPAR density49. This might relate to differences in the timing of protein dysfunction (from early neurodevelopment vs later in adult life) or to other in vivo roles of LGI1 and its targeting by autoantibodies that have not yet been clearly defined.Interestingly, in humans, LGI1 gene mutations lead to autosomal dominant lateral temporal lobe epilepsy characterised by frequent partial seizures often with auditory auras (rather than FBDS) but typically without cognitive impairment50. Correspondingly, MR imaging in the acute phase of LGI1 or CASPR2 encephalitis is usually abnormal, typically showing inflammation in MTL, an area critically involved in memory processing52. At follow-up, almost all patients show some degree of hippocampal atrophy, which is often bilateral, with loss seen particularly in the CA3 subfields53. The volumetric atrophy also correlates with the severity of episodic autobiographical and verbal memory impairments53. The microstructural integrity of the hippocampus is also impaired on a wider level which associates with both disease severity and memory function; functional connectivity of the remaining hippocampus correlates closely with the degree of memory impairment54. Interestingly, functional connectivity analyses also show characteristic alterations in several large-scale networks, suggesting that LGI1 encephalitis is not confined to the limbic system. Increased connectivity in the ventral and dorsal default-mode network is associated with improved memory performance, indicating a compensatory mechanism, while connectivity in the salience network is reduced and correlated with impaired memory function55. These network changes indicate cognitive deficits beyond mere memory impairment in LGI1 encephalitis, suggesting brain-wide alteration of the connectome triggered by focal hippocampal damage.In the rodent, LGI1 and CASPR2 are preferentially expressed in CA3 and CA1 hippocampal subfields56. Among the different CASPR2-related syndromes, limbic encephalitis and Morvan’s syndrome have a particular impact on cognitive functions. CASPR2 encephalitis, similarly to LGI1 encephalitis57, is characterised by limbic dysfunction, with temporal seizures, memory impairment and frontal dysfunction36. Morvan’s syndrome, a rarer disorder, is characterised by peripheral nerve hyperexcitability and encephalopathy, in addition to sleep disturbance, hallucinations, dysautonomia and pain. Whether Morvan’s syndrome is a distinct entity or merely a combination of autoimmune encephalitis and peripheral nerve hyperexcitability is unclear, but some suggest there are sufficient differences to consider it a distinct syndrome. Limbic dysfunction, such as temporal seizures, anterograde amnesia or hyperintensities in the MRI are uncommon in Morvan’s syndrome, except in patients who are both CASPR2 and LGI1-antibody positive. Both CASPR2 limbic encephalitis and Morvan’s syndrome are more common in elderly male patients, but association with thymoma and other autoimmune diseases is much more common in Morvan’s syndrome.CASPR2 antibodies associate with a wide range of neurological syndromes, which often overlap in the same patient. Manifestations include peripheral nerve hyperexcitability (often referred as neuromyotonia), neuropathic pain, paroxysmal movement disorders and limbic encephalitis57. However, relapses are frequent, particularly in the form of increased seizure activity, and are usually steroid-responsive.Data on long term outcomes, particularly on cognitive sequelae, are remarkably scarce in CASPR2 encephalitis and Morvan’s syndrome, and it is often pooled together with that of LGI1 encephalitis or under the previous umbrella designation “VGKC encephalitis”. Partial or full recovery after immunosuppression is usually the norm in non-paraneoplastic CASPR2 encephalitis or Morvan’s syndrome59. CASPR2 stabilises the VGKCs such that antibody-mediated disruption of this protein causes peripheral hyperexcitability syndromes. The role of CASPR2 at the CNS synapse, however, is not well known, but the bulk of data suggests important functions in synaptic processes and neuronal activity. CASPR2 was implicated in the trafficking of AMPA receptors to the synaptic membrane61, suggesting that glutamatergic transmission dysfunction could underpin the cognitive impairment seen in CASPR2-mediated CNS disease. Others suggest that CASPR2 has a role in inhibitory hippocampal synapses, and that the antibody-mediated perturbation of inhibitory interneuron activity could lead to increased neuronal hyperexcitability and ultimately to the seizures suffered by these patients63.CASPR2, a member of the neurexin family, is a cell adhesion transmembrane protein first identified in the VGKC clusters (mainly Kv1.1 and Kv1.2) at the juxtaparanodes of myelinated neurons64. The mechanisms by which these antibodies cause disease, however, are still not completely understood and there have been conflicting results in the literature. While some report absence of CASPR2 internalisation65, others suggest that CASPR2 is indeed internalised66 by the antibodies. It is possible that the IgG subclass (IgG1 vs IgG4) titre differences in the CASPR2-IgG preparations used in these studies, as well as differences in the in vitro systems used, partially account for these contradicting results. Those groups that found no evidence for antibody-mediated internalisation suggest that CASPR2-antibodies exert their function through interference with the interaction between CASPR2 and TAG-165. Finally, CASPR2-antibodies may also exert their pathogenicity by altering the protein’s known function in AMPAR synaptic traffic. A recent study showed significant synaptic loss of AMPARs in cortical neurons incubated with IgG1 and IgG4 CASPR2 antibodies, while in vivo injection of the same antibodies in the mouse visual cortex significantly decreased AMPAR-mediated currents67.Like LGI1-antibodies, CASPR2-antibodies are predominantly of the IgG4 subclass, although IgG1 antibodies can also be present and are potential contributors to pathology. They too target multiple epitopes, with the protein’s N-terminal discoidin-like and laminin G1 domains being obligatory epitopesThese results suggest that CASPR2 likely has different functions in different synapses, which would imply different but synergistic effects of the CASPR2-antibodies in the pathophysiology of encephalitis or Morvan’s syndrome. More studies on the pathogenic mechanisms of CASPR2-antibodies are necessary.68. First described in a series of ten patients, it was reported to mostly affect older women, often with an underlying malignancy and high rates of relapse69. The phenotype has since widened with marked heterogeneity in presentation observed70. However, cognitive dysfunction remains a universally prominent feature and isolated amnesic syndromes have also been observed with a focal impairment to anterograde memory71. Indeed, in a review of 18 cases there was evidence of cognitive impairment in all, ranging from anterograde memory impairments and executive dysfunction to generalised confusion68. A recent systematic review identified 55 patients with AMPAR encephalitis; a diverse phenotype was observed but amnesia was recognised as the most common clinical symptom71. However, amnesia at onset was also associated with greater diagnostic delays, highlighting the need for improved recognition of the symptomatic profile associated with AMPAR encephalitis.Due to its rarity, the clinical course of AMPAR encephalitis is not yet well characterised. Most studies describe limbic dysfunction at onset characterised by anterograde and retrograde amnesia, confusion, psychiatric symptoms and seizures72. Although in general outcomes appear favourable, psychiatric symptoms or fulminant encephalopathy at onset is associated with poor prognosis at follow up72. In a case report, significant memory impairment was found to persist 1 year after disease onset, accompanied by hippocampal atrophy, persistent hippocampal hypermetabolism in 18FDG PET imaging and ongoing epileptic activity on EEG73.There are limited data available on the neuropsychological outcomes of AMPAR encephalitis patients. Case reports have described considerable neurocognitive improvement at follow-up with a third achieving complete recovery, but this has yet to be characterised quantitatively74. The composition of subunits has important consequences for the role of AMPAR in synaptic plasticity and typically AMPAR antibodies target GluA1 and GluA2 subunits75. AMPAR antibodies cause reductions in AMPAR expression with changes to their synaptic localisation through receptor internalisation and degradation76. Reductions in AMPAR-mediated currents are seen with alterations in the patterns of action potential firing and an increase in intrinsic excitability of neurons likely due to a compensatory decrease in inhibitory synaptic transmission76. Haselmann et al. demonstrated antibody-mediated internalisation of GluA2-containing AMPARs with compensatory insertion of mostly GluA1-containing AMPARs77. The subsequent LTP impairments were hypothesised to be secondary to the reduced availability of extrasynaptic AMPAR, on which LTP is dependent78. Alongside the LTP changes, they found in vivo impairments to learning and memory; this was the first animal model to recapitulate the severe memory impairments typical of AMPAR encephalitis77.AMPARs are glutamate-gated ion channels composed of combinations of the tetrameric subunits GluA1–4. AMPARs mediate much of the rapid, excitatory neurotransmission in the brain and are integral to LTP69. Indeed, in the vast majority brain MRI is abnormal in the acute phase, often showing bilateral temporal lobe enhancement, reflecting areas of greatest AMPAR density71. Given AMPARs are found throughout the brain, albeit at lower concentrations than in the limbic regions, autoantibody binding in other regions could account for the marked heterogeneity seen in clinical profile, with the more generalised distribution also underpinning the global atrophy and hypometabolism reported in some cases of AMPAR encephalitis79.Although ubiquitous, GluA1/2 and GluA2/3 are particularly expressed in the hippocampal and limbic regions, and as such these regions are particular targets for AMPAR antibodiesAR antibody-mediated encephalitis has a broad clinical phenotype affecting all ages of both sexes81. The largest case series to date confirmed seizures as the most frequent symptom with altered cognition evident in two-thirds of patients81. While the clinical phenotype has yet to be fully characterised, greater variability in presentation is evident with memory deficits not ubiquitous and cases without seizures also described83. It is possible that the variation in clinical presentation of GABAAR antibody encephalitis across these series may be due to the differences in subunit specificity of cell-based assays used to detect the antibodies. Studies using the α1 and β3 subunits84 have tended to find a more restricted phenotype than those using the α1, β2 and γ2 subunits82, although differences may also reflect the different nature of the patient populations whose samples were tested in these studies.GABAA receptors are ligandgated chloride ion channels, underpinning fast synaptic inhibition, while GABAB receptors are G-protein coupled receptors modulating slower inhibitory transmission. Autoantibodies to GABAARs are generally IgG1 and those which target the extracellular epitope of the γ2, α1 and β3 subunit cause reduced synaptic and extrasynaptic GABAAR with consequent reductions in inhibitory postsynaptic currents in vitro85. Mutations to the GABAAR reducing expression levels cause generalised epilepsies but there are, as yet, no animal studies which demonstrate the impact of GABAAR antibodies in vivo86. However, in the acute phase brain MR imaging is commonly abnormal; 77% show multifocal, asynchronous grey and white matter changes most often in the temporal and frontal lobes81. These widespread changes reflect the extensive distribution of GABAAR which, along with the likely presence of additional anti-neuronal antibodies, may underpin the heterogeneity in presentation of GABAAR encephalitis87.GABA receptors are the major mediator of inhibitory synaptic transmission in the CNS. GABABR encephalitis was first described in a case series of 15 patients characterised by seizures and memory deficits88. Older adults are most affected and there is a strong association with small cell lung cancer, occurring in up to 50% of patients and associated with poorer outcomes89. While more recent clinical descriptions have expanded the clinical phenotype, cognitive impairment and seizures remain the central symptoms, almost universally affecting patients in the acute phase but the nature of neuropsychological impairments have not been examined in detail90. Interestingly, this recent case series identified a subset of patients with GABABR encephalitis presenting with a “rapidly progressive dementia” with subacute cognitive impairment in the absence of seizures90. Prognosis is often poor, with a median survival of 17 months, and the long term outcomes for GABABR encephalitis have yet to be studied90.GABABR encephalitis are predominantly of the IgG1 subclass targeting the extracellular domain of the B1 subunit88. Autoantibodies to the GABABR act to inhibit channel function rather than internalise or deplete cell surface receptor levels91. In line with the clinical phenotype, knockout GABAB1R mice exhibit spontaneous seizures with marked memory impairment92. The GABAB receptor is mainly expressed in hippocampus, amygdala, thalamus, and cerebellum reflecting the common MTL abnormalities seen in imaging during the acute phase of encephalitis93.Antibodies associated with GABA94. However, this is not equivalent to asserting that all such antibodies are potentially encephalitogenic and this distinction must be held in mind95.While the impact of neuronal autoantibodies on cognition is well established within each encephalitic syndrome, their role outside this context is less clear. However, there is accumulating evidence to indicate these antibodies (particularly NMDAR antibodies) may be of relevance outside clinically defined encephalitis, in patients without evidence of frank encephalopathy. All NMDAR antibodies, irrespective of immunoglobulin class and donor source, demonstrate pathological potential; in vitro  instigating NMDAR internalisation and dysfunctional glutamatergic signalling96. Notably, different assays also appear to have different sensitivities for detection of neuronal autoantibodies. Live cell-based assays, in which sera or CSF is applied to recombinant HEK cells expressing the antigen of interest before fixation, appear to detect many more positive specimens than do fixed assays, in which the sera or CSF is applied after fixation. Nonetheless, while live CBAs do detect antibodies that demonstrably bind their target, the clinical relevance of the results is less clear—that is, the increased analytical sensitivity of these assays may come at the expense of clinical specificity98.Serum NMDAR antibodies, of uncertain clinical relevance, are found in appreciable numbers in healthy controls, which is a challenge to the view that they are universally pathological. In one such study serum NMDAR antibodies of all isotypes occurred at an overall frequency of about 10%, increasing with age but not differing according to disease status; NMDAR IgG remained relatively rare however, detectable in around 1%. Similar results were reported for antibodies to other antigens, although these were much less common than NMDAR antibodies99 or for autoimmune psychosis100 can a positive serum antibody on its own lead to a diagnosis of probable antibody-mediated disorder.In part because of the issue of non-specificity of serum autoantibodies to neuronal antigens, diagnostic criteria for autoimmune CNS disorders place much emphasis on paraclinical investigations which are required to determine the clinical relevance of a positive antibody, such as MRI or EEG. Neither in consensus criteria for autoimmune encephalitis103. Indeed, in SLE rodent models, infusion of NMDAR antibodies (targeting the NR2 rather than NR1 subunit) only caused cognitive impairment where the blood-brain barrier was disrupted104. It also remains possible that impaired blood-brain barrier integrity, or other mechanisms, could allow formation of antibodies in patients with neuronal decline via recognition of these neuronal antigens for the first time105. We review the evidence for a role of autoantibody-associated cognitive impairment in various common disorders.It has been suggested that some aspect of blood-brain barrier permeability is one factor which can determine the clinical relevance of a positive serum antibody, with numerous studies showing autoantibody-mediated neuropsychiatric symptoms to be dependent on blood-brain barrier permeability106. The antibodies most commonly identified were IgA or IgM NMDAR antibody, and the level of cognitive impairment was related to the degree of blood-CSF barrier disruption as indexed by the cerebrospinal fluid/serum albumin quotient106. These findings were then replicated and extended in a prospective study of melanoma patients107. Importantly, all patients underwent a comprehensive cognitive assessment that was performed blinded to antibody status. Melanoma patients with neuronal autoantibodies (mostly serum IgA and IgM NMDAR antibodies) showed more than threefold higher odds for cognitive impairment than melanoma patients without antibodies. Furthermore, the degree of cognitive impairment was correlated with the titre of NMDAR IgM or IgA antibody107. Affected cognitive domains included memory, attention and executive function indicating neuronal autoantibodies may have a role as both a pathophysiological factor and a potential biomarker for cognitive impairment107. However, future studies are needed to determine whether the observed cognitive impairments in antibody-positive patients are specific to cancer and whether antibodies themselves are pathogenic or rather indicate pathophysiological states leading to cognitive decline.Paraneoplastic neurological syndromes are immune-mediated disorders triggered by tumours driving the immunization process. Many of the autoimmune encephalitis-associated antigens are expressed by tumours, and paraneoplastic neurological syndromes can be associated with neuronal autoantibodies targeting these tumour-expressed neuronal antigens (such as NMDAR antibodies with teratomas and CASPR2 antibodies with thymomas). However, the association between tumours, neuronal antibodies and cognition has recently been explored beyond the context of limbic encephalitis. In a retrospective study, neuronal antibodies were observed in almost a quarter of cancer patients tested and cognitive deficits were found to be significantly more common in those with a positive serum neuronal antibody109. In up to 90% of so-called ‘relapses’ of HSE, where the clinical picture is frequently dominated by cognitive dysfunction, the aetiology is now understood to represent a “secondary autoimmune encephalitis” responsive to immunotherapy; indeed autoimmune encephalitis is thought to occur in around a third of HSE patients58. One obvious factor potentially responsible for initiation of autoimmunity is the gross neuronal destruction and subsequent epitope exposure caused by HSV infection. Indeed, other CNS viral infections are also known to initiate neuronal autoantibody production109. However, history of non-encephalitic HSV infection is also more common in NMDAR encephalitis110 suggesting molecular mimicry may also play a role111. Interestingly in one study, even in patients who did not develop frank encephalopathy after HSV infection, CSF NMDAR antibodies were interpreted to be predictive of the degree of improvement in cognitive function in the recovery phase of HSE112. However, in this study, there was no difference in cognitive performance between NMDAR antibody positive and negative patients at any time during follow-up; rather, there was only a significantly greater improvement of cognitive scores in the antibody negative group, driven by recovery from a worse baseline performance for these patients compared to that of the NMDAR antibody positive patients. Moreover, the relatively impaired performance of NMDAR antibody negative patients at baseline was driven by four outliers with particularly impaired performance112. Given the significant impact of post-HSE cognitive impairment on functioning and quality of life, and the lack of clarity of these results, attempts at clarifying the possible prognostic significance of NMDAR antibodies in this patient group are of continued interest. Overall it appears that secondary neuronal autoimmunity following other kinds of brain tissue damage could have a role in shaping the extent of cognitive dysfunction following an acute event. One such obvious example of brain tissue damage, amenable to study by virtue of its frequency, is stroke.It is now established that neuronal autoimmunity—principally to NMDAR but other antigens have also been implicated—can be initiated by herpes simplex encephalitis (HSE)113. However, there is as yet no consensus regarding the relevance of these antibodies in this clinical population. One recent study did not find any association of serum neuronal antibody seropositivity with functional outcome or clinical features in acute stroke114. Another large study did not find an association with seroprevalence per se, unless the group was stratified by NMDAR antibody titre, when high antibody titre was found to correlate with poor functional outcomes115. In addition, NMDAR antibody seropositive patients had an increased risk of secondary vascular events or death. The integrity of the blood-brain barrier may also be relevant in stroke: in patients with acute ischaemic stroke, NMDAR antibodies were associated with larger stroke lesions in patients with a “leaky” blood-brain barrier, as indicated by APOE4 status, and conversely, in patients with an intact blood-brain barrier NMDAR antibodies were associated with smaller stroke lesion size113. Thus, the relationship between autoantibodies and outcomes following stroke is not linear but serum NMDAR antibodies may be of particular relevance where they are found at high titre and/or with a compromised blood-brain barrier. Although functional outcomes are in part driven by cognitive status following stroke, to date no study has investigated an association between antibody seropositivity and quantitative cognitive outcomes following stroke. This would be methodologically challenging due to the inherent variability in cognitive outcomes with lesion heterogeneity but could offer the potential for new insights into the impact of neuronal antibodies outside encephalitis.Serum neuronal antibodies are detected in up to one-fifth of patients following acute stroke116. This has caused considerable interest in investigating a potential role for neuronal antibodies in the pathogenesis of psychiatric syndromes. Various case-control studies have produced conflicting results, but a systematic review and meta-analysis found that serum NMDAR antibodies were three times as common in patients with schizophrenia, schizoaffective disorder, bipolar affective disorder or major depressive disorder compared to controls118. This finding was supported by a more recent large case-control study indicating that serum NMDAR antibodies were more prevalent in patients with first episode psychosis than in the healthy controls, although this was not the case for the other antibodies tested; LGI1, GABAAR and VGKC-complex119. However, in the psychiatric patient population, the clinical characteristics between antibody positive and antibody negative patients appear similar and to date the clinical significance of this increased prevalence is unclear120.Psychiatric symptoms are hallmarks of many of the autoimmune encephalitic syndromes, in some cases occurring in the absence of the other clinical symptoms121. These deficits occur before the onset of psychosis and remain stable throughout the course of the disease123. Glutamate receptor hypofunction has been hypothesised to underlie this cognitive impairment and more recently NMDAR autoimmunity has been implicated. Indeed, a recent study found that first episode psychosis patients with schizophrenia who had a positive serum NMDAR antibody exhibited greater cognitive impairments in all domains relative to controls124. Furthermore, serum antibody level was inversely correlated with scores in verbal and learning memory, working memory and speed of processing. In this clinical population, the pathogenicity of NMDAR antibodies also appears to relate to blood-brain barrier integrity125. Increased blood-brain barrier permeability is known to associate with Toxoplasma gondii exposure in human cohorts and NMDAR antibody seropositivity (to the NR2 subunit) in schizophrenia was associated with higher degrees of cognitive impairment where it coexisted with Toxoplasma gondii exposure125. While these findings are of great interest, no firm conclusions can be made without replication on a larger scale.Cognitive impairment is a central feature of schizophrenia with the dysfunction extending across domains of memory, attention and executive function126. While it remains unclear whether these autoantibodies have a primary pathogenic role or reflect a response to neuronal damage, there is some evidence to support a possible role in mediating cognitive symptoms. Patients with neurodegenerative disease such as Parkinson’s disease have been found to have serum NMDAR antibody frequencies in the range of controls unless there is evidence of dementia e.g., dementia with Lewy bodies or Parkinson’s disease dementia126. Although no such association was found in a more recent study of Parkinson’s disease with dementia, given the cognitive impairment in the studied group was modest (mean MMSE 25) this merits replication127.Neuronal autoantibodies have also been detected at relatively high frequencies in a number of dementia syndromes; in one study increased prevalence of NMDAR antibodies, predominantly IgA and IgM, was demonstrated across all types of dementia (16% vs 2% in controls)126. These patients frequently had subacute onset with rapid progression or fluctuation and an inflammatory CSF, often showing reversibility when treated with immunotherapy128. In a recent meta-analysis, we reported that both IgG and IgA/M serum NMDAR antibodies were more prevalent in atypical dementias vs healthy controls, while there was no difference for all-cause dementia. However, the total number of studies was small and “atypicality” was inconsistently defined and in some studies may have been done so post-hoc, necessitating caution in interpretation of this intriguing result129.The prevalence of serum NMDAR antibodies is not uniformly distributed across dementia subtypes. Disproportionately high frequencies of positive antibodies (>60%) are found in ‘unclassified’ or ‘atypical’ dementias130. In a study of 56 patients, a third of those who responded to immunotherapy, with notable improvements in all cognitive domains, had been initially diagnosed with a neurodegenerative or prion disorder131. Numerous case reports have illustrated the potential for antibodies to produce a phenocopy of established dementia syndromes, with misdiagnoses seen in cases of both NMDAR and LGI1 encephalitis mimicking atypical neurodegenerative dementias37. Given the reversible nature of autoimmune dementia, potential misdiagnoses of this nature could be catastrophic and all efforts to avoid them must be made.The term “autoimmune dementia” has been proposed to describe this subacute cognitive impairment responsive to immunotherapy and, while its prevalence is unclear, there is growing suspicion that many cases may go undiagnosed, overlooked as primary neurodegenerative dementias133. It is therefore perhaps unsurprising that in utero autoantibody exposure has been implicated neurodevelopmentally as a pathogenic factor altering the cognitive development of the foetus. CASPR2 is known to have a critical role in neurodevelopment, is highly expressed in the proliferating zones and is necessary for dendritic spine development and arborisation, integral to neural circuit assembly134. In CNTNAP2 knockout mice, which lack the predominant CASPR2 isoform, there are abnormalities in neuronal migration and reduced inhibitory GABAergic neurons causing a typical autistic phenotype with spontaneous seizures135. This phenotype is mirrored in paediatric patients with homozygous CNTNAP2 mutations who lack CASPR2136. Mice exposed to CASPR2 antibody in utero show abnormal cortical migration and development with reduced glutamatergic synapses, increased microglial activation and decreased hippocampal inhibitory neurons138. The offspring showed subsequent long-term behavioural sequelae with repetitive behaviour and impairments in sociability and flexible learning138. Prevalence of CASPR2 antibodies was markedly higher in a subgroup of mothers with autistic children (37%) than in the control groups (8–12%)137. These results were not replicated in a Danish cohort study but CASPR2 antibodies were found more frequently in the mothers of children with “mental retardation or disorders of psychological development”139. This study did not find a significant association between maternal NMDAR antibodies and child cognitive development but a murine model has demonstrated reduced density of NMDAR in neonates of mothers with NMDAR antibodies with associated neuropathological changes and greater postnatal mortality and chronic increased hyperactivity140. An association between maternal lupus, with anti-dsDNA antibodies cross-reacting with the NR2A/NR2B subunits of the NMDAR, and neurocognitive problems in the offspring has been reported, with the offspring showing deficits in behaviour, memory and learning141. dsDNA-specific NMDAR antibodies injected into pregnant dams caused thinned, disorganised cortex in offspring with subsequent cognitive impairments142. While there is no evidence of overlap with the NR1 NMDAR antibodies seen in encephalitis it illustrates the potential for these antibodies to also exert an effect on neurodevelopment143.Placental transfer of IgG antibodies during gestation is a well-established phenomenon, with these antibodies also having the potential to penetrate the foetal blood-brain barrier during specific developmental windowsEach neuronal antibody exerts a distinct mechanistic effect and while the downstream effects all include cognitive dysfunction, the affected domains vary between subtype. However, there is much work to be done in fully characterising both the acute and chronic impairments of the encephalitic syndromes; current descriptions tend to be mostly qualitative and for the less common subtypes data is sparse.144. It is not at all clear that the same process is occurring in the non-encephalitic situations described in this review. Indeed, blood-brain barrier disruption might be an important factor in some of the situations described above precisely because there is no intrathecal production of pathogenic antibodies. Furthermore, the description of unmutated yet functional  NMDAR antibodies raises the possibility that, outside of the encephalitis context, the clinical relevance and/or pathogenicity of neuronal autoantibodies might in fact arise from the so-called ‘healthy’ naïve B cell repertoire145. Differences in epitope specificity, antibody titres and duration of interaction and initial immunising stimulus may also all have relevance in distinguishing encephalitis cases from non-encephalitis cases where the antibodies nonetheless may have some pathogenic role. An alternative, and underexplored, perspective is that neuronal autoantibodies  could have an adaptive physiological role; recent animal (and to some extent human) work suggests that NMDAR antibodies could be produced in response to stress as a mechanism to reduce anxiety/depressive behaviours, possibly via NMDAR antagonism146. It is conceivable that there is an analogous role in preventing, for example, excitotoxicity-mediated neuronal damage and cognitive impairment in some circumstances, although this remains to be explored.Outside of encephalitis, pathogenicity of the neuronal antibodies may be contingent on factors including the integrity of the blood-brain barrier. Where this is compromised there is often evidence of secondary cognitive dysfunction; we postulate that blood-brain barrier disruption modulates much of the heterogeneity seen in the impact of neuronal antibodies outside of encephalitis. The site of autoantibody production may be equally important. In autoimmune encephalitis, it is likely that ongoing peripheral germinal centre reactions generate antigen-specific B cells. Subsequently, antigen-secreting cells that have differentiated from these B cells likely access the CNS, resulting in intrathecal production of pathogenic IgG131. The possibility that specific immunotherapies could have a role in treating cancer-associated cognitive impairment, or as a treatment to prevent the progression of cognitive impairment in neuronal antibody-positive post-HSV encephalitis patients, for example, warrants further evaluation.Crucially, there is very little evidence currently for or against the possibility that immunotherapy could be an effective treatment in any of these non-encephalitis situations; the main notable exceptions are in cases of atypical or “autoimmune dementia”—and in some of these cases the demarcation from autoimmune encephalitis is far from clear94, the clinical consequence may vary with isotype. It is well established that the IgG isotype can cause NMDAR encephalitis while IgA NMDAR antibodies have been implicated in a more insidious cognitive impairment128. Indeed, the frequencies of NMDAR antibodies detected in dementia, cancer and stroke are more than twofold greater for IgA or IgM isotypes than IgG113. We suggest that these isotypes may be relevant for understanding cognitive impairment outside of encephalitis. While the potential exists for all the neuronal antibodies described to impact cognition, further work is needed to characterise this. In the future this could pave the way for novel, immunologically-based therapeutic options to treat cognitive impairment, with potentially transformative implications.Non-IgG NMDAR antibodies, in particular, have been implicated in the cognitive impairment seen in a multitude of disorders including cancer, dementia and schizophrenia. While NMDAR antibodies have been shown to universally have pathogenic potential


Dopamine, one of the main neurotransmitters in the mammalian brain, has been implicated in the coding of prediction errors that govern reward learning as well as fear extinction learning. Psychotherapy too can be viewed as a form of error-based learning, because it challenges erroneous beliefs and behavioral patterns in order to induce long-term changes in emotions, cognitions, and behaviors. Exposure therapy, for example, relies in part on fear extinction principles to violate erroneous expectancies of danger and induce novel safety learning that inhibits and therefore reduces fear in the long term. As most forms of psychotherapy, however, exposure therapy suffers from non-response, dropout, and relapse. This narrative review focuses on the role of midbrain and prefrontal dopamine in novel safety learning and investigates possible pathways through which dopamine-based interventions could be used as an adjunct to improve both the response and the long-term effects of the therapy. Convincing evidence exists for an involvement of the midbrain dopamine system in the acquisition of new, safe memories. Additionally, prefrontal dopamine is emerging as a key ingredient for the consolidation of fear extinction. We propose that applying a dopamine prediction error perspective to psychotherapy can inspire both pharmacological and non-pharmacological studies aimed at discovering innovative ways to enhance the acquisition of safety memories. Additionally, we call for further empirical investigations on dopamine-oriented drugs that might be able to maximize consolidation of successful fear extinction and its long-term retention after therapy, and we propose to also include investigations on non-pharmacological interventions with putative prefrontal dopaminergic effects, like working memory training. Prediction errors occur when there is a mismatch between the expected state and the actual state of the world: they serve as a signal that current expectations are inaccurate and should be updated. Formalized theories of learning specify how prediction errors govern the updating of expectations in a changing environment, and decades of experimental research have established a major role for dopamine signaling in this error-based learning process and in keeping updated expectations and other mental representations stable over time3. Thus, dopamine is emerging as a central neuromodulator in the flexible guidance of adaptive behavior.It is becoming increasingly clear that prediction errors have a central role in the shaping of our actions and expectations6. In this review, we propose that the violation of dysfunctional expectations in psychotherapy shares important similarities with dopamine-based prediction errors. This approach lays the ground for a mechanistic understanding of psychotherapy in terms of formal learning theory and cognitive neuroscience. In addition, because dopamine is also involved in keeping updated expectations stable over time, we propose that its role may expand to maintaining treatment gains over the long-term and decreasing the risk of relapse.Psychopathology, on the other hand, is typically characterized by maladaptive behaviors that are inflexible and resistant to change. Recent theories in this domain propose that dysfunctional expectations are at the heart of maladaptive behaviors and it is argued that effective psychotherapies work by violating and updating such dysfunctional expectationsWe start this review by discussing the role of dysfunctional expectations in psychopathology and the importance of expectancy violation for psychotherapeutic change. We then elaborate on the proposed link between expectancy violations and prediction errors, and we review the evidence for an involvement of mesolimbic dopamine signaling. As a case in point, we emphasize recent work on the involvement of dopamine-based prediction errors in the acquisition of fear extinction as a model of exposure-based psychotherapy for pathological anxiety based on safety learning. We furthermore report evidence that hippocampal and prefrontal dopamine is important for the consolidation of fear extinction memories. Finally, we integrate several lines of research on meso-cortical dopamine signaling, fear extinction, and working memory, and we propose that non-pharmacological interventions such as working memory training should be considered in future empirical research as a way to modulate dopamine levels and contribute to long-term gains of psychotherapy.8 and motivate disabling avoidance behaviors9. For example, following an embarrassing moment in a group conversation, a socially anxious individual may develop an exaggerated expectation that conversations lead to embarrassment, and therefore avoid speaking up during group conversations. Depressed patients, on the other hand, have pessimistic expectations about their self and their future5. Experiences of failures lead to the exaggerated expectation that anything they do will become a failure. Accordingly, dysfunctional expectations may drive many psychopathological symptoms and are a prime target for psychotherapies.Learning to predict when important events will occur is crucial for survival. Accurate expectations about rewards guide appropriate approach behaviors to collect and consume recompenses, while accurate expectations about threat guide appropriate avoidance behaviors to prevent dangerous encounters. Many of these expectations are triggered by cues or actions that reliably preceded important outcomes in the past. Arguably, these past experiences laid down in memory an association between the cue/action and the outcome, so that future occurrences of the cue/action trigger the expectation of the outcome. The challenge for adaptive learning is to arrive at accurate expectation values from only a limited set of contingency experiences. Sometimes, this expectation learning process goes awry. In anxiety patients, erroneous expectations of danger trigger excessive fear levels11. Such effect relies on disconfirmation processes, where the expectation toward a specific stimulus is violated by the surprising absence of the anticipated threat12. Because exposure therapy is so explicitly focused on violating expectancies, much of the research on dysfunctional expectations has been done in the context of fear and anxiety.In psychopathology, dysfunctional expectations tend to persist. Consequently, this tendency needs to be challenged by experiences that ostensibly violate erroneous expectations. Psychotherapies provide such experiences. Particularly, exposure-based therapies directly target the exact situation/object of the dysfunctional expectation. In exposure therapy, anticipatory anxiety is triggered by the guided exposure of the patient to a threatening stimulus/situation. By means of exposures, current expectations are challenged: a threat signal loses its predictive value and the behavioral response toward the feared stimulus decreases13. However, other work has shown that the amount of fear reduction obtained by the end of extinction training or exposure treatment is not predictive of fear levels at follow-up15. A clinical improvement in contamination fear after exposure treatment, for instance, is not predicted by the degree of behavioral or physiological fear at the end of such treatment14.The positive effects of exposure-based therapies on fear levels can readily be modeled experimentally using extinction learning paradigms. Within these paradigms, first a conditioned stimulus (CS) is repeatedly presented in association with an aversive unconditioned stimulus (US). These contingencies will promote the acquisition of a CS–US association, such that the presentation of the CS alone becomes sufficient to elicit a conditioned (fearful) response (CR). Next, this fear can be extinguished by the repeated presentation of the same CS in the absence of the US. Historically, these procedures have been used to investigate the potential processes underpinning fear reduction. In particular, fear reduction during repeated exposures to the CS  has long been explained by reference to habituation processes. Habituation models consider fear reduction as an essential precursor of long-term therapeutic benefit. In line with this notion, compared to animals that extinguish fear quickly, slow extinguishers are more vulnerable to relapse16, and its long-term retention17. This learning process is elicited by novel experiences of the previously conditioned stimulus and the (unexpected) absence of the feared outcome . These experiences allow for the formation of a new safety memory regarding the CS, so that the CS now signals the absence of the aversive US (CS → noUS). This new memory representation will henceforth compete with the original threat memory (CS → US) and inhibit fear responding18. Likewise, exposure treatment is now thought to lay down a novel safety memory that associates a feared situation with the absence of danger. Accordingly, exposure therapy outcomes seem to benefit more from strategies that maximize safety learning processes than from those that promote habituation to threat20.More recently, novel safety learning emerged as an additional process underpinning fear extinction22. Accordingly, within exposure-based therapies, only about 49.5% of anxious patients show an improvement at the end of treatment23. These data suggest that some patients might have difficulties acquiring the information conveyed by the therapeutic experience that the feared stimulus or situation is actually ‘safe’ . Arguably, this lack of extinction learning may reflect a deficit in the prediction-error driven learning process that is critical for the acquisition of safety.Studies on fear extinction highlight that many anxiety patients or anxious subjects show delayed extinction learning24. Fear extinction memories have to be stored in long-term memory (consolidation) and activated when needed ; a failure in either of these processes may lead to a return of fear. But, even in healthy individuals, a return of fear can be observed under certain test conditions26. A return of extinguished fear responding can occur when the time has passed since the end of extinction training27; when the extinction context changes to a new or back to the conditioning context25; when an unexpected and unsignalled US is presented after extinction (reinstatement)26.Studies on fear extinction have also indicated that the ability to retrieve fear extinction memories in anxious patients is impaired28. However, in clinical anxiety such characteristic might be exacerbated. Accordingly, many patients show a relapse in symptomatology after therapy, which has been attributed to difficulties in ‘retrieving’ extinction in addition to ‘acquiring’ extinction11.Arguably, this characteristic of fear extinction memories has evolutionary benefits, as it makes an individual be cautious rather than rash, in line with a ‘better-safe-than-sorry’ strategyClearly, delayed acquisition and impaired retrieval of fear extinction memories point to a necessity to develop new strategies to induce effective and more durable behavioral change. In this review, we address this challenge by considering the similarities between expectancy violation processes and prediction error signaling, in order to merge recent trends in these clinical and fundamental fields of research and to inspire the development of novel adjuncts to psychotherapy.29, and reinforcement learning theories1 highlight how the ‘surprising’ aspect of these violations represents the key ingredient of a learning process. The level of surprise, or the prediction error (PE) signal, has been mathematically formalized as the mismatch between experienced and expected outcome. The RW model describes the result of such a mismatch as a change in the associative strength between two stimuli (CS → US), while reinforcement learning theories extend this notion to ‘action values’ that denote the associative strength between an action and its outcome (action → outcome). Both explain how mismatches induce new learning, lead to change behaviors, and to the updating of expectations. In the present manuscript, we focus on the stimulus → outcome association, applying the basic RW model to fear extinction learning.Formal learning theories specify how violations of prior expectations lead to updating of expectation values and induce behavior change. Pavlovian learning theories, such as the Rescorla–Wagner (RW) modelV = α(λ − VΣ)29. In this formula, ∆V specifies the change in associative strength on a particular learning episode or trial; λ refers to the maximum magnitude of the US; VΣ is the sum of the associative strengths of all the CSs present on that particular trial, and α is a learning-rate parameter . In the specific case of fear extinction, an expected US is suddenly omitted after the presentation of the CS, which triggers a negative PE that counters the previously learned CS → US association. The mismatch between actual absence and expected delivery of the US is captured in the Rescorla–Wagner model by subtracting the CS → US associative strength from a zero value . This negatively signed PE serves to decreases the strength of the CS → US association in the original RW model; however, later developments in fear extinction learning have yielded a reformulation of the model in which the unexpected absence of the US triggers a novel memory representation of ‘noUS’ and a corresponding positive (reward) PE signal. This positive PE governs the development of a separate, CS → noUS association that competes with the CS → US association for behavioral control30. This adjustment to the original RW model allows explaining how fear of a CS can return after extinction, because retrieval of the CS → US association is suppressed but the strength of the association as such remains unchanged. Within the context of exposure-based therapy, repeated exposure to a feared situation in the absence of expected harm may generate a positive PE (expectancy violation) that drives new safety learning and counters the erroneous expectation that underlies dysfunctional fear and avoidance behaviors. Thus, the success of safety learning depends on the level of expectancy violation and positively signed PE achieved during the exposure intervention. It is for this reason that expectancy violation strategies aim to keep threat expectancy levels high during each exposure trial, which is radically different from older habituation strategies that strive to reduce fear and threat expectancy during exposure20.The RW model is classically used to explain the strengthening and weakening of stimulus → outcome associations, which is particularly relevant within the context of fear reduction. The RW equation is expressed by the formula: ∆31. Especially the mesolimbic dopaminergic pathway shapes learning processes by coding PE signals. Extensive research on reward learning highlights dopamine as a neurotransmitter carrying information related to expectations1 and to the outcome value of rewards32. Numerous studies have indicated that rewarding stimuli, like food delivery, trigger a phasic burst of activity in dopaminergic neurons within the ventral tegmental area (VTA) and the subsequent release of dopamine in the nucleus accumbens (NAcc). Critically, the degree of this dopaminergic activity corresponds to the magnitude of the mismatch between expected and received reward, and to other properties of reward processing 1,35. Conversely, a decrease in PE  goes along with a decrease in phasic mesolimbic dopamine response36. Here we focus mostly on the role of phasic dopaminergic signaling in encoding the magnitude of the PE, because it is this property that reflects the level of ‘surprise’ associated with specific unexpected events, such as disconfirmations in psychotherapy.Error-based learning is strongly modulated by specific neurotransmitters in our nervous system, in particular dopamine38. Given that many forms of psychotherapy rely on violating erroneous expectations of negative outcomes, these results suggest that dopamine could also play a role in the learning processes that mediate behavioral change in psychotherapy. Therefore, we review in detail the animal and human studies that have implicated dopamine in fear extinction learning and we investigate its specific role in PE signaling.Critically, recent studies have shown that mesolimbic DA is not only involved in coding unexpected rewards but also unexpected omissions of punishment which is directly relevant for fear extinction learning39. Accordingly, other studies showed that, similarly to positive rewards40, increased release of mesolimbic dopamine has been shown during instrumental avoidance learning41 and when a punishment (pain) terminates42. A recent study pinned down the critical role of dopamine by demonstrating that dopaminergic neurons in VTA detect the omission of an expected unpleasant US. Optogenetic inhibition of DA neurons at the time of US omission disrupted extinction, demonstrating that especially VTA neurons projecting to the NAcc shell are necessary for the acquisition of fear extinction38. This finding is in line with a parallel study that demonstrated how firing of DA neurons during early omission in VTA is both necessary and sufficient for fear extinction learning: whereas optogenetic inhibition of VTA DA neurons at the time of US omission prevents fear extinction acquisition, activation of the same neurons accelerates it37. This is in line with the idea that unexpected omissions of aversive events are rewarding and coded as a positive PE by dopaminergic neurons, which then produces new learning about when omission of the US can be expected . Taken together, these results provide important insights into the role of mesolimbic dopamine in the generation of new safety memories43. Obviously, this evidence carries important clinical implications for therapies based on expectancy violation procedures, as it suggests that pharmacological manipulations of phasic dopamine levels might enhance the acquisition of fear extinction.Studies in rodents have indicated a time-dependent effect of DA on the acquisition of fear extinction learning. In-vivo microdialysis shows a basal increase in DA and noradrenaline in mPFC during the first phase of extinction training44. In rodents, a systemic increase in extracellular DA-levels through the administration of methylphenidate hydrochloride (MPH), a DA transporter blocker , can promote fear reduction during extinction sessions (indicative of enhanced acquisition of a novel safety memory) when administered before extinction training, and increase extinction retention when administered either before or after the training45. Similarly, pre-extinction hippocampal CA1 infusion of MPH in rats that otherwise do not show fear extinction, boosts fear reduction during extinction and enhances its retention through β-adrenergic and D1 receptors46. The post-extinction administration of MPH in CA1 does not enhance extinction retention, suggesting that MPH modulates acquisition rather than consolidation of novel safety memories46.A multitude of animal studies indicates that the consolidation of fear extinction is mediated by DA-levels in the amygdala (AMY) and its intercalated neurons, the medial prefrontal cortex (mPFC), and the hippocampusl-DOPA. l-DOPA is an indirect dopamine precursor that, different than dopamine itself, can cross the blood–brain barrier; in the brain, l-DOPA is converted into dopamine by the enzyme aromatic l-amino acid decarboxylase47. It has been shown that in animals, the post-extinction administration of l-DOPA reduces the return of fear and promotes elevated vmPFC and reduced AMY activity during a delayed test48. This is in line with a recent animal study showing that temporarily inhibiting downstream IL-BLA projections during the acquisition of fear extinction impairs extinction memory retrieval40. In mice expressing extinction deficits , systemic injection of L-DOPA improves the acquisition of fear extinction (if administered before extinction training) and its consolidation (if administered after extinction training)49. Additionally, once fear extinction is acquired, dopamine-related activity in mPFC (and not in NAcc) underlies its long-term retention50. Accordingly, rats with a lesion at the level of the vmPFC have been shown to be unable to express fear extinction during a subsequent test , while increased levels of bursting of IL neurons were observed to be correlated with extinction recall in extinguished rats51.Several studies also investigated the effects of other DA enhancers on fear extinction learning, such as 52. Although such results have to be taken with caution (see Box 1), it seems that increasing tonic dopamine levels, especially in prefrontal regions of the brain, could potentially facilitate the acquisition and consolidation of extinction, also in the presence of a fear extinction deficit. Crucially, however, it is still unknown if DA-based interventions might produce different effects when the baseline DA is higher. Consequently, the individual profile in basal tonic DA must be considered if pharmacological dopaminergic interventions can be efficacy used to increase fear extinction in the context of anxiety. Finally, it remains presently unclear whether a pre-retrieval pharmacological manipulation of dopamine levels might modulate the capacity to retrieve fear extinction.Of note, studies in stressed rodents (which are often used to simulate anxiety disorders) have shown that fear extinction retrieval deficits following stress are associated with the presence of low extracellular dopamine levels in fear-related circuits53, and D2 and especially D1 receptor genes are highly expressed in PFC54 and hippocampus55. Several studies investigated the contribution of these DA receptors in fear extinction.D1 and D2 G-protein-coupled receptors  govern a large number of DA-dependent learning processes via long-term potentiation and depotentiation and therefore, via neuroplasticity. Their density varies along the dopamine system: mRNA encoding D2R (conveying genetic information from DNA to the ribosome) is highly present in VTA56. Additionally, pre-extinction injection of a D2R antagonist (raclopride) in IL impairs later retrieval of fear extinction in rodents without affecting its acquisition57. A similar study in rodents, however, highlights how effects of pharmacological manipulations of DA receptors in IL are age-dependent, with quinpirole effects on long-term fear extinction present only during youth58. Similarly, pre-extinction blocking of D2R decreases the positive effect of glucocorticoids on fear memory extinction59 although in another study it facilitates extinction 24 h after conditioning60.Animal findings show that pre-extinction injection of D2R antagonists (sulpiride) in the basolateral amygdala (BLA) delay fear extinction during training and its long-term retention, while pre-extinction infusion of D2R agonists (quinpirole) in the same brain area increases the acquisition of fear extinction and its long-term retention61. Hence, it seems that (still unspecified) changes in D2R-related DA activity interact with the acquisition of fear extinction and possibly with its consolidation and retrieval; yet, such changes might exert opposite effects depending on whether pharmacological manipulations are administrated locally  or systemically. Furthermore, especially from a translational view, it is important to bear in mind the possibility that agonistic and antagonistic effects on D2R (or other DA receptors) might have different consequences on the consolidation and retrieval of fear extinction when administered pre- versus post-extinction learning.More evidence for an involvement of D2R comes from a study that shows that extinction training in mice decreases D2R mRNA in the IL62 and a pre-training blockade (antagonist SCH23390) of D1R in BLA reduces the acquisition of fear extinction but not its consolidation63. On the other hand, fear extinction consolidation decreases when the pre- or post-administration of the same antagonist is applied in IL63. A previous study also found that activating D1 receptors in the dorsal striatum and the substantia nigra during fear extinction enhances exclusively its consolidation64. Given that D1R in the hippocampus is also involved in the formation of long-term fear memory (LTM)65 and in contextual fear conditioning67, recent studies have investigated the role of D1R in the context of fear extinction and its consolidation. Results from these studies show that facilitation of fear extinction (by novelty) is mediated by D1R dopamine-dependent hippocampal activity68, and that pre-extinction blockade of these receptors reverses the positive effects that pre-extinction infusion of MPH exerts on contextual fear extinction learning and (possibly) its retention46. The role of hippocampal D1R in consolidating fear extinction memories is further supported by the fact that long-term memory (LTM) of fearful experiences depends on activation of VTA/CA1 hippocampus dopaminergic connections, mainly involving D1R and mediated by brain-derived neurotrophic factor69 , methylphenidate, or sulpiride (for an overview see ref. 76). These dopaminergic manipulations could be then potentially used during or after therapy to enhance the long-term effects of exposures exercises. On the other hand, it remains still unknown if dopaminergic manipulations via systemic administrations can interfere with DA transmission in other brain regions, causing alterations (side effects) in other DA-dependent cognitive functions.In summary, the specific roles of D1R, D2R, and D3R in fear extinction remain to be further clarified (Box 1). However, in rodents, mesolimbic and prefrontal DA levels modulate the acquisition and consolidation of fear extinction learning; on the one hand, this evidence supports the great utility in studying the effects of dopamine-based pharmacological interventions to boost fear extinction, such as Box 1 Fear extinction and dopamine: current challenges from animal research.53;the brain area(s) under investigation (DA receptors might play a different role depending on their location in the brain)the age of the sample under analysis ;the still unknown actions exerted by D1R, D2R, and D3R in different parts of the brain;the individual basic level of DA;the different density and localization of D1R, D2R, and D3R receptors across the brain;time-dependent effects of the pharmacological manipulation ;the confounding effects on other neurobiological systems .The effects of DA-receptor manipulation on the acquisition and consolidation of fear extinction might vary in relation to:119 and the motivation to move120, in particular when injections are systemic and therefore also reach dopamine-related motor circuits . Although recent studies have attempted to those confounds into account , this still represents a potential issue in interpreting and translating animal findings and in interpreting human findings.This last point represents an important issue given that DA interacts with a variety of other neurotransmitters. As a clear example, the progress of fear extinction is typically evaluated through behavioral changes in animals, often involving movement . However, induced changes in dopamine levels strongly affect the locomotor systemα in the RW equation) and higher hemodynamic responses to US omissions in the ventral striatum than non-9R carriers77. This result adds credit to the application of the dopaminergic theory of PE1 to the acquisition of fear extinction in humans. Other findings indicate that subjects carrying two met alleles of the gene codifying the enzyme catechol-O-methyltransferase  and therefore displaying a higher extracellular DA profile especially in pre-frontal regions, also fail to extinguish fear78. Theoretical models of the COMT met allele describe this condition as involving a reduction in phasic DA in sub-cortical regions , coupled with a higher tonic extracellular prefrontal DA level and an increased D1 cortical activity 79. Consequently, on the basis of the available findings on fear extinction and theoretical DA models regarding COMT and DAT polymorphisms, we suggest that reduced striatal DA activity might impair the acquisition of fear extinction in humans. This suggestion, however, remains speculative and rests merely on the complex DA profile associated with these polymorphisms.Few studies have investigated how DA signaling mediates fear extinction learning in humans. In line with the animal findings, a functional polymorphism in the DA transporter (DAT) gene, , affects the acquisition of fear extinction, with DAT1 9R carriers showing a higher extinction learning rate . Specifically, optimal D1R stimulation in PFC networks is thought to facilitate cognitive stability by maintaining information ‘online’ and protecting this information against interfering experiences82. Conversely, val carriers have been suggested to display better performance in tasks requiring cognitive flexibility , increased D2-mediated phasic DA transmission, and decreased D1-mediated cortical DA transmission79. Given that fear extinction learning first requires the processing of expectancy violation  for its acquisition and subsequently requires cognitive stability for its consolidation and retrieval, the COMTval158met polymorphism might hold promise for future research on DA-based mechanisms of fear extinction learning. As an example, it would be interesting to investigate whether COMT val or val/val carriers may have an intact capacity to acquire extinction but a reduced capacity to consolidate and retrieve fear extinction.Complementary to reduced striatal DA transmission, a met allele advantage for tasks requiring cognitive stability  has sometimes been reportedl-DOPA decreases the later return of fear85. Furthermore, the degree of spontaneous replay of activation patterns observed during US-omission in vmPFC predicts extinction memory retrieval, an effect that is enhanced by the post-extinction administration of l-DOPA86. Of note, compared to a control group, elevated vmPFC neural activity (but not fear reduction in skin conductance) during a return-of-fear test was found 1 week after post-extinction l-DOPA administration87, indicating that l-DOPA has long-term effects on the activity of brain areas involved in fear extinction retention. Importantly, post-extinction l-DOPA administration successfully reduces fear levels during a later retrieval test only if extinction is effective 85.The evidence for an involvement of prefrontal dopamine in the consolidation of fear extinction is more straightforward. Like in animals, the post-extinction administration of To summarize, further studies should investigate whether the modulation of phasic DA levels can influence the acquisition of fear extinction also in humans. Meanwhile, dopamine-based interventions do clearly emerge as potential adjuncts for long-term gains after successful psychotherapy. To sharpen the scientific knowledge that could support the application of DA-enhancers in exposure treatment, future pharmacological studies on fear extinction should additionally investigate whether the acquisition of fear extinction and/or its consolidation is impaired in the case of aberrant basal dopaminergic activity in PFC, as might be present in psychiatric disorders (Box 2).85.The studies described above carry implications for psychotherapies that use expectancy violation techniques to change maladaptive behaviors. Most of the experiments described indeed mimic the dynamic of a classical exposure exercise. Given the central role of mesolimbic dopamine signaling in processing PEs and updating expectations, and given that prefrontal dopamine seems to be linked to the successful consolidation of fear extinction memories, we propose that expectancy violation techniques in psychotherapy might benefit from including DA-based interventions in three different moments: during the acquisition of new safe memories (at the moment of PE), during the subsequent consolidation, and at the time of intended retrieval of those memories17. Additionally, within the animal research, emerging results seem to indicate that diet manipulations  might also serve to increase phasic dopaminergic outcome in reward-learning areas, such as VTA and NAcc91. For example, food restriction has been shown to increase mRNA levels of tyrosine hydroxylase (an enzyme involved in the synthesis of DA) and DA transporter in VTA of male rats90, suggesting that food restriction might sensitize the mesolimbic system.With regard to the acquisition, such interventions could take the form of the administrations of drugs that modulate phasic dopamine at the moment of scheduled expectancy violation. In exposure treatment, this can be accomplished by guiding a patient through a feared situation in the absence of the expected aversive event. From the basic research described above, we expect that targeted pharmacological interventions may lead to stronger acquisition of the new safety experiences that can then more strongly counter the existing fear associations. Such interventions fit with an inhibitory model of fear extinction, according to which fear reduction from exposure treatment is mainly obtained through novel safety learning. However, it remains unknown which agonist and/or antagonist would exclusively target phasic dopamine in the VTA/ventral striatum during exposure. To date, pharmacological manipulations of dopamine levels in humans influence both phasic and tonic dopamine signaling, making it impossible to separately optimize striatal and prefrontal dopaminergic fear extinction processes involved in acquisition and consolidation, respectively. On the basis of the current knowledge base on dopaminergic signaling, therefore, we here emphasize behavioral options for keeping US-expectancy levels high during exposure, in order to maximize surprise (unexpected US omissions) and enhance the phasic release of DA Fig. . Inducinl-DOPA administration after therapy, effectively reduce the return of fear in healthy individuals and are promising for clinical use. However, the effects of l-DOPA might occur only if substantial fear extinction has been achieved during the session85. Also, clinical trials testing the effects of l-DOPA in the presence of pre-existing alterations in PFC dopamine activity are urgently needed. This is an important step to bridge the current evidence in healthy humans and future clinical application of DA-enhancers in patients, given that in clinical conditions DA levels may not correspond to a functional DA profile (Box 2). In those circumstances, l-DOPA administration might even affect treatment outcomes negatively. Furthermore, studies should investigate the effect of l-DOPA specifically on the ability to retrieve fear extinction memories and to counteract the retrieval of prior threat expectations.With regard to the consolidation and later retrieval of safe memories, dopamine-based pharmacological interventions, such as Box 2 Dopamine and fear extinction learning in anxiety: a problem in the acquisition or in the consolidation of fear extinction?125. A recent review of these molecular neuroimaging studies points to the presence of an alteration in striatal DA functioning in anxious patients, although the findings are not always consistent across studies126. In light of the role that striatal DA has in the acquisition of fear extinction in animals, we suggest that altered striatal DA functioning in anxious patients may be associated with a potential decrease in their ability to learn from errors . Future studies in clinical (anxious) populations should therefore examine whether the presence of striatal DA alterations is associated with difficulties in the acquisition of fear extinction . In this respect, results from a recent fMRI study involving individuals with a diagnosis of specific phobia showed how high vmPFC activation during US-omissions (together with a trend found in the NAcc during the same conditions) was predictive of a reduction in clinical symptoms after exposure therapy127. These results indicate that, in the presence of clinical anxiety, higher prediction-error-related signaling  is associated with better therapeutic outcome. Additionally, it has been recently shown that a reduction in anxiety-related symptoms after cognitive behavioral therapy (CBT) was negatively associated with increased D2R receptor binding in the mPFC and hippocampus of individuals with social anxiety128, and elevated D2R receptor availability was found in the OFC and dlPFC of patients with the same diagnosis129. These results indicate the presence of a prefrontal DA alteration in anxious individuals. On the basis of the evidence that DA transmission in prefrontal regions of the brain is crucial for the consolidation of fear extinction, we suggest that future studies on clinical anxiety should also investigate whether the presence of aberrant prefrontal DA activity is associated with the impaired ability to consolidate and retrieve fear extinction memories and alterations in WM capacity.Positron emission tomography (PET) and single photon emission computerized tomography (SPECT) studies investigated the role of DA in clinical anxiety, often in patients with symptoms of social anxiety92 for other important brain functions: reinforcement learning, motivation, executive functions, motor control, arousal, and reward, just to name a few. Consequently, developing behavioral strategies to optimize prefrontal dopamine modulation during fear extinction could provide safer and more specific advantages as adjuncts to psychotherapy93.Although prefrontal dopaminergic manipulations after successful fear extinction procedures are emerging as a promising adjunction to maintain the long-term outcomes of exposure, these systemic manipulations do not guarantee specificity in their effects. This is particularly true in light of the fact that dopamine acts as a neuromodulator96, relates to dopaminergic activity in PFC97. Some evidence that WM training increases activity in prefrontal regions of the brain98 and cortical DA already exists (see below)99. Especially, the lPFC is a brain region rich in DA projections and described by influential neural models of cognition as heavily involved in attention and working memory capacity101. Additionally, human theories of fear emotion regulation suggest that the lateral PFC could enhance the inhibitory effects that the vmPFC exerts on fear levels during extinction 102. Since the presence of an elevated functional connectivity between lPFC and vmPFC, lPFC has been recently used as a target in transcranial magnetic stimulation (TMS) during extinction learning. The results from this TMS-study showed an enhancement of fear extinction recall 1 day after the intervention103. Based on these emerging lines of evidence, we propose that behavioral strategies that enhance WM capacity could serve to optimize dopamine-related activity in lateral PFC and, consequently, improve long-term fear extinction retrieval.Positron emission tomography studies indicate that working memory (WM), the capacity to retrieve and keep goal-relevant information online and to use it to guide adaptive behavior104; second, subjects high in anxiety show poor safety learning and concomitant low memory capacity105; third, a tendency in anxious individuals to misallocate WM resources to threatening distractors has been linked to enhanced reactivity of amygdala nuclei106; finally, pathological anxiety has been linked to meso-corticolimbic DA alterations107. In the next paragraphs, on the base of recent influential dopamine-based models of cognition, we develop a theoretical framework that can set the stage for future studies to elucidate the potential role of WM in maintaining long-term gains of exposure therapies.To date, no study has investigated a direct dopaminergic link between WM capacity and the ability to retrieve safe memories. Nevertheless, such dopaminergic link is suggested by indirect evidence. First, WM capacity is positively associated to a higher fear inhibition109. Indeed, D1-receptor antagonists cause impaired performance during delayed response tasks that measure the ability to keep goal-relevant information online109. Moreover, as within the context of impaired fear extinction retrieval, this impairment can be reversed by l-DOPA administration70. Additionally, positron emission tomography studies show that striatal110 and prefrontal dopaminergic functions111 are related to and predicted by the individual WM profile. Consequently, WM capacity seems to reflects this complex (midbrain–PFC) oppositional dopaminergic dual system.A hypothetical link between working memory, meso-corticolimbic DA, and individual ability in retrieving fear extinction might be re-framed within recent theoretical models of dopamine-action on other cognitive domains. For these models, the midbrain–PFC system seems to be involved in maintaining an equilibrium between ‘updating’ representations in working memory (via PE-related midbrain phasic dopamine and D2R) and keeping such PFC-representations ‘stable’ in the WM buffer  despite distractions3. Additionally, prefrontal and midbrain dopamine interacts with tonic hippocampal DA (mainly mediated by D1R) for the generation of long-term episodic memory. Consequently, it is not surprising that WM also promotes the formation of long-term episodic memories113 via lateral PFC and hippocampus activations113. However, although dopamine seems to ‘tune’ learning across different brain areas (from updating till long-term storage of information), the specific steps of such learning mechanism remain uncertain, especially within the context of fear extinction. Crucially, future studies aiming to improve exposure-based therapies, should investigate whether potential positive effects of WM training on prefrontal dopamine and fear extinction consolidation exist.Given this link, working memory capacity is also emerging as a potentially useful proxy for DA functioning—measuring prefrontal DA levels directly is very challenging99. Interestingly, training of WM and other basic cognitive processes of executive functions has already been adopted successfully to increase response inhibition in obesity, resulting in increased retention of weight loss after a cognitive behavioral therapy-based weight loss program115. Given that extinction retrieval, like any other form of episodic memory retrieval, is mediated by working memory activity, WM training could be used to enhance the ability to recall and maintain ‘online’ the extinction memory for the time that is needed to positively influence decision-making.It remains unknown whether non-pharmacological interventions aimed at strengthening WM might also help to improve the ability to retrieve safe memories when needed. This ability is crucial, since it might reduce negative expectations and rigid behaviors like excessive avoidance, and hence favor long-term therapeutic gains. Within an exposure therapy context, it is noteworthy that some evidence suggests that WM capacity is trainable, with such training (35 min daily for 5 weeks) yielding changes in cortical DR1, as measured via PET before and after training116. Therefore, WM training procedures may be most successful if they specifically target processes that are relevant for and experiential contents that the patient (successfully) acquired during the exposure sessions. Relevant procedures for this may include tasks that require frequent memory updating, affective procedures to enhance retrieval ability, rehearsal exercises, and others117. Finally, we argue that WM training might improve fear extinction retrieval via prefrontal dopamine modulation. WM training may generate safer effects compared to post-extinction pharmacological DA manipulations (such as l-DOPA85). Additionally, by interfering with prefrontal DA-activity in mPFC and lPFC, WM training may generate more specific effects than other non-pharmacological interventions, such as physical activity ; the latter seems indeed to primarily increase DA-related activity in the dorsal striatum and basal ganglia118, although it remains unclear whether more skilled and motor learning-based activities  might actually induce changes also in prefrontal DA transmission.Critically, performance gains in tasks involving short-term or WM components following WM training seem mostly restricted to ‘near-transfer effects’Many forms of psychotherapy involve expectancy violation to induce new learning and behavioral change. Formal learning theory conceptualizes expectancy violation as prediction errors, and empirical studies have linked prediction error-based learning convincingly to mesolimbic dopaminergic signaling. Strategies that maximize dopamine-mediated prediction error signaling might therefore enhance the encoding of new learning experiences in psychotherapy, to change maladaptive behaviors. We propose that to facilitate a patient’s retrieval of beneficial memories laid down in psychotherapy, the effects of dopamine-related interventions (including working memory training) after a successful therapy should be investigated in future clinical trials.