The time course of pattern discrimination in the human brain.
In electrophysiological experiments on visual pattern discrimination, decision difficulty was manipulated either via the physical characteristics of the test stimuli, or by changing the instruction given to the observer. Visual stimuli were rectangular matrices each composed of 100 Gabor patches having different orientations. Matrices differed in the number of Gabor patches with vertical, or horizontal, orientation. The observers' task was either to discriminate the dominant orientation or to detect collinear elements in the matrix. Relating task difficulty to performance, in the first experimental paradigm (detection of orientation) we obtained the conventional S-like psychometric function but in the second (detection of collinearity) the psychometric function showed a complicated U-curve. Matching between electrophysiological and psychophysical data and image statistical functions allowed us to establish the relative timing of the cortical processes underlying perception and decision making in relation to textural features. In the first 170ms after stimulus onset coding of the low-level properties of the image takes place. In the time interval 170-400ms, ERP amplitude correlated only with complex image properties, but not with task difficulty. The first effects arising from decision difficulty were observable at 400ms after stimulus onset, and therefore this is probably the earliest electrophysiological signature of the decision making processes, in the given experimental paradigm.
['Harauzov|A K|AK|', 'Shelepin|Y E|YE|', 'Noskov|Y A|YA|', 'Vasilev|P P|PP|', 'Foreman|N P|NP|']
D000293:Adolescent D000328:Adult D001921:Brain D004569:Electroencephalography D005260:Female D006801:Humans D008297:Male D009949:Orientation D010364:Pattern Recognition, Visual D011601:Psychophysics D011930:Reaction Time D013997:Time Factors D055815:Young Adult
Decision difficulty Instructions ERP Gabor gratings Orientation discrimination