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.

[STUDYING THE INTERACTION OF RHESUS MONKEYS WITH THE TACTILE MONITOR DURING THEIR OBSERVATION OF LOW-FREQUENCY VISUAL TEST IMAGES].

In behavioral experiments rhesus macaque monkeys were trained to interact with the computer using a tactile display. We used grayscale Gabor patches of low spatial frequency as stimuli. Monkeys' task was to touch the screen with his hand in the area of the target stimulus, followed by automatic food or juice reinforcement. After two successive correct answers, stimulus contrast gradually decreased. Using a two-alternative forced choice method the contrast threshold was measured within which monkeys can detect the appearance of low-frequency images. It was shown that the contrast sensitivity decreased with the decrease of stimulus spatial frequency, while the reaction time increased. The findings extend our knowledge of the primates' activity in the virtual environment and open new possibilities for modeling and studying various human diseases.

[Visual Priming and Perception of the Small Pictures in Scene with Multiscale Objects.]

Biederman and co-authors [1, 2], have shown that the priming effect in the long-tirn priming paradigm does not depend on the difference between the angular sizes of the test stimulus and the primer. However, these two and other similar works (both with long-time and short-fime priming paradigms) studied a small range of the angular sizes of stimuli. In Vakhrameeva et al. [3], it has been shown that there exist two perceptionally different size ranges: perception of the objects with angular size varying from 1-1.5 to 50 deg was found to be invariant, but for the objects which angular size is less than 1-1.5 deg (depending on object class and task) their perception is no longer invariant. In this work we have investigated the presence of priming effect in match-to-sample task with such a difference in the angular sizes of a primer and a test stimuli, when the sizes of the primer (about 4 deg) and the test stimulus (about 0.5 deg) belong to those different physiological size ranges. The sample stimulus was presented with and without the noise superposition. It has been shown that the priming effect is suppressed when the size difference between the primer and the test stimulus is large. A congruent primer can give a positive impact on the recognition of the test objects, but this takes place under viewing conditions complicated by the noise superposition.