[Electrophysiological investigation of the texture discrimination mechanisms]

Abstract

In electrophysiological and psychophysical experiments, we investigated mechanisms of the visual system underlying local and global texture processing. Textures included rectangular matrixes composed of Gabor patches (sine wave grating windowed by a Gaussian envelope). Orientation of each grating varied from 0 to 165 degrees with the step of 15 degrees. Matrixes differed by the amount of Gabor patches with vertical or horizontal orientation. The observers' task was to discriminate the dominant orientation. The advantage of such stimuli involved a possibility to calculate global statistics of the textures, which we considered as the difference between whole amount of vertical and horizontal orientations in the stimulus irrespective of their location. The local statistics was calculated as relative amount of spatially organized nearby gratings (i. e. collinear contours). The subjects' accuracy was low in discriminating less organized textures and gradually improved with the amount of vertically of horizontally oriented Gabor patches, while the reaction time decreased. Visual evoked potentials (VEPs) recorded from occipital lobes revealed different dependencies of their components' magnitude on the amount of equally oriented gratings. Amplitude of the late positive component P3 with latency 400 ms directly depended on the texture discriminability, and N2 wave with latency 180 ms had an S-like dependence. Opposite to that, the magnitude of P2 wave with latency 260 ms was maximal in response to less organized textures and gradually decreased with the amount of equally oriented gratings. The dependencies received were compared with the textures' statistics. Data analysis allowed us to suppose that, in the conditions of our experimental paradigm, two mechanisms were involved in discrimination of the textures--the local and the global processing. We believe that by recording VEPs one can separately investigate activity of these two processes.