The Perception of the Orientationally Modulated Textures and its Expression in the Visual Evoked Potentials

The paper investigates the psychophysiological mechanisms of the texture perception when the stimuli differ in axial ratios of the carrier (texture elements) and the envelope (orientational modulation). The human visual system has a number of preattentive mechanisms performing spatial grouping and segmentation of the scene. The perception of texture most likely carried out by the several “early” mechanisms, but at certain stages their activity can be modulated by top-down influences. We consider the second-order visual filters and the contour integration processes as the candidates that provide the perception of spatial orientational modulations, as well as analyze the results of our electrophysiological study aimed at the brain mechanisms of the orientational modulations perception. According to A. Schofield and T. Yates (2005), when the texture is sinusoidally modulated by the orientation, a number of different perceptual effects can be obtained. If the orientations of the modulation and the carrier are orthogonal, the texture pattern forms a “corrugation” and the effect of depth appears. The “herringbone” is observed then the orientation of the modulation and the carrier are parallel, otherwise the “continuous structure” is observed. The contours formed by the textural elements seem like continuous in the orthogonal cases, and they are segmented in the parallel cases. These perceptual phenomena can be explained by the participation of the different mechanisms in the processing of visual textures. Thus, when the carrier and the envelope are orthogonal, the role of the contour integration mechanisms can be increased. We used the method of visual evoked potentials for studying the sychophysiological mechanisms of the orientation-modulated stimuli perception. 15 people voluntarily participated in the investigation. The VEPs elicited by the modulated (“corrugation”, “herring-bone”) textures and unmodulated control stimuli were recorded using 19 electrodes. It was found that the mismatches between VEPs elicited by the experimental and control stimuli have strong spatial differences, respectively, for the textures modulated orthogonally or in parallel to the carrier direction. This result may give the evidence that different brain mechanisms are involved in the perception of the orientationally modulated textures. That is also confirmed by the source localization of difference waves calculated by subtracting the evoked responses to the test and control stimuli (using sLoreta).