VEPs elicited by local correlations and global symmetry: characteristics and interactions

Psychophysical and fMRI studies have indicated that visual processing of global symmetry has distinctive scaling properties, and proceeds more slowly than analysis of contrast, spatial frequency, and texture. We therefore undertook a visual evoked potential (VEP) study to directly compare the dynamics of symmetry and texture processing, and to determine the extent to which they interact. Stimuli consisted of interchange between structured and random black-and-white checkerboard stimuli. For symmetry, structured stimuli were colored with 2-fold symmetry (horizontal or vertical mirror), 4-fold symmetry (both mirror axes), and 8-fold symmetry (oblique mirror axes added). For texture, structured stimuli were colored according to the "even" isodipole texture [Julesz, B., Gilbert, E. N., & Victor, J. D. (1978). Visual discrimination of textures with identical third-order statistics. Biological Cybernetics, 31, 137-140]. Thus, all stimuli had the same contrast, and check size, but differed substantially in correlation structure. To separate components of the VEP related to symmetry and texture from components that could be generated by local luminance and contrast changes, we extracted the odd-harmonic components of the VEP (recorded at C(z)-O(z), C(z)-O(1), C(z)-O(2), C(z)-P(z)) elicited by structured-random interchange. Responses to symmetry were largest for the 8-fold patterns, and progressively smaller for 4-fold, vertical, and horizontal symmetry patterns. Eightfold patterns were therefore used in the remainder of the study. The symmetry response is shifted to larger checks and lower temporal frequencies compared to the response to texture, and its temporal tuning is broader. Processing of symmetry makes use of neural mechanisms with larger receptive fields, and slower, more sustained temporal tuning characteristics than those involved in the analysis of texture. Sparse stimuli were used to dissociate check size and check density. VEP responses to sparse symmetry stimuli showed that there is no difference between first- and second-order symmetry for densities less than 12.5%. We discuss these findings in relation to local and global visual processes.