Parametric responses to rotation symmetry in mid-level visual cortex.

Symmetry is a fundamental principle of perceptual organization that contributes to multiple aspects of vision, running the gamut from judgments of aesthetics and mate selection, to shape processing and surface orientation. Crystallographic group theory categorizes all possible two-dimensional repetitive patterns into 17 wallpaper groups, as unique combinations of the four fundamental symmetries: mirror reflection, translation, rotation and glide reflection (Liu et al., 2010). We have developed an algorithm that can generate a near-infinite number of well-controlled exemplars belonging to each of the 17 wallpaper groups. Here, we focus on four wallpaper groups that all contain rotation symmetry exclusively - only differing in the maximum order of rotation symmetry found in each group. We use an fMRI block design that isolates the symmetry-specific BOLD response by alternating group exemplar images with matched control images. Using scalp EEG, we have previously shown that the neural responses to the groups increase linearly with maximum order of rotation symmetry (Kohler et al., VSS 2014), indicating that rotation is represented parametrically. Our fMRI experiment (n=12) localizes this parametric effect in several extra-striate areas of human visual cortex (V4, VO1, LOC), while other visual areas have little response to rotation symmetry (e.g. MT). Interestingly, the parametric response is also seen in V3, an area that has previously been found to have little or no response to mirror symmetry (Sasaki et al 2005). The fact that V1 and V2 do not show the effect suggests a functional distinction between V3 and earlier visual areas. Neurons in intermediate-level visual areas like V4 have complex and diverse response properties (Roe et al., 2012) that are difficult to characterize systematically (Gallant et al., 1996). These results present a novel, well-defined stimulus set, the wallpaper groups, that can drive V4 and other mid-level visual areas in a robust and systematic way. Meeting abstract presented at VSS 2015.