Symmetry activates extrastriate visual cortex in human and nonhuman primates.

Humans often create and appreciate visual symmetry in their environment, and the underlying brain mechanisms have been a topic of increasing interest. Here, symmetric versus random dot stimuli produced robust functional MRI (fMRI) activity in higher-order regions of human visual cortex (especially areas V3A, V4, V7, and LO) but little activity elsewhere in brain. This fMRI response was found both with and without attention controls. Moreover, it was highly correlated with the psychophysical perception of symmetry. Similar symmetry responses were found by using line-based and dot stimuli and were found at a wide range of stimulus sizes and geometric configurations. Weaker symmetry responses were found in analogous regions of macaque visual cortex by using fMRI techniques with higher sensitivity. This evidence suggests that visual symmetry is specifically enhanced in the human brain, but that the underlying neural mechanisms may nevertheless be resolvable in nonhuman primates.