Brain Activity in Response to Visual Symmetry.

Research on the neural basis of symmetry perception is making rapid progress. ERP studies reliably show a sustained posterior negativity (SPN), with lower amplitude for symmetrical than random patterns at occipital electrodes, beginning around 250 ms after stimulus onset. Functional MRI shows activity in extrastriate visual areas and in the lateral occipital complex (LOC). We summarise and integrate the evidence by answering six questions. (1) Is there an automatic and sustained response to symmetry in visual areas? Yes, and because this is broadly unaffected by the task it suggests automatic processing of symmetry. (2) Which brain areas are involved in symmetry perception? There is an extended symmetry-sensitive network in extrastriate visual areas and the LOC. (3) Is bilateral reflection special? Reflection is the optimal stimulus for a general regularity-sensitive network that responds also to rotation and translation. (4) Is the response to symmetry independent of view angle? When people classify patterns as symmetrical or random, the response to symmetry is view-invariant. When people attend to other dimensions, the network responds to residual regularity in the image. (5) Does the neural response to symmetry scale with degree of regularity? Yes, the proportion of symmetrically positioned elements predicts the size of SPN and fMRI responses (see attached Figure). Finally, (6) Are connections between the hemispheres especially important for symmetry perception? No. SPN amplitude increases with the number of axes, and is comparable for horizontal and vertical symmetry. Overall, these studies of the brain mechanisms involved in symmetry perception show a consistent link between brain activity and measures of sensitivity from psychophysical studies. Meeting abstract presented at VSS 2015.