Скачать или смотреть Marco Bertamini - 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). I will 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) Is bilateral reflection special? Reflection is the optimal stimulus for a general regularity-sensitive network that responds also to rotation and translation. There is no unique area for bilateral symmetry.


(3) 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.

(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.

(6) Is perception of symmetry dependent on figure-ground assignment? Yes, despite the fact that regularity in the image is what predicts activation independently of task, contour ownership modulates this activity. In summary, we have learned a lot recently about how the visual system respond to with symmetry and it appears that while the system is tuned to regularity, it does so by involving a number of visual areas and there is an interaction between perception of symmetry and figure-ground organisation.