10-24 – When does perceptual organization happen?

Reflectional (mirror) symmetry is an important cue for visual perception. The visual brain processes symmetry efficiently. We know that symmetry activates the extrastriate cortex and generates an event related potential (ERP) called the Sustained Posterior Negativity (SPN). Previous work suggests that no tasks completely block symmetry processing and prevent SPN generation. We tested this with eight new Electroencephalography (EEG) experiments (344 participants in total). All experiments used the same symmetrical or asymmetrical arrangements of 40 dots. When participants attended to regularity in Experiment 1, there was a large SPN (Mean amplitude -2.423 microvolts). The SPN was reduced, but not abolished, when participants discriminated luminance in Experiments 2 and 3 (-.835 and -1.410 microvolts) or the aspect ratio of a superimposed cross in Experiments 4 and 5 (-.722 and -.601 mV). The SPN also remained when the background pattern was potentially disruptive to the primary task in Experiment 6 (-1.358 mV) and when participants classified negative superimposed words in Experiment 7 (-.510 mV). Finally, the SPN was not abolished when participants classified to the orientation of a diagonal line in Experiment 8 (-.589 mV). While task manipulations can turn down the extrastriate symmetry response, they cannot render the system completely symmetry-blind. Permanent readiness to detect reflectional symmetry in the foveal region could be an evolved adaptation.