Scaling of the extrastriate neural response to symmetry

- The sustained posterior negativity (SPN) is an ERP generated by visual symmetry.
- SPN amplitude increases parametrically with proportion of symmetrical dots.
- The lateral occipital complex (LOC) probably generates the SPN.
- Participants made binary judgements ('some regularity' or 'no regularity').
- SPN amplitude predicted judgement, but there were also cases of independence.

Neuroimaging work has shown that visual symmetry activates extrastriate brain areas, most consistently the lateral occipital complex (LOC). LOC activation increases with proportion of symmetrical dots (pSymm) in a degraded display. In the current work, we recorded a posterior ERP called the sustained posterior negativity (SPN), which is relatively negative for symmetrical compared to random patterns. We predicted that SPN would also scale with pSymm, because it is probably generated by the LOC. Twenty-four participants viewed dot patterns with different levels of regularity: 0% regularity (full random configuration) 20%, 40%, 60%, 80%, and 100% (full reflection symmetry). Participants judged if the pattern contained “some regularity” or “no regularity”. As expected, the SPN amplitude increased with pSymm, while the latency and duration was the same in all conditions. The SPN was independent of the participant's decision, and it was present on some trials where people reported 'no-regularity'. We conclude that the SPN is generated at an intermediate stage of visual processing, probably in the LOC, where perceptual goodness is represented. This comes after initial visual analysis, but before subsequent decision stages, which apply a threshold to the analog LOC response.