Polygons have a small facilitatory effect on extraretinal symmetry perception.

Symmetrical objects only project a symmetrical image onto the retina when viewed from certain angles. Previous work has investigated the ERP response to visual symmetry in frontoparallel and perspective views. When participants are attending to regularity, the ERPs are the same. When participants are attending to colour, the response to perspective symmetry is reduced. We term this reduction 'perspective cost'. We predicted that perspective cost would be lessened if the stimuli were polygons rather than dot patterns. This prediction was confirmed in a new experiment. This result suggests some stimuli may support automatic 3D interpretation better than others. However, the facilitatory effect of polygons was relatively small, and perspective cost was not eliminated. Furthermore, this study also revealed that attention to symmetry is not always sufficient to eliminate perspective cost.