Electrophysiological Evidence of the Amodal Representation of Symmetry in Extrastriate Areas

Abstract

Extrastriate visual areas are strongly activated by image symmetry. Less is known about symmetry representation at object-, rather than image-, level. Here we investigated electrophysiological responses to symmetry, generated by amodal completion of partially-occluded polygon shapes. We used a similar paradigm in four experiments (N=112). A fully-visible abstract shape (either symmetric or asymmetric) was presented for 250ms (t0). A large rectangle covered it entirely for 250ms (t1) and then moved to one side to reveal half of the shape hidden behind (t2, 1000ms). Note that at t2 no symmetry could be inferred from retinal image information. In half of the trials the shape was the same as previously presented, in the other trials it was replaced by a novel shape. Participants matched shapes similarity (Exp. 1 and Exp. 2), or their colour (Exp. 3) or the orientation of a triangle superimposed to the shapes (Exp. 4). The fully-visible shapes (t0-t1) elicited automatic symmetry-specific ERP responses in all experiments. Importantly, there was an exposure-dependent symmetry-response to the occluded shapes that were recognised as previously seen (t2). Exp. 2 and Exp.4 confirmed this second ERP (t2) did not reflect a reinforcement of a residual carry-over response from t0. We conclude that the extrastriate symmetry-network can achieve amodal representation of symmetry from occluded objects that have been previously experienced as wholes.