Temporal dissociation of spatial attention and competition effects between face representations

Abstract

Scalp event-related potential (ERP) studies in humans indicate that the processing of faces differs from other categories between 130 and 170 ms after stimulus onset, peaking at the occipito-temporal N170. We have recently shown that the N170 response to a face stimulus is strongly reduced when another face - or a non-face object of expertise (Rossion, Kung & Tarr, 2004) - is processed concurrently (Jacques & Rossion, 2004), suggesting that neural representations of individual faces compete in the occipito-temporal cortex. However, this amplitude reduction of the N170 may be related to spatial attentional modulations rather than to a competition between face representations. To test this alternative hypothesis, ERP responses to a lateralized face were recorded while subjects were processing a centrally presented stimulus, either a face or a phase-scrambled face stimulus. Subjects performed either a low-level matching task on the central stimulus - thus allocating minimal attention to the lateralized face -, or a task in which they had to detect the occurrence of the lateralized faces. The results are twofold: first, replicating our previous results, the N170 to the lateralized face was largely reduced, starting at about 130 ms, when the central stimulus was a face relative to the control stimulus. Importantly, this competition effect was of similar amplitude in both attentional conditions. Second, when subjects concentrated on the central stimulus, the N170 to the lateral face was strongly reduced, whether the central stimulus was a face or a scrambled face, i.e. the effects of competition and spatial attention were additive on the N170. Moreover and most importantly, this effect of spatial attention started at about 80 ms after stimulus onset, on the visual P1, much earlier than the competition effect. These results indicate that spatial attention and object competition are distinct mechanisms that both strongly shape the processing of faces in extrastriate cortex.