Abstract
A key to understanding visual cognition is to determine when and how brain responses are sensitive to the specific visual information underlying categorization behavior. We know that the N170 is the first brain response coding such diagnostic information (Schyns et al., Current Biology 2007), as recently shown in Rousselet et al.’s experiment (Journal of Vision 2014). Using Bubbles (Gosselin & Schyns, Vision Research 2001), we found that the eyes are diagnostic for face detection and are coded in latency and amplitude modulations of the N170. Here, with new analyses we show that diagnostic coding of the eyes in the N170 involves cross-hemispheric transfer. We proceed in three steps. First, we show that the eye contralateral to the recording electrode (i.e. Occipito-Temporal Left, or Right, OTL, OTR) modulates the N170 latencies, whereas the ipsilateral eye modulates the later N170 amplitudes. Second, we show that single-trial N170 latencies and amplitudes fully account for all the coding of the eye that can be extracted from the EEG signal, at any time point. Finally, we show an important effect of the temporal ordering of the left and right N170s (on OTL and OTR) on coding of the eyes. Specifically, an earlier N170 on OTL (initially coding the right eye) has a strong effect on the later coding of the right eye on OTR (initially coding the left eye), suggesting causal transfer of the right eye across hemispheres. To summarize, in a face detection task where both eyes are diagnostic, the N170 ERP initially codes the contralateral eye, closely followed by the ipsilateral eye that is transferred from the earlier N170 in the opposite hemisphere. Our results suggest that the deceptively simple N170 ERP hides a complex mechanism of diagnostic visual feature coding involving cross-hemispheric feature transfer.
Meeting abstract presented at VSS 2015