Temporal Dynamics of Sensitivity to Second-Order Facial Relations: Evidence from a Rapid ERP Adaptation Paradigm

Second-order facial relations-the metric spacing among internal facial features-contribute to face individuation, but the temporal stages at which the brain differentiates repeatedly from changed relational information remain unclear. Using a rapid-adaptation EEG paradigm, seventy adults viewed S1-S2 face pairs derived from the same source identity, in which the spacing of the eyes and mouth was either repeated or changed while first-order facial layout was preserved. ERPs to S2 were analysed in the P1, N170, P2, and N250 time windows and complemented by cluster-based permutation tests. Repeated versus changed spacing did not reliably modulate the P1 or N170. Instead, a sustained posterior condition difference emerged after the N170 and extended across the conventionally defined P2 and N250 time ranges, with repeated spacing eliciting a more positive-going response than changed spacing across this interval. Cluster-based analyses confirmed that this difference extended across contiguous post-N170 time points and posterior electrodes. Under the present rapid-adaptation conditions, neural differentiation of repeated versus changed second-order relations was therefore most reliably expressed in later P2 and N250 activity. These findings do not imply that second-order information is absent from earlier processing stages, but identify a sustained post-N170 interval in which repeated and changed second-order relations were reliably differentiated.