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.