Cross-validating the electrophysiological markers of early face categorization.

Human face categorization has been extensively studied using event-related potentials (ERPs), positing the N170 ERP component as a robust neural marker of face categorization. Recently, the fast periodic visual stimulation (FPVS) approach relying on steady-state visual evoked potentials (SSVEPs) has also been used to investigate face categorization. FPVS studies consistently report strong bilateral SSVEP face categorization responses over the occipito-temporal cortex, with a right hemispheric dominance, closely mirroring the N170 scalp topography. However, it remains unclear whether SSVEP responses can be considered a proxy for the N170 or are driven by different components. To address this question, we recorded electrophysiological signals from observers viewing face and object images during FPVS and ERP paradigms. We quantified the FPVS response in the frequency domain and extracted ERP components, including the P1, N170, and P2, from both the FPVS time domain and ERP paradigms. Our results revealed little relationship between any single ERP component and the FPVS frequency response. Only the peak-to-peak differences between N170 and P2 components explained the FPVS frequency response. Our data show that the FPVS frequency response reflects a later complex neural integration rather than any isolated ERP component, such as the N170. These findings raise important methodological and theoretical considerations regarding the relationship between SSVEPs and transient ERPs. While both markers are indicative of human face categorization, they appear to capture different stages of this cognitive process. Our study untangles the very nature of the electrophysiological neural responses of face categorization. We recorded and directly compared steady-state visual evoked potentials (SSVEPs) with transient event related potentials (ERP) evoked by faces and objects in human observers. Contrary to the assumption associating SSVEPs with the early N170 ERP component, we found that the N170-P2 difference was consistently associated with the SSVEPs. This finding suggests that SSVEPs in fast periodic visual stimulation (FPVS) may reflect later stages of neural processing. Our findings invite to caution when interpreting SSVEP responses, avoiding premature assumptions about their relationship with ERPs. This work highlights the need for integrated research approaches to better understand the complex interplay between SSVEPs and ERPs across different cognitive domains.