The N170, not the P1, indexes the earliest time for categorical perception of faces, regardless of interstimulus variance.

ABSTRACT

A negative event-related potential (ERP) at occipito-temporal sites peaking around 150-170 ms after stimulus onset (N170) is typically larger for faces than other object categories. Most theories interpret this finding as due to face-selective processing in occipito-temporal and temporal cortex. However, a controversial account recently attributed the N170 effect to differences in interstimulus variance (ISV) among the images typically used for face and object conditions and proposed that the earlier P1 instead indexes the categorical processes generally attributed to the N170. This ERP study aimed to test this account definitively by using conditions in which the same face and object were shown repeatedly, eliminating both physical and perceptual ISV. Fourier amplitude spectra of faces and objects were matched to equate basic low-level visual properties that may affect early ERPs such as the P1. Results demonstrate that i) face selectivity of the N170 is largely preserved across many object categories after abolishing ISV, and ii) stimulus category does not modulate the P1. This conclusively refutes the ISV account while strongly supporting category as a critical factor driving N170 face selectivity.