Examining the neural correlates of within-category discrimination in face and non-face expert recognition.

While expert face discrimination develops naturally in humans, expert discrimination in non-face object categories, such as birds, cars and dogs, is acquired through years of experience and explicit practice. The current study used an implicit visual discrimination paradigm and electroencephalography (EEG) - Fast Periodic Visual Stimulation - to examine whether within-category discrimination of faces and non-face objects of expertise rely on shared mechanisms despite their distinct learning histories. Electroencephalogram was recorded while bird experts and bird novices viewed 60s sequences of bird images or face images presented at a periodic rate of six images per second (i.e., 6.0Hz). In the sequence, an adapting base image of a family-level bird (e.g., robin), a species-level bird (e.g., purple finch) or a face (e.g., Face A) was presented repeatedly for four consecutive cycles, followed by a different within-category "oddball" image at every fifth cycle (e.g., warbler, house finch, Face B). A differential response between the adapting base and the oddball images (6.0Hz/fifth cycle = 1.20Hz) provided an index of within-category discriminability. The results showed that both experts and novices demonstrated a robust EEG signal of equal magnitudes to the 6.00Hz base face and bird images at medial-occipital channels and to the oddball 1.20Hz face and bird images at the more anterior occipito-temporal channels. To examine whether the responses to faces and birds were generated by shared neural mechanisms, we correlated the responses to birds and faces at the participant-level. For the base signal at medial-occipital channels, all object categories positively correlated in both the experts and the novices, as expected given that the base signal indexes visual responses that are shared by all object categories (e.g., low-level). In contrast, for the discrimination signal at the more anterior occipito-temporal channels, the response to family- and species-level birds positively correlated with faces for the experts, but no face-bird association was found for the novices. These findings indicate the existence of partially shared neural mechanisms for within-category discrimination of faces and birds in the experts, but not in the novices.