Neurophysiological markers of the global/local biases in face perception

Global/local biases in the visual processing of structurally complex stimuli occur under certain conditions of the beholder. Previous experiments using hierarchical letters (large letters made of small ones) have reported a global precedence in young adults. Here, we aimed to define neurophysiological markers of a possible global/local bias during the implicit processing of new faces. We performed two ERP experiments on young adults using hierarchical/compound facial stimuli, either congruent (eyes replaced by small faces) or incongruent (objects as eyes), with peripheral small objects and faces, respectively, grafted as distracters. In Experiment 1, the face-sensitive N170/VPP was largest for regular faces and progressively decreased through congruent and incongruent hierarchical faces, evidencing disruption of the global bias. In Experiment 2, N170/VPP was similar between both hierarchical faces and a new objects condition, likely due to the visual angle increase used to prevent a global bias. Moreover, an N200-350 with maximal amplitude for incongruent faces and objects appeared in both experiments, suggesting a perceptive mismatch with a canonical face template. Experiment 2 also revealed a posterior negativity ∼220-330 msec for objects and faces, likely indicating access to known categories, as well as an object-specific late positivity that could reflect pre-semantic, visual recognition of common objects. Source reconstruction highlighted the significant involvement of ventral (predominantly right-sided) visual regions in these ERP modulations. This experimental design can be useful to detect global/local biases and several markers of face and object processing related to individual differences and neurodegenerative diseases even before they manifest overtly.