Abstract
This study investigates categorization of human and ape faces in 9-month-olds using a Fast Periodic Visual Stimulation (FPVS) paradigm while measuring EEG. Categorization responses are elicited only if infants discriminate between different categories and generalize across exemplars within each category. In study 1, human or ape faces were presented as standard and deviant stimuli in upright and inverted trials. Upright ape faces presented among humans elicited strong categorization responses, whereas responses for upright human faces and for inverted ape faces were smaller. Deviant inverted human faces did not elicit categorization. Data were best explained by a model with main effects of species and orientation. However, variance of low-level image characteristics was higher for the ape than the human category. Variance was matched to replicate this finding in an independent sample (study 2). Both human and ape faces elicited categorization in upright and inverted conditions, but upright ape faces elicited the strongest responses. Again, data were best explained by a model of two main effects. These experiments demonstrate that 9-month-olds rapidly categorize faces, and unfamiliar faces presented among human faces elicit increased categorization responses. This likely reflects habituation for the familiar standard category, and stronger release for the unfamiliar category deviants.
Highlights
Several studies have compared the N170 in response to human and ape faces[14,15,16]
The categorization response was observable in the grand-averaged data when upright ape faces were presented as deviant stimuli among human faces (SNR 1.37, Z > 3.11, p < 0.01; see Fig. 2 and Table 1)
Infants showed a strong categorization response for upright ape faces presented among human faces, which was spread over the occipital cortex
Summary
Several studies have compared the N170 in response to human and ape faces[14,15,16]. Carmel and Bentin[14] observed shorter N170 peak latencies for human than ape faces. Broad categorical repetition effects (face/non-face) were observed on the level of the early visual P1 component, which was elicited with increased amplitude and decreased latency for all faces following house fronts compared to faces. A species-specific repetition effect was observed on the level of the N290: N290 amplitude and latency were enhanced for human targets following ape face adaptors, whereas amplitude and latency were decreased for ape targets following human face adaptors This was taken to indicate that the N290 reflects activation of basic-level representations. A human face-specific increase in N290 amplitude for inverted faces has been obtained only in 12-month-olds[25] Several challenges make it difficult to draw conclusions from infant ERP studies measuring average responses to human and ape faces[15,25]. To clearly demonstrate perceptual categorization, a paradigm is required that tests both discrimination between exemplars belonging to different categories, and generalization across exemplars belonging to the same category
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