The current study investigates how infants categorize human compared to ape faces. Nine-month-old infants were presented with priming stimuli related to human (N = 24) or ape (N = 25) face targets on different levels of categorization. Event-related potentials were recorded during a passive-looking rapid repetition paradigm. In a within-subjects design, priming effects of the same faces, different faces from the same basic-level category, different faces from the other basic-level category (human/ape faces), and house fronts were examined. Human and ape faces were first categorized on a superordinate level ("faces"), as indicated by enhanced P1 amplitude and reduced P1 latency for faces primed by any faces. Then, human and ape faces were categorized on a basic level. N290 amplitude and latency were larger for human and monkey targets primed by human faces. Neither human nor ape faces were categorized on the individual level.

We examined the processing of upright and inverted faces and cars in 3-month-old infants applying an event-related-potentials paradigm. The current study is the first to contrast human faces with an object category, cars, in a within-subjects design with infants. N290 amplitude was larger for inverted than upright faces, whereas no inversion effect was observed for cars. Moreover, N290 latency was enhanced for inverted faces and cars. This indicates that neural processing may already be partly face-specific in young infants.

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.

Abstract:In the present article we review behavioral and neurophysiological studies on face processing in adults and in early development. From the existing empirical and theoretical literature we derive three aspects that distinguish face processing from the processing of other visual object categories. Each of these aspects is discussed from a developmental perspective. First, faces are recognized and represented at the individual level rather than at the basic level. Second, humans typically acquire extensive expertise in individuating faces from early on in development. And third, more than other objects, faces are processed holistically. There is a quantitative difference in the amount of visual experience for faces and other object categories in that the amount of expertise typically acquired for faces is greater than that for other object categories. In addition, we discuss possible qualitative differences in experience for faces and objects. For instance, there is evidence for a sensitive period in infancy for building up a holistic face representation and for perceptual narrowing for faces of one's own species and race. We conclude our literature review with questions for future research, for instance, regarding the exact relationship between behavioral and neuronal markers of face processing across development.