Elucidating the Neural Representation and the Processing Dynamics of Face Ensembles.

Extensive behavioral work has documented the ability of the human visual system to extract summary representations from face ensembles (e.g., the average identity of a crowd of faces). Yet, the nature of such representations, their underlying neural mechanisms, and their temporal dynamics await elucidation. Here, we examine summary representations of facial identity in human adults (of both sexes) with the aid of pattern analyses, as applied to EEG data, along with behavioral testing. Our findings confirm the ability of the visual system to form such representations both explicitly and implicitly (i.e., with or without the use of specific instructions). We show that summary representations, rather than individual ensemble constituents, can be decoded from neural signals elicited by ensemble perception, we describe the properties of such representations by appeal to multidimensional face space constructs, and we visualize their content through neural-based image reconstruction. Further, we show that the temporal profile of ensemble processing diverges systematically from that of single faces consistent with a slower, more gradual accumulation of perceptual information. Thus, our findings reveal the representational basis of ensemble processing, its fine-grained visual content, and its neural dynamics.SIGNIFICANCE STATEMENT Humans encounter groups of faces, or ensembles, in a variety of environments. Previous behavioral research has investigated how humans process face ensembles as well as the types of summary representations that can be derived from them, such as average emotion, gender, and identity. However, the neural mechanisms mediating these processes are unclear. Here, we demonstrate that ensemble representations, with different facial identity summaries, can be decoded and even visualized from neural data through multivariate analyses. These results provide, to our knowledge, the first detailed investigation into the status and the visual content of neural ensemble representations of faces. Further, the current findings shed light on the temporal dynamics of face ensembles and its relationship with single-face processing.