The neural dynamics of face ensemble and central face processing

Abstract

Extensive work has investigated the neural processing of single faces, including the role of shape and surface properties. However, much less is known about the neural basis of face ensemble perception (e.g., simultaneously viewing several faces in a crowd). Importantly, the contribution of shape and surface properties have not been elucidated in face ensemble processing. Furthermore, how single central faces are processed within the context of an ensemble remains unclear. Here, we probe the neural dynamics of ensemble representation using pattern analyses as applied to electrophysiology (EEG) data in healthy adults (seven males, nine females). Our investigation relies on a unique set of stimuli, depicting different facial identities, which vary parametrically and independently along their shape and surface properties. These stimuli were organized into ensemble displays consisting of six surround faces arranged in a circle around one central face. Overall, our results indicate that both shape and surface properties play a significant role in face ensemble encoding, with the latter demonstrating a more pronounced contribution. Importantly, we find that the neural processing of the center face precedes that of the surround faces in an ensemble. Further, the temporal profile of center face decoding is similar to that of single faces, while those of single faces and face ensembles diverge extensively from each other. Thus, our work capitalizes on a new center-surround paradigm to elucidate the neural dynamics of ensemble processing and the information that underpins it. Critically, our results serve to bridge the study of single and ensemble face perception.Significance StatementWe often view groups of faces, called a “face ensemble”, whenever we encounter multiple people (e.g., on a crowded subway). Yet, little is known about how the brain represents ensembles and how attention to a single individual (e.g., during a face-to-face conversation in a crowd) impacts ensemble representations. Here, we show that shape and surface properties contribute to ensemble processing, with shape exhibiting a different neural profile for single versus ensemble face representations. We also show that the visual system processes central faces within an ensemble differently from unattended faces. These results indicate that the encoding of ensemble and single faces is mediated by distinct neural mechanisms, highlighting the importance of studying both as well as their relationship.