Face Processing in Natural Scenes: A Posterior-to-anterior Gradient of Sensitivity to “Faceness”

Abstract

Faces are among the most important visual stimuli that we encounter every day. A glance at a face can often tell us their identity, sex, age, race, emotional expression, and direction of gaze (Tsao & Livingstone, 2008). Previous fMRI studies in macaques have shown that face selectivity increases from the posterior to the anterior regions of face areas (Freiwald, 2020; Weiner & Grill-Spector, 2010). However, the neural basis of face processing in humans remains unknown, particularly in a naturalistic context. Here we tested whether there is a similar posterior-to-anterior gradient of sensitivity to faces in the human brain. Ten participants viewed 1000 natural scene images under central fixation and made a judgment on their level of face perception for each image. We asked participants to judge as quickly and accurately as they can based on whether (1) there is at least one face that they can identify (if they see it again), (2) there is at least one face with some facial features (that they can see clearly), (3) there is at least one head of an animate thing, and (4) there is none of the above. The “faceness” ratings obtained were used to analyze fMRI responses of face-selective regions as provided by the Natural Scenes Dataset (NSD) (Allen et al., 2022). We found an increasing, posterior-to-anterior gradient of sensitivity to “faceness” along the face-selective regions of the lateral ventral temporal cortex. Our results suggest that (a) face-selective regions are involved in different functional components of face processing and that (b) broadly-tuned face detection begins in the anterior regions and increasingly becomes fine-tuned for face recognition in the posterior regions.