Neural correlates of categorical face perception

Human observers excel at categorical face perception and can accurately and consistently distinguish true faces from very face-like non-face objects. The neural correlates of such categoricity are unclear, although it has been well demonstrated that the fusiform gyrus is involved in visual face processing. Here we report neuroimaging evidence suggesting that the left and right fusiform areas behave quite differently in terms of their facial analyses. Our experimental strategy involved compiling a set of 300 images, extracted from natural scenes, ordered to gradually become increasingly face-like, and thereby span a continuum of facial similarity from non-faces to genuine faces. We then measured brain activity corresponding to each of the 300 images using fMRI. We find that the pattern of activity in the left fusiform changes in a graded fashion as the image stimuli become increasingly face-like, while the pattern of activity in the right fusiform shows a step-like response corresponding to a categorical difference between faces and non-faces. Besides improving our understanding of the organization of face processing in the adult human brain, this evidence of an anatomical dissociation between categorical face processing and image level facial similarity processing might help account for the previously unexplained lateral differences in face processing, and also provide nominal references against which to compare brain activation patterns of children and patients with face-perception deficits.