Object-centered encoding by face-selective neurons in the cortex in the superior temporal sulcus of the monkey.

Neurophysiological studies have shown that some neurons in the cortex in the superior temporal sulcus and in the inferior temporal cortex respond to faces. To determine if some face responsive neurons encode stimuli in an object-centered coordinate system rather than a viewer-centered coordinate system, a large number of neurons were tested for sensitivity to head movement in 3 macaque monkeys. Ten neurons responded only when a head undergoing rotatory movements was shown. All of these responded to a particular movement independently of the orientation of the moving head in relation to the viewer, maintaining specificity even when the moving head was inverted or shown from the back, thereby reversing viewer-centered movement vectors. This was taken as evidence that the movement was encoded in object-centered coordinates. In tests of whether there are neurons in this area which respond differently to the faces of different individuals relatively independently of viewing angle, it was found that a further 18 neurons responded more to one static face than another across different views. However, for 16 of these 18 cells there was still some modulation of the neuronal response with viewing angle. These 16 neurons thus did not respond perfectly in relation to the object shown independently of viewing angle, and may represent an intermediate stage between a viewer-centered and an object-centered representation. In the same area as these neurons, other cells were found which responded on the basis of viewer-centered coordinates. These neurophysiological findings provide evidence that some neurons in the inferior temporal visual cortex respond to faces (or heads) on the basis of object-centered coordinates, and that others have responses which are intermediate between object-centered and viewer-centered representations. The results are consistent with the hypothesis that object-centered representations are built in the inferior temporal visual cortex.