Exploring face representation in humans and monkeys by using high-level aftereffects

As a first step toward investigating the neural encoding of faces and other complex objects, we examined the effects of visual adaptation on the perception of human faces. We found that following a few seconds of exposure to one face, the perceived identity of a second face was systematically distorted along a specific trajectory in multidimensional 'face space'. This trajectory passed through the central tendency of all faces, and its direction thus defined a particular identity. The results suggested that the visual system considers the average prototype face to be a reference point in its representation of faces, and led us to speculate that neural decoding of faces is a fundamentally comparative process. Such a scheme might constitute a fast and economical storage strategy for the brain to contend with a myriad of very similar shapes. With the aim of investigating this hypothesis more directly by neurophysiological methods, we recently trained a monkey to perform the same task, again with human faces. We found that, while the monkey's identification thresholds were slightly higher than the mean threshold for humans, his perception was affected by adaptation in exactly the same way as that of the human subjects