Beyond opponent coding of facial identity: Evidence for an additional channel tuned to the average face

Face identity can be represented in a multidimensional space centered on the average. It has been argued that the average acts as a perceptual norm, with the norm coded implicitly by balanced activation in pairs of channels that respond to opposite extremes of face dimensions (two-channel model). In Experiment 1 we used face identity aftereffects to distinguish this model from a narrow-band multichannel model with no norm. We show that as adaptors become more extreme, aftereffects initially increase sharply and then plateau. Crucially there is no decrease, ruling out narrow-band multichannel coding, but consistent with a two-channel norm-based model. However, these results leave open the possibility that there may be a third channel, tuned explicitly to the norm (three-channel model). In Experiment 2 we show that alternating adaptation widens the range identified as the average whereas adaptation to the average narrows the range, consistent with the three-channel model. Explicit modeling confirmed the three-channel model as the best fit for the combined data from both experiments. However, a two-channel model with decision criteria allowed to vary between adapting conditions, also provided a very good fit. These results support opponent, norm-based coding of face identity with additional explicit coding of the norm.