How is facial expression coded?

Facial expression is theorized to be visually represented in a multidimensional expression space, relative to a norm. This norm-based coding is typically argued to be implemented by a two-pool opponent coding system. However, the evidence supporting the opponent coding of expression cannot rule out the presence of a third channel tuned to the center of each coded dimension. Here we used a paradigm not previously applied to facial expression to determine whether a central-channel model is necessary to explain expression coding. Participants identified expressions taken from a fear/antifear trajectory, first at baseline and then in two adaptation conditions. In one condition, participants adapted to the expression at the center of the trajectory. In the other condition, participants adapted to alternating images from the two ends of the trajectory. The range of expressions that participants perceived as lying at the center of the trajectory narrowed in both conditions, a pattern that is not predicted by the central-channel model but can be explained by the opponent-coding model. Adaptation to the center of the trajectory also increased identification of both fear and antifear, which may indicate a functional benefit for adaptive coding of facial expression.