Intensity dependence in high-level facial expression adaptation aftereffect.

Perception of a facial expression can be altered or biased by a prolonged viewing of other facial expressions, known as the facial expression adaptation aftereffect (FEAA). Recent studies using antiexpressions have demonstrated a monotonic relation between the magnitude of the FEAA and adaptor extremity, suggesting that facial expressions are opponent coded and represented continuously from one expression to its antiexpression. However, it is unclear whether the opponent-coding scheme can account for the FEAA between two facial expressions. In the current study, we demonstrated that the magnitude of the FEAA between two facial expressions increased monotonically as a function of the intensity of adapting facial expressions, consistent with the predictions based on the opponent-coding model. Further, the monotonic increase in the FEAA occurred even when the intensity of an adapting face was too weak for its expression to be recognized. These results together suggest that multiple facial expressions are encoded and represented by balanced activity of neural populations tuned to different facial expressions.