Morphing between expressions dissociates continuous from categorical representations of facial expression in the human brain

Whether the brain represents facial expressions as perceptual continua or as emotion categories remains controversial. Here, we measured the neural response to morphed images to directly address how facial expressions of emotion are represented in the brain. We found that face-selective regions in the posterior superior temporal sulcus and the amygdala responded selectively to changes in facial expression, independent of changes in identity. We then asked whether the responses in these regions reflected categorical or continuous neural representations of facial expression. Participants viewed images from continua generated by morphing between faces posing different expressions such that the expression could be the same, could involve a physical change but convey the same emotion, or could differ by the same physical amount but be perceived as two different emotions. We found that the posterior superior temporal sulcus was equally sensitive to all changes in facial expression, consistent with a continuous representation. In contrast, the amygdala was only sensitive to changes in expression that altered the perceived emotion, demonstrating a more categorical representation. These results offer a resolution to the controversy about how facial expression is processed in the brain by showing that both continuous and categorical representations underlie our ability to extract this important social cue.