Categorical perception of facial expressions: Categories and their internal structure

The question whether judgements of facial expression show the typical pattern of categorical perception was examined using three sets of 11 photographs, each constituting an Ii-step continuum extending between two extreme protypical exemplars: angry-sad, happy-sad and angry-afraid, respectively. For each continuum, intermediate exemplars were created using a morphing procedure. Subjects first identified all faces in each continuum in terms of the extreme expressions, and then performed an ABX discrimination task on pairs of faces two steps (Experiments 1 and 2) or three steps (Experiment 3) apart. The classical categorical perception prediction that discrimination performance must peak around the point on the continuum at which identification reaches 50% was tested not on group means, as in earlier studies, but on a subject-by-subject basis. It was supported by the results for both adults (Experiment 1) and 9- to 10-year-children (Experiment 3). For adults, two noncategorical interpretations of the main finding were discarded by showing that it was not replicated with the same material presented upside down (Experiment 2).