Categorical perception of morphed facial expressions

Using computer-generated line-drawings, Etcoff and Magee (1992) found evidence of categorical perception of facial expressions. We report four experiments that replicated and extended Etcoff and Magee's findings with photographic-quality stimuli. Experiments 1 and 2 measured identification of the individual stimuli falling along particular expression continua (e.g. from happiness to sadness) and discrimination of these stimuli with an ABX task in which stimuli A, B, and X were presented sequentially; subjects had to decide whether X was the same as A or B. Our identification data showed that each expression continuum was perceived as two distinct sections separated by a category boundary. From these identification data we were able to predict subjects' performance in the ABX discrimination task and to demonstrate better discrimination of cross-boundary than within-category pairs; that is, two faces identified as different expressions (e.g. happy and sad) were easier to discriminate than two faces of equal physical difference identified as the same expression (e.g. both happy).