Categorical perception of faces and facial expressions: the age factor.

Effects of normal aging on categorical perception (CP) of facial identities and facial emotional expressions were investigated. The stimuli were morphs of three identities and of three expressions. In Experiment 1, 38 healthy participants (22 to 79 years old; three age groups) had to identify morphed identities (three continua) and expressions (three continua) varying with respect to their distance from the original photographs. This experiment allowed the computation of the boundary between categories (expressions and identities). The results showed that the locus of this boundary was not affected by age (except for the continuum disgust-happiness), but that the rate of intrusions and latencies of responses increased with age. In addition, CP was suggested, as latencies increased as a function of the distance between the displayed morph and the original, unmorphed photograph. CP predicts an easier discrimination of two morphs located on both sides of the boundary than discrimination of two morphs located on the same side. This was investigated in Experiment 2 (48 new participants; three age groups). For expressions, the prediction was verified in all groups for both accuracy and correct latency; in addition, an effect of age was observed only for morphs located on both sides of the boundary. For identities, the same pattern of results emerged, with one exception: for accuracy, the predicted advantage of pairs located on both sides of the boundary was not verified in the oldest group. Thus, CP of facial identities seems to vanish with normal aging, whereas CP of facial expressions appears to be more robust.