Hand movements reveal the time-course of shape and pigmentation processing in face categorization

Although the roles of shape and pigmentation cues in face categorization have been studied in detail, the time-course of their processing has remained elusive. We measured participants' hand movements via the computer mouse en route to male or female responses (gender task) or young or old responses (age task) on the screen. Participants were presented with male and female faces (gender task) or with young and old faces (age task) that were typical, shape-atypical, or pigmentation-atypical. Before settling into correct categorizations, the processing of atypical cues led hand trajectories to deviate toward the opposite gender or age category. A temporal analysis of these trajectory deviations revealed dissociable dynamics in shape and pigmentation processing. Pigmentation had a privileged, early role during gender categorization, preceding shape effects by approximately 50 ms and preceding pigmentation effects in age categorization by 100 ms. In age categorization, however, pigmentation had a simultaneous onset of influence as shape. It also had a more dominant influence than shape throughout the gender and age categorization process. The results reveal the time-course of shape and pigmentation processing in gender and age categorization.