Categorical perception of conspecific faces in rhesus macaques

Previous studies indicate humans perceive faces categorically, particularly when the faces are familiar. Categorical perception is traditionally defined by positive results on two psychophysical tasks: an identification and a discrimination task. Whether nonhuman primates demonstrate the same phenomenon has not yet been explored. This study bridges this gap in the literature by exploring categorical perception of familiar and unfamiliar conspecific faces in two rhesus macaques using computer-generated morph line continua similar to those used in previous face categorization studies. Evidence of both hallmarks of categorical perception was found, demonstrating that rhesus macaques perceive conspecific faces in a categorical manner. This phenomenon becomes more distinct when one, but not both, of the conspecific faces are personally familiar to the individual. Inter-trial adaptation effects cannot account for these results. This study has laid down the behavioural foundation for future exploration of the neural underpinnings of the phenomenon known as categorical perception.