Amygdala damage eliminates monkeys' viewing preference for real and illusory faces.

Abstract

Rhesus monkeys (Macaca mulatta), like humans, look longer at images of faces than at non-face objects in a viewing preference task. We recently demonstrated that monkeys also exhibit a viewing preference for photographs of objects that elicited face pareidolia in human subjects over photographs of similar objects that did not elicit illusory faces. We interpreted this preference as evidence that monkeys experience face pareidolia, the illusion of facial structure in an otherwise inanimate object (Taubert et al., 2017, Current Biology). Here, we evaluated viewing preferences for face stimuli, both real and illusory, in three monkeys with selective, bilateral amygdala lesions. The amygdala is thought to be part of the extended face-processing system in the primate brain, but its exact role in face perception is not well understood. In this experiment, we used three types of stimuli: 15 conspecific faces, 15 illusory faces and 15 non-face objects. We presented each subject (N = 3) with all possible pairs of these 45 images, resulting in 1980 trials. Each pair was presented for four seconds. We collected two dependent variables: how long a subject looked at each of the images in a given pair, and where they fixated on each image. Unlike normal monkeys, who show robust preferences for both real and illusory faces, monkeys with selective amygdala damage showed no preference for real faces over non-face objects or for illusory faces over non-face objects. Further, whereas normal monkeys show classic face viewing patterns prioritizing discrete facial features such as the eyes and mouth, monkeys with amygdala damage showed disorganized viewing patterns. Our results thus indicate that the amygdala plays a critical role in spontaneous behavior towards face stimuli (i.e. the speed and ease with which we detect faces, and prioritize them over non-face objects).

Meeting abstract presented at VSS 2018