Abstract
The primate amygdala is thought to play a critical role in social and emotional processing. Consequently, the response of the amygdala to visually presented faces has received a disproportionate level of attention. However, to better inform theories of amygdala function, our aim was to characterize the amygdala’s response to a broader range of visual stimuli varying in both social relevance and emotional valence. We used event-related fMRI to measure amygdala activity in four adult macaques. This design permitted us to investigate both neural tuning and representational geometry. The univariate results revealed that at the population level amygdala activity is moderated by both social relevance and emotional valence. As expected, the average fMRI signal was greater for valent stimuli than neutral stimuli. Surprisingly, however, the non-social stimuli drove amygdala activity more so than the social stimuli. Despite the univariate results, we use multivariate analyses to show that neither social relevance nor emotional valence are adequate models for characterizing the amygdala’s entire representational space. The amygdala was driven the most by an assortment of different stimuli, including aggressive social interactions, the faces of conspecific infants, bird eggs, snakes and familiar objects such as medical syringes. Meanwhile, cortical regions in the ventral visual pathway (two face-selective regions and one object-selective region) were found to be sensitive to the distinction between social and non-social stimuli; activity in the face patches was greatest for the social stimuli than the non-social stimuli. In sum our findings suggest that the visual responses of the macaque amygdala are not easily explained by concepts such as social relevance or emotional valence (as defined by human researchers), and that amygdala function extends beyond the recognition of facial expressions.