A category-specific response to animals in the right human amygdala

Abstract

The amygdala is important in emotion, but it remains unknown whether it is specialized for certain stimulus categories. We analyzed responses recorded from 489 single neurons in the amygdalae of 41 neurosurgical patients and found a categorical selectivity for pictures of animals in the right amygdala. This selectivity appeared to be independent of emotional valence or arousal and may reflect the importance that animals held throughout our evolutionary past.

Figure 1

Amygdala neurons respond preferentially to animal pictures. (a) Response probabilities of neurons in different MTL regions to different stimulus categories revealed significant preferences in the amygdala (P < 10⁻¹⁵, main effect of increased responses to animals at ~1%) and entorhinal cortex (P < 0.03, main effect of decreased responses to persons), but not in the hippocampus. (b) Mean response magnitudes of all responsive neurons showed increased response activity of amygdala neurons to animals (P < 10⁻⁵). (c, d) The animal preference in both response probability and magnitude was seen only in the right amygdala (P < 10⁻¹⁵ and P < 0.0005, respectively). Error bars denote binomial 68% confidence intervals (a, b) and s.e.m. (c, d). *P < 0.05, ***P < 0.001.

Figure 2

A specific category response to animals in the right amygdala at the population level. (a) For a set of 201 amygdala units (96 left, 105 right) that were all presented with the same 57 stimuli (23 persons, 16 animals, 18 landmarks), we constructed representational dissimilarity matrices by determining the dissimilarity in evoked response patterns for each pair of stimuli (as 1 – r from the Pearson correlation across units). (b) Hierarchical cluster analysis automatically grouped stimuli with similar response patterns together into clusters. In the right amygdala, this unsupervised procedure yielded a cluster that contained all animal stimuli, whereas no such category effect was found in the left amygdala.