Individual neurons in amygdala and hippocampus encode visual features that help recognize faces, study finds

Humans are innately capable of recognizing other people they have seen before. This capability ultimately allows them to build meaningful social connections, develop their sense of identity, better cooperate with others, and identify individuals who could pose a risk to their safety.

Several past studies rooted in neuroscience, psychology and behavioral science have tried to shed light on the neural processes underlying the ability to encode other people’s identities. Most findings collected so far suggest that the identity of others is encoded by neurons in the amygdala and hippocampus, two brain regions known to support the processing of emotions and the encoding of memories, respectively.

Based on evidence collected in the past, researchers have concluded that neurons in these two brain regions respond in specific ways when we meet a person we are acquainted with, irrespective of visual features (i.e., how their face looks). A recent paper published in Nature Human Behaviour, however, suggests that this might not be the case, and that individual neurons in the amygdala encode and represent facial features, ultimately supporting the identification of others.

The authors of the paper, based at Washington University in St Louis, West Virginia University, and other institutes, carried out four experiments involving 19 neurological patients. The patients had electrodes implanted in their brains as part of their treatment for epilepsy, and the researchers used these electrodes to record the activity of over 3,500 individual neurons in the amygdala and hippocampus.

“Neurons in the human amygdala and hippocampus are classically thought to encode a person’s identity invariant to visual features,” wrote Runnan Cao, Jinge Wang and their colleagues in their paper. “However, it remains largely unknown how visual information from higher visual cortical areas is translated into such a semantic representation of an individual person.

“Across four experiments (3,581 neurons from 19 neurosurgical patients over 111 sessions), we demonstrate a region-based feature code for faces, where neurons encode faces on the basis of shared visual features rather than associations of known concepts, contrary to prevailing views.”

The patients who participated in the team’s experiment were shown images of different people’s faces, some of whom were more or less familiar to them. While they looked at the images, the electrodes implanted in their brains recorded the activity of individual neurons in the hippocampus and amygdala.

The researchers then analyzed the recordings they had collected and found that some neurons appeared to consistently respond to specific face features, irrespective of the extent to which a person was familiar to a participant. This appears to contradict earlier theories, suggesting that some neurons in the hippocampus and amygdala do in fact encode visual features and could also play a part in the human ability to recognize specific people.

“Feature neurons encode groups of faces regardless of their identity, broad semantic categories or familiarity; and the coding regions (that is, receptive fields) predict feature neurons’ response to new face stimuli,” wrote Cao, Wang and their colleagues.

“Together, our results reveal a new class of neurons that bridge perception-driven representation of facial features with mnemonic semantic representations, which may form the basis for declarative memory.”

The new insights gathered by Cao, Wang and their colleagues could deepen the present understanding of identity recognition. In the future, it could inspire further research focusing on the class of neurons identified by the researchers, while also potentially pinpointing neural processes that are disrupted in individuals who struggle to recognize faces, such as those diagnosed with the cognitive disorder prosopagnosia.

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