Researchers from California Institute of Technology and UCLA reported that neurons throughout the amygdala, a center in the brain known for processing emotional reactions, respond preferentially to images of animals.
Their findings are described in a paper published online in the journal Nature Neuroscience.
The collaborative research team was responsible for recruiting 41 epilepsy patients at the Ronald Reagan UCLA Medical Center. These patients were already being monitored for brain activity related to seizures. Using electrodes already in place, the team recorded single-neuron responses in the amygdala as study participants viewed images of people, animals, landmarks or objects. The amygdalae are two almond-shaped clusters of neurons — cells that are core components of the nervous system — located deep in the medial temporal lobe of the brain.
“Our study shows that neurons in the human amygdala respond preferentially to pictures of animals, meaning that we saw the most amount of activity in cells when the patients looked at cats or snakes versus buildings or people,” says Florian Mormann, lead author on the paper and a former postdoctoral scholar in the Division of Biology at Caltech. “This preference extends to cute as well as ugly or dangerous animals and appears to be independent of the emotional contents of the pictures. Remarkably, we find this response behavior only in the right and not in the left amygdala.”
Mormann says this striking hemispheric asymmetry helps strengthen previous findings supporting the idea that, early on in vertebrate evolution, the right hemisphere became specialized in dealing with unexpected and biologically relevant stimuli, or with changes in the environment. “In terms of brain evolution, the amygdala is a very old structure, and throughout our biological history, animals—which could represent either predators or prey—were a highly relevant class of stimuli,” he says.
“This is a pretty novel finding, since most amygdala research in the past was usually about faces of people and emotions related to fear rather than pictures of animals,” adds Ralph Adolphs. “Nobody would have guessed that cells in the amygdala respond more to animals than they do to human faces, and in particular that they respond to all kinds of animals, not just dangerous ones. I think this will stimulate more research and has the potential to help us better understand phobias of animals.”
The study is also a clear illustration of how scientists doing basic research can benefit from working with collaborators in a clinical setting and vice versa.
