A study found that the amydgala, a brain center involved in emotional learning and memory, responds preferentially and more rapidly to images of animals. These findings indicate that the amygdala is hardwired to detect nonhuman animals and may explain why some people are drawn to furry animals while others develop phobias of spiders.
Researchers from the California Institute of Technology and UCLA recorded single-neuron responses in the amygdalas of 41 epilepsy patients as they viewed images of people, animals, landmarks or inanimate objects. The patients already had electrodes in place because they were being monitored for seizure-related brain activity at the Ronald Reagan UCLA Medical Center.
Florian Mormann, lead researcher and a former postdoctoral scholar at Caltech, said that the neurons in the human amygdala responded preferentially to pictures of animals. When the patients looked at pictures of cats or snakes, they showed greater cell activity in their amygdalas compared to when they looked at pictures of people or inanimate objects. The animal-related areas were unrelated to the patients' epilepsy.
Given the prominent role of the amygdala in fear conditioning, Mormann and his colleagues expected stronger responses to dangerous animals such as snakes. However, cute animals, as well as those regarded by the patient as ugly and dangerous, elicited the same level of activity in the amygdala and appeared to be independent of the emotional contents of the pictures.
As controls, the researchers also monitored the hippocampus and entorhinal cortex. There were no significant responses or stimulus preferences in these regions.
Notably, the response behaviors were detected in the right but not the left amygdala. The right amydgala showed not only greater activity, but also faster neural responses to the animal pictures. Follow-up studies in people not suffering from epilepsy yielded the same results.
The amygalae are two almond-shaped clusters of neurons (core components of the nervous system) located in the medial temporal lobe of the brain. As research on the amydgala had previously focused on human faces and fear, Ralph Adolphus, a coauthor of the paper and Caltech professor, was surprised that the team found a generalized and hemispheric response to animals. He believes that these findings will stimulate more research and could potentially be used to better understand animal phobias.
According to Mormann, the amygdala is a very old structure in terms of brain evolution. Its hemispheric asymmetry reinforces previous findings that the right hemisphere of the brain became specialized in dealing with unexpected and biologically relevant stimuli or with changes in the environment early on in vertebrate evolution.
Throughout human biological history, animals, which could represent either predators or prey, were a highly relevant class of stimuli. This is in line with findings that the amygdala is also involved in reward processing and vigilance, because early vertebrates and humans had to remain aware of other animals in their environment.
The amygdala is still important today, partly because we need to distinguish between harmless and potentially dangerous animals. However, it remains surprising that the intensity of brain activity would be similar for a dog and a wolf.
Itzhak Fried, coauthor of the study and a neurosurgeon at UCLA, says that the study clearly illustrates how scientists doing basic research can benefit from collaborating in a clinical setting, and vice versa. Specifically, the study showed how special situations in neurosurgery, such as patients being treated for epilepsy, could give insight into the function of the human mind.
The paper, titled "A category-specific response to animals in the right human amygdala," was published online as part of Nature Neuroscience on August 28.
