Whether cute and cuddly or fierce and frightening, animals affect the brain in ways scientists are just starting to appreciate. In a study of people who had electrodes implanted in their brain for the treatment of epilepsy, an international team discovered neurons that respond specifically to animals. The 41 individuals in the study were shown picturesof recognizable landmarks, objects, animals and people for about one second each as tiny electrodes measured the activity of individual neurons in three regions of their brain. When the researchers analyzed the electrical data from the 400 to 550 neurons in each region, they found a marked jump in the activity of neurons in the right amygdala that was not seen in the other brain regions tested—and only after viewing the pictures of animals. The report by senior author Christof Koch, a neuroscientist at the Allen Institute for Brain Science, and his colleagues appeared this past August online in Nature Neuroscience. (Koch also writes the monthly column Consciousness Redux for Scientific American Mind.)

Previous studies in animals hinted that the right hemi­sphere might be specialized for detecting prey or threats. Given the amygdala’s proposed role in emotion and arousal, this finding led the team to speculate that the right-amygdala response might have evolutionary roots. More broadly, the fact that only the right side of the amygdala responds specifically to animals is tantalizing, Koch explains, because it is the first time this kind of hemispheric asymmetry has been found at the cellular level in the human brain. Imaging studies can detect only much larger shifts in activity. In this case, the patients being treated for epilepsy offered scientists a unique opportunity to examine such subtle brain responses.