A part of the human brain that's involved in emotion gets particularly excited at the sight of animals, a new study has shown. The brain structure in question is the amygdala: that almond-shaped, sub-cortical bundle of nuclei that used to be considered the brain's fear centre, but which is now known to be involved in many aspects of emotional learning.
Florian Mormann and his colleagues didn't use a brain scanner for their main study. Instead they inserted electrodes directly into the brains of 41 patients with epilepsy, who were undergoing neurosurgery as part of their treatment. This allowed the researchers to present the patients with different pictures and to record the resulting activity of nearly 1,500 individual brain cells, located in the amygdala, hippocampus, and entorhinal cortex (all regions are found in the medial temporal lobe; the latter two are involved in memory).
The dramatic result was that cells in the right-sided amygdala, but not the other regions, were far more likely to respond to pictures of animals, and to be aroused more powerfully by them, as compared with pictures of people (mostly celebrities), landmarks and objects (e.g. food and tools). By contrast, hippocampus cells responded similarly to the different picture categories, whilst the entorhinal cortex cells showed a reduced likelihood of response to pictures of people.
Cells in the right-sided amygdala weren't only more likely to respond to the sight of animals than other pictures, and to do so more powerfully, they also did so extra fast, with a mean latency of 324ms. This wasn't true for the other brain regions. Although this suggests the sight of animals is processed with extra efficiency by the amygdala, the latency is not so short as to suggest bypassing of the cortex (the crumpled, outer layer of the brain associated with conscious processing).
Because the amygdala is involved in fear learning, among other functions, it's tempting to interpret these findings alongside fossil evidence showing that early hominids were preyed on by carnivores, and alongside findings relating to "prepared learning" - this is our innate or early predisposition to have our attention grabbed by threats, such as snakes, faced by our ancestors rather than by contemporary threats like guns. Other research shows that animals are more likely to be detected, than vehicles or even buildings, in change blindness tasks, in which an object or animal appears in a scene that remains otherwise unchanged. However, Mormann's team noted that there was no relation between the likelihood or speed of response of amygdala cells and the nature of the animal pictures as either threatening or harmless.
The researchers said the differential response to animals by amygdala cells is "truly categorical" and "argues in favour of a domain-specific mechanism for processing this biologically important class of stimuli.
"A plausible evolutionary explanation," they continued, "is that the phylogenetic importance of animals, which could represent either predators or prey, has resulted in neural adaptations for the dedicated processing of these biologically salient stimuli."
F. Mormann, J. Dubois and 10 others (2011). A category-specific response to animals in the right human amygdala. Nature Neuroscience, In Press.