As part of a cool project in blogging on Google+ ("plogging"), Nature editor Noah Gray writes about a recent experiment that found that specific neurons in the human amygdala respond instantly to images of animals. These responses were stronger and faster than when other neurons responded to those images, and stronger and faster than when the animal-centric neurons responded to other types of images.
The amygdala is well known to be involved in fear modulation and memory, as well as influencing other types of emotional processing. So is it expected that cells in this structure would respond so strongly to the sight of animals? There is a moderate precedent from the non-human primate literature. Studies in macaques have revealed strong firing of amygdalar neurons to faces, so categorical responses aren't unique in the amygdala. This is true in humans as well, but humans also maintain a different dedicated brain region for face processing, perhaps opening up some portions of the amygdala to take on additional, different roles. But why would we need a dedicated system for animal imagery, elevating this particular stimulus to such an important position in our recognition system? Well this is all speculation, but it isn't difficult to state the obvious and stress that animals were critical as prey for our ancient ancestors, as well as potential threats. Thus, early man may have developed a system to speed our reaction times to such an important category as the landscape was visually scanned for information. Placing this system in a brain region critical to emotion processing could have also more-easily mobilized action through a rapid activation of attack or flight responses.