You’re calmly walking along in the forest and all of the sudden you jump and scream at the sight of a snake... that is actually only a stick. It’s this type of split-second reaction that an international team of scientists wanted to study, under the hypothesis that the fear response could be activated in our brains even before we’re aware of what triggered it. What happens in our heads? Let’s find out.

To explain the “false scare,” neurobiologist Constantino Méndez-Bértolo and colleagues from the universities of Madrid and Geneva (the study was directed by researchers at the Campus de Excelencia Internaciol Moncloa) hypothesized (on the basis of a previous study on rats) that the amygdala plays a key role due to its function in processing emotions. They reasoned as follows: if the detection of a threatening stimulus is faster than conscious perception, this means that the amygdala must be activated before other areas of the brain (including the visual cortex) that allow us to actually see a potential threat. C. Méndez-Bértolo says that the amygdala (which is part of the limbic system) is one of the best-connected structures, capable of sending and receiving "projections from brain areas at different levels" while at the same time indirectly triggering “physiological changes and responses from the autonomic nervous system.”

For their study published in Nature Neuroscience, the researchers analyzed the amygdala activity of patients who had had electrodes implanted in this area of their brain (along with the visual cortex) for medical reasons (for better diagnosing epilepsy). Two experiments were performed. In the first, the patients were asked to indicate by pressing a button whether the images presented to them (faces expressing fear, happiness, or a neutral expression) were of a man or a woman. Among the visual stimuli, the experimenters also included blurry photos (an image made up of low spatial frequency components), for which it was possible to distinguish whether the eyes or the mouth were open, but no other details. The result of the experiment validated the initial hypothesis, establishing that the amygdala detects fearful faces in 70 milliseconds, while the visual cortex requires over 100 milliseconds to respond and then activate ("watch out, it really is a snake!") or inhibit ("don't panic, it's just a stick!") the amygdala. It’s this delay, necessary for your visual cortex to reassure your amygdala, that causes these false scares.

Indeed, visual information can travel down two different paths: An express route that goes directly from the thalamus to the amygdala, that only the low spatial frequency components follow (thus, very basic visual information may be enough to activate the amygdala) and a longer route that connects the thalamus to the occipital cortex, where traditional visual processing begins (that of high and low frequencies). In the second experiment, instead of faces, complex images that were either neutral or extremely unpleasant were presented to the same patients who were asked to determine whether it was an indoor or outdoor scene. Compared to the previous results, the researchers observed that fast emotional reactions (the express route) were not present for more complex visual stimuli.

This Spanish-Swiss research can help us better understand phobias and anxiety. According to Dr. Bryan A. Strange: “the responses in the amygdala are so fast that they could reflect an automatic or unconscious visual process, which might explain why fear can sometimes feel out of our voluntary control.”