Identifying safe or dangerous situations are essential for survival. A child may be fearful of crossing a road alone, but he will be completely relaxed while crossing the same road with his mom. The inability to discriminate between dangerous and safe situations produces responses that may lead to anxiety disorders or post-traumatic stress disorder (Pitman et al., 2012). The fear circuitry in the brain has been extensively mapped out with many studies focusing on the amygdala as the primary player in fear regulation (LeDoux, 2000). In fact, many undergraduate students in psychology and physiology are taught that the amygdala is the “fear center” of the brain. However, recent experiments demonstrate that the amygdala is responsible for regulating safety, too!
Almost one year ago, scientists found “safety signals” in the rat amygdala (Sangha et al., 2013). To do this, electrodes that recorded neurological activity were implanted into the amygdala. As shown in the animation below, the rat received either an auditory tone and a foot shock that signaled fear, or a visual light with no foot shock that signaled safety. In addition to the distinct fear and safety signals, there was a third signal, in which the fear signal (tone) and the safety signal (light) occurred at the same time but without a foot shock (the rat was safe). This “safety + fear” signal is analogous to the child crossing a road; if the child crosses a road alone, it elicits fear, whereas if the same child crosses the road with his mom, it elicits safety.
A few months ago, another research group did a similar experiment in the primate (monkey) amygdala and also found “safety signals” (Genud-Gabai et al., 2013). In these experiments, two distinct auditory tones were used to signify either fear or safety. One tone came in conjunction with an air puff, which elicited fear, while the second tone was not paired with an air puff and elicited safety.
In both experiments in rats and monkeys, the neurons in the amygdala responded to the fear signal. Surprisingly, they also responded to all the measures of the safe signal: the safety signal alone as well as the “fear + safety” signal. Does this mean that the amygdala signifies both fearful and safe situations?
Interestingly, the neurons in the amygdala that respond to fear are not the same as the ones that are active during the safe signal. This suggests there may be a separate neuronal circuitry within the amygdala to signal dangerous and safe situations. Perhaps the results of these studies will guide novel therapies for people with anxiety disorders and post-traumatic-stress disorder, since the amygdala plays a role in the expression of both disorders. Further experiments may reveal the specific circuitry involved with normal and abnormal learning of fearful and safe situations. In the meantime, these studies have induced a paradigm shift, in which the amygdala cannot be simply defined as the brain’s “fear center.”
Written by Don Davies.
LeDoux J.E. (2000). Emotion Circuits in the Brain, Annual Review of Neuroscience, 23 (1) 155-184. DOI: 10.1146/annurev.neuro.23.1.155
Pitman R.K., Rasmusson A.M., Koenen K.C., Shin L.M., Orr S.P., Gilbertson M.W., Milad M.R. & Liberzon I. (2012). Biological studies of post-traumatic stress disorder, Nature Reviews Neuroscience, 13 (11) 769-787. DOI: 10.1038/nrn3339