What are you afraid of? The Neuroscience of Fear
With Halloween fast approaching, people are going to get scared. Zombies, ghosts, and werewolves will soon be stalking the streets of Boston, frightening innocent college students. Yet, when we are jumping back in fright from costumed pranksters, what is really happening inside of our brains? For years, it was considered fact that the amygdala, a part of the limbic system in our brain that processes components of emotion, was solely responsible for this reaction. Yet, this simplistic explanation doesn’t truly explain was happens inside our brains every time we feel fear. To investigate what really happens, we need to first talk about anxiety.
Anxiety is similar to fear, though some experts say that anxiety is less centralized and has less of a focus. There are chemicals, called anxiogenics, in the brain that can create a heightened anxious response in the brains of humans and animals. Conversely, there are neurotransmitters known as anxiolytics that help to reduce anxiety. Benzodiazepines, a class of drug often prescribed for anti-anxiety, are actually exogenous forms of natural anxiolytics.
When a frightening stimulus is first presented, it has been postulated that it is not the amygdala that first responds, but rather a small area of the pons called the locus cereleus. Activity in this region activates a stress response that sends messages to other parts of the brain, including the hypothalamus, the hippocampus, the amygdala, and the prefrontal cortex. This has beed dubbed the “fear circuit.”
The pathways of this circuit have been studied using methods such as electrical brain stimulation and investigations into the deficits found when brain areas are lesions. The pathways have been mapped as such: as the locus cereleus receives sensory information from the thalamus, it sends out stress signals to the other parts of the brain, including the hippocampus. These stress signals help to activate different neural pathways in the amygdala that produce a number of natural fear responses- the “freezing” in fright behavior, adrenocortical hormone activation, and a number of other autonomic nervous system responses to threatening stimuli. In addition, the amygdala contains the fear chain by sending signals to the prefrontal cortex (PFC) and hypothalamus.
The prefrontal cortex, in the forebrain, is mostly responsible for “higher thinking” or the capabilities we consider to be purely human. Yet, it is also clear that the prefrontal cortex is also highly involved in emotional processing. Lesion studies have shown that the right (but not nearly as much in the left) PFC is involved with anxiogenesis and it is thought that the PFCis responsible for the mental interpretations of fear.
The hypothalamus, often thought of as the director of whole-body endocrine effects, is the next major stop in the fear circuit. A specific area of the amygdala called the BNST sends out messages to the lateral hypothalamus and other areas that activates specific panic responses, part of sympathetic nervous system response to threatening stimuli. Given the answers we have now, this completes the major immediate processes of the fear circuit- in actuality, there are surely much more complex, long-lasting effects of fear-feeling in the brain, already a sophisticated field of study very appropriate to this time of year. Happy halloween kids, post in the comments below costume ideas that you think will elicit the best fear-response!
The biology of fear- and anxiety-related behaviors – Dialogues in Clinical Neuroscience