With the Halloween season in full swing, horror movies, haunted houses, and jumpscares are all terrifying us. But have you ever wondered how we feel fear in the first place? How come your brain knows what to be afraid of and what’s going on in your brain when you’re having a fight-or-flight response?
The way different brain systems interact in the moment in response to a threatening context has been a research topic for many neuroscientists and psychologists. Fear is described as a motivational state caused by certain stimuli that cause us to act defensively or flee the threat, i.e. fight or flee.
When you’re turning the corner in a haunted house and a “ghost” jumps at you, your brain takes those quick movements and loud noises as enough stimulus to trigger your fear response, causing you to scream or freeze.
At its core, fear experiences have a physical or spatial context; All threatening experiences have taken place in a specific place and time. Our brain uses these contexts and memories to inform future expressions of fear.
To examine the importance of context in experiencing fear, some labs use a technique known as fear conditioning. Fear conditioning repeatedly pairs neutral and aversive stimuli over time, which can result in fearful responses to the neutral stimulus—a stimulus that previously would not have elicited a response in the subject. Accordingly, certain contexts can elicit fear if previously paired with something aversive.
The fear conditioning technique is a valuable behavioral paradigm for understanding the neural basis of human psychiatric disorders related to anxiety and fear.
Studies using this technique and others identified the hippocampus as central to context representation and memory formation. The two-process theory suggests that both the hippocampus and the amygdala are involved in contextual fears, with the former being involved in storing the context of the traumatic event and the latter being involved in storing the experience. When faced with a threatening event, the co-activation of these two brain areas drives physiological and psychological responses to fear, such as increased breathing, increased heart rate, release of stress hormones, and feelings of fear and anxiety.
When presented with a situation resembling a previously experienced threatening context, the hippocampus decides between processes called pattern completion and pattern separation to determine an outcome.
“If a context is sufficiently similar to the memory of the threatening context, the hippocampus will automatically complete and retrieve the memory of the threatening context,” explains Robert Rozeske, an assistant professor at the University of Toronto who studies neural mechanisms of anxiety and fear, in an email to The University. “When a context bears little resemblance to memory [of] the threat context, the hippocampus classifies this context as different, thus separating the current context from the threat context [in] Memory.”
He further explained that in cases where context is ambiguous, it is believed that the prefrontal cortex is essential to resolve ambiguities related to context. This process underscores the importance of having different brain regions work together to achieve the same goal: understanding fear.
One problem these systems and fear responses face is overgeneralization. While the brain is largely efficient at predicting the outcomes of current situations based on past experiences on a daily basis, this efficiency can lead to over-generalization. “When sufficiently intense trauma occurs, a person in an otherwise non-threatening environment may generalize about their current situation to closely resemble a traumatic memory,” Rozeske wrote. Post-Traumatic Stress Disorder is one such consequence, which theoretically occurs due to impairment of a circuit consisting of the hippocampus and the prefrontal cortex for context processing.
Using rodents as model organisms, Rozeske is interested in the dynamic expression of emotional behaviors, using the power of techniques such as optogenetics – the use of light to activate or deactivate neurons – to decipher whether the activity of specific neurons is essential for expression behavior is required by fear. His recent work found that the prefrontal cortex in mice presents contexts differently after the mice were subjected to contextual fear conditioning, which converts those contexts from neutral to threatening and vice versa. His team also identified a circuit that activates when mice move from a threatening to a neutral context. The circuit contains the prefrontal cortex and a midbrain structure called the periaqueductal gray.
Future studies of fear should examine how we indirectly experience fear and how the brain perceives and responds to the fear expressed by others. Understanding these mechanisms will give us a better understanding of fear and social expressions in general.
So the next time you’re wandering the unknown in a haunted house when someone runs at you with a saw, you’ll know exactly why you want to sprint away at full speed!