Neuroscience of Fear Circuitry

Introduction

How exactly do our experiences shape our brains and behavior? What is the relationship between our subjective feelings and the connections in our brain? One of the best ways to answer these questions is to look at a common feeling all of us have: fear. This topic is divided in several parts: The first blog will discuss about what ‘fear’ looks like in our brains and how living through traumatic experiences impacts the way our brains learn fear. The next blog will showcase two papers from our lab that provide evidence for brain related and fear learning changes due to trauma exposure. The upcoming blogs are intended to showcase how important childhood experience is in shaping the brain’s ability to learn fear appropriately. If you haven’t already, check out our previous ‘Critical Periods Study’ blog as that describes the aims of our study and gives more background info on how we can measure and study trauma.

What do our brains do and how does that relate to our subjective feelings?

All of us have very complicated, yet coordinated and regulated brains that function to learn information from the environment and produce appropriate responses. The brain serves as a central processing unit that organizes inputs and coordinates outputs. Yet, there are millions of nerves that connect our central nervous system (brain) to other body parts. In this way, our brain processes emotions based on how the environment affects our body. The body has nerves that signal this information as electrical activity to brain. The more signals the brain gets from a particular event, the stronger the connection between neurons that are responsible for generating particular emotions. If the event is positive, the neural connection will represent pleasure. But if the event is negative the signaling will represent pain and aversion. Repeated negative feelings can impact our decision making and make us behave more anxiously even when there is no threatening event. For example, you may have heard the phrase: Neurons that fire together, wire together. This is exactly what is happening in the brain as it gets repeated information about fearful events. Once the fear connections are established early in brain development, it becomes very hard to change those brain states and behaviors later in adulthood. Our lab uses this connection between brain and behavior to study how brain activity relates to specific feelings and emotions so that our research can help clinicians identify and treat the effects of trauma at the best times.

To better understand how trauma affects our brains, we first need to learn about three simplified ‘systems’ our brains use to control our emotions and behavior.

1. The ‘reptilian system’ is primarily associated with the brain stem and is responsible for basic survival. This system is shared between humans, mammals, and reptiles such as turtles and lizards. The brain stem controls our autonomic functions such as breathing, blood pressure, digestion, and other muscle reflexes. These functions work in the background to keep us alive and fluctuate based on physical and emotional stress.

2. The ‘limbic system’ contains brain regions such as the amygdala and hippocampus and is responsible for producing emotions. These regions are shared amongst most mammals such as dogs and cats, who might also express emotions. When we define emotion, we talk about two qualities: valence and arousal. Valence refers to whether an experience is positive or negative, and arousal refers to how intense that feeling is. In our Critical Periods Study, we studied these aspects of emotion while our participants looked at pictures in the brain scanner.

3. The ‘cognitive system’ is the most complex system and is associated with the cortex, which is very advanced in primates. Primates such as chimpanzees and gorillas have extensive cortex similar to humans. Though these non-human primates have a cortex, its complexity doesn’t compare to that of the human cortex by any stretch (See the image below for a comparison). Studies have shown that higher surface area and volume of prefrontal cortices indicate a greater emotion regulation abilities across species. This emotion regulation is a key feature of resilience after trauma, and we will see more about this in the next paragraph.

How does trauma change these brain systems?

Now that we understand the three broad systems of our central nervous system, we can start to think about how trauma might affect each of them. It is important to note that these systems are interconnected, for instance, we know that stress not only impacts our emotional brain circuits, but it can also hinder our cognitive functioning. Another instance is when our body uses stress hormones to instruct our cells to use energy for fighting or running away during a dangerous event. Yet, over production of these stress hormones can lead to brain changes that then affect the way we understand and process trauma-related cues. These changes include 1. smaller hippocampus, a part of the emotional system that is responsible for memory and emotional learning; 2. increased activation of the amygdala, another part of the emotional system involved with the fight or flight behavior; 3. decreased prefrontal cortex (higher cognition region) regulation of the amygdala.

Both the amygdala and hippocampus are regulated by the prefrontal cortex, so at times of extreme stress such as trauma, neurons in cognitive regions will serve as brakes for emotional centers. Yet, exposure to trauma can decrease the ability of these cognitive regions to control the emotional stress response. The lack of regulation from higher regions alongside the primitive response from brain stem (increased heart rate, skin conductance, and rapid respiration) can often lead to very unpleasant feelings. So, when the emotional system is imbalanced due to excessive stress hormones, it negatively impacts the communication between other brain systems. Each traumatic event can have additive effects on this miscommunication. The good news is that there is a lot of innate neuroplasticity (natural brain changes) that allows us to withstand traumatic events and rewire the neurons in our brains when treatment is provided.

Trauma exposure during particular sensitive periods of childhood can have very negative effects in adulthood—that is why treatment is important to use early. One form of treatment is exposure therapy where the person gradually talks about the memories of the traumatic event and is reassured about their safety. More research is being done on new types of treatment directly on the brain, such as transcranial magnetic stimulation, or rTMS. This treatment uses non-surgical magnetic stimulation to activate the prefrontal cortex and helps restore the braking system on emotional over-reactions. This can help control fight-flight emotions during anxiety episodes.

At this point, you must be excited to look at our lab’s findings and how they give evidence to the three-system model of brain-behavior proposed earlier. In our next blogs, we will discuss two papers: first one discussing about the emotional center, amygdala, and how that affects fear learning. And the second one will be on how childhood trauma impacts the ability of higher cognitive regions such as prefrontal cortex regulate lower emotional centers. Together, these papers will strongly argue that fear, as a subjective feeling, is highly correlated with communication between distinct brain systems.