In this paper, we review changes in the brain that may underlie how people with classic PTSD and the dissociative subtype of PTSD experience sensations from the outside world (e.g., touch, auditory, and visual sensations) and from the internal world of the body (e.g., visceral sensations, physical sensations associated with feeling states). We describe how sensory processing occurs in healthy people. Then, we show that people with PTSD have changes in the parts of the brain that overlap with the neural pathways that are important for processing sensations. We propose that changes in these neural pathways may have cascading effects on the ability of people with PTSD to perform cognitive functions, including emotion regulation, social cognition, and taking action towards one’s goals. Finally, we introduce a model based on how the brain processes sensory information that helps to conceptualize how people with PTSD experience altered sensory processing, how this relates to symptoms in PTSD, and how we can help people with PTSD to process trauma. 

What parts of the brain process sensory information in healthy people? The brainstem is responsible for receiving incoming sensations from the external environment (for example, seeing a bear) and simultaneous sensations from within the body (for example, fear). The brainstem takes in sensory information and sends it to the thalamus, which affects where our attention goes in response to sensory information, and acts as a gate to send sensory information between the brainstem and the cortex, including the insula. The insula helps put the sensory information in context (such as feeling fear at the same time as seeing a bear), which helps guide our behavior. The insula sends this information to parts of the cortex, which are involved in more complex cognitive processes such as emotion regulation, and how we socially engage and communicate with others. This multi-sensory integration is critical for interpreting incoming sensory information and provides context to a sensory experience, which informs how we feel and how we react.

So how are these brain areas different in people with PTSD? Our research has identified that there are two types of PTSD: Classic PTSD, and the Dissociative Subtype of PTSD. Individuals with the classic form of PTSD may experience intrusive memories of past traumatic experiences and may show persistent hypervigilance of their surroundings, even in the absence of threat. Importantly, approximately 14–30% of traumatized individuals present with the dissociative subtype of PTSD, in which people experience depersonalization (feeling disconnected from oneself) and derealization (feeling as though their surroundings are not real), and emotional detachment.

Research shows that people with classic PTSD consistently show hyperactivation of the brainstem, which plays a large role in controlling one’s alertness and hypervigilance. People with classic PTSD also show increases in activity in a part of the brain called the periaqueductal gray (PAG), along with decreases in activity in the prefrontal cortex when there is an imminent threat. The PAG integrates information about how you feel about sensory information, and the prefrontal cortex controls complex cognitive processes like how we think and behave. There are also changes in connections between the PAG and the cortex in people with classic PTSD, which could change how sensory information is relayed in the brain and could contribute to people with PTSD feeling constantly on alert. People with classic PTSD also show hyperactivation of the innate alarm system, including the PAG and the superior colliculus (which is involved in turning your eyes towards a threat).

As well, another part of the brain called the insula is involved in identifying how you feel in your body and emotional awareness. Brain scans have shown that people with PTSD experience increased activation of the insula, which could lead to them feeling constantly on alert. Lastly, changes in the prefrontal cortex are commonly found in people with PTSD, which affects people’s ability to regulate their emotions, inhibit their behavior, and work towards goals.

Interestingly, the PTSD Dissociative Subtype has distinct changes in the brain compared to the classic presentation of PTSD. People with the dissociative subtype of PTSD have shown changes in how the brainstem is connected to the PAG, which could contribute to emotional detachment and depersonalization. People with dissociative PTSD also show hypoactivation of the insula, which could decrease emotional awareness. Lastly, people with dissociative PTSD experience changes in the cortex that inhibit other lower parts of the brain’s innate alarm system, which could contribute to depersonalization, derealization, and emotional detachment.

We now provide a model using a hierarchy that incorporates the brainstem, the insula and the prefrontal cortex to show how sensory processing is impacted in PTSD. The brainstem takes in information about sensory information from inside the body (Interoceptive Sensations) and from the external world (Exteroceptive Sensations). This information is relayed to the insula, which provides awareness of the sensory experience, including about emotions. Then this information is sent to the prefrontal cortex, which provides context about the sensory experience. All of these brain areas show changes in people with PTSD, which could lead to altered sensory perceptions, hypervigilance or hypo-awareness of sensory information (such as sounds, sights or smells that are triggering), and disrupted emotion processing in response to stimuli. Once we integrate this sensory information into our sense of self and sense of the world, it creates an embodied experience. Embodiment refers to how one’s perception of the world affects their sense of self and how their body interacts with the world. When one has an embodied self, they have the ability to coordinate and control their behavioral responses and reassess their thoughts about traumatic memories.