Neuroscience of OCD
The neuroscience of OCD is still not completely understood. Various factors, such as age and the severity and type of OCD symptoms, could affect what specific neural circuits are involved. However, understanding the neural circuitry behind OCD that is understood can help us understand why obsessions and compulsions emerge. The first concept to understand is that the brain has two primary functions. The first is to manage essential bodily functions, such as heart rate and digestion. The second is to act as a prediction machine, predicting future events based on past experiences using memory systems. This second primary function plays a significant role in OCD.
Cortico-Striato-Thalamo-Cortical Loop
Magnetic resonance imaging (MRI) and positron emission tomography (PET) imaging have shown that there is a particular neural loop in the brain that is very active in those who suffer from OCD. This loop is called the cortico-striato-thalamo-cortical (CSTC) loop, consisting of the cortex, striatum, and thalamus. In people with OCD, there is more metabolic activity, more blood flow, and more neural activity in this CSTC loop. It is believed that dysfunction in this loop accentuates OCD.
Source: Psych Scene Hub
Components
Source: ResearchGate
The cortex is the outermost layer of the brain and is involved in the perception of what is happening around us. The prefrontal cortex is at the front of the brain, and the orbitofrontal cortex (OFC) is at the bottom of the prefrontal cortex. The OFC integrates sensory information and calculates rewards, thereby playing a crucial role in controlling our impulsivity and decision-making. Moreover, the OFC is vital to adapting behavior in response to fluctuating environmental conditions and incentive-driven situations. People view the OFC as the primary hub for emotional processing.
The OFC is crucial for anxiety and decision-making, and the development of the obsessive thoughts that define OCD is associated with increased activity in the OFC. For instance, imagine you were to leave on a road trip, and an hour into the trip, you think to yourself, Did I turn off the oven at home? When you think about this, the OFC sends a signal that something is wrong. The signal's significance lies in our response to it, which determines whether we dismiss the thought or feel compelled to return home and verify whether the oven is off. Of course, someone with OCD who has a checking compulsion would likely feel compelled to return home and check the oven.
Furthermore, the compulsive behaviors exhibited in those with OCD may be in part due to serotonin depletion in the OFC, as this depletion can lead to a disruption in the normal function of the OFC.
The striatum, which consists of two main parts: the caudate nucleus and the putamen, is part of the basal ganglia. The basal ganglia comprise structures such as the caudate nucleus, putamen, globus pallidus, substantia nigra, and subthalamic nucleus. The basal ganglia are located deep within the cerebral hemispheres and brainstem, adjacent to the thalamus. The basal ganglia receive input from the cerebral cortex, which is primarily received first by the caudate and putamen, which serve as the main input nuclei. Output from the basal ganglia is mainly through the globus pallidus and substantia nigra, reaching the cerebral cortex, mainly via the thalamus, and to brainstem nuclei.
Activity within the basal ganglia does not directly initiate movement; rather, it influences other parts of the brain, such as the motor cortex, to influence movement. This indirect influence on movement occurs through the direct and indirect pathways. Put simply, the direct pathway facilitates movement by disinhibiting thalamic neurons involved in motor control. Conversely, the indirect pathway suppresses unwanted movement by inhibiting thalamic activity through a series of inhibitory linkages involving the subthalamic nuclei and the external segment of the globus pallidus.
The direct and indirect pathway circuits produce what are known as “go” and “no-go” actions. Picking up your cell phone when it is ringing to answer a call is an example of a "go" action. This requires the initiation and execution of a voluntary movement, and the direct pathway facilitates this action. Stopping yourself from touching a burning stove after noticing that it would be dangerous is an example of a "no-go" action. This requires the suppression of an involuntary movement, and the indirect pathway helps inhibit this action.
Someone with OCD has an imbalance between the direct and indirect pathways. Someone with OCD will have an overexcitable direct pathway, which will overpower the activity of the indirect pathway. This results in the person struggling to turn their focus away from the obsessions that cause discomfort.
Source: PsyPost
The last main component of the CSTC loop is the thalamus. The thalamus is egg-shaped and situated near the center of the brain, right above the brainstem. The thalamus routes signals from various brain regions to the appropriate areas for interpretation, such as the OFC. The thalamus has various channels through it to relay sensory information from the environment to the cortex, making some sensory experiences apparent while suppressing others. The thalamic reticular nucleus surrounds the thalamus, serving as a gatekeeper for sensory information. The inhibitory neurotransmitter GABA heavily regulates the thalamic reticular nucleus by shutting down or suppressing the activity of other neurons. For example, while reading the information on this website, the thalamic reticular nucleus, with the help of GABA, suppresses auditory information from the environment. Additionally, the thalamic reticular nucleus plays a crucial role in determining which thoughts are allowed to reach our conscious perception. The thalamus collects and processes all our sensory experiences in parallel, except smell, and the thalamic reticular nucleus makes it so only relevant information passes through to our conscious understanding and perception.
Source: Flint Rehab
Another part of the brain that is important for OCD and interacts with components of the CSTC loop is the amygdala. When we face something frightening or when things go wrong, the amygdala triggers anxiety and panic responses. The amygdala is likely hyperactive in those with OCD.
The Loop
When the brain recognizes something as a potential danger or threat, the OFC is activated and sends a signal to the thalamus. The OFC is more active in those with OCD, meaning people with OCD perceive more things as a danger or threat. For instance, let’s say someone with OCD has a fear of germs or contamination, and after shaking someone’s hand, they think, Should I wash my hands to eradicate the germs that could make me sick? This individual perceived shaking the other person's hand as a potential danger. People with OCD will have more of these thoughts.
Between the OFC and the thalamus is the striatum, which is part of the basal ganglia. On the way from the OFC to the thalamus, the initial signal will reach the striatum, which contains the main input nuclei: the caudate nucleus and the putamen. The basal ganglia contains the excitatory direct pathway and the inhibitory indirect pathway. The indirect pathway would suppress the signal, preventing hyperactivity in the thalamus. Conversely, the direct pathway would not suppress the signal, allowing the thalamus to become overly active. Someone without OCD would have a proper balance between the two pathways. People with OCD have an overexcitable direct pathway, making it more active than necessary. In the “washing hands” scenario, the basal ganglia would be involved in deciding whether washing hands would be a “go” or “no-go” action. Because the person has OCD, activity in the direct pathway would likely drown out activity in the indirect pathway, thereby making the action a “go” action and allowing the signal to reach the thalamus and become overly excited. This would essentially amplify the intrusive thought of washing one's hands, making the action of washing hands in this scenario very forced and almost automatic.
When the signal reaches the thalamus, the thalamus can become overexcited, transmitting strong signals back to the OFC. The thalamic reticular nucleus would contribute to making the thought of washing one’s hand basically the only thought allowed to reach one’s conscious perception. When the signal is returned to the OFC, the CSTC loop will repeat and could repeat until the action is completed. While this loop is happening, the overly sensitive amygdala in those with OCD becomes activated, triggering anxiety responses. The person would feel the necessity of washing their hands to achieve a break from the anxiety. The person would find it very difficult to shift their focus, and they would be obsessed with the thought of washing their hands. The more the person indulges in the compulsion of washing their hands after shaking someone’s hand, the stronger the intrusive thoughts become and the more automatic this compulsion becomes in the future.
Exposure and Response Prevention Therapy
People with OCD have an overly sensitive amygdala, as it is overly activated and activated very quickly, leading to more prominent anxiety responses. Put simply, the OFC tells the amygdala whether to activate or not. Usually, the OFC is inhibitory and will tell the amygdala not to activate, therefore stopping any anxiety response. However, in someone with OCD, the OFC is less inhibitory than it should be.
In exposure and response prevention therapy (ERP), the patient is exposed to extremely anxiety-provoking situations in an attempt to quiet down the amygdala. ERP engages the OFC and trains it to tell the amygdala not to activate. The amygdala gradually adapts to the situations and learns to become less overreactive. If ERP is successful, the patient will not feel as anxious as they did before, in the same anxiety-provoking situations.
Source: MantraCare