ADHD vs. OCD: Brain regions and bloodflow patterns

ADHD and OCD (obsessive compulsive disorder) are two disorders that often fall at the opposite end of the neurochemical spectrum. However, the two disorders, which are sometimes comorbid (occur alongside one another), actually share a fair degree of similarity with regards to underlying causes. It is believed that both disorders are the result of chemical imbalances in similar brain regions. One of these brain regions is the prefrontal cortex (orange region in figure below, please click here for original image source):

In the brain diagram above (side view, the left is the front of the head), the area highlighted in orange constitutes what is referred to as the prefrontal cortex region. We have previously alluded to the connection between the prefrontal cortex region and ADHD. It is believed that levels of the free signaling neurotransmitter dopamine are significantly lower in this region of the brain in ADHD individuals.

In addition, evidence strongly suggests a reduction in blood flow in the prefrontal cortex for ADHD individuals. This region can be likened to a "braking" region in the brain, in which inhibitory judgment and control of behaviors is thought to occur. Therefore, under activity in this critical brain region, either via deficiency in blood flow or chemical signaling agents and processes is thought to reduce the ability of the individual to inhibit unwanted responses and behaviors. As a result, impulsive behavior, which is a hallmark characteristic of ADHD is more likely to occur if this brain region is underactive.

On the other hand, this brain region has also been implicated as a critical brain region in cases of Obsessive Compulsive Disorder (OCD). However, unlike ADHD, where the prefrontal cortex region of the brain is believed to be underactive, in cases of OCD, the prefrontal cortex activity is thought to be overactive.

An increasingly popular method of determining brain activity is performed by using a process called SPECT. SPECT, which is short for Single Photon Emission Computed Tomography, utilizes a radioactive tracer of a compound which involves a radioactive isotope of the element Technetium. This chemical complex of Technetium can be used as a tracer to measure blood flow patterns to the brain, which is a method of detecting overall brain activity. In general, the higher the level of this "marked" blood, the more active a particular brain region is thought to be. This can be especially useful in determining which brain regions are used for specific tasks, such as problem solving or exercises involving heavy concentration. It is often the change in blood flow patterns between "resting" or "relaxed" states vs. cognitive tasks which can often give a clear picture of information as to how "hard" a specific brain region is working to complete the specific task.

While the above process is relatively safe and non-invasive, the idea of doing brain scans involving radioactive tracers and gamma radiation detection methods have significantly limited the use of this method of brain imaging in children and adolescents. However, since both ADHD and OCD often present themselves as disorders originating in early childhood or adolescence, relatively few studies have been done on SPECT-derived brain images in these individuals.

Returning to the ADHD vs. OCD topic of discussion, a relatively recent study was performed using children and adolescents with one of these two disorders, making it one of the few reported studies of its kind involving this particular age group of the population. Although relatively small in the number of test subjects, the study did confirm earlier presumptions regarding blood flow and brain activity of these two disorders, such as in a recent SPECT study on adults with ADHD.

The study found that there was a significantly lower level of blood flow (and therefore brain activity) in specific brain regions for the ADHD children vs. those seen in the OCD children. A strong attempt was made to compare images of ADHD vs. OCD children of the same age and gender in order to reduce the impact of developmental differences.

Below is rough sketch of some of the brain regions compared between the ADHD and OCD children from the study (for original image source, please click here). In this diagram, we are looking at the brain from the right side of an individual facing to the right. The term "cortex", used throughout this post, refers to the outer layers of a particular region. The term "prefrontal" cortex, as seen in the previous diagram, typically refers more toward the outer layer of the brain, right behind the forehead region:

Using the above diagram as a reference, here are some of the findings by Oner and coworkers regarding the differences in cerebral blood flow between ADHD and OCD children:

A quick word of caution: I am not going to go over the statistical methods used in the study in detail. However, given the relatively small sample size and numerical "cutoffs" for a difference to be statistically significant (as opposed to getting a difference just because of random chance due to natural variations with regards to sample sizes), the only region which met the criteria of being statistically significant in this study was the right prefrontal cortex. Nevertheless, there were some differences in blood flow patterns for some other brain regions, which, while not statistically "significant", were still somewhat noteworthy. The left prefrontal cortex should be noted in particular. Keep in mind that for this region it appears that activity is higher in ADHD than OCD, while the opposite is true for the right prefrontal cortex. I thought this difference was worth at least a mention in this post.


A few things worth noting from these differences in brain function between OCD and ADHD individuals:

• Both ADHD and OCD are believed to be disorders associated with the glutamatergic system. While we will not go into too much detail here, glutamatergic activity involves glutamate, which is a form of one of the common amino acids and is a major neurotransmitting (a signaling process between cells in the nervous system) agent. ADHD is believed to be a hypoglutamatergic disorder (lower than normal activity of the glutamatergic system) while OCD is believed to be hyperglutamatergic (higher than normal activity of the glutamatergic system). In other words, ADHD and OCD are two disorders which are both believed to be imbalances of the same signaling or neurotransmitting system, but on opposite sides of the spectrum.

• Gender may play a role in the magnitude of difference between metabolic activities in the brains of ADHD individuals. A study by Zametkin and coworkers found a more pronounced difference in brain metabolism in girls with ADHD. A similar finding was seen in a study by Ernst on reduced brain metabolism in hyperactive girls.

• ADHD, OCD and Tourette's Syndrome may all share a common pathway involving a group of brain regions called the CSTC (which is short for cortico-striato-thalamo-cortical pathway). While I will not go into any more detail here, and save this for future discussion, this potential connection is worth mentioning because these three disorders often have a relatively high degree of overlap. We have already investigated some of the overlap between ADHD and Tourette's in previous posts.

• Additionally, ADHD, OCD, autism and schizophrenia have all been connected to the frontostriatal region of the brain. The frontostriatal brain system is comprised of multiple brain regions, one of them being part of the prefrontal cortex.

***Please note: I do not want to open the door of erroneously linking multiple unrelated disorders together. I believe that it is one of the negative tendencies of researchers to attempt to link multiple disorders together based on insufficient evidence in an attempt to find some sort of unified underlying cause to everything. While I admit that I myself am susceptible to this natural bias as well, I try to avoid making these types of false conclusions as much as possible. Nevertheless, the last point was meant more to illustrate that a number of disorders which have been frequently listed as comorbid to ADHD do tend to exhibit differences in overlapping brain regions, especially the prefrontal cortex. In my opinion, the prefrontal cortex is, therefore, potentially the most critical brain region to study when investigating ADHD comorbid disorders.

While the prefrontal cortex region is a crucially important brain region with regards to ADHD and related disorders, it is by no means the only one involved in these processes. We will investigate some of these other key brain regions in posts in the near future.