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The Neurobiology of ADHD

                                                                                                                                            

It is generally agreed among the scientific community that ADHD is a biological condition resulting from disfunction in the brain. Gradually, neurologists and neuroscientists are uncovering the neural mechanisms that underlie ADHD. One way in which they are doing this involves utilising brain imaging techniques.

 

This is a picture of a comparing the brain of an ADHD child and a non-ADHD child (PET scan). From Zametkins et al. 1990

 

 

 

Brain imaging

 

Brain imaging provides us with a means of photographing the brain and watching it as it works. There are several different methods of imaging:

 

• Computerized tomography (CT)

• Magnetic Resonance Imaging(MRI)

• Functional Magnetic Resonance Imaging(fMRI)

• Positron Emission tomography (PET)

• Single Photon Emission tomography (SPECT)

 

Many studies using these imaging techniques have been conducted with ADHD children...

 

So what do we know about ADHD from these brain imaging techniques?

 

Using the methods presented above, researchers have begun to find differences between ADHD children and non-ADHD children. Up until recently, most of the imaging studies have been structural (CT and MRI scans), this means that they take static pictures of the brain.

 

Brain Imaging - Structural

 

The evidence has highlighted structural differences between ADHD and non-ADHD children. The majority of this evidence suggests that a region known as the frontal cortex is involved in ADHD (the orange coloured area in the picture below right).

The frontal cortex is part of the cerebral cortex (or the outer surface of the brain). The cerebral cortex is broken up into 4 lobes:

 

• The Temporal lobe

• The Occipital lobe

• The Parietal lobe

• The Frontal lobe

 

 

(All of these areas are highlighted in the picture below)

 

The frontal cortex is involved mainly with executive functions. These include:

 

• problem solving

• attention

• reasoning

• planning

 

Note that ADHD suffers usually have deficits in these functions. These deficits become obvious in tests that are used during diagnosis, such as the Stroop test.

The brain has two hemispheres. The left is responsible for language and calculation, while the right is involved in attention.

 

Some evidence has suggested that right frontal lobe is smaller in children with ADHD than non-ADHD children (the right side of the brain is generally considered to be involved in attention processes). This data was supported by the close similarities in symptoms between ADHD and people who have suffered frontal lobe damage or right hemisphere damage through illness or accident.

Brain imaging - functional

 

New technology has allowed us to advance from looking at the brain through static imaging techniques (such as CT scans< and MRI scans). Now we have functional imaging techniques (SPECT & fMRI) which allow researchers to view the brain while is works - a moving picture if you like.

Functional studies have exposed a decrease in the metabolic activity in the right frontal lobe, but also in an area known as the basal ganglia (this region is responsible for regulating movement and is connected with the frontal lobe region).

Recently these functional imaging techniques have pointed at 3 areas closely related to the basal ganglia, believed to be responsible for the symptoms of ADHD:

 

Some researchers believe that problems in the circuit between these three regions are the underlying mechanisms that cause ADHD symptoms.

The prefrontal cortex is thought to be the brain's "command center" while the other two parts translate the commands into action.

 

How does Ritalin and other medication help?

 

Despite it's wide spread therapeutic use over the last 50 years, little is known about how Ritalin works. New research, however, has provided us with insight into the mechanisms of Ritalin in the brain.

 

One more thing:

 

The evidence suggested here is simply an introduction. Unfortunately there is a lot of variance in ADHD data (some imaging studies have found left hemisphere disfunction rather than right) and it is difficult to know what to believe.
For further articles on the neurobiology of ADHD, including a review of the imaging studies..
This page is continually being updated as new data comes to hand. We will make you aware of any future developments.

 

Hey kids, check out this cool web site about the brain! And parents, for a great article on the neurobiology of ADHD by Dr. Castellanos (a pediatrician/child psychiatrist and ADHD specialist) Click here.

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