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Brain Differences in ADHD

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Modern technology has given us several ways to learn about the brain:

  • Magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI) are examples of structural imaging. Structural imaging gives a two- or three-dimensional picture of the brain. However, it does not give any information about brain activity or how the abnormalities affect the child’s functioning.
  • Functional magnetic resonance imaging (fMRI), electroencephalography (EEG), and magnetoencephalograpy (MEG) are examples of functional imaging. Functional imaging gives information about activity in specific areas of the child's brain while the child is performing certain tasks.

At present, these techniques cannot be used to help diagnose ADHD. They are only used for research. However, future advances will give us more information about brain structure and function in ADHD. It is possible that these advances may help us to diagnose ADHD as well.

Several recent studies using these techniques have given us information about subtle differences in the brains of people with ADHD. These brain abnormalities probably contribute to the behaviour symptoms and cognitive problems of people with ADHD.

So far, there is no evidence that treatment with stimulant medication causes the brain abnormalities seen in ADHD.

Brain size and structure and ADHD

One study looked at children with and without ADHD over a 10-year period. At various ages, children's brains were scanned using magnetic resonance imaging (MRI). The researchers found that:

  • The brains of boys and girls with ADHD were 3% to 4% smaller than the brains of children without ADHD.
  • Children with more severe ADHD symptoms had smaller frontal lobes, temporal grey matter, caudate nucleus, and cerebellum. These brain regions are involved in concentration, impulse control, inhibition, and motor activity, which are all problem areas for children with ADHD. They are discussed in more detail below.
  • The course of brain development in children with and without ADHD was similar. This suggests that changes in the brain happen early in development.

Other studies have used functional MRI (fMRI) to measure brain activation while children are performing various tasks. These studies have shown that reductions in brain volume in children with ADHD are associated with poorer performance on:

  • tests of attention and inhibition
  • measures of behaviour

White matter and ADHD

A new technique called diffusion tensor imaging (DTI) lets scientists look at the white matter in children's brains in more detail. White matter consists of axons (nerve fibres) covered by myelin sheaths. DTI lets us look at the nerve pathways between different areas of the brain.

Grey Matter and White Matter
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The outer layer (or cortex) of the brain is called grey matter, and is made mostly of neuron cell bodies and synapses. White matter is underneath the grey matter, and is made up of long neuronal axons covered by a fatty sheath called myelin. Myelin insulates the axons and facilitates the conduction of electrical impulses.
A recent study using DTI found abnormalities in the fibre pathways in the frontal cortex, basal ganglia, brainstem, and cerebellum. These areas are involved in attention, impulsive behaviour, inhibition, and motor activity. They are discussed in more detail below.

The findings suggest that specific brain circuits that connect different areas of the brain may be altered in people with ADHD. It is not surprising, therefore, to find that people with ADHD often have problems with regulating attention, behaviour, and learning.

An introduction to the brain

There are three major parts of the brain: the cerebrum, the cerebellum, and the brainstem.

Brain Areas Involved in ADHD
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The cerebrum

The cerebrum is the largest part of the brain and fills most of the upper skull. The cerebrum uses information from our five senses to help us understand what is happening around us. Then it tells our body how to respond. It also controls our emotions and our ability to talk, think, read, and learn. The surface of the cerebrum is called the cerebral cortex or “grey matter.”

The cerebral cortex is divided into the right and left cerebral hemispheres. The two hemispheres are connected by a thick band of nerve fibres called the corpus callosum, which allows them to communicate and share information.

Each hemisphere is divided into lobes: frontal, temporal, parietal, and occipital lobes.

  • The frontal lobes are large, complex structures. They contain the motor cortex, which controls movement. They are also important for speech, planning, problem solving, social and emotional behaviour, self-awareness, and self-control.
  • The temporal lobes are the main area responsible for memory about facts and events. Together with the limbic system, they help us express emotions and understand the emotions of other people. They seem to have an effect on personality. They are also very important for hearing, and help us understand language and sounds such as music. Children with more severe ADHD symptoms had less temporal grey matter in one study.
  • The parietal lobes interpret sensations and messages from other parts of the brain. They make connections between the information from all the different senses and store memories. These lobes interpret touch, temperature, pain, sounds, and visual information about objects and the environment. They help us understand shape, size, texture, and direction.
  • The occipital lobes contain the primary vision centres, as well as areas that help us visually recognize objects and understand what written words mean.

Underneath the surface of the cerebrum is the “white matter” and deeper structures: the basal ganglia and the limbic system, which are closely connected.

  • The basal ganglia are a group of structures around the thalamus, which include the putamen, globus pallidus, and caudate nucleus. The basal ganglia are important for voluntary movement and contribute to learning skills. They control our response to reinforcement or rewards.
  • The limbic system is a complex network of brain areas that includes the amygdala and the hippocampus, as well as the interior parts of the temporal, frontal, and parietal lobes. The limbic system is the “primitive” or “animal” part of our brain. It controls our immediate, automatic responses to stimuli  -- our “gut reactions.”

The cerebellum

The cerebellum is located under the cerebrum at the back of the brain. It coordinates our balance and complex movements. For example, actions such as walking or playing the piano are coordinated by the cerebellum. It contributes to the control of speech, and also participates in many of the functions controlled by the cerebrum in ways that are not understood fully.

The brainstem

The brainstem connects the brain and the spinal cord. It passes messages back and forth between parts of the body and the brain. The brainstem controls functions such as breathing, blood pressure, body temperature, heart rhythms, hunger and thirst, and sleep patterns.

Peter Chaban, MA, MEd

Rosemary Tannock, PhD