To understand research about ADHD and how it implicates specific areas of the brain, it is essential to know about the structure of the brain. The brain has basically three main parts:

1. The cerebrum or thinking part of the brain, characterized by folds and wrinkles

2. The cerebellum or part that controls some motor functions and looks like a small head of cauliflower at the base of the cerebellum

3. The brain stem, a structure that connects the brain to the spinal cord and controls automatic systems such as breathing and digestion.

Each one of these three parts is made of multiple regions. The brain has two halves called hemispheres, which are joined by a tough band of fibers called the corpus callosum. The brain weighs about 3 pounds and has the consistency of thick custard.

Some scientists trace the impairments of ADHD to deficits in the brain. One area of interest is called the frontal lobe of the cerebrum. This area, located to the front and upper part of the brain, allows people to solve problems, plan ahead, understand the behavior of other, and restrain impulses.

A second area of interest is the basal ganglia, interconnected gray masses deep in the cerebrum, which connect the cerebrum and the cerebellum. The cerebellum is responsible for motor coordination and is divided into three parts. The middle part is called the vermis.

Scientists use imaging to look inside the brain. These methods include functional magnetic imaging (fMRI), positron emission tomography (PET), and single-photon emission computed tomography (SPECT). Imaging has allowed scientists to link certain parts of the brain with certain behaviors related to ADHD. Although imaging tools are important for studying the brain, they cannot be used for diagnosis.

Weighing about 3 pounds, the human brain is composed of about 100 billion neurons, the building blocks of the brain. Electrical messages progress from neuron to neuron across a gap called the synapse and are assisted by chemicals called neurotransmitters. Several centers in the brain process, organize, and regulate this constant flow of electrical communication. Three basic types of areas operate to control this communication between the neurons: local centers, regional centers, and integrative centers.

- Local centers. Located in the rear of the brain, local centers process specific types of information, such as perceptions from the senses, seeing, smelling, taste, and touch. The sense organs pick up the stimuli and transmit it to the appropriate area of the brain. These bits of information are only fragmented.

- Regional centers. Regional centers pull all the fragment bits of information to create more complex informational maps. An example is when the visual area of the brain receives the fragmented images of objects from the retina. The visual association cortex assembles the fragments into a more coherent and recognizable picture of what is being looked at during each moment.

- Integrative centers. The integrative centers are not just located in one area like the sensory receptors but are linked throughout the brain to allow for the flow of information from one network to another. These areas assemble data from vision, smell, and hearing, to create a multimedia experience with the outside world.

The networks operate also on both the left and right sides or hemispheres of the brain. Traditional thinking recognized the left hemisphere as dealing with language data, whereas the right hemisphere dealt primarily with visual and spatial operations. Robert Ornstein in 1997 presented research that suggests that the right hemisphere tends to deal more with getting the ''big picture,'' whereas the left side works with details and focuses information. The corpus callosum mediates these messages. Thus, the brain works not only from local and regional centers but also from circuits that crisscross hemispheres.

With all the complex flow of information, a managerial system is essential. Some networks monitor, coordinate, and manage other networks. These systems stop, start, and put everything together and are the networks that are most implicated in ADHD. Areas of the brain include the prefrontal cortex, where working memory circuits are located; the hippocampus, which is responsible for long-term memories; the amygdala; and the cerebellum. The circuits from these areas interact with many others.

The prefrontal cortex, a relatively small area, is located just behind the forehead and takes up slightly less than one-third of the brain's total volume. This area controls working memory circuits and is the only segment fully connected with every functional unit of the brain. Working memory is the ability to take the active thoughts of the moment and link them with stored memories, allowing the person to string together experiences that make sense of what is perceived and to act on those thoughts. Working memory allows the person not only to live in the present but to make sense of the streams from the past. In 1987, Richard Levy and Patricia Goldman-Rakic found specific cells in the prefrontal cortex for spatial working memory in a tiny area of the prefrontal cortex.

Although working memory functions like RAM on a computer, files are not saved unless the command is given to save them. In the brain the hippocampus processes working memory into long-term memory. The brain produces specific proteins that make the connections in a process called long-term potentiation (LTP). If the hippocampus is damaged, the individual does not hold new memories.

However, not all information that is in short-term memory is stored permanently. What determines which information will be stored and held onto and what will fade out? What can be called into conscious attention when it is needed? What drives the ''file manager'' and ''search engines'' of the brain so the information will be there when needed? One of the key elements is emotion.

Circuits in the limbic region, a center beneath the cortex of the brain, are critical in assigning emotional importance and priorities to incoming perceptions. For example, the area tells the brain to act quickly on conditions that are threatening. The amygdala, a tiny structure in the midbrain, screens incoming perceptions for any sign of danger. The danger is not from physical danger like that of an oncoming car but from other situations that might cause pain such as ridicule, disgust, or rejection from others. These experiences are highly individualized. Although some fear reactions are based on instinct, many are based on memories. However, the amygdala not only scans for dangers but also indications that something will be particularly rewarding. The brain uses the neurotransmitter dopamine to highlight important stimuli.

At the back of the brain are centers that regulate alertness and fine-tune cognitive processes. Two structures at the back of the brain regulate stages of sleep and alertness, or vigilance:

- The reticular formation in the middle of the brain stem that has widespread connections throughout the brain and spinal cord

- The locus coeruleus that is important in the regulation of sleep and wakefulness

The primary neurotransmitter chemical in the reticular system and locus coeruleus is norepinephrine. Lack of such firing and reduced norepinephrine is associated with inattention, increased drowsiness, and sleep.

The cerebellum, another structure at the back of the brain, makes up about 10 percent of the total brain volume and has extensive loops to every part of the brain. The cerebellum has such wide functions as correlating which verbs go with certain nouns and may be involved in adjusting social behaviors, such as laughing or crying, to specific occasions.

All the circuits that are involved in brain management do not work in isolation and are interactive. Multiple circuits integrate perceptions, assign importance, facilitate memories, regulate alertness, and control emotion. They interact continuously to maintain the activities of daily life.

Bibliography:

1) Barkley, Russell. 2006. Attention-deficit hyperactive disorder: A handbook for diagnosis and treatment. 3rd ed. New York: Guilford Press

2) Brown, Thomas E. 2005. Attention deficit disorder: The unfocused mind in children and adults. New Haven: Yale University Press

3) Levy, R., and Goldman-Rakic, P. S. 1999. Association of storage and processing functions in the dorsolateral prefrontal cortext of the nonhuman primate. Journal of Neuroscience, 19:5149-58

4) Ornstein, Robert. 1997. The right mind: Making sense of hemispheres. New York: Harcourt Brace; Piano, Marina. 2003. Scientists Use MRIs to study ADHD, depression in children. San Antonio Express-News (Texas), May 19:F1

5) Shaw, Philip et al. 2007. Attention-deficit/hyperactivity disorder is characterized by a delay in cortical maturation. Proceedings of the National Academy of Sciences USA 104:19649-54.

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