COMMENTARY

The Neurobiology of ADHD

Robert D. Hunt, MD

Disclosures

September 25, 2006

In This Article

Prerequisites of Attention

Arousal and alertness are essential to maintaining vigilance, to scanning the environment both inside the cockpit and outside the aircraft. Sensory processing of both visual and verbal information, as well as their correct interpretation, is essential in order for the pilot to determine his position, altitude, pitch, and direction. The pilot continually shifts his attention by changing focus from flight to engine instruments, from visual to auditory information. Even though he hears conversations with all aircraft on his frequency via air traffic control, he is constantly filtering auditory signals in order to respond only to those with his call letters, 864JWN (Juliet, Whisky, November). One of the tasks of a pilot is to be highly selective in the information he transmits to flight control, thus exemplifying selective expression. He must inhibit his response to both extraneous instructions directed to other aircraft and unessential information that would clutter the airways and consume valuable air traffic control time. Much of initial flight training addresses communication that is highly effective and teaching a would-be pilot to provide only ended responses to direction and inhibit unnecessary chatter. All of these processes consume working memory, which is capacity-limited by how much information one can hold in memory at one moment (such as the length of digit span forward or backwards).

Effective attention requires sufficient alertness to mobilize filtering and sustain connection and interpretation of information. The ability to pay attention depends on maintaining adequate arousal to support adequate alertness in order to process information effectively. It is hard to sustain attention when we are fatigued or depressed. Attention involves emotion as well as cognition and reflects the sensorial strength of the external stimulus and the salience or degree of interest a person has.

At many levels, attention is like a dance requiring the coordination of cortical and subcortical functioning.[9] Spatial attention, for example, involves subcortical modulation of sensory experience.[10]

The neurobiological substrate of arousal encompasses several brain systems. The most critical structures for maintaining alertness include the reticular activating system. This system, beginning in the brain stem, is primarily activated by norepinephrine (NE) arousing from the locus coeruleus (LC) located in the area of the pons. The LC sends diffuse projections throughout the cortex and cerebellum, extending to the limbic system and down the spinal cord. These projections regulate both tonic (baseline) level of arousal and phasic (episodic) or event-activated arousal.[11,12,13] Tonic arousal helps us to stay awake while driving at night, even when we are tired. Phasic arousal is activated when the traffic suddenly stops on the freeway, and when, facilitated by a sudden NE surge, are able to quickly slam on the brakes and swerve.

Orientation. This NE surge, or burst of energy, enables us to orient to a situation, respond immediately, ie, to come "alive" and react quickly. The diffuse localization of NE neurons facilitates this broad impact on behavior and thought essential for survival in the jungle, or in an environment filled with surprise and danger. The spatial localization of events essential to orientation involves the parietal lobe.

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