The Neurobiology of ADHD

Robert D. Hunt, MD


September 25, 2006

In This Article

Sensory Attention: Working Memory Arousal, Filtering, Inhibition

Sensory processing. The ability to selectively process incoming information requires both specific and more generalized neurotransmission. The generalized components of sensation require noradrenergic activation of attention. Once we are engaged, the process of focusing requires attachment and filtering: the subjective amplification of stimuli that are important and the inhibition or downregulation of stimuli that are incidental. Attention is essential to processing language involved in the visual attention applied to reading; it is also essential in auditory attention when we are engaged in conversation or are listening to a symphony.

The extent to which we must apply effortful attention depends in part on the characteristics of the stimulus -- whether the stimulus or object of our attention is interesting, colorful, and intrinsically engages our curiosity. The cognitive effort for discrimination partially depends on how distinct the relevant stimuli are from background noise. It is much easier to process information that is moving, changing, and multisensory, such as watching TV or a movie. The attentional burden greatly increases if the stimulus is static, black on white, and does not come to life until we pay attention to it. No wonder many more people would rather see the movie than read the book! In an age of multisensory stimulation and omnipresent entertainment, shifting attention internally towards quiet contemplation or values formation or personal goals and strategies, or even internal problem solving, becomes more difficult.[6]

Inhibition. Our ability to selectively ignore repetitive or nonessential stimuli begins at the sensory, subcortical, and preconscious levels. Our capacity to listen carefully and selectively, to focus on a quiet but informative conversation in a noisy room, for example, requires noradrenergically mediated auditory discrimination.

Our internal emotional state colors the way we process information.[14] When we are depressed, our sensory sensitivity may be dampened and we may more often notice the objects that fit our mood. Similarly, anxiety can also blunt attention by preempting our focus by our internal anxiety, leading to distraction.[15]

Interest. Our ability to pay attention also depends on personal characteristics, such as our degree of alertness, our subjective interest in the subject, and the importance or motivation it has for us at the moment. We often pay attention to stimuli that are not intrinsically engaging, but which reflect our sense of purpose, priority, and importance. For example, we pay attention to taxes, even if we do not find the process or stimuli intrinsically interesting. A premed student may focus intensely on learning organic chemistry even if he or she is bored by the subject.


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