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Understanding ADHD
Understanding ADHD

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2.1 ADHD and the brain

Cognitive tests indicate that individuals with ADHD exhibit poorer performance in a range of cognitive tasks compared with neurotypical controls (Frazier et al., 2004) and seven different cognitive functions have been found to be disrupted in ADHD (Mueller et al., 2017). These include working memory, response inhibition and cognitive flexibility. Collectively these functions are known as ‘executive functions’.

Working memory refers to an ability to preserve a representation of information over short periods of times (seconds). Response inhibition refers to an ability to suppress actions that are inappropriate for a given task. Finally, cognitive flexibility refers to an ability to switch between tasks without significant loss of performance.

As well as showing impairment in these so-called executive functions, Mueller et al. (2017) suggest that individuals with ADHD show impairments in:

  • Selective attention – the ability to preferentially process one stimulus in the presence of other potentially distracting stimuli.
  • Sustained attention – the ability to continuously perform a task over a prolonged period without decline in performance.
  • Response precision – temporal and/or spatial precision in behavioural responses to stimuli.
  • Temporal information processing – the ability to accurately recognise or reproduce time intervals.

To cover the entire brain basis of all of these domains would be a whole course in itself, so this section shall focus on selective attention, where much of the research has been conducted in ADHD.

Selective attention can be driven by two main processes (illustrated in Figure 6):

  • in endogenous attention, we wilfully attend to something based on our current goals (e.g. hunger motivating us to seek food)
  • in exogenous attention, our attention is drawn to a stimulus based on its characteristics (e.g. the appearance or smell of food).
In the top image, endogenous attention, there is a female thinking about a hamburger on the left and the same female eating a hamburger on the right. The bottom image, exogenous attention, shows a female looking at an advert for a hamburger and the same female eating a hamburger on the right.
Figure 6 The two main drivers of selective attention: (top) endogenous attention, (bottom) exogenous attention
  • Take a moment to reflect on your selective attention when you are studying this course. Can you think of an example of an endogenous and exogenous driver of attention in this context?
  • Hopefully you are driven to focus on the course reading because your goal is to learn more about ADHD. When you are directing your attention to the course reading resources; this is an example of endogenous attention. However, if your phone rings or your email notification pops up, you may direct your selective attention towards those stimuli even though you were not intending to do so. This would be an example of exogenous attention.

In ADHD disruptions may be found to both types of selective attention. For example, an endogenous selective attention impairment might be expressed as a difficulty in attending to the specific voice of a teacher because of disruption by intrusive thoughts. By contrast, an exogenous impairment could be expressed through increased sensitivity to irrelevant but salient (most notable) stimuli, such as loud noises outside the classroom.

Various tasks exist that can assess endogenous and exogenous selective attention in people, as well as non-human primates and rodents. By using these tasks in combination with methods that collect information about the brain, such as structural and functional brain imaging techniques, it is possible to unpick which parts of the brain may be involved in different types of selective attention.