Cognitive and neural basis of ADHKD

AD-HKD is an aetiologically heterogeneous disorder caused by a range of biological, psychological, and social factors that most likely interact to increase the risk of having AD-HKD. Presumably, these factors have their effects on the neural substrate of cognition—for example, on executive functions, self-regulation, arousal, and motivation. These are the delaying, future-oriented, and intentional control processes of the cognitive system that guide action and behaviour. (63) They help maintain alertness, sustain attention and effort, inhibit inappropriate actions, and adjust behaviour when errors are made and circumstances change. Deficits in these processes result in the poorly regulated behaviour that is characteristic of AD-HKD in everyday life and during laboratory tasks. (64> These deficits produce a cascade of secondary impairments in behaviour, working memory, self-regulation of affect, internalization of speech, and development of conscience. (6>

Executive functions depend on a distributed neural system involving the prefrontal cortex and associated subcortical structures. (6 ,,66> These neurones are rich in noradrenaline and dopamine—neurotransmitters that are known to play an important role in attention. (6 68) The involvement of these neural substrates in executive function is confirmed by primate lesion studies, (69,70) as well as by human imaging, electrophysiological, and neuropathological studies. (7 72)

In children with AD-HKD, neuroimaging studies confirm abnormalities in those regions of the brain that are implicated in executive function. (73> These studies report significantly smaller asymmetrical prefrontal and basal ganglia structures, particularly on the right side, in children with AD-HKD than in matched controls. (7 75) Correlations of magnetic resonance imaging-based anatomical measures and specific-task performance in children with a diagnosis of AD-HKD suggest that the right prefrontal cortex is involved in inhibiting attentional and behavioural responses, whereas the basal ganglia seem involved in the execution of these responses. (76)

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