Contents

1 Cognitive Effects of Methylphenidate

Attentional impairments are seen in a variety of conditions, obviously including attention-deficit/hyperactivity disorder (ADHD), but also schizophrenia and Alzheimer’s disease, which have pronounced attentional impairments as a component of the syndrome [6]. Cognitive enhancing drugs are also used in these disorders to reverse the deficits, and to improve learning, memory and attention in these patients. Attention is a hypothetical construct that cannot be measured directly, but is experimentally inferred from behavior in controlled environments. Selective attention is thought to be engaged when an animal faces multiple stimuli and chooses among them. Novelty plays an important role in determining the behavior of the animal and thus, its attentional selection [6]. Sustained attention, by contrast, is thought to be engaged when an animal’s behavior is controlled by a single stimulus that occurs unpredictably in time or space. For example, a light flash or auditory chirp may signal the availability of food and guide the animal’s choice of responses required to obtain that food [6].

1.1 Cognitive Effects of Psychostimulants in ADHD

The complex relationship between performance and psychostimulant medication has been interpreted in accordance with a hypothesized inverted U-shaped function, whereby optimal catecholamine levels determine optimal performance and catecholamine levels along the curve at either side of the optimum are associated with impaired performance [1, 7, 10]. Across well-controlled studies of individuals with ADHD, stimulant-related cognitive enhancements were more prominent on tasks without an executive function component than on tasks with an executive function component [9]. Dose-response studies of stimulant medications suggest that the optimal dose varies across individuals and depends somewhat on the domain of function, with high doses tending to produce greater enhancement on some aspects (e.g. attention, vigilance, memory, and working memory) but not others (e.g. planning, cognitive flexibility, inhibitory control, naming, and motor speed) [3]. MPH treatment was associated with improved spatial working memory and response inhibition in the prefrontal cortex in ADHD children and adults [2]. In terms of academic achievement, evidence suggests that stimulants improve acute academic performance of children with ADHD, but that long-term effects have not been supported [4]. For example, the The Multimodal Treatment Study of Children with ADHD Cooperative Group (MTA) demonstrated treatment with stimulant medications over the 14-month trial resulted in significant improvement of achievement scores in math and reading on the Wechsler Individual Achievement Test (WIAT) immediately post-treatment [8]. However, differences between groups were no longer significant at the 3-year follow up assessment, suggesting that any relative cognitive enhancement may not be sustained [5].

1.2 Affecting the Whole Brain

The ascending adrenergic systems appear to maintain arousal, modulate ‘response vigor’, and reduce the influence of distracting events, although inconsistent effects of manipulations of these systems do not afford firm conclusions [6]. Attenuation of striatal dopaminergic systems primarily slows responding, with little effect on accuracy; however, dopaminergic lesions restricted to the prefrontal cortex impair accuracy without response latency effects [6].

Acronyms

ADHD

attention-deficit/hyperactivity disorder

MTA

The Multimodal Treatment Study of Children with ADHD Cooperative Group

WIAT

Wechsler Individual Achievement Test

References