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Executive function refers to a set of cognitive abilities that facilitate control over voluntary behaviors, including attention control, working memory, and cognitive flexibility. While executive functions are critical for complex tasks such as planning, they are also mentally taxing. Without sufficient motivation, people with poor executive function may struggle to meet goals. Researchers report their findings that dopamine signaling is responsible for the effects of Ritalin and other stimulant medications on motivation and executive function.
Dopamine is one of the most abundant neurotransmitters in the brain and is involved in reward-motivated behavior, learning, and memory. Activities that provide a reward (e.g., food, money) increase dopamine levels, causing a sensation of pleasure that enhances learning by deeply encoding memories related to rewarding activities. Motivation to complete a task is based, in part, on whether a task is judged to provide sufficient pleasure relative to the cost of its required effort. Capacity to synthesize dopamine varies from person to person; however, lower dopamine levels in key brain areas are associated with attention deficit hyperactivity disorder (ADHD), substance use disorders, and Parkinson’s disease. Drugs such as methylphenidate (i.e., Ritalin), a medication used to treat ADHD, and sulpiride, a medication used to treat schizophrenia and depression, interact with dopamine receptors in the brain and can increase motivation.
The authors recruited 50 healthy adults (ages, 18 to 43 years). Participants completed a test called a cognitive effort-discounting paradigm. In this test, participants are asked how much money they would want to receive in exchange for completing tasks of varying difficulty. The authors measured the estimated effort cost as the amount of money necessary to make participants willing to perform a cognitively difficult working memory task. Participants completed effort-discounting tasks under the influence of 20 milligrams of methylphenidate, 400 milligrams of sulpiride, or a placebo on three separate testing days. The researchers used a positron emission tomography (PET) scan to measure dopamine synthesis capacity in the caudate nucleus, a brain region responsible for reward-based learning. Finally, the researchers used a statistical model based on the effort-discounting task to further explore the effects of methylphenidate and sulpiride on motivation.
While on the placebo treatment, participants’ willingness to expend cognitive effort increased as their baseline dopamine synthesis capacity increased. Notably, while performance on the working memory task decreased with difficulty, there was no relationship between task performance and dopamine levels. Both methylphenidate and sulpiride increased willingness to expend cognitive effort, but only in participants with low baseline dopamine synthesis capacity. Using their computer model, the investigators found that methylphenidate increased feelings of reward while sulpiride decreased effort cost. Further, they found that the cost-benefit analysis involved in the decision to expend effort occurs early in the decision-making process and can be measured by patterns in gaze (focusing on a reward or cost of a task) during cognitive testing. While higher baseline dopamine synthesis capacity and drug administration did not affect gaze patterns directy, higher dopamine levels strengthened the impact of gaze and attention to the benefits versus the costs of a decision.
These findings indicate that Ritalin and other attention-enhancing drugs work by increasing willingness to attempt cognitively-difficult tasks, not the ability.
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