In a recent study published in the Journal of Neuroscience, researchers found that individuals with Attention-Deficit/Hyperactivity Disorder (ADHD) show reduced motivation to engage in effortful activities, both cognitive and physical, which can be significantly improved with amphetamine-based medications.
ADHD, a common behavioral disorder, is often associated with difficulty in maintaining attention, hyperactivity, and impulsiveness. Traditionally, ADHD has been viewed primarily through the lens of these symptoms. However, recent research suggests that motivation, particularly the willingness to exert effort, plays a crucial role in ADHD. This new study was conducted to explore how individuals with ADHD differ in their motivation for effortful tasks compared to those without the disorder, and to assess the effectiveness of commonly prescribed ADHD medications in addressing these differences.
“A hallmark of ADHD is thought to be reduced levels of motivation – in particular, a lower willingness to invest effort,” said study author Trevor Chong, an Australian Research Council Future Fellow, head of the Cognitive Neurology Laboratory, and an associate professor at Monash University.
“Effort can be experienced across multiple domains – for example, it can be perceived cognitively (such as when studying for an exam), or physically (such as when training for a race). Importantly, despite the importance of motivation to current frameworks of ADHD, very few studies have examined the willingness of individuals with ADHD to exert cognitively or physically effortful behaviour. We therefore designed a task to specifically test whether motivation is indeed lower in ADHD and, if so, whether amphetamine-based medications that are commonly used to treat ADHD can restore it.”
The study involved 44 participants: 20 individuals diagnosed with ADHD and 24 without the condition (referred to as the control group). The ADHD group included individuals who were being treated with amphetamine-based drugs like dexamfetamine or its prodrug, lisdexamfetamine. The participants with ADHD were tested twice – once when they were on their regular medication and once after a 72-hour period without medication, to understand the effect of these drugs on their motivation levels.
To measure motivation, the researchers developed a unique experiment divided into two main parts: a training (or reinforcement) phase and a choice phase. In the reinforcement phase, participants were trained in tasks that required either cognitive (mental) or physical effort.
For instance, the cognitive task involved detecting specific letters in rapidly changing sequences, which increased in difficulty. The physical task required participants to exert varying levels of force using a hand-held device. After training, the choice phase measured the participants’ willingness to engage in these effortful tasks by offering them a choice between a low-effort/low-reward option and a variable high-effort/high-reward option.
In the cognitive effort task, the researchers found no significant performance differences between the ADHD participants and the control group, or within the ADHD group when on or off medication. This implies that ADHD does not inherently affect the ability to perform cognitive tasks. However, in the physical effort task, those with ADHD showed a greater ability to sustain effort when on medication compared to off medication. Despite this, there was no significant difference in the ability to obtain rewards, suggesting that the medication did not simply make the task easier to perform.
The choice phase results were particularly striking. When off their medication, individuals with ADHD showed less motivation to invest effort in both cognitive and physical tasks compared to the control group. This effect was especially pronounced at higher levels of cognitive effort and lower levels of physical effort. On the other hand, when on medication, the motivation of the ADHD group increased significantly, aligning closely with the motivation levels of the control group.
“Some authors have postulated that the cognitive symptoms of ADHD may be driven by a lower willingness to engage in cognitively effortful behavior,” Chong said. “Our data confirmed that cognitive motivation is indeed lower in ADHD relative to controls, but also showed that ADHD was associated with lower levels of motivation in the physical domain.”
These results provide concrete evidence that motivation, particularly the willingness to exert effort, is a key component of ADHD, and not merely a side effect of other symptoms. Furthermore, it demonstrates that amphetamine-based medications, commonly prescribed for ADHD, are effective in enhancing motivation in these individuals.
“The motivation to invest cognitive and physical effort is lower in individuals with ADHD relative to those without the condition,” Chong told PsyPost. “Reassuringly, this reduction in motivation can be improved with currently available stimulant medications.”
However, it’s important to note certain limitations of the study. First, the sample size was relatively small, and larger studies are needed to confirm these findings. Additionally, the study primarily focused on the effects of amphetamine-based medications, and it’s unclear if similar results would be observed with other types of ADHD medications. The study also did not delve deeply into the psychological aspects of motivation, such as the role of personal interests or the impact of long-term goals, which could be important factors in understanding motivation in ADHD.
“Amphetamines are stimulant medications that increase the activity of dopamine and noradrenaline in the brain,” Chong explained. “The fact that amphetamines improve the willingness to invest effort is evidence that these neurotransmitters play an important role in motivated behaviour. However, motivation is a complex neurobiological process, and the role of other neurotransmitters, such as serotonin, remains to be clarified.”
The study, “Amphetamines Improve the Motivation to Invest Effort in Attention-Deficit/Hyperactivity Disorder“, was authored by Trevor T.-J. Chong, Erika Fortunato, and Mark A. Bellgrove.
