Prescription Stimulants' Effects on Healthy Inhibitory Control, Working Memory, and Episodic Memory: A Meta-analysis

Abstract

The use of prescription stimulants to enhance healthy cognition has significant social, ethical, and public health implications. The large number of enhancement users across various ages and occupations emphasizes the importance of examining these drugs' efficacy in a nonclinical sample. The present meta-analysis was conducted to estimate the magnitude of the effects of methylphenidate and amphetamine on cognitive functions central to academic and occupational functioning, including inhibitory control, working memory, short-term episodic memory, and delayed episodic memory. In addition, we examined the evidence for publication bias. Forty-eight studies (total of 1,409 participants) were included in the analyses. We found evidence for small but significant stimulant enhancement effects on inhibitory control and short-term episodic memory. Small effects on working memory reached significance, based on one of our two analytical approaches. Effects on delayed episodic memory were medium in size. However, because the effects on longterm and working memory were qualified by evidence for publication bias, we conclude that the effect of amphetamine and methylphenidate on the examined facets of healthy cognition is probably modest overall. In some situations, a small advantage may be valuable, although it is also possible that healthy users resort to stimulants to enhance their energy and motivation more than their cognition. The large number of enhancement users across various ages and occupations emphasizes the importance of examining these drugs' efficacy in a nonclinical sample. The present meta-analysis was conducted to estimate the magnitude of the effects of methylphenidate and amphetamine on cognitive functions central to academic and occupational functioning, including inhibitory control, working memory, short-term episodic memory, and delayed episodic memory. In addition, we examined the evidence for publication bias. Forty-eight studies (total of 1,409 participants) were included in the analyses. We found evidence for small but significant stimulant enhancement effects on inhibitory control and short-term episodic memory. Small effects on working memory reached significance, based on one of our two analytical approaches. Effects on delayed episodic memory were medium in size. However, because the effects on long-term and working memory were qualified by evidence for publication bias, we conclude that the effect of amphetamine and methylphenidate on the examined facets of healthy cognition is probably modest overall. In some situations, a small advantage may be valuable, although it is also possible that healthy users resort to stimulants to enhance their energy and motivation more than their cognition. ■

Does Transcranial Direct Current Stimulation Improve Healthy Working Memory?: A Meta-analytic Review

“…WM is essential for performing many cognitive tasks ( Wiley & Jarosz, 2012), and individual differences in WM capacity are correlated with individual differences in intelligence (Engle, Tuholski, Laughlin, & Conway, 1999). In part for this reason, the feasibility of enhancing WM has been explored with training (MelbyLervåg & Hulme, 2013), stimulant medications (Ilieva, Hook, & Farah, 2015), and transcranial direct current stimulation (tDCS; Brunoni & Vanderhasselt, 2014). Here, we look more closely into the tDCS and WM enhancement literature.…”

“…Third, by excluding studies for which active and sham stimulation sessions were not counterbalanced, we increased the quality of the analyzed research. Fourth, we selected the most appropriate dependent variable from each study and combined them meta-analytically, following Ilieva et al (2015). As discussed in more detail in the Methods section, under Dependent variables, we used a priori criteria to select dependent variables, rather than separately meta-analyzing accuracy and RT measures and selecting the specific measure of accuracy or RT emphasized by the authors of the original study.…”

Methylphenidate does not enhance visual working memory but benefits motivation in macaque monkeys

“…Nevertheless, it is unclear whether ADHD patients are responsive to MPH because of higher DA transporter density (see for review Fusar-Poli et al, 2012); no significant change in NE transporter density have been found (Vanicek et al, 2014). To the best of our knowledge, the monkeys in this study have unimpaired catecholamine systems, but understanding the effects of MPH in healthy subjects is an important step, as stimulants are increasingly being used recreationally as putative cognitive enhancers (McCabe et al, 2005(McCabe et al, , 2014Smith and Farah, 2011;Ilieva et al, 2015). Thus far, evidence for MPH enhancing working memory in the healthy brain is limited, and that its effects on cognition seem modest at most (see for review Ilieva et al, 2015).…”

“…To the best of our knowledge, the monkeys in this study have unimpaired catecholamine systems, but understanding the effects of MPH in healthy subjects is an important step, as stimulants are increasingly being used recreationally as putative cognitive enhancers (McCabe et al, 2005(McCabe et al, , 2014Smith and Farah, 2011;Ilieva et al, 2015). Thus far, evidence for MPH enhancing working memory in the healthy brain is limited, and that its effects on cognition seem modest at most (see for review Ilieva et al, 2015). Our findings that MPH has some beneficial effects on task engagement is consistent with the report from Ilieva and Farah (2013), who suggested that the stimulant-induced enhancement perceived by healthy users has a significant motivational component.…”

“…One potential reason for such equivocal results is the unaddressed possibility that catecholamine effects are most apparent when tasks are made more difficult by increasing mnemonic demands. Although there is a literature on the effects of MPH on healthy adult humans performing tasks requiring retention of multiple memoranda, evidence suggests marginal benefits (see for review Linssen et al, 2014;Ilieva et al, 2015), even though MPH continues to be used recreationally as a potential cognitive enhancer (McCabe et al, 2005(McCabe et al, , 2014Smith and Farah, 2011;Ilieva et al, 2015). A major limitation of human studies is that MPH doses were generally not varied.…”