Brief reportCaffeine enhances working memory for extraverts
Introduction
Caffeine increases subjective experience of concentration and alertness (e.g., Peeling and Dawson, 2007), which may account for its widespread use in cognitively demanding situations (e.g., learning and achievement contexts). However, the effect caffeine has on key cognitive functions, such as working memory (WM; Baddeley, 2007), is unclear. For instance, caffeine modulates neuronal processing during WM paradigms (Koppelstaetter et al., 2008) and yet often has no effect on WM and related tests of cognitive performance (e.g., Childs and de Wit, 2006, Hameleers et al., 2000, Nehlig, 2010). Individual differences in response to caffeine may partly explain these inconsistent findings.
Although caffeine has diffuse psychobiological effects, research has attributed much of these to indirect potentiation of dopamine (DA) activity (Fredholm et al., 1999, Garrett and Griffiths, 1997). Specifically, caffeine may facilitate DA release through blockade of adenosine, a nucleoside that inhibits release of catecholamines and other neurotransmitters. Caffeine blocks adenosine release at A1 and A2a receptor sites (Basheer et al., 2004, Garrett and Griffiths, 1997), the latter of which are concentrated in dopamine (DA) rich brain regions, including the nucleus accumbens and striatum (Moreau and Huber, 1999, Nehlig, 1999). The view that caffeine may potentiate DA activity through antagonistic action at A2a–D2 receptor sites is consistent, for instance, with similarities between the effects of caffeine and that of selective DA agonists (Fredholm et al., 1999).
One might expect individual differences in response to caffeine to correspond with DA-related person characteristics. One such variable is extraversion, a personality dimension reflecting sociable, bold and outgoing behaviour (Wilt and Revelle, 2009). Extraversion is highly heritable (Bouchard, 1994), and several biologically-inspired theories link this phenotype with variation in DA functioning (Depue and Collins, 1999, Pickering, 2004, Rammsayer, 1998). In support of this, studies have linked extraverted personality with hormone response to DA agonists and antagonists (e.g., Reuter et al., 2002) and genetic markers of DA function (e.g., Smillie et al., 2010).
Interestingly, although not motivated by a potential dopaminergic mechanism, research has indeed demonstrated that some effects of caffeine are moderated by extraversion (Gupta, 1988, Revelle et al., 1976). These studies show that caffeine improves performance on verbal ability tests for extraverts but not their introverted counterparts. Such effects may have been driven by an effect of caffeine on working memory (WM), which is dependent upon dopaminergic modulation of pre-frontal cortex function (Barch, 2004) and contributes strongly to performance on cognitive ability tests (Daneman and Merikle, 1996, Salthouse and Pink, 2008). Although this possibility has not been investigated, recent work shows that a selective DA antagonist impaired WM performance for extraverts only (Wacker et al., 2006, Chavanon et al., 2007). To the extent that we can consider caffeine an indirect DA agonist, we might anticipate complementary effects to be observed for this widely-used stimulant.
In sum, extraversion-dependent effects of caffeine on WM are suggested by a common thread linking (a) indirect potentiation of DA by caffeine, (b) covariation of extraverted personality with DA function, and (c) the functional role of DA in WM. We investigated the interactive effect of caffeine and Extraversion on WM, assessed using the widely-known N-Back task. This task has the advantage of dissociating basic perceptual-motor performance (target/non-target identification) from WM load (complex span); WM involvement is indicated only by load-dependent effects.
Section snippets
Method
Sixty healthy adults were recruited via a psychology research participation scheme and via advertisements placed around Goldsmiths, University of London. One participant's data were excluded due to very low performance in both sessions (Final N = 59, 66% female; aged 18–40; M = 23.95, SD = 5.64).
All participants attended two separate sessions (5–12 days apart) and were instructed to abstain from caffeinated consumables (e.g., coffee, chocolate) on both days of testing. Due to potential time-of-day
Results
A 2 (drug) × 3 (N-Back load) repeated measures ANOVA predicting WM performance (percent correct) was conducted with extraversion included as a continuous variable within the factorial design. Covariates included to maximise statistical control were age, sex, weekly caffeine intake, and counterbalance order (see Table 1). There was no main effect of drug, F(1,53) = 1.05, p = 0.31, a significant interaction between drug and extraversion, F(1,53) = 4.12, p = 0.05, d = 0.48, and a significant interaction
Discussion
This study reveals that caffeine facilitates WM performance for extraverts but not introverts, and, intriguingly, that the effect of caffeine on WM is entirely dependent upon extraverted personality. Results may clarify previous investigations that did not take into account key individual differences constructs. Data may also throw new light on earlier research: Classic studies showing a differential impact of caffeine on cognitive ability test performance as a function of extraversion (e.g.,
Acknowledgement
We would like to thank Tarik Ali and Fiona Murray for their assistance with data collection.Funding: The first author would like to acknowledge financial support from the British Academy (PDF/2006/291).
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