Article
Effects of caffeine, practice, and mode of presentation on stroop task performance

https://doi.org/10.1016/0091-3057(95)02116-7Get rights and content

Abstract

A series of experiments were designed to investigate the effects of 125 and 250 mg caffeine, or placebo, on performance of the Stroop task. Caffeine had no effect on performance of either the classic colour-word version or a numerical version of the task, either using computerised presentation of the stimuli or a traditional card version. However, significant practice effects were found using a withinsubjects design with the card version of the task, and differences were found between performance in the card and computerised versions of the task. It is concluded that at these doses, caffeine does not significantly affect Stroop performance. It is also suggested that practice effects in studies using within-subject designs may be a problem when attempting to detect subtle effects of drugs on cognitive performance, and that the computerised version of the Stroop task may not be an exact analogue of the traditional card version.

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    Stroop also seems to be susceptible to a substantial learning effect, demonstrated in our study by significant improvements in speed from day 1 to day 2. A practice-related reduction in size of interference effect has also been reported in previous studies (Beglinger et al., 2005; Chen et al., 2013; Davidson et al., 2003; Edwards et al., 1996). Practice effects could mask the impact of heat exposure on performance, decreasing the usefulness of this test for studies in which participants are required to perform cognitive tests multiple times.

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    There exists a number of studies focusing on practice effects in experimental paradigms tapping on conflict processing. For example, color Stroop task practice results in an increased practice-related reduction of reaction times (RTs) in incongruent trials versus RTs in congruent trial and thus a reduction of the color Stroop effect (e.g., Davidson, Zacks, & Williams, 2003; Dotson, Sozda, Marsiske, & Perlstein, 2013; Dulaney & Rogers, 1994; Edwards, Brice, Craig, & Penri-Jones, 1996; Ellis, Woodley-Zanthos, Dulaney, & Palmer, 1989; Flowers & Stoup, 1977; Macleod, 1998; MacLeod & Dunbar, 1988; Reisberg, Baron, & Kemler, 1980; Roe, Wilsoncroft, & Griffiths, 1980; Rogers & Fisk, 1991; Wilkinson & Yang, 2012). A comparable pattern of results (i.e., increased practice-related reduction of incongruent trial RTs than on congruent trial RTs) was also evident in a number version of the Stroop task (Bush et al., 1998).

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    However, there was no significant interaction between condition and congruency suggesting that this effect relates more to an increase in basic processing speed or reaction time, rather than a specific enhancement of the Stroop interference effect. This is consistent with Edwards et al. (1996) who failed to find an effect of caffeine using both the computerised and traditional card versions of the Stroop task, but differs from results found by others (e.g. Dawkins, Shahzad, Ahmed, & Edmonds, 2011; Hasenfratz & Battig, 1992; Kenemans et al., 1999). Such discrepancies in findings could be related to task presentation (Kenemans et al., 1999) and practice effects (Edwards et al., 1996), or even the validity of the test itself (Jansari et al., 2014).

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    Interestingly, the pattern of practice-related improvement in interference was similar to the one observed in the present study for seriation and repetition measures, with the greatest effect occurring early in practice (Table 1: Quadratic on-line effect column and Interaction column). While the impact of practice has been widely demonstrated in Stroop task performance (Edwards, Brice, Craig, & Penri-Jones, 1996), only few studies have investigated such impact on RNG task. A practice-related effect was showed by Evans and Graham (1980).

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