Effects of caffeine on reaction time are mediated by attentional rather
than motor processes
Christopher W. N. Saville
H. M. de Morree
Neil M. Dundon
S. M. Marcora
Received: 30 August 2017 / Accepted: 13 November 2017 / Published online: 23 December 2017
The Author(s) 2017. This article is an open access publication
Background Caffeine has a well-established effect on reaction times (RTs) but the neurocognitive mechanisms underlying this
Methods In the present study, 15 female participants performed an oddball task after ingesting caffeine or a placebo, and
electroencephalographic data were obtained. Single-trial P3b latencies locked to the stimulus and to the response were extracted
and mediation models were fitted to the data to test whether caffeine’s effect on RTs was mediated by its effect on either type of
Results Stimulus-locked latencies showed clear evidence of mediation, with approximately a third of the effect of caffeine on RTs
running through the processes measured by stimulus-locked latencies. Caffeine did not affect response-locked latencies, so could
not mediate the effect.
Discussion These findings are consistent with caffeine’s effect on RTs being a result of its effect on perceptual-attentional
processes, rather than motor processes. The study is the first to apply mediation analysis to single-trial P3b data and this technique
holds promise for mental chronometric studies into the effects of psychopharmacological agents. The R code for performing the
single trial analysis and mediation analysis are included as supplementary materials.
Single trial analysis
Caffeine has been the subject of great interest as a possible
cognitive enhancer. One of its most consistently replicated
cognitive effects is its reduction of reaction times (RTs) in
speeded tasks (e.g. Childs and De Wit 2006; Haskell et al.
2005;Heatherleyetal.2005;Mclellanetal.2016). What is
less clear, however, is what neurocognitive mechanisms are
behind this effect.
RTs measure the total duration of orienting to stimuli, iden-
tifying said stimulus, choosing the appropriate response,
converting this response into a motor plan, and executing
the motor plan. Caffeine’s influence on RTs could result from
accelerating any, or all, of these stages.
On a neurochemical level, the psychomotor effects of
caffeine have been shown to be mediated by its antagonis-
tic binding to adenosine receptor sites (Snyder et al. 1981).
Pharmacological studies in rats suggest that its effect on
RTs may be due to its antagonistic effect on A
receptors specifically (Higgins et al. 2007). However,
again in rats, adenosine receptor antagonists have been
shown to both stimulate motor behaviour (Karcz-Kubicha
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s00213-017-4790-7) contains supplementary
material, which is available to authorized users.
* Christopher W. N. Saville
North Wales Clinical Psychology Programme, School of Psychology,
Bangor University, Adeilad Brigantia, Ffordd Penrallt,
Bangor, Gwynedd, Wales LL57 2AS, UK
Personal Health Department, Philips Research,
Eindhoven, The Netherlands
Department of Child and Adolescent Psychiatry, Psychotherapy, and
Psychosomatics, Medical Faculty, University of Freiburg,
School of Sport and Exercise Science, University of Kent,
Department of Child and Adolescent Psychiatry, Psychosomatics and
Psychotherapy, Medical Faculty, University of Cologne,
Psychopharmacology (2018) 235:749–759