Effects of caffeine on reaction time are mediated by attentional rather than motor processes

Background

Caffeine has a well-established effect on reaction times (RTs) but the neurocognitive mechanisms underlying this

are unclear.

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

P3b latencies.

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.