Caffeine has no effect on measures of cortical excitability

Objective To assess the effect of caffeine on motor thresholds, short interval intra-cortical inhibition (SICI), intra-cortical facilitation (ICF) and cortical silent periods in a placebo controlled double-blinded trial. Methods In eleven healthy non-smoking subjects the following parameters were measured using transcranial magnetic stimulation (TMS): motor thresholds (rest, RMT and active, AMT), SICI and ICF at different conditioning stimulus intensities (60, 70, 80, 90% AMT), cortical silent periods at 130, 150 and 175% AMT, and size of motor evoked potential at rest at 110, 125 and 150% RMT. Measurements were repeated after one cup of decaffeinated coffee. On another day, measurements were obtained before and after one cup of decaffeinated coffee that contained caffeine (3 mg/kg bodyweight). Caffeine concentrations were measured in serum before and after experiments. Experiments were conducted and data were evaluated blinded to the experimental condition. Results The results of repeated measurements of all parameters were similar comparing experiments on each day, or when comparing the caffeine arm of the study with the placebo arm. Conclusions Caffeine in a concentration similar to that in a strong cup of coffee does not have a major effect on TMS measures of motor cortex excitability. Significance In healthy controls, the design of TMS experiments that investigate the parameters assessed in this TMS study does not need to control for caffeine.