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.