Effects of a competitive stressor on motor cortex excitability: a pilot study

Several intervening variables may influence motor evoked potentials (MEP) from transcranial magnetic stimulation (TMS). Among the most common are motor (e.g. executed or intended muscle contraction) and sensory factors. However, little is known about the effects of a well defined stressor on MEPs and thus on central motor control. We studied 11 healthy right-handed volunteers (five men and six women), aged 21-32 years (mean 25.2 years. SD = 3.5). All subjects underwent a 20 min rest and a 20 min stress period (sequence-controlled, cross-over design). The stressor employed a competitive videogame. MEPs from the left abductor digiti minimi muscle were obtained immediately after rest and stress. We found that six (54.5 per cent) subjects presented a significant increase (p < 0.05) of MEP amplitudes from rest to stress, three (27.3 per cent; p < 0.05) decreased, and two (18.2 per cent) did not change, F- and M-waves did not change significantly. Psychological testing (state and trait anxiety inventory, STAI) revealed that individuals who presented an increase in MEP latency scored lower in state and trait anxiety than others, probably mdicating a lower vulnerability to stress. We believe that the observed increase in MEP amplitudes, combined with a decrease in MEP latency, can be regarded as neurophysiological evidence of stress-induced facilitation related to higher excitability of the corticospinal system and/or presynaptic neurons.