The human hand motor area is transiently suppressed by an unexpected auditory stimulus

Objective: To study the effect of a loud auditory stimulus on the excitabil ity of the human motor cortex. Methods: Ten normal volunteers participated in this study. The size of resp onses to transcranial magnetic or electrical cortical stimulation (TMS or T ES) given at different times (ISIs) after a loud sound were compared with t hose to TMS or TES alone (control response). Different intensities and dura tions of sound were used at several intertrial intervals (ITIs). In additio n, we examined how the presence of a preceding click modulated the effect o f a loud sound (prepulse inhibition). The incidence of startle response evo ked by various stimuli was also studied. Results: A loud auditory stimulus suppressed EMG responses to TMS when it p receded the magnetic stimulus by 30-60 ms, whereas it did not affect respon ses to TES. This suggests that the suppression occurred at a cortical level . Significant suppression was evoked only when the sound was louder than 80 dB and longer than 50 ms in duration. Such stimuli frequently elicited a s tartle response when given alone. The effect was not evoked if the ITI was 5 s, but was evoked when it was longer than 20 s. A preceding click reduced the suppression elicited by loud sounds. Conclusions: Auditory stimuli that produced the greatest effect on response s to TMS had the same characteristics as those which yielded the most consi stent auditory startle. We suggest that modulation of cortical excitability occurs in parallel with the auditory startle and both may arise from the s ame region of the brain-stem. (C) 2000 Elsevier Science Ireland Ltd. All ri ghts reserved.