The auditory-evoked arousal modulates motor cortex excitability.

Arousal enhances the readiness to process sensory information and respond to it. Rapid increment of arousal, referred to as arousal reaction or startle, increases the level of attention and the chance of survival. Arousal reaction is known to originate from the brainstem ascending reticular activating system and to modulate neuronal activity throughout the central nervous system. In the present study we investigated the effect of arousal on the central motor system by synchronizing transcranial magnetic stimulation (TMS) with acoustically evoked N100 potential. Because of the widespread cortical distribution of N100 to a sudden acoustic stimulus it is thought to be related to arousal reaction. Eight healthy subjects participated in this study. TMS was focused on the primary motor cortex utilizing neuronavigation. Trains of four identical loud tones repeated at 1-s intervals were delivered to the right ear and TMS was randomly placed after one tone in the train. The motor-evoked potentials (MEPs) were measured from the contralateral first dorsal interosseous muscle. The MEPs evoked by TMS timed at N100 after the first tone in train were significantly (p<.001) larger in comparison with the control stimulation without a preceding sound or stimulation placed after the N100, i.e., 120% of the N100 interstimulus interval. Also, the MEPs following the second tone were significantly weaker (p<.05) when compared with the MEPs following the first tone. Our findings suggest that acoustic arousal reaction facilitates, not only the activation of sensory cortices, but also simultaneously the central motor system.