Increased auditory stimulus intensity results in an earlier and faster rise in corticospinal excitability

Increasing the intensity of auditory stimuli has been shown to produce faster simple reaction times (RTs). Typical explanations for this effect involve earlier detection of the more intense stimulus; however, these explanations fail to consider how stimulus intensity may impact response initiation processing. To investigate the mechanism responsible for the auditory stimulus intensity effect, transcranial magnetic stimulation (TMS) was applied at various times during the simple RT interval (equivalent to 0, 30, 45, 60, and 75% of baseline RT) to examine changes in corticospinal excitability after a go-signal of varying intensity (60, 70, 80, or 90 dB). Premotor RT data confirmed a stimulus intensity effect whereby the 90 dB stimulus resulted in faster RTs than all other intensities. Analysis of motor evoked potential (MEP) amplitude elicited by TMS across stimulus intensity conditions revealed that in the 80 dB and 90 dB conditions, corticospinal excitability began to increase earlier from baseline (pre-stimulus) levels, supporting the detection hypothesis. In addition, MEP amplitude increased at a greater rate during the RT interval for the 90 dB condition, indicative of impacts on response initiation. These results indicate that stimulus intensity effects result from a combination of earlier detection and faster initiation.