Modulatory influence of continuous tone, tone offset, and tone onset on the human acoustic startle response.

Startle modulation in young adult men, by continuous background tone and its offset, a 2-s sustained tone and its offset, and the onset of a 25-ms tone pip were compared. Tone (75dB 1000 Hz) offset and onset occurred either 2000 ms or 100-120 ms before the startle stimuli (104dB (SPL), 50-ms white noise bursts). Blink amplitude and latency were unaffected by continuous background tone. Blink amplitude was reliably inhibited by 100-ms offset of both the continuous background tone and the 2-s sustained tone or 120-ms onset of the tone pip, whereas effects on latency were more variable. Facilitation of blink amplitude and latency was significant but weak and only following the 2-s sustained tone, and only with respect to one of two experimental contexts. These findings support those of others and suggest that startle inhibition results from activation of neurons responding to transient environmental changes. The degree of inhibition appears to be related to stimulus value. Startle amplitude facilitation following long sustained prestimulation intervals is dependent on experimental context. Overall latency and amplitude modulation tend to be concordant, leading to the conclusion that the mechanism(s) underlying both are context dependent and linked in the adult human.