Bidirectional influences of vibrotactile stimuli on modification of the human acoustic startle reflex

Abstract

The present study deals with the effects of a vibrotactile prepulse on the acoustic startle reflex. An 85-dB(A) broadband noise was used to elicit the startle reflex, and the amplitude, latency, and probability of integrated electromyographic activity of the muscle beneath the eye were measured. Experiment 1 involved the presentation of vibrotactile prepulses at 20-dB SL to the hand, at lead times (stimulus onset asynchronies) of 0 to 400 msec before the acoustic startle stimulus. When compared to control (no prepulse) responding, a vibrotactile prepulse with a lead time of 25 msec caused a significant increase in response amplitude, while prepulses at lead times of 150 to 400 msec inhibited response amplitude. Response latency was significantly reduced (there was facilitation) in comparison with results for the control condition, at lead times of 25, 50, 75, 100, 200, and 350 msec. Experiment 2 involved presenting subjects with two prepulses (at lead times of 25 and 200 msec) on the same trial or on separate trials. A prepulse at a lead time of 25 msec caused an increase in response amplitude, but had no effect on response latency or probability. A prepulse at a lead time of 200 msec caused a decrease in response probability, but had no effect on response amplitude or latency. Response amplitude and probability were larger when both prepulses were presented on the same trial than they were when the 200-msec lead time prepulse was presented alone. These data support the hypothesis that the mechanisms for inhibition and facilitation of the human startle reflex are partially independent, and that crossmodal modification of the startle response may involve different mechanisms than those responsible for modification within a single sensory system.