Temporal integration of acoustic stimulation obtained in reflex inhibition in rats and humans.

An acoustic stimulus (S1) presented just before reflex elicitation inhibits reflex expression. The present studies questioned whether inhibition provided by initial stimuli of various durations conforms to established temporal integration functions. Initial stimuli were noise bursts varying in duration (2, 20, or 200 msec) and intensity (55 or 85 dB). Eliciting stimuli (S2) for rats were intense tone bursts, which elicited the acoustic startle reflex, and for humans electrotactile stimuli to the forehead, which elicited the eye blink. Findings revealed that inhibition was greater with the 85-dB S1 stimulus and increased linearly with log increases in duration. These latter data suggest that the acoustic substrate for reflex inhibition has a long-time constant. There was one exception to this general finding. For seven (of nine) human subjects, inhibition declined when the duration of the 85-dB S1 was increased from 20 to 200 msec. Postexperimental questioning and video monitoring suggest that this anomaly resulted from a reflex enhancing arousal process.