Role of Weak Signals in Acoustic Startle

When a relatively weak pulse of noise precedes an intense (startle eliciting) stimulus by 5 msec, the latency of the rat's startle reaction to the intense signal is reduced. When, however, the same pulse of noise precedes the intense stimulus by 150 msec, the startle response to the intense stimulus is inhibited. The present research examined the thresholds for these effects using stimulus configurations in which the weak pulse had intensities as low as 10 dB (re 0.0002 dyn/cm2). Both response inhibition and latency reduction were detected when the weak signal had an intensity of only 35 dB, and the magnitude of the two effects increased with increases in pulse intensity. Since, for signals comparable to the pulses used here, the rat's threshold for hearing is approximately 35 dB, it is concluded that the brain system that mediates these effects is probably as sensitive as the auditory system itself.