Herring (Clupea harengus L.) show a marked ‘startle’ response when subjected to sound stimuli from a vibrating source (Blaxter, Gray & Denton, 1981c). The response, which occurs at a consistent threshold, is elicited by the first cycle of a tone burst or noise, the threshold to one cycle being the same as for a burst of many cycles. Furthermore, if the onset of the tone burst is ‘ramped’ so that it takes several cycles for the full amplitude to be reached, the threshold is markedly raised. Under some conditions the fish move away from the source, showing that they can determine its direction even though the stimulus is confined to a single cycle. The relative roles of sound pressure and particle velocity in providing information about the amplitude and direction of the noise have not been firmly established. Present evidence suggests that pressure is important in triggering the response and probably in measuring the strength of the stimulus. A vector such as particle velocity provides the directional information used in the response. The response can be very fast with a latency of about 25 msec and a contraction time of about 20 msec (Blaxter, Denton & Gray, 1981 b). In these very fast reactions the Mauthner cells probably constitute the motor pathway (Diamond, 1971).
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