Many organisms, including humans, exhibit some form of startle reaction when exposed to sudden, intense, and unexpected stimulation. Depending on the circumstances of its elicitation, this reaction can include many muscle groups, as in the reflexive “body jerk” that characterizes the massive flexor response to an unexpected gunshot, or it can include only a few muscle groups, as in the eyeblink that sometimes occurs to a weaker sound, or to a more intense stimulus that has undergone considerable habituation. In either case, the eliciting stimulus must reach a minimum intensity within a min imum amount of time if any response is to occur at all. Even the most intense sound will fail to generate a body jerk or even an eyeblink if the rise time is slow enough. Fleshier (1965) found that in the rat, an acoustic signal had to reach an intensity of about 90 dB (re .0002 dyne/cm2) within no more than 12 ms if a measurable reaction was to occur. When the stimulus failed to meet this dual criterion, startle did not occur even though the signal might even tually reach a level of 140 dB. Similar effects have been seen in adult humans for rise time and stimulus duration, although here the “critical duration” may extend to at least 30 ms (Berg, 1973).