The phoneme is most often taken to be the smallest unit of speech that can, by itself, distinguish one utterance from another as to meaning. Thus, the existence of the two words bill and dill makes it clear that b and d are different phonemes in English. It should be emphasized that the phoneme is not a single sound, but is, rather, a class which can and usually does include a great many sounds that differ from each other in various ways without causing any change in meaning.

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Several acoustic and temporal variables responsible for the elicitation of the rat's startle reaction were examined by measuring the subject's overt movement under each of a variety of stimulus conditions. When startle was repeatedly evoked, the response decreased in amplitude, but adaptation was slow and was not complete after 675 stimulus presentations. The threshold for startle evocation increased with repeated stimulation, but partial recovery occurred during interpolated rest periods. Startle reactions were amplified when testing occurred in a background of steady noise, but the effect was independent of the frequency components of the background stimulation. Moreover, although a relatively weak signal could inhibit the reaction to a subsequently presented intense signal, the effect was independent of the frequency characteristics of either signal. Finally, it was found that relatively weak signals could either inhibit the startle reaction to a subsequently presented intense signal or reduce response latency, depending upon the temporal relationships between the two signals. It was concluded that weak signals can activate the neural mechanisms responsible for the startle reaction, and that the occurrence of acoustic startle along with its associated inhibitory and facilitory effects is dependent upon the sensation levels of the signals, but not upon their frequency characteristics.

Special equipment was used to assess abrupt movements which characterize mammalian startle reactions to sudden acoustic inputs. Presence of background stimulation (random noise either at 50, 70, or 90 db. re .0002 dynes/ cm 2 ) facilitated startle to an intense tone (13,250 ops 117 db.). Other experiments indicated that startle to an intense pulse of noise (approximately 120 db.) is attenuated when a weaker pulse (approximately 80 db.) precedes primary stimulus by approximately 100 msec. Results indicate that startle is sensitive to sounds with .intensities comparable to those encountered in a normal acoustic environment, and suggest that mechanisms involved in startle are more or less continually active even though overt reactions seldom occur.