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.

Eighteen Peking ducklings were imprinted and tested for their tendency to emit distress calls during the presentation and withdrawal of the imprinted stimulus. A subsequent arrangement in which each distress vocalization led to a 5-second presentation of the imprinted stimulus resulted in an enhanced tendency to emit distress calls.

Newly hatched ducklings were exposed to imprinting procedures and subsequently trained to peck a key by presenting the imprinting stimulus as the reinforcing (response contingent) event. It was found that the key peck was learned only when imprinting procedures were initiated during the first 6 to 8 hr after hatch. Additional studies revealed that: (1) the duckling's distress vocalizations were reduced in the presence of the imprinting stimulus and enhanced in its absence; (2) when the ducklings had constant access to the imprinted stimulus (via a key peck), pecking responses occurred in bursts and relatively few distress vocalizations occurred; (3) the initial effect of extinction procedures was an increase in key peck rate. When, however, repeated key pecks failed to produce the imprinted stimulus, distress vocalization ensued and peck rate declined; (4) both the presentation of an unfamiliar mechanical figure and delivery of electrical shock enhanced distress vocalization and key pecks; (5) for some ducklings, certain familiar objects in the environment influenced distress calls in a manner comparable to the imprinted stimulus in that distress calls increased when these objects were removed.

No abstract