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Six rats were exposed to a sequence of gunshot-like acoustical bursts during silence, during steady noise, and during pulsed noise. Assessment of their startle reactions to the bursts revealed that a background of steady noise enhanced the response, whereas a background of pulsed noise produced suppression of response. It is hypothesized that pulsed noise causes a relative refractory state in the mechanisms responsible for startle and that steady noise may enhance startle by masking uncontrolled punctiform acoustic stimuli.

In this research, we investigated a form of the avoidance paradigm in which impending shock is signalled by a warning stimulus and the avoidance response is a bar press that terminates this warning signal and prevents the occurrence of the associated shock.One purpose of the research was to document the course of avoidance over an extended number of sessions. The second purpose derives from the observation that some animals do poorly on the avoidance task. Personal communication with other investigators (0. Ray, L. Stein, & G. Heise) and our own laboratory experiences indicated large differences in the performance levels of individual animals. In the present work, we attempted to examine the conditions responsible for these differences.The research involved four related experiments. In the initial experiment, several rats were subjected to an extensive program of avoidance conditioning. During this period, a criterion for well-developed avoidance was in effect, and, as animals met criterion, their training was discontinued. On the other hand, the animals which failed to reach criterion were retained and run for a sufficient number of sessions until it was clear that continued training was not likely to overcome the deficiency in their performances. They were then subjected to a sequence of experiments in an effort to isolate the variables responsible for their relatively depressed performances.The technique of performing experiments upon animals which have previously met a given behavorial criterion has a well-established history in experimental psychology and needs no discussion at this time. The present approach, which focuses upon the animals which fail to reach criterion, differs only in detail. Its historical antecedent is in the work of the pathologist; as in pathology, the technique provides an opportunity to examine processes which are often refractory to investigation by more usual methods. METHOD SubjectsThe Ss were 10 female rats of Sprague-Dawley stock. They were approximately 90 days old at the start of the experiment and were maintained on a free-feeding diet. ApparatusThe experimental chamber consisted of a sound- was a bar of-0.25-by 0.75-inch aluminum, which projected 1.5 inches into the test chamber and was mounted 1.5 inches above the grid floor. A pressure of 20 grams was required to actuate the microswitch associated with the bar. Acoustic signals were delivered through a 5-inch speaker mounted against the back wall of the chamber. In all experiments, the warning signal was a pure tone at 3500 cycles per second, with an i-ntensity of 80 decibels (reference, 0.0002 dyne per centimeter squared) when measured in front of the speaker. Tones were programmed through a Magnecord PT 6 A tape recorder. Tape recording of the signals made it possible (through appropriate splicing techniques) to arrange the tones so that their onsets were free of transients.A series of timers, steppers, and relays was used to establish the several stimulus-response contingencies which the research demanded. The action o...

The absolute threshold for startle was obtained as a function of primary dimensions of pure ; tone: frequency, rise time, and duration. Individual rat's thresholds were measured using the up-down technique. Response latency (16 msec.) exhibited little variability under all experimental conditions. Startle sensitivity increased with tone frequency; this function seemed to reflect the sensitivity of the rat's ear. From the results of the rise time and duration experiments, it was concluded that the tone's peak intensity within the 1st 12 msec., irrespective of the shape of its intensity envelope is the critical dimension in startle evocation.

When a previously neutral stimulus has gained control over a given segment of behavior, stimuli which are like the stimulus, but not identical to it, frequently will exhibit control over that behavior. In the present sequence of studies, we attempted to examine the phenomenon of stimulus generalization when the behavior consisted of a reduction in the rate of ongoing responses. This latter process, conditioned suppression, is one of the by-products of a training history in which a neutral stimulus has typically preceded the occurrence of an unavoidable noxious event (for example, electrical shock).The behavior resulting from the joint action of stimulus generalization and conditioned suppression is of particular interest to the student of aversive controls because it represents a process by which the effects of aversive training may come to encompass large segments of an organism's behavior. An analysis of this process may help to clarify certain of the mechanisms involved in the behavioral phenomena of anxiety, conflict, and avoidance.We SubjectsThe Ss were four naive, domestic, White Carneaux pigeons. They were adapted to individual cages for 3 weeks and then maintained by restricted feeding at 80% of their free-feeding weights throughout the experiment. The pigeon feed was a grain combination recommended by Ferster and Skinner (1957), with a supplement of Vitamins A and D. ApparatusThe experimehtal chamber was a modification of the standard Foringer pigeon box. On the front wall of the chamber were two recesses and two translucent keys. Water was continuously available in the recess on the left, whereas reinforcement was delivered through a recess below and halfway between,the two keys. The right key, illuminated from behind by a red "An abbreviated version of a portion of this work has appeared in Science, 1961, 133, 753-755. This research was supported by USPHS Grant M-2433. light, was the only one used. Reinforcement consisted of a 7-second presentation of grain. During reinforcement, several events occurred: (1) The red key-light went out; (2) a solenoid-operated food tray lifted into the recess; and (3) a white light over the food tray came on. General illumination was provided by houselights at the top of the back wall. Mounted behind this wall was a 5-inch Quam audio speaker.Shock was administered to the pigeons via permanently fastened wing bands. Each band consisted of a length of standard chrome-plated, 0.156-inch-diameter bead chain which was wrapped twice around the base of each wing. Shock was delivered to the bird via a flexible connector fastened to the bands at the start of each experimental session. (See Hoffman, 1960, for a more detailed discussion of this method of administering shock to pigeons.) Shock of an average level of 10 volts was supplied by a Variac without external series resistance.The pigeon chamber was housed in a large, insulated double chamber which provided 45 decibels of sound attenuation and was fitted with a one-way-vision observation port. The acoustical stimuli were g...