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Though avoidance learning poses critical problems for S-R theories of learning, parametric studies of avoidance are notably few in number. Particularly, the effects of varying the time interval between the conditioned and the unconditioned stimulus are not well known. The early study by Warner (IS) indicated that rats require more trials to achieve an avoidance criterion as CS-US interval is lengthened. The rats, however, had been pretrained to escape shock. There was no study of extinction, and many characteristics of acquisition were not reported. The present experiment explores the effects of CS-US interval on the acquisition and extinction of a shuttle-box avoidance response by dogs. METHOD Subjects and ApparatusThe Ss were 50 experimentally naive mongrel dogs. They ranged in age from about one to six years, and in weight from about 9 to 13 kg. The animals were housed in laboratory cages, with food and water freely available at all times. The data were obtained while the dogs were in apparent good health.The apparatus used was much the same as that described by Solomon and Wynne (14). The shuttle box consisted of two identically built compartments, separated by a barrier which was adjustable in height. There was no drop-gate to prevent retracing. Thus, the dogs were always free to jump from one compartment to the other. The inside dimensions of each compartment were: length 45 in., width 24 in., height 40 in. The plywood walls were sheathed on the inner surface by aluminum painted a flat black. The barrier between compartments was built of aluminum-sheathed plywood slats slipped one on top of another. The compartments had hinged doors which could be locked by slide bolts, The ceiling was composed of hardware cloth. There were two 40-w. lights above each compartment, and two layers of cheesecloth were draped over scaffolding above the apparatus to serve as a crude one-way screen for E.The floor of the apparatus consisted of 1-in. stainless steel bars M in. apart. The grid bars were individually wired to a commutator. The method of construction 1 This paper is based upon a Harvard Ph.D. thesis.

3 experiments, with 232 rats, attempted to clarify the shape of the retention curve for an incompletely learned shuttle box avoidance response. In agreement with earlier work, the amount retained at first declined with time, but then increased. The curvilinear function appears to be a composite of 2 factors : a monotonic warm-up decrement increasing with time, plus some other factor which maximally impedes performance about 1 hr. after original learning. The shape of the retention function observed during early relearning trials is difficult to predict without knowledge of the factors affecting magnitude of warm-up decrement.

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Using rats in a CER paradigm, a truly random (TR) control procedure was found to produce reliable suppression during testing. The suppression could not be attributed to a nonassociative effect based simply on prior experience with both stimuli, independent of temporal spacing, as a group receiving the same number of CSs and USs "explicitly unpaired" showed little suppression. Further, in a second experiment, unlike at least some nonassociative effects, TR-produced suppression was retained intact over a long retention interval. In a final experiment, the outcome of a TR schedule was shown to be sensitively related to average CS-US interval, despite the absence of any contingency.