The acquisition of auditory intensity discriminations in rats trained on multiple variableinterval extinction schedules was studied as a function of some of the variables that contribute to the speed of development of differential responding and the final level attained. The effects of three variables were isolated and studied in detail: (1) the decibel difference between the discriminative stimuli (intensity difference); (2) the intensity relationship between the stimuli (relative intensity); and (3) the position of the stimuli on the intensity continuum (absolute intensity). Each of the three variables generated orderly relationships and interacted with one another to produce complex effects upon differential responding.The present investigations examined the effects of three aspects of stimulus intensity upon the development of an auditory discrimination. Comparable experimental conditions obtained in each of the studies, excepting the manipulation of the variable under investigation.After Pavlov developed the "method of contrasts", many procedures, devices, and response measures were invented to explore the stimulus control of differential responding (see Warden, Jenkins, and Warner, 1935;Woodworth and Schlosberg, 1954). It would appear that on occasion the technique itself has nIot only been a tool for research but has also defined the problems to be studied. This early literature contains generous amounts of information, much of which is difficult to interpret or impossible to integrate, and it sometimes produced irrelevant feuds arising out of the lack of standardized technique.In

Two groups of four rats each were trained to bar press on a variable-interval 2-min schedule. During training, either 3, 5, or 9 auditory stimuli of various intensities were randomly presented. A direct relationship between stimulus intensity and rate of responding was obtained, but it was more consistent in the group trained initially with three stimuli than for the group that started with nine stimuli. The results are related to the concept of stimulus intensity dynamism and the necessary conditions for the acquisition of stimulus control.

In order to assess possible confounding of discriminative stimulus effects with those produced by the reinforcing stimulus, three groups of four rats each were trained for 45 hr on a variable-interval 1-min reinforcement program. Two groups were run on a multiple variableinterval extinction schedule in which the reinforcement stimulus (SD) and the nonreinforcement stimulus (SI) were two intensities of a 4-kHz (cps) tone separated by 40 or 10 db. The third group was run on a mixed schedule with a single intensity constantly present. The mixedschedule animals showed no discrimination of the reinforcement program. Under the multiple schedule, the highest SI rates were obtained after SD intervals, regardless of the reinforcement availability in the SD interval. These local rate variations in SA were small in proportion to those produiced by the SD versus SI intensities. Jenkins (1965) has criticized much of the previous operant discrimination work on the basis that the effects produced by the discriminative stimulus are confounded with effects produced by the stimulus used as a reinforcer.

Using two counterpart pairs of groups, rats were trained to discriminate between 4 kHz tones spaced 5 dB apart at the high or low ends of an intensity continuum. The major findings agree with those of Sadowsky (1966) who, using 10 dB separations, found a large amount of dynamism at high intensities, but none at low intensities. However, some details of the present results differ and may be related to the SD-S~ separation variable.