Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)




Conditioned suppression describes the attenuation of an ongoing operant (e.g., lever press or key peck) when a warning stimulus precedes unavoidable electric shock. First reported by Estes and Skinner (1941), the procedure has been refined by Kamin (1961, 1965) and other investigators (Geller, 1963, 1964; Hunt and Brady, 1955; and James and Mostoway, 1968). Although many of the parameters of conditioned suppression have been evaluated, relatively little attention has been given to interstimulus intervals. In the only parametric study of interstimulus intervals, Libby (1951) investigated seven short intervals ranging from 0 to 30 seconds. He reported that suppression increased as the interstimulus interval increased from 0 to 10 seconds and showed a slight decrease from 10 to 30 seconds.

Using a modification of the Estes and Skinner (1941) procedure, the present study replicated that of Libby (1951). The present investigation employed a consummatory lick response as an operant. Since the lick response is emitted at a higher rate than is the bar press response, this procedure permitted a more reliable assessment of the effect of interstimulus intervals upon acquisition of conditioned suppression. Thirty-two Sprague-Dawley rats were randomly assigned to one of eight groups differing in the interstimulus interval at which they were trained (1-, 3-, 7-, and 30-seconds) and tested (3- and 30- seconds). Following habituation to the 1000 Hz tone CS, Ss received one training trial and one test trial daily for 16 sessions. During extinction all Ss received two 30-second test trials with UCS omitted. Extinction was continued until each S's responding had recovered to the pre-suppression training level. No significant differences in either acquisition, or extinction were found between the various interstimulus intervals. However, certain regularities in the data suggest that the 3- and 7-second interstimulus intervals produce greater suppression. Although this differs from Libby’s (1951) finding of increased suppression up to 10 seconds, the intervals at which conditioning is best in both studies is within the range suggested from classical heart and lick conditioning studies. Several explanations for failure to replicate Libby’s (1951) results were offered. These explanations involved analysis of the different methodologies employed in the two studies.