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Ss were given 1200 classical eyelid conditioning trials over 12 sessions of 100 trials each. The manipulated independent variable was interstimulus interval, the specific values employed being 200, 500, and 800 msec. Depending upon the treatment combination, some Ss received only 1 interstimulus interval throughout training, while others were shifted back and forth between 2 interstimulus interval values. The major results were (a) response frequency continued to increase beyond 400 training trials, though differences as a function of interstimulus interval were not statistically significant; (b) both mean and SD of latency varied directly with interstimulus interval and changes in interstimulus interval; and (c) both mean and SD of latency decreased across training sessions. The data were interpreted in terms of an assumed reinforcement related to the temporal disparity between UCR and UCS onsets.

The performances of 3 groups of human Ss in a classical eyelid conditioning situation were compared at an interstimulus interval (ISI) of 2.497 sec. 1 group received all training at an ISI of 2.497, a 2nd group received prior training at an ISI of .63 and was switched to one of 2.497, while the 3rd group began training at .63 and was gradually changed to one of 2.497 in 6 steps. The frequency of response was greater in the gradually shifted group than in the other 2 groups. It was concluded (a) that whether or not high response frequencies were obtained at long ISIs depends upon the specific training regimen; (b) that a time discrimination, in the form of response latency, played an important role in the results; and (c) that asymptotic performance at one ISI is not independent of experience with other ISIs.

2 levels of intertrial interval (40 sec. vs. 20 sec.) were investigated in a GSR conditioning situation, with controls for sensitization incorporated. The dependent variables reported were: base, magnitude, latency, and recruitment. The primary results in acquisition were: (a) the conditioning operations led to greater magnitude and recruitment than did the sensitization operations; (b) greater magnitude and recruitment were associated with spaced, as opposed to massed, practice; and (c) sensitization and conditioning operations were differentiated across trials by a decrease in magnitude in the sensitization groups rather than by an increase in the conditioning groups. The primary results in extinction were: (a) the decrease in magnitude across extinction trials; (b) the longer latency of sensitized, as opposed to conditioned, responses; and (c) the greater magnitude associated with spaced as opposed to massed practice.The results of several recent studies (Kimmel, 1959;Stewart, Stern, Winokur, & Fredman, 1961) have drawn attention to the role of sensitization in classical GSR conditioning. Of particular concern are the conditions under which the effects specific to CS-UCS pairings can be separated from the influences of sensitization. Kimmel (1959) found that, with an intense auditory stimulus as CS, conditioning performance was no different from that obtained under sensitization procedures, i.e., performance to a tone when unpaired tone and shock are presented in an unpredictable order. In contast, with a low-intensity auditory CS, evidence of the effects of CS-UCS pairings was illustrated by the superior performance of the conditioning groups to its

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