The variable of CS duration preceding unavoidable electric shock was investigated with rats as Ss. For the 6 durations examined from 3.75 to 60 sec, suppression of positively reinforced bar-pressing in the presence of CS was complete. While response suppression between CS presentations was initially severe, it gradually dissipated as the experiment progressed. A separate group of rats received unavoidable shocks not preceded by a stimulus. General suppression of responding throughout the session was more severe than with the cued-shock groups, and recovery was slower. The difference in the effects of uncued shock and cued shock was confirmed when stimulus presentation conditions were reversed between the subject groups.
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