Ss received series of electric shocks to the forearm in which the temporal schedule and the sequence of shock intensities were associated with varying levels of randomness or uncertainty. The Ss were permitted an instrumental response which produced, in advance, information concerning 1 or both of these aspects of the shocks. Such information functioned as a strong positive reinforcement for most Ss. Instrumental response rate was a significant increasing function of the degree of uncertainty

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... sociated with the shock series, and occurred significantly more often for information concerning the temporal occurrence of shock than for its intensity.