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Only the function relating the required pulse frequency to the interburst interval discriminates between the priming effect and the rewarding effect: Separating two short bursts of pulses has no effect on their combined rewarding effect, but it enhances their priming effect. There are significant nonscalar differences between animals in the number-current and charge-duration functions for priming and reward, but there are no significant differences in these functions between effects within animals. Substantially more electrical-stimulation is required to produce good priming than to produce a reward that sustains maximum possible levels of performance. These findings suggest that the priming effect is a transient excitement generated by the receipt of one or more very large rewards. The discussion elaborates a model that reconciles this hypothesis with the earlier finding that the rewarding effect may be blocked pharmacologically without blocking priming.

Temporal summation of the rewarding effects of medial forebrain stimulation in the rat was investigated by varying the interval separating the two short bursts of stimulation given as a reward in a runway. One finding--that the reinforcing effect of the two bursts is independent of interburst interval--supports a model in which there is perfect summation of the portion of the reward signal that exceeds some threshold. However, the constant-threshold form of this model is not reconcilable with the results of a second experiment, which shows that charge-duration functions obtained with different levels of performance differ by a multiplicative (scalar) factor, that is, the ratio between the values of the two functions is everywhere the same. (The charge-duration function gives the charge required as a function of the train duration.) None of the models of postsynaptic integration so far suggested is capable of explaining simultaneously the fact that the strength-duration function is a perfect hyperbola that has nearly reached its rheobase at a train duration no greater than 2 s, that there is no statistically detectable effect of interburst interval on summation between bursts separated by intervals up to 2 s and longer, and that the strength-duration functions (or, equivalently, the charge-duration functions) derived by using different performance criteria differ by a multiplicative factor.