We investigated temporal summation of the rewarding effects of medial forebrain stimulation in 4 male Sprague-Dawley rats by varying the interval separating the 2 short bursts of stimulation given as a reward in a runway. One finding of Exp I—that the reinforcing effect of the 2 bursts was 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, Exp II (with the subjects from Exp I) showed that charge–duration functions obtained with different levels of performance differed by a multiplicative (scalar) factor—that is, the ratio between the values of the 2 functions was everywhere the same. The models of postsynaptic integration are not capable of explaining simultaneously the fact (a) that the strength–duration function is a perfect hyperbola that has nearly reached its rheobase at a train duration no greater than 2 sec; (b) that there is no detectable effect of interburst interval on summation between bursts separated by intervals up to 2 sec and longer; and (c) that the strength–duration functions (or, equivalently, the charge–duration functions) derived by using different performance criteria differ by a multiplicative factor.
Co-author: C. R. Gallistel.
Published in Behavioral Neuroscience, volume 98, pp. 467 – 478, March 1984.
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