Behavioral methods for inferring anatomical linkage between rewarding brain stimulation sites.

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Abstract
Rats lever pressed for concurrent electrical stimulation of the lateral hypothalamus and ventral tegmentum. The pulse-pair stimulation technique was used, with the 1st pulse of each pair applied to 1 electrode and the 2nd pulse to the other electrode; the intrapair interval was varied. The effectiveness of stimulation, measured behaviorally, increased abruptly (within 0.4 ms) as the intrapair interval was increased in the range from 1.0-2.0 ms. These results, which do not resemble single-electrode refractory period results, are interpreted as evidence of collision in the directly stimulated, reward-related neurons linking the 2 sites. Self-stimulation of the medial forebrain bundle involves the direct activation of long-axon, longitudinal pathways. Estimates of the conduction velocity in the fibers subserving the collision-like effects are consistent with the properties of small myelinated axons but not central monoaminergic fibers.