MAPPING THE SUBSTRATE FOR BRAIN-STIMULATION REWARD BY MEANS OF CURRENT-NUMBER TRADE-OFF FUNCTIONS

In previous studies using moveable electrodes to map the substrate for brain stimulation reward, reward effectiveness was estimated by varyi ng either the current or the number of pulses per train. The resulting relationship between reward effectiveness and stimulation site is ind ependent of the fixed parameter only if the current-number trade-off f unctions (TOFs) for all sites are parallel. This assumption was tested in the present experiment. Movement of an electrode through or near t he ventral tegmental area did not always shift the TOFs in a parallel manner and sometimes caused them to intersect. To describe such effect s, a current-number TOF must be obtained at each site in lieu of a sin gle estimate of the required stimulation strength. Across-site changes in the parameters of these TOFs are interpreted in terms of differenc es in the density, excitability, or postsynaptic impact of the directl y stimulated reward neurons.