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 varying either the current or the number of pulses per train. The resulting relationship between reward effectiveness and stimulation site is independent of the fixed parameter only if the current-number trade-off functions (TOFs) for all sites are parallel. This assumption was tested in the present experiment. Movement of an electrode through or near the ventral tegmental area did not always shift the TOFs in a parallel manner and sometimes caused them to intersect. To describe such effects, a current-number TOF must be obtained at each site in lieu of a single estimate of the required stimulation strength. Across-site changes in the parameters of these TOFs are interpreted in terms of differences in the density, excitability, or postsynaptic impact of the directly stimulated reward neurons.