DOES THE MFB CONVEY FUNCTIONALLY DIFFERENT REWARD SIGNALS

Pulse frequencies that sustain the same % of the maximum self-stimulat ion rate (equipotent frequencies) are equipreferred by rats because th ey elicit identical reward signals(7). However, when two equipotent fr equencies are delivered through different electrodes, lack of equipref erence can be expected if the resulting neural signals belong to diffe rent rewarding processes and if these signals are differentially ranke d on the animal's decisional scale. We used this rationale to test the possibility that the MFB conveys functionally different signals. Rats were implanted with an electrode near the anterior MFB (aMFB) and ano ther near the posterior MFB (pMFB). The rate of self-stimulation, as a function of the pulse Frequency, was first obtained for each electrod e, separately. Rats were then allowed to press for aMFB or pMFB stimul ation in a double-lever box. One of the levers delivered a fixed aMFB frequency whereas the other delivered a variable pMFB frequency. In th e following session, this situation was reversed. The time spent barpr essing for each stimulus was plotted as a function of the variable fre quency. Equipreference for equipotent stimuli (i.e for frequencies tha t supported the same %, of the maximum rate in the single-lever box) w as noted for 6 out of 11 electrode pairs. However, in 3 cases, the sub jects preferred the pMFB stimulus over an aMFB equipotent stimulus and in two other cases they preferred the aMFB stimulus. The data from th ese five subjects suggest the presence of functional heterogeneity wit hin the MFB reward pathway, a view already supported by a variety of o ther studies.