DOES THE MFB CONVEY FUNCTIONALLY DIFFERENT REWARD SIGNALS

Pulse frequencies that sustain the same % of the maximum self-stimulation rate (equipotent frequencies) are equipreferred by rats because they elicit identical reward signals(7). However, when two equipotent frequencies are delivered through different electrodes, lack of equipreference can be expected if the resulting neural signals belong to different rewarding processes and if these signals are differentially ranked 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 another near the posterior MFB (pMFB). The rate of self-stimulation, as a function of the pulse Frequency, was first obtained for each electrode, separately. Rats were then allowed to press for aMFB or pMFB stimulation in a double-lever box. One of the levers delivered a fixed aMFB frequency whereas the other delivered a variable pMFB frequency. In the following session, this situation was reversed. The time spent barpressing for each stimulus was plotted as a function of the variable frequency. Equipreference for equipotent stimuli (i.e for frequencies that supported the same %, of the maximum rate in the single-lever box) was noted for 6 out of 11 electrode pairs. However, in 3 cases, the subjects preferred the pMFB stimulus over an aMFB equipotent stimulus and in two other cases they preferred the aMFB stimulus. The data from these five subjects suggest the presence of functional heterogeneity within the MFB reward pathway, a view already supported by a variety of other studies.