Summation and saturation properties in the rewarding effect of brain stimulation

Prolonged rewarding stimulation of the rat medial forebrain bundle: Neurochemical and behavioral consequences.

“…Depending on the nature of the weight-changing function, the weights would thus either climb towards infinity or saturate. Such outcomes are inconsistent with voluminous data showing that graded adjustment of the strength of MFB stimulation over a substantial range produces correspondingly graded changes in reward strength (Gallistel & Leon, 1991;Miliaressis & Malette, 1987;Simmons & Gallistel, 1994). The model can be rescued from this unrealistic prediction by assuming that the electrode activates the DA neurons trans-synaptically (as in the "series model"), a proposition that is on firmer empirical ground.…”

Ventral pallidum self-stimulation induces stimulus dependent increase in c-fos expression in reward-related brain regions

“…Nevertheless, VHF resulted in appreciably larger counts of immunoreactive nuclei than HF, in all positive regions. Self-stimulation studies involving various brain regions using a two-lever paradigm, 29,42 revealed that SS-related reward does not saturate at the frequency that elicits maximal SS rate (the HF) but rather grows exponentially until this frequency value is at least doubled. If this phenomenon holds also for VP self-stimulation then, the present data would indicate that Fos immunoreactivity correlated well with reward magnitude, but not with bar-pressing behaviour.…”

Effect of current on the maximum possible reward.

“…If the subjective reward produced by one lever is substantially greater than that produced by the other, one might expect the rat to prefer the lever that yielded the greater reward even when both are large enough to sustain the maximum possible rate of pressing in a single-lever task. Miliaressis and Malette (1987) and Waraczynski, Stellar, and Gallistel (1987) used choice paradigms to show that (a) the underlying rewarding effect of stimulation does saturate, usually at some pulse frequency between 100 and 500 pps, (b) raising the current generally lowers the pulse frequency at which the rewarding effect saturates, and (c) at higher currents, the frequency at which the rewarding effect saturates is substantially higher than the frequency at which performance (rate of pressing) saturates. What this third finding means is that, generally speaking, the maximum possible subjective reward has not been attained at the rather low pulse frequencies that sustain the maximum possible rate of pressing.…”

“…In Experiment 3, the current-dependent pulse frequencies at which reward saturated were determined with greater precision using the A frequency-versus-S frequency method first used by Miliaressis and Malette (1987) and Waraczynski et al (1987). In this method, the pulse frequency on the S lever varies between sessions, while the pulse frequency on the A lever varies between trials within sessions.…”