Further evidence against adaptation of prolonged electrical self-stimulation reward

Using a Y-maze preference test paradigm, we examined the temporal characteristics of the neural network subserving self-stimulation reward. The first part of the experiment demonstrated that when prolonged electrical brain stimulation is initially delivered with a low pulse frequency (100 Hz), rats prefer an increase over either a decrease or no change in the pulse frequency of subsequent stimulation. However, the second part showed that when prolonged brain stimulation is initially delivered with a high pulse frequency (250 Hz), an increase is not preferred. The data are inconsistent with an adaptation model of summation. These results are explained in terms of an improved model of summation involving two integrators and fatigue.