Psychophysical mapping of reward system operating characteristics: Utility transforms of stimulation duration

Abstract

By applying signal detection theory to the utility scaling of rewarding brain stimulation, we have obtained a psychophysical function which maps duration of medial forebrain bundle (MFB) stimulation to reward value. Six 120 day-old Charles River rats, implanted with bipolar electrodes directed at the MFB, were trained to discriminate between high and low frequency tones for brain stimulation durations which ranged from .2 sec to 8.5 sec. The ratios of manipulated reward durations formed between the two types of correct response produced changes in the animal's response bias, as measured by beta, which were used to derive utility transforms of brain stimulation duration. We propose an optimizing differential equation system, based on the derived utility function, which models the total utility of rewarding electrical stimulation as subject to its marginal utility. Our results suggest that the brain scales activity in the reward pathway as a stimulus falling on a prothetic continuum, thereby allowing fine distinctions to be made between motivational stimuli across a wide range of stimulation.