Subjective reward magnitude of medial forebrain stimulation as a function of train duration and pulse frequency.

The growth of the subjective reward magnitude of medial forebrain bundle stimulation in the rat (Sprague-Dawley) as a function of train duration and pulse frequency was measured in 2 ways: (a) a titration method, which used differences in rate of reward on 2 levers to compensate for differences in the magnitude of the rewards; and (b) a direct method, in which the ratio of the reward magnitudes at the 2 levers was assumed to be given by the ratio of times spent on each lever. The results of the 2 methods agree. Reward magnitude grows as approximately a power function of train duration up to train durations of about 1 s, then declines somewhat over the interval from 2-20 s. The exponent of growth varies from 0.4 to 2.3. With stronger stimulation (higher pulse frequency), peak reward magnitude is bigger, but the saturating train duration is approximately the same.