Intermittent rewards raise self-stimulation thresholds

Rats implanted with lateral hypothalamic electrodes pressed a lever to obtain 0.5 s bursts of pulses under the four combinations of fixed or variable, interval or ratio, schedules of reinforcement. Along with continuous reinforcement schedules, intervals of 1, 2, 5, and 10 s, or ratios of 2, 5, and 10 responses per stimulation were used in sessions wherein the frequency thresholds, defined as pulse frequencies that would support half-maximum rates of response, were estimated. Thresholds rose systematically under both ratio and interval schedules of reinforcement, and there was no difference between fixed and variable variations. When normalized data were plotted as a function of reinforcement density, thresholds from both interval and ratio schedules followed the same pattern, suggesting that the common factor was time between rewards. An increase in the current with a corresponding decrease in pulse frequency increased the rate at which time between rewards augmented thresholds.