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 c ontinuous 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 w ould support half-maximum rates of response, were estimated. Threshold s rose systematically under both ratio and interval schedules of reinf orcement, and there was no difference between fixed and variable varia tions. When normalized data were plotted as a function of reinforcemen t density, thresholds from both interval and ratio schedules followed the same pattern, suggesting that the common factor was time between r ewards. An increase in the current with a corresponding decrease in pu lse frequency increased the rate at which time between rewards augment ed thresholds.