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At least 1,117 records · Page 62Linked to original sources
PubMed1996

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.

Animals↗
PubMed1999

Effects of hippocampal damage on reward threshold and response rate during self-stimulation of the ventral tegmental area in the rat.

The main purpose of this study was to explore the role of the hippocampus in motivated behavior. Rats with bilateral excitotoxic lesions of the hippocampus and controls were trained to lever press for electrical stimulation of the ventral tegmental area. Rate intensity functions were generated from an ascending and descending series of current intensities. Lesion-induced changes in sensitivity to reward were distinguished from enhancements in motor output by calculating reward thresholds and maximal response rates from the rate-intensity functions. Rats with hippocampal damage showed lower reward thresholds and higher maximal response rates than controls. These results provide further evidence of hippocampal modulation of the nucleus accumbens, suggesting that lesions of this structure enhance sensitivity to reward and increase motor output.

Animals↗