Effects of delay to reinforcement on the choice between cocaine and food in rhesus monkeys
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- Woolverton, W.L. & Anderso, K.G. Psychopharmacology (2006) 186: 99. doi:10.1007/s00213-006-0355-x
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Although a delay between behavior and reinforcer has been shown to weaken behavior, little is known about the effects of delay on drug choice.
The present study examined effects of delay between lever press and reinforcer presentation on the choice between a drug and non-drug reinforcer and between different drug doses.
Materials and methods
Monkeys (n=4) were allowed to choose 32 times/day between cocaine and four food pellets. The delay between lever press and a preferred dose of cocaine (0.05 mg/kg/injection) was increased systematically from 0 to 240 s, while the delay to food remained at 0 s. A second group of monkeys (n=4) was allowed to choose between 0.05 mg/kg/injection and a lower dose of cocaine (0.025 mg/kg/injection). Next, a delay that resulted in less than 20% choice of 0.05 mg/kg/injection cocaine was selected and delay to the alternative was varied.
Results were similar across groups. The choice of 0.05 mg/kg/injection approximated 100% at 0 delay and decreased to near 0 as delay increased. As the delay to alternative was subsequently increased from 0 to 240 s, choice of 0.05 mg/kg/injection increased, though full cocaine choice was not generally restored. The delay estimated to maintain 50% choice (indifference point) was lower for the cocaine-food choice (mean=64 s) than for the cocaine–cocaine choice (mean=207 s).
This experiment demonstrates that the choice between cocaine and a non-drug or drug alternative can be modified by increasing the interval between behavior and drug injection. Overall, the results are consistent with a temporal discounting model of drug choice.