A delay discounting procedure for rhesus monkeys

Abstract

Delay discounting is important in drug abuse research as drug abusers often discount delayed rewards more than non drug users; however, ethical limitations in human research necessitate complimentary studies with other species. An advantage of studying delay discounting in monkeys is that current technologies allow for imaging monkeys while they are performing behavioral tasks. Thus it would be possible to detect delay discounting related brain activation patterns and observe how behavior and activation patterns change as a result of drug exposure, provided suitable tasks are available. Two adult rhesus monkeys have been tested on a delay discounting task in which reinforcer contingencies (i.e. changes in delay to reinforcer delivery) are systematically altered. Subjects were first put on a 2‐choice task where responding on one lever produced a large juice reinforcer (1.25 ml) and responding on the other lever produced a small juice reinforcer (0.125 or 0.25 ml). Once subjects made at least 90% of responses on the lever associated with the larger reinforcer, lever contingencies were switched across days until subjects reliably made at least 90% of responses on the lever that produced that large reinforcer. Next, subjects were tested on a delay discounting task with repeated opportunities to respond for an immediate small juice reinforcer or a large juice reinforcer that was delivered after an increasing delay (0–60 s). Responding for the large reinforcer decreased with increasing delays to its delivery; both monkeys responded predominantly for the large reinforcer more when delays were removed for individual daily sessions. This procedure appears suitable for future studies to assess the effects of drugs of abuse on delay discounting and its underlying neural circuitry.