Delay discounting in rhesus monkeys: Equivalent discounting of more and less preferred sucrose concentrations

Abstract

Humans discount larger amounts of a delayed reinforcer less steeply than smaller amounts, but studies with pigeons and rats have yet to reveal such a magnitude effect, suggesting that the effect may be unique to humans. The present study examined whether the magnitude effect is observed in a species phylogenetically closer to humans, by comparing the rates at which rhesus monkeys discounted 10% and 20% concentrations of sucrose. There were no systematic differences in the rates at which the monkeys discounted the two sucrose concentrations, despite the fact that they strongly preferred the 20% concentration. Interestingly, the monkeys discounted delayed sucrose at a rate higher than was observed with delayed cocaine, and lower than was observed with delayed saccharin in previous studies (Freeman et al. Behavioural Processes, 82, 214-218, 2009; Woolverton et al. Experimental and Clinical Psychopharmacology, 15, 238-244, 2007). Taken together, these findings suggest that although both quantitative and qualitative differences can affect monkeys' preferences between immediate reinforcers, qualitative differences between types of reinforcers (e.g., sucrose vs. cocaine) can affect monkeys' discounting rates in a way that quantitative differences within a reinforcer (e.g., 10% vs. 20% sucrose) do not.

Delay discounting of food by rhesus monkeys: Cocaine and food choice in isomorphic and allomorphic situations.

“…Four subjects (314116, DK1M, MR4324, RQ661) were experimentally naïve at the beginning of the experiment. The other four subjects (CK6R, M163, M341, RO105) had experimental histories, most recently with ropinirole self-administration (CK6R; Freeman, Heal, McCreary, and Woolverton, 2012), delay discounting of sucrose solutions (M163 and RO105; Freeman et al, 2012), and delayed punishment of cocaine self-administration with histamine (M341; Woolverton, Freeman, Myerson, and Green, 2012). Subjects were given unlimited access to water and were maintained at stable body weights by food provided during the session as well as supplemental feeding (Teklad 25% Monkey Diet, Harlan/Teklad, Madison, WI).…”

“…The procedure was similar to previous delay-discounting studies from our laboratory (Freeman et al, 2009; 2012; Woolverton et al, 2007). Sessions were conducted daily, beginning at 12:00 p.m., and were signaled by illumination of the white lights to the left or right of the response lever.…”

When is it worth waiting for? Food quantity, but not food quality, affects delay tolerance in tufted capuchin monkeys

“…Although all rats showed clear food and liquid preferences, their discounting rate was similar across all rewards. On the contrary, rhesus macaques (Macaca mulatta) tested in a delay choice task using saccharin and cocaine (Freeman et al 2009;Woolverton et al 2007) discounted delayed cocaine less steeply than delayed saccharine (Freeman et al 2012). However, since cocaine is an addictive substance, its shallower discounting might have depended on its addictive nature, rather than on its quality.…”

William L. Woolverton: A case history in unraveling the behavioral pharmacology of stimulants

“…This led him to conduct a series of studies involving food-drug choice conditions in which he systematically manipulated environmental variables (e.g., Nader and Woolverton, 1991, 1992a,b; Woolverton and Rowlett, 1998; Anderson et al, 2002; Woolverton, 2003; Woolverton et al, 2007, 2012; Freeman et al, 2012). Increases in the magnitude of food reinforcers could decrease cocaine choice (Nader and Woolverton, 1991), as could increases in the “cost” (FR value) of cocaine (Nader and Woolverton, 1992a; Negus, 2003).…”