Locey, M. L., & Dallery, J. (2009).
Isolating behavioral mechanisms of inter-temporal choice: Nicotine effects on delay discounting and amount sensitivity.
Journal of the Experimental Analysis of Behavior, 91, 213-223.
Many drugs of abuse produce changes in impulsive choice, that is, choice for a smallerŠsooner
reinforcer over a largerŠlater reinforcer. Because the alternatives differ in both delay and amount, it is
not clear whether these drug effects are due to the differences in reinforcer delay or amount. To isolate
the effects of delay, we used a titrating delay procedure. In phase 1, 9 rats made discrete choices
between variable delays (1 or 19 s, equal probability of each) and a delay to a single food pellet. The
computer titrated the delay to a single food pellet until the rats were indifferent between the two
options. This indifference delay was used as the starting value for the titrating delay for all future
sessions. We next evaluated the acute effects of nicotine (subcutaneous 1.0, 0.3, 0.1, and 0.03 mg/kg)
on choice. If nicotine increases delay discounting, it should have increased preference for the variable
delay. Instead, nicotine had very little effect on choice. In a second phase, the titrated delay alternative
produced three food pellets instead of one, which was again produced by the variable delay (1 s or 19 s)
alternative. Under this procedure, nicotine increased preference for the one pellet alternative.
Nicotine-induced changes in impulsive choice are therefore likely due to differences in reinforcer
amount rather than differences in reinforcer delay. In addition, it may be necessary to include an
amount sensitivity parameter in any mathematical model of choice when the alternatives differ in
reinforcer amount.
Key words: risk, impulsive choice, reinforer delay, reinforcer amount, nicotine, lever press, rats