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Delay discounting in Lewis and Fischer 344 was compared using a novel concurrent-chains procedure. In the initial-link, two levers were available and a random-interval schedule was in effect. One terminal link provided a smaller reinforcer (1 pellet) and the other terminal provided a larger reinforcer (4 pellets). Terminal-link entry was scheduled randomly, with the constraint that entries to each occurred equally often. The terminal link associated with the smaller reinforcer was always a fixed-interval (FI) 5-s (i.e., the delay to the smaller reinforce was always 5 s). Delay discount functions were obtained by varying the terminal-link FI associated with the larger reinforcer across blocks of cycles within each session. The order of the delays (5, 10, 20, 40, and 80 s) within each session was randomly determined to minimize carryover effects. Graded delay-discount functions were obtained which were well described by a hyperbolic-delay equation. Initially, the Lewis rats were more sensitive to the effects of delay to larger reinforcer than the Fischer 344 rats. With extended training, however, the Fischer 344 rats showed a greater change in sensitivity to the delayed larger reinforcer, achieving levels of impulsivity comparable to those reached by the Lewis rats. These data suggest that a) the concurrent-chains procedure arranged here provides an efficient method for producing within-sessions graded discount functions in individual subjects, and b) differences in delay discounting between Lewis and Fischer rats often reported (and attributed to neurobiological differences) are not apparent after extended training.
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