Impulsivity and sensitivity to amount and delay of reinforcement in an animal model of ADHD

• Spontaneously hypertensive rats (SHR) and Wistar rats were evaluated in three choice tasks.
• Impulsivity, sensitivity to delay and sensitivity to amount of reinforcement were measured.
• SHR were found to be more impulsive, and more sensitive to delay.
• SHR and Wistar rats were found to be equally sensitive to amount of reinforcement.

Previous research has been inconclusive about the degree of impulsivity displayed by spontaneously hypertensive rats (SHR), an animal model of Attention Deficit Hyperactivity Disorder (ADHD). In the present set of experiments, concurrent-chains schedules were employed in order to explore SHR's impulsivity, sensitivity to delay, and sensitivity to amount of reinforcement; Wistar rats (WIS) were used as comparison group. In the three experiments – performed with different subjects – non-independent variable interval 30 s schedules were presented in the initial links; the difference between experiments was in the terminal links. For exploring impulsivity, one of the terminal links (SS) was associated to a short delay (2 s) and a small reinforcer (1 pellet), whereas the other terminal link (LL) was associated to a longer delay (28 s) and a larger reinforcer (4 pellets). The results indicated a remarkably higher impulsivity in SHR. Because this impulsivity may have as potential mechanisms an increased sensitivity to delay and/or a decreased sensitivity to the amount of reinforcement, in experiments 2 and 3 these possibilities were examined. For assessing sensitivity to delay, the following pairs of fixed interval (FI) schedules were used in the terminal links in five conditions: 2–28, 6–24, 15–15, 24–6, 28–2 s; the magnitude of reinforcement was 1 pellet in all conditions for both alternatives. For assessing sensitivity to amount, in five conditions the alternatives were associated with different magnitudes of reinforcement: 1–5 pellets, 2–4, 3–3, 4–2 and 5–1 in left–right alternatives, respectively; the delay to reinforcement was controlled by a FI 15 s in all conditions and for both alternatives. The sensitivity to delay and the sensitivity to amount were calculated according to the Generalized Matching Law. The results indicated a higher sensitivity to delay in SHR, and the same sensitivity to amount in SHR and WIS rats. These results suggest that the increased sensitivity to delay influences the high level of impulsivity observed in SHR.