Date of Award


Degree Name

Doctor of Philosophy




Previous studies (e.g., Hoffman, Branch, & Sizemore, 1987) have demonstrated that greater drug tolerance develops in responding under fixed-ratio (FR) schedules of reinforcement when the schedule parameter is small than when it is large; little research has been done on related schedules such as variable-ratio (VR). In Experiment 1, three pigeons responded under a multiple FR 5 FR 125 VR 5 VR 125. A range of prechronic doses of cocaine produced dose-dependent reductions in response rates under the component schedules. Following chronic dosing with daily administrations of 5.6 mg/kg, similar dose-response curves were derived by substitutions of the usual dose. In Experiment 2, the procedure was similar but the chronic dose was adjusted to ensure reductions of response rate and reinforcers earned within the range of doses tested. Dose-response curves were similarly derived following chronic dosing with daily administrations of a dose that initially produced substantial decreases in responding and reinforcement. The results replicated earlier findings for FR, with less tolerance under the larger response requirement than the smaller. Tolerance under VR schedules was similar to that under FR schedules of the same schedule parameter. These results favor the response strength hypothesis, which posits greater recovery in situations associated with greater reinforcement rate. Directions for future research on tolerance are discussed.

Access Setting

Dissertation-Open Access