Christine E. Hughes

Rapid acquisition of preference in concurrent schedules: Effects of d-amphetamine on sensitivity to reinforcement amount

In the present study, effects of d-amphetamine on sensitivity to reinforcement amount under concurrent schedules were examined using a rapid-acquisition procedure. Four pigeons key pecked under single concurrent variable-interval 30-s schedules of grain presentation. Two different reinforcer-amount ratios (7:1 and 1:7) changed across sessions according to a 31-step pseudo-random binary sequence (PRBS). After at least four times through the PRBS, response ratios generally tracked the session-to-session changes in amount ratios; estimates of sensitivity ranged from 0.26 to 0.31 across the four pigeons. Effects of a range of doses of d-amphetamine (0.3-5.6mg/kg) then were determined. For 3 of 4 pigeons, at least one dose, which did not dramatically alter overall response output or bias, decreased sensitivity to reinforcement amount. These results suggest that reducing sensitivity of responding to reinforcement amount may be one behavioral mechanism of stimulants, which may have implications for interpreting drug effects on self-control.

DEVELOPMENT AND MAINTENANCE OF CHOICE IN A DYNAMIC ENVIRONMENT

Four pigeons were exposed to a concurrent procedure similar to that used by Davison, Baum, and colleagues (e.g., Davison & Baum, 2000, 2006) in which seven components were arranged in a mixed schedule, and each programmed a different left∶right reinforcer ratio (1∶27, 1∶9, 1∶3, 1∶1, 3∶1, 9∶1, 27∶1). Components within each session were presented randomly, lasted for 10 reinforcers each, and were separated by 10-s blackouts. These conditions were in effect for 100 sessions. When data were aggregated over Sessions 16-50, the present results were similar to those reported by Davison, Baum, and colleagues: (a) preference adjusted rapidly (i.e., sensitivity to reinforcement increased) within components; (b) preference for a given alternative increased with successive reinforcers delivered via that alternative (continuations), but was substantially attenuated following a reinforcer on the other alternative (a discontinuation); and (c) food deliveries produced preference pulses (immediate, local, increases in preference for the just-reinforced alternative). The same analyses were conducted across 10-session blocks for Sessions 1-100. In general, the basic structure of choice revealed by analyses of data from Sessions 16-50 was preserved at a smaller level of aggregation (10 sessions), and it developed rapidly (within the first 10 sessions). Some characteristics of choice, however, changed systematically across sessions. For example, effects of successive reinforcers within a component tended to increase across sessions, as did the magnitude and length of the preference pulses. Thus, models of choice under these conditions may need to take into account variations in behavior allocation that are not captured completely when data are aggregated over large numbers of sessions.

Effects of d-amphetamine on behavior maintained by signaled and unsignaled delays to reinforcement.

Four pigeons responded under a progressive-delay procedure. In a signaled-delay condition, a chained variable interval (VI) 30-s progressive time (PT) 4-s schedule was arranged; in an unsignaled-delay condition, a tandem VI 30-s PT 4-s schedule was arranged. Two pigeons experienced a signaled-unsignaled-signaled sequence; whereas, two pigeons experienced an unsignaled-signaled-unsignaled sequence. Effects of saline and d-amphetamine were determined under each condition. At intermediate doses (1.0 and 1.78m/kg) delay functions were shallower, area under the curve was increased, and, when possible, break points were increased compared to saline; these effects were not systematically related to signaling conditions. These effects on control by delay often were accompanied by decreased response rates at 0s. These results suggest that stimulus conditions associated with the delay may not play a crucial role in effects of d-amphetamine and other stimulants on behavior controlled by reinforcement delay.