No abstract

Four rats responded under a "self-control" procedure designed to obtain delay-discount functions within sessions. Each session consisted of seven blocks, with seven trials within each block. Each block consisted of two initial forced-choice trials followed by five free-choice trials. On choice trials, the rats could press either of two retractable levers. A press on one lever was followed by presentation of a smaller reinforcer (a single dipper presentation of a sucrose solution); a press on the other lever was followed by presentation of a larger reinforcer (four consecutive dipper presentations). The delay associated with the smaller reinforcer always was 0 s, whereas the signaled delay associated with the larger reinforcer increased across blocks (from 0 to 50 s). Under these conditions, the percentage of choices of the larger reinforcer decreased across blocks, and relatively reliable delay-discount functions were obtained within sessions. Doses of methylphenidate (1.0 to 17.0 mg/kg) and morphine (0.3 to 17.0 mg/kg) were then administered prior to selected sessions. Typically, intermediate doses of methylphenidate shifted the discount functions to the right (increased choices of the larger reinforcer). For 2 of the rats, this effect was pronounced; for the other 2 rats, this effect occurred after the range of delays for the larger reinforcer was decreased (0 to 20 s). On the other hand, in most cases morphine produced a slight leftward shift in the discount function (decreased choices of the larger reinforcer). The present procedure appears to be a useful and efficient method to characterize drug effects on an entire delay-discount function. As with many procedures used to study self-control choices, however, sources of control other than reinforcement delay and amount may have been operating in the present study, and these sources must be considered when interpreting drug effects.

Preference pulses are thought to represent strong, short-term effects of reinforcers on preference in concurrent schedules. However, the general shape of preference pulses is substantially determined by the distributions of responses-per-visit (visit lengths) for the two choice alternatives. In several series of simulations, we varied the means and standard deviations of distributions describing visits to two concurrently available response alternatives, arranged "reinforcers" according to concurrent variable-interval schedules, and found a range of different preference pulses. Because characteristics of these distributions describe global aspects of behavior, and the simulations assumed no local effects of reinforcement, these preference pulses derive from the visit structure alone. This strongly questions whether preference pulses should continue to be interpreted as representing local effects of reinforcement. We suggest an alternative approach whereby local effects are assessed by subtracting the artifactual part, which derives from visit structure, from the observed preference pulses. This yields "residual" preference pulses. We illustrate this method in application to published data from mixed dependent concurrent schedules, revealing evidence that the delivery of reinforcers had modest lengthening effects on the duration of the current visit, a conclusion that is quantitatively consistent with early research on short-term effects of reinforcement.

Identity matching-to-sample has been difficult to demonstrate in rats, but most studies have used visual stimuli. There is evidence that rats can acquire complex forms of olfactory stimulus control, and the present study explored the possibility that identity matching might be facilitated in rats if olfactory stimuli were used. Four rats were trained on an identity match-to-sample procedure with odorants mixed in cups of sand as stimuli. Digging in the sample cup produced two comparison cups, and digging in the comparison cup that contained the same scent as the sample was reinforced. When criterion accuracy levels were reached, novel stimuli were added to the baseline training regimen. All 4 rats reached terminal performance of above 90% correct matching with more than 20 different baseline stimuli and matched novel stimulus combinations with above-chance accuracy; 3 of the 4 rats matched novel stimuli at levels significantly above chance. Accurate matching performance was demonstrated both with 2- and 3-comparison procedures. These results suggest that generalized matching-to-sample can be observed in rats when olfactory stimuli are used and, furthermore, that multiple-exemplar training may be important for its emergence.

Extended pausing during discriminable transitions from rich-to-lean conditions can be viewed as escape (i.e., rich-to-lean transitions function aversively). In the current experiments, pigeons' key pecking was maintained by a multiple fixed-ratio fixed-ratio schedule of rich or lean reinforcers. Pigeons then were provided with another, explicit, mechanism of escape by changing the stimulus from the transition-specific stimulus used in the multiple schedule to a mixed-schedule stimulus (Experiment 1) or by producing a period of timeout in which the stimulus was turned off and the schedule was suspended (Experiment 2). Overall, escape was under joint control of past and upcoming reinforcer magnitudes, such that responses on the escape key were most likely during rich-to-lean transitions, and second-most likely during lean-to-lean transitions. Even though pigeons pecked the escape key, they paused before doing so, and the latency to begin the fixed ratio (i.e., the pause) remained extended during rich-to-lean transitions. These findings suggest that although the stimulus associated with rich-to-lean transitions functioned aversively, pausing is more than simply escape responding from the stimulus.

Log-survivor analyses of interresponse times suggest that the behavior of rats responding under single variable-interval schedules is organized into bouts (i.e., periods of engagement and disengagement). Attempts to generalize this analysis to the key pecking in pigeons, however, have failed to produce the characteristic broken-stick appearance typically obtained with rats. This failure may be due to a relatively low rate of reinforcement for engaging in alternative behavior experienced by pigeons. The present study tested this hypothesis by exposing four pigeons to concurrent schedules of reinforcement for key pecking, first without a changeover delay (COD) and then with a COD. In this arrangement, one of the concurrent options was treated as the target response and the rate of reinforcement for that option was manipulated across conditions. The other option provided explicit reinforcement for engaging in an alternative response (i.e., explicit reinforcement for disengaging from the target response). In the absence of a COD, log-survivor plots for three of the pigeons were approximately linear, thus providing no evidence that responding was organized into bouts. When a COD was present, plots were broken stick in appearance, indicating a bout structure had been generated in the pigeons' behavior. Both bout length and the rate of bout initiations were a function of differences in rate of reinforcement. These data suggest that behavior may become organized into bouts when contingencies create sufficiently long visits to both the target behavior and the extraneous behavior. Fits of a double-exponential model deviated systematically from the actual plots due to the presence of a plateau between the two limbs. An alternative, double-gamma, model was explored, and it provided a considerably better fit than did the double-exponential.

The purpose of this study was to examine effects of d-amphetamine on choice controlled by reinforcement delay. Eight pigeons responded under a concurrent-chains procedure in which one terminal-link schedule was always fixed-interval 8 s, and the other terminal-link schedule changed from session to session between fixed-interval 4 s and fixed-interval 16 s according to a 31-step pseudorandom binary sequence. After sufficient exposure to these contingencies (at least once through the pseudorandom binary sequence), the pigeons acquired a preference for the shorter reinforcement delay within each session. Estimates of the sensitivity to reinforcement immediacy were similar to those obtained in previous studies. For all pigeons, at least one dose of d-amphetamine attenuated preference and, hence, decreased estimates of sensitivity to reinforcement immediacy; in most cases, this effect occurred without a change in overall response rates. In many cases, the reduced sensitivity to reinforcement delay produced by d-amphetamine resulted primarily from a decrease in the asymptotic level of preference achieved within the session; in some cases, d-amphetamine produced complete indifference. These findings suggest that a reduction in the sensitivity to reinforcement delay may be an important behavioral mechanism of the effects of psychomotor stimulants.

Repeated-acquisition procedures that include performance controls for effects not specific to acquisition permit the assessment of drug effects on learning on a within-subject, within-session basis. Despite the advantages of this methodology, few studies have examined effects of psychomotor stimulants on repeated acquisition in rodents. The purpose of the present study was to investigate the effects of methylenedioxymethamphetamine (MDMA, 0.3-10 mg/kg), methamphetamine (MA, 0.1-3 mg/kg) and methylphenidate (MPD,1-17 mg/kg) using repeated-acquisition procedures with performance controls in rats using a touch-screen apparatus. Rats were presented a 2 × 3 array of stimuli using a computer touch-screen and nose-pokes to target locations within the array were reinforced. In the acquisition component, the correct location changed across sessions, whereas during the performance component, the correct location was constant across sessions. All three drugs reduced accuracy of responding to target locations in a dose-dependent fashion. None of the compounds enhanced learning at any dose. MPD and MA produced significant disruptions of acquisition accuracy only at doses that also disrupted performance, but the 3 mg/kg dose of MDMA impaired acquisition of target responding without affecting performance. The selective impairment of acquisition found in the present study adds to the evidence of learning and memory disruption produced by acute MDMA administration and raise questions about the mechanisms for these actions.