Human observing: maintained by negative informative stimuli only if correlated with improvement in response efficiency.

Two experiments investigated the effect of observing responses that enabled college students to emit more efficient distributions of reinforced responses. In Experiment 1, the gains of response efficiency enabled by observing were minimized through use of identical low-effort response requirements in two alternating variable-interval schedules. These comprised a mixed schedule of reinforcement; they differed in the number of money-backed points per reinforcer. In each of three choices between two stimuli that varied in their correlation with the variable-interval schedules, the results showed that subjects preferred stimuli that were correlated with the larger average amount of reinforcement. This is consistent with a conditioned-reinforcement hypothesis. Negative informative stimuli--that is, stimuli correlated with the smaller of two rewards--did not maintain as much observing as stimuli that were uncorrelated with amount of reward. In Experiment 2, savings in effort made possible by producing S- were varied within subjects by alternately removing and reinstating the response-reinforcement contingency in a mixed variable-interval/extinction schedule of reinforcement. Preference for an uncorrelated stimulus compared to a negative informative stimulus (S-) decreased for each of six subjects, and usually reversed when observing permitted a more efficient temporal distribution of the responses required for reinforcement; in this case, the responses were pulls on a relatively high-effort plunger. When observing the S- could not improve response efficiency, subjects again chose the control stimulus. All of these results were inconsistent with the uncertainty-reduction hypothesis.     

Selective punishment of interresponse times: The roles of shock intensity and scheduling

Two experiments investigated the roles of shock intensity and scheduling in selective punishment of interresponse times. In each experiment the punishment contingencies were imposed on a background of rats' responding maintained by a variable-interval schedule of food presentation. In Experiment 1 all interresponse times greater than 8 seconds produced shock. In Experiment 2 all interresponse times greater than 8 seconds but less than 12 seconds produced shock. In each experiment shock intensity was initially 0.3 milliamperes (mA) and then was varied through an ascending sequence ranging from 0.1 mA to 0.4 mA, in 0.1-mA increments. Experiment 1 produced response-rate increases at low intensities (0.1 and 0.2 mA) but eliminated responding at the remaining intensities. Experiment 2 produced response-rate increases only with 0.1-mA shock, although responding was maintained at all shock parameters investigated. Analysis of the interresponse times per opportunity showed differential suppression of the targeted responses in all cases except the high-intensity shock phases of Experiment 1. The current data support and extend previous studies of selective interresponse-time-dependent shock schedules but suggest that response-rate increases are not a necessary outcome of this type of procedure. The view that variable-interval schedules of shock presentation selectively target long interresponse times was also supported.     

SUPPRESSIVE AND FACILITATIVE EFFECTS OF SHOCK INTENSITY AND INTERRESPONSE TIMES FOLLOWED BY SHOCK

Although response‐dependent shock often suppresses responding, response facilitation can occur. In two experiments, we examined the suppressive and facilitative effects of shock by manipulating shock intensity and the interresponse times that produced shock. Rats' lever presses were reinforced on a variable‐interval 40‐s schedule of food presentation. Shock followed either long or short interresponse times. Shock intensity was raised from 0.05 mA to 0.4 mA or 0.8 mA. Overall, shock contingent on long interresponse times punished long interresponse times and increased response rates. Shock contingent on short interresponse times punished short interresponse times and decreased response rates. In Experiment 1, raising the range of interresponse times that produced shock enhanced these effects. In Experiment 2, the effects of shock intensity depended on the interresponse times that produced shock. When long interresponse times produced shock, low intensities increased response rates. High intensities decreased response rates. When short interresponse times produced shock, high shock intensities punished short interresponse times and decreased response rates more than low intensities. The results may explain why punishment procedures occasionally facilitate responding and establish parameters for future studies of punishment.     

Rapid acquisition of bias in signal detection: dynamics of effective reinforcement allocation

We investigated changes in bias (preference for one response alternative) in signal detection when relative reinforcer frequency for correct responses varied across sessions. In Experiment 1, 4 rats responded in a two-stimulus, two-response identification procedure employing temporal stimuli (short vs. long houselight presentations). Relative reinforcer frequency varied according to a 31-step pseudorandom binary sequence and stimulus duration difference varied over two values across conditions. In Experiment 2, 3 rats responded in a five-stimulus, two-response classification procedure employing temporal stimuli. Relative reinforcer frequency was varied according to a 36-step pseudorandom ternary sequence. Results of both experiments were analyzed according to a behavioral model of detection. The model was extended to incorporate the effects of current and previous session reinforcer frequency ratios on current-session performance. Similar to findings with concurrent schedules, effects on bias of relative reinforcer frequency were highest for the current session. However, carryover from reinforcer ratios of previous sessions was evident. Generally, the results indicate that bias can come under control of frequent changes in relative reinforcer frequency in both identification and classification procedures.     

Exclusive preference develops less readily on concurrent ratio schedules with wheel-running than with sucrose reinforcement

Previous research suggested that allocation of responses on concurrent schedules of wheel‐running reinforcement was less sensitive to schedule differences than typically observed with more conventional reinforcers. To assess this possibility, 16 female Long Evans rats were exposed to concurrent FR FR schedules of reinforcement and the schedule value on one alternative was systematically increased. In one condition, the reinforcer on both alternatives was .1 ml of 7.5% sucrose solution; in the other, it was a 30‐s opportunity to run in a wheel. Results showed that the average ratio at which greater than 90% of responses were allocated to the unchanged alternative was higher with wheel‐running reinforcement. As the ratio requirement was initially increased, responding strongly shifted toward the unchanged alternative with sucrose, but not with wheel running. Instead, responding initially increased on both alternatives, then subsequently shifted toward the unchanged alternative. Furthermore, changeover responses as a percentage of total responses decreased with sucrose, but not wheel‐running reinforcement. Finally, for some animals, responding on the increasing ratio alternative decreased as the ratio requirement increased, but then stopped and did not decline with further increments. The implications of these results for theories of choice are discussed.     

Delay discounting of qualitatively different reinforcers in rats

Humans discount larger delayed rewards less steeply than smaller rewards, whereas no such magnitude effect has been observed in rats (and pigeons). It remains possible that rats' discounting is sensitive to differences in the quality of the delayed reinforcer even though it is not sensitive to amount. To evaluate this possibility, Experiment 1 examined discounting of qualitatively different food reinforcers: highly preferred versus nonpreferred food pellets. Similarly, Experiment 2 examined discounting of highly preferred versus nonpreferred liquid reinforcers. In both experiments, an adjusting-amount procedure was used to determine the amount of immediate reinforcer that was judged to be of equal subjective value to the delayed reinforcer. The amount and quality of the delayed reinforcer were varied across conditions. Discounting was well described by a hyperbolic function, but no systematic effects of the quantity or the quality of the delayed reinforcer were observed.     

The Acquisition of Time Properties Associated with a Sequential Motor Skill

Three experiments are reported that examined the relative importance of phasing and duration training in the motor learning of a sequential task. In all three experiments, the task involved knocking down three barriers in a specified order. The Phasing task required the subject to contact each of the barriers in a particular goal time interval, that is, each segment had a particular movement-time goal. The Duration task required the subject to contact the final barrier in a total elapsed-time goal defined by the experimenter. Following training, half of the subjects in each training condition transferred to either a novel Duration or a novel Phasing task. Phasing-trained subjects, compared to Duration-trained subjects, produced equivalent transfer performance on the Duration transfer task but superior performance on the Phasing transfer task. These results suggest that phasing serves as a higher-order source of information for the performer in a sequential motor task. In addition, these experiments complement and extend previous work by Shapiro (1977) and Summers (1975) which demonstrated that learned phasing patterns were not modified despite changes in the overall rate of performing a motor sequence. Our experiments indicate that phasing training increases the performer’s sensitivity to phasing patterns such that novel temporal patterns can be produced when they are well-defined.     

The role of temporal intervals on reinforcer accumulation

Animals accumulate reinforcers when they forgo the opportunity to consume available food in favor of acquiring additional food for later consumption. Laboratory research has shown that reinforcer accumulation is facilitated when an interval (either spatial or temporal) separates earning from consuming reinforcers. However, there has been no systematic investigation on the interval separating consuming reinforcers from earning additional reinforcers. This oversight is problematic because this second interval is an integral part of much of the previous research on reinforcer accumulation. The purpose of the current study was to determine the independent contributions of these two temporal intervals on reinforcer accumulation in rats. Each left lever press earned a single food pellet; delivery of the accumulated pellet(s) occurred upon a right lever press. Conditions varied based on the presence of either an intertrial interval (ITI) that separated pellet delivery from the further opportunity to accumulate more pellets, or a delay‐to‐reinforcement that separated the right lever press from the delivery of the accumulated pellet(s). Delay and ITI values of 0, 5, 10 and 20 s were investigated. The delay‐to‐reinforcement conditions produced greater accumulation relative to the ITI conditions, despite accumulation increasing the density of reinforcement more substantially in the ITI conditions. This finding suggests that the temporal separation between reinforcer accumulation and subsequent delivery and consumption was a more critical variable in controlling reinforcer accumulation.