Sensitivity to reward delay in children with attention deficit hyperactivity disorder (ADHD).

The present study compared the sensitivity to pre- and post-reward delays of children with and without DSM-IV combined type ADHD. Thirty-six children with DSM-IV combined type ADHD and 36 controls completed a signal-detection task. Correct identification of one stimulus produced an immediate reward and then a 3.5 s delay before the next trial (immediate reward). Correct identification of the other stimulus was associated with a 3.5 s delay before reward was delivered (delayed reward). Group differences emerged in response bias toward the immediately rewarded alternative. Children in the ADHD group showed a greater bias toward immediate reward than the controls. Children in the control group showed different patterns of response bias and discriminability following immediate and delayed rewards. For these children discriminability was lower and response bias was greater on trials following delayed reward. Both groups responded more slowly on trials following delayed reward. These findings support the hypothesis that children with ADHD are unusually sensitive to pre-reward delays.

Effects of response disparity on stimulus and reinforcer control in human detection tasks.

In two detection experiments, university students reported whether the second of two sequentially presented tones was longer or shorter than the first by responding to stimuli presented on a touch screen. Stimulus disparity and response disparity were manipulated to compare their effects on measures of discrimination and response bias when the reinforcement ratio for correct responses was asymmetric. Choice stimuli consisted of squares filled with different pixel densities. Response disparity was manipulated by varying the difference in density between the two choice stimuli. In both experiments, decreasing stimulus disparity reduced discrimination but had no consistent effect on bias. Decreasing response disparity also reduced discrimination in both experiments, and often reduced estimates of bias. The effects of response disparity on bias were most clear in Experiment 2, in which a greater overall level of response disparity was arranged. The data show that, like corresponding research with pigeons, detection performance of human subjects can be conceptualized as discriminated operants.

Reinforcer control and human signal-detection performance.

Eight humans participated in a two-choice signal-detection task in which stimulus disparity was varied over four levels. Two procedures arranged asymmetrical numbers of reinforcers received for correct left- and right-key responses (the reinforcer ratio). The controlled procedure ensured that the obtained reinforcer ratio remained constant over changes in stimulus disparity, irrespective of subjects' performances. In the uncontrolled procedure, the asymmetrical reinforcer ratio could covary with subjects' performances. The receiver operating characteristic (ROC) patterns obtained from the controlled procedure approximated isobias functions predicted by criterion location measures of bias. The uncontrolled procedure produced variable ROC patterns that were somewhat like the isobias predictions made by likelihood ratio measures of bias; however, the obtained reinforcer ratio became more extreme as discriminability decreased. The obtained pattern of bias was directly related to the obtained reinforcer ratio. This research indicates that criterion location measures seem to be preferable indices of response bias.

Sensitivity to reward frequency in boys with attention deficit hyperactivity disorder.

Compared the sensitivity of boys with and without attention deficit hyperactivity disorder (ADHD) to differences in reward frequency. Fifteen boys with ADHD as diagnosed in the Diagnostic and Statistical Manual of Mental Disorders (3rd ed., rev.; American Psychiatric Association, 1987) and a matched control group completed a signal-detection task in which correct identification of 1 stimulus was rewarded 3 times as often as correct identification of the other. Boys in the ADHD group completed the task twice, on and off medication. Group differences emerged in response bias toward the more frequently rewarded alternative. Boys in the control group showed a stable pattern of response bias, irrespective of which alternative they were last rewarded on. Boys in the ADHD group showed different patterns of response bias following rewards on the 2 alternatives. These results suggest children with ADHD were more sensitive to individual instances of reward compared with controls, whose response bias is governed more by their reinforcement history. Methylphenidate improved discriminability and reduced sensitivity to individual instances of reward in the boys with ADHD.

The effects of hippocampal and area parahippocampalis lesions in pigeons: I. Delayed matching to sample.

Four experiments were conducted to determine the effects of bilateral damage to the hippocampus and area parahippocampalis (Hp-APH) on visual memory in pigeons using the delayed matching-to-sample (DMS) procedure. In Experiment 1, we generated visual retention gradients with delays of 0, 1.5, 3, 6, and 12 sec both preoperatively and postoperatively in three pigeons with considerable preoperative visual DMS experience. Bilateral Hp-APH lesions had no effect whatsoever on visual retention. In Experiment 2, we examined the effects of Hp-APH lesions on both the acquisition of a visual DMS task with a 0-sec delay, and the subsequent retention performance with delays of 0, 3, 6, 12, and 24 sec. There was no difference between unoperated control pigeons and Hp-APH pigeons either in terms of the number of sessions required to learn the visual DMS task or in terms of their subsequent visual retention performance levels. In Experiments 3 and 4, we examined whether Hp-APH pigeons might be more sensitive than control pigeons to the effects of proactive interference (by reducing the duration of the intertrial interval) and retroactive interference (by introducing delay-interval illumination). Although reducing the duration of the intertrial interval and increasing the level of delay-interval illumination both resulted in lower performance levels on the visual DMS task, there was no indication that the Hp-APH pigeons were any more affected by the changes in interference levels than were unoperated control pigeons. These findings support the view that the Hp-APH in pigeons plays little role in the processing and retention of purely visual information.

Types of responding in a signal-detection task.

Four pigeons were trained to discriminate between two line orientations in a two-alternative forced-choice procedure. The distribution of reinforcers for the two types of correct response was varied across conditions. Performance on each trial was recorded separately, including the time taken to make a choice response. Discriminability and response-bias measures were calculated for overall performance, and, following a median split of the data from each condition, for faster and slower choice responses in each condition. Discriminability between the stimuli did not vary systematically as a function of choice latency. Variations of the reinforcer distributions produced larger response biases for the faster responses than for the slower responses. Responses on trials following reinforcers were faster and showed a greater effect of the reinforcer distribution than did other responses. Behavioral models of signal detection should consider the speed of the choice response as a factor modulating the effects of reinforcer distributions.

Concurrent choice: Effects of overall reinforcer rate and the temporal distribution of reinforcers.

Six pigeons responded on a series of concurrent exponential variable-interval schedules, offering a within-subject comparison with previously published data from concurrent arithmetic variable-interval schedules. Both relative and overall reinforcer rates were varied between conditions. The generalized matching law described the data well, with undermatching much more frequent than strict matching. Time-allocation sensitivity consistently exceeded response-allocation sensitivity for both schedule types, and exponential-schedule sensitivity exceeded arithmetic-schedule sensitivity for both measures of choice. A further set of conditions using variable-interval schedules whose shortest interval was correlated with the mean interval, like arithmetic schedules, but that provided a constant conditional probability of reinforcement, like exponential schedules, produced sensitivities between those produced by conventional arithmetic and exponential schedules. Unlike previous arithmetic-schedule results, exponential sensitivity changed nonmonotonically with changes in overall reinforcer rate. The results clarify our knowledge of the effects of arithmetic and exponential schedules but confuse our understanding of the effects of overall reinforcer rate on concurrent choice.

Human symbolic matching-to-sample performance: Effects of reinforcer and sample-stimulus probabilities.

Four experiments, each with 6 human subjects, varied the distribution of reinforcers for correct responses and the probability of sample-stimulus presentation in symbolic matching-to-sample procedures. Experiment 1 held the sample-stimulus probability constant and varied the ratio of reinforcers obtained for correct responses on the two alternatives across conditions. There was a positive relation between measures of response bias and the ratio of reinforcers. Experiment 2 held the ratio of reinforcers constant and varied the sample-stimulus probability across conditions. Unlike previous studies that used pigeons as subjects, there was a negative relation between bias and the ratio of sample-stimulus presentations. In Experiment 3, the sample-stimulus probability and the reinforcer ratio covaried across conditions. Response bias did not vary systematically across conditions. In Experiments 1 to 3, correct responses were reinforced intermittently. Experiment 4 used the same procedure as Experiment 3, but all correct responses now produced some scheduled consequence. There was a positive relation between response bias and the ratio of reinforcers. The results suggest that human performance in these tasks was controlled by both the relative frequency of reinforced responses and the relative frequency of nonreinforced responses.

Cued and uncued terminal links in concurrent-chains schedules.

Pigeons were trained on a concurrent-chains schedule. The initial links were concurrent variable-interval schedules arranged on two side keys. Each terminal link was a fixed-interval schedule arranged on the center key. In cued conditions, different center-key colors signaled the two terminal-link schedules. In uncued conditions, the same center-key color appeared for both terminal links. Experiment 1 arranged unequal initial links and equal terminal links. Preference for the shorter initial-link schedule was greater when the terminal links were uncued. Experiment 2 arranged equal initial links and unequal terminal links. Preference for the shorter terminal-link schedule was greater when the terminal links were cued. Although the results of Experiment 2 successfully replicated previous research, the results of Experiment 1 are not easily reconciled with conditioned-reinforcement or discriminative-stimulus accounts of the role of terminal-link cues. Rather, terminal-link cues appear to decrease sensitivity to initial-link contingencies.

Effects of differences between stimuli, responses, and reinforcer rates on conditional discrimination performance.

In a discrete-trial conditional discrimination procedure, 4 pigeons obtained food reinforcers by pecking a key with a short latency on trials signaled by one stimulus and by pecking the same key with a long latency on trials signaled by a second stimulus. The physical difference between the two stimuli and the temporal separation between the latency values required for reinforcement were varied factorially over four sets of conditions, and the ratio of reinforcer rates for short and long latencies was varied within each set of conditions. Stimulus discrimination varied directly with both stimulus and response differences and was unaffected by the reinforcer ratio. Sensitivity to reinforcement, estimated by generalized-matching-law fits to the data within each set of conditions, varied directly with the response difference but inversely with the stimulus difference arranged between sets of conditions. Because variations in stimulus differences, response differences, and reinforcer differences did not have equivalent effects, these findings question the functional equivalence of the three terms of the discriminated operant: antecedent stimuli, behavior, and consequences.

Prototype identification and categorization of incomplete figures by pigeons.

Previous research has shown that pigeons, unlike humans, cannot identify prototypes after training with corrupted exemplars. In the present experiment, pigeons were initially trained to discriminate between examples of a degraded square and a degraded triangle. The pigeons then received a transfer test in extinction comprised of novel examples of the degraded square and triangle, and the prototype square and triangle. The pigeons successfully categorized both the novel examples of the degraded figures and the underlying prototypes. These results suggest that aspects of categorization and prototype identification may be shared across species.

Successive independence and behavioral contrast in a closed economy.

Two pigeons had access to multiple concurrent schedules of reinforcement for 24 hours per day in their home cages. The variable-interval schedules comprising the multiple concurrent schedules were varied across 16 conditions. In three sets of conditions, one schedule was varied while its concurrent alternative and the concurrent schedules in the other component were held constant. Behavioral contrast was observed; that is, as the rate of reinforcement arranged by the varied schedule decreased, response rates on the constant schedules typically increased. These conditions formed part of two larger sets of conditions in which the concurrent schedules in one multiple-schedule component remained constant while the concurrent schedules in the other component were varied. Successive independence was found, in that behavior allocation during the constant component did not vary as a function of the reinforcer ratios in the varied component. Successive independence between components in multiple concurrent schedules is a robust result that occurs in closed economies and under conditions that promote behavioral contrast.

Discriminability between alternatives in a switching-key concurrent schedule.

Six pigeons were trained to discriminate between two intensities of white light in a symbolic matching-to-sample procedure. These stimuli were then used to signal which schedule was available on the main key in a switching-key concurrent schedule. The concurrent schedules led to a symbolic matching-to-sample phase in which the subject identified the concurrent schedule to which it last responded before a reinforcer could be obtained. The concurrent schedules were varied across conditions. Discriminability, measured during the symbolic matching-to-sample performance, was high throughout and did not differ across the two procedures. Performance in the concurrent schedules was like that typically obtained using these schedules. Delays were then arranged between completion of the concurrent schedules and presentations of the symbolic matching-to-sample phase. A series of conditions with an intervening delay of 10 s showed that both concurrent-schedule performance and symbolic matching-to-sample performance were affected by the delay in a similar way; that is, choice responding was closer to indifference.

Effects of varying stimulus disparity and the reinforcer ratio in concurrent-schedule and signal-detection procedures.

The present study measured the effects of stimulus and reinforcer variations on pigeons' behavior in two different choice procedures. Two intensities of white light were presented as the stimuli on the main key in a switching-key concurrent schedule and as the sample stimuli in a signal-detection procedure. Under both procedures, the scheduled rate of reinforcement was varied across conditions to produce various ratios of obtained reinforcement. These ratios were obtained for seven pairs of light intensities. In the concurrent schedules, the effects of reinforcer-ratio variations were positively correlated with the physical disparity between the two light intensities. In the signal-detection procedure, changes in the reinforcer ratio produced greater effects on performance when stimulus disparity was very low or very high compared to those found at intermediate levels of stimulus disparity. This discrepancy creates a dilemma for existing behavioral models of signal-detection performance.

Behavior-dependent reinforcer-rate changes in concurrent schedules: A further analysis.

Six pigeons were trained on concurrent variable-interval schedules in three different procedures. The first procedure was a standard concurrent schedule, and the relative reinforcer frequency for responding was varied. The second was a schedule in which a relative left-key response rate (over a fixed period of time) exceeding .75 produced, in the next identical time period a higher reinforcer rate on the right key. If this criterion was not exceeded, equal reinforcer rates were arranged on the two keys in this period. This was the dependent procedure. In the third (independent) procedure, the periods of higher right-key reinforcer rates occurred with the same probability as in the second procedure, but occurred independently of behavior. In the second and third procedures, the fixed-time period (window) was varied from 5 s to 60 s, and to 240 s in the second procedure only. Performance on the two keys was similar in the concurrent and independent procedures. The procedure used in the dependent conditions generally affected performance when the windows were shorter than about 30 s. Models of performance that assume that subjects do not discriminate changes in local relative reinforcer rates cannot account for the data. Moreover, existing models are inherently unable to account for the effects of contingencies of reinforcement between responding on one alternative and gaining reinforcers on another that are arranged or that emerge as a result of time allocated to alternative schedules. Undermatching on concurrent variable-interval schedules may result from such emergent contingencies.

Concurrent-chain performance: Effects of absolute and relative terminal-link entry frequency.

Six pigeons were trained in a concurrent-chain procedure with constant variable-interval 6-s variable-interval 12-s terminal links. Five groups of conditions were arranged. Within a group of conditions, the duration of one initial-link schedule was held constant and the duration of the other initial link was varied. The duration of the varied initial link was always longer than, or equal to, the constant initial-link duration. The duration of the shorter initial link was varied across groups of conditions from 5 s to 70 s. The data from each group were well described by the generalized matching law. Sensitivity (a) to the terminal-link entry ratio increased as the shorter initial-link duration increased, but appeared to reach an asymptote at shorter initial-link durations greater than 32 s. Terminal-link bias did not change with changes in shorter initial-link duration for the response-allocation data, but showed a small increase with increasing shorter initial-link duration for the time-allocation data.

Concurrent-schedule performance: Effects of relative and overall reinforcer rate.

Six pigeons were trained to respond on two keys, each of which provided reinforcers on an arithmetic variable-interval schedule. These concurrent schedules ran nonindependently with a 2-s changeover delay. Six sets of conditions were conducted. Within each set of conditions the ratio of reinforcers available on the two alternatives was varied, but the arranged overall reinforcer rate remained constant. Each set of conditions used a different overall reinforcer rate, ranging from 0.22 reinforcers per minute to 10 reinforcers per minute. The generalized matching law fit the data from each set of conditions, but sensitivity to reinforcer frequency (a) decreased as the overall reinforcer rate decreased for both time allocation and response allocation based analyses of the data. Overall response rates did not vary with changes in relative reinforcer rate, but decreased with decreases in overall reinforcer rate. Changeover rates varied as a function of both relative and overall reinforcer rates. However, as explanations based on changeover rate seem unable to deal with the changes in generalized matching sensitivity, discrimination accounts of choice may offer a more promising interpretation.

Preference for multiple versus mixed schedules of reinforcement.

Five pigeons were trained in a concurrent-chain procedure. In the initial links, equal nonindependent variable-interval schedules were available concurrently on two keys. Completing the schedule on either key led to exclusive presentation of one of two further variable-interval schedules for a fixed period of time. During these terminal links, as many reinforcers as were scheduled could be obtained. If the response producing this terminal link occurred on one key, differential stimuli signaled which variable-interval schedule had been produced. If the response producing the terminal link occurred on the other key, no such differential stimuli were available. Once the fixed period of time elapsed, the initial links were reinstated. In Experiment 1, the period of time for which the terminal links were available was always 10 s and the absolute duration of the initial links was varied. Subjects preferred the alternative leading to the multiple schedule when the initial-link duration was short, but preferred the alternative leading to the mixed schedule when the initial-link durations were longer. In Experiment 2, both the initial-link duration and the duration of the terminal links were varied. The effect of initial-link duration was identical to that in Experiment 1 and there was no systematic effect of varying the terminal-link duration.