Disrupted waiting behavior in ADHD: exploring the impact of reward availability and predictive cues.

Altered motivational processing is purported to contribute to ADHD symptoms. A stronger preference for immediate over delayed reward is well documented in ADHD. However, little attention has been paid to children's capacity to withhold responding until a "better" reward becomes available, and their actions while waiting. Using a novel computer task, we examine the ability of children with and without ADHD to wait to collect a large reward in the presence of a small available reward. The effects of a reward-predicting cue on response times and response choices are also explored. Data from 136 children (6-12 years), 90 with ADHD and 46 typically developing (TD) children, are included. The children could collect a small immediately available reward or wait to access a larger reward after a variable delay, its imminent availability sometimes signaled by a cue. Subsequent probe trials explored the effects of longer waiting times and disruption of the cue-reward association. As expected, children with ADHD collected the small immediately available reward more often than TD children. Importantly, they were more likely to terminate waiting once commenced, collecting the small reward or attempting to collect the large reward early. The cue decreased their response time but disrupted their waiting when it no longer consistently predicted reward. Children with ADHD were more likely to abandon efforts to wait, especially when wait times were extended and when expected rewards failed to appear. Behavioral interventions for ADHD should take into account reduced waiting capacity that extends beyond children's preference for immediate reward.

Is increased sensitivity to punishment a common characteristic of attention deficit/hyperactivity disorder? An experimental study of response allocation in Japanese children.

Research on motivational processes in attention deficit/hyperactivity disorder (ADHD) focuses on reward. Studies with punishment are limited and findings mixed. This study evaluated the effects of punishment on response allocation in Japanese children with and without ADHD. Thirty-four children meeting DSM-IV criteria for ADHD and 59 typically developing control-group children completed an operant task in which they choose between playing two simultaneously available games. Reward was arranged symmetrically across the games under concurrent variable interval schedules. Asymmetric punishment schedules were superimposed with responses on one game punished four times as often as responses on the other. Children with ADHD showed greater behavioral sensitivity to punishment than controls. They allocated significantly more responses to the less frequently punished alternative and were more likely to play this game on consecutive trials and responded more slowly to the more punished game. Control group children allocated their responses evenly across games. Punishment exerted greater control over the behavior of Japanese children with ADHD than controls, similar to findings with children from Western countries, suggesting this is a common characteristic of the disorder. The behavior of typically developing Japanese children, while demonstrating awareness of punishment, was not controlled by the frequency of its occurrence.

Behavioral adjustment to asymmetric reward availability among children with and without ADHD: effects of past and current reinforcement contingencies.

Altered reinforcement sensitivity is hypothesized to underlie symptoms of attention deficit hyperactivity disorder (ADHD). Here we evaluate the behavioral sensitivity of Brazilian children with and without ADHD to a change in reward availability. Forty typically developing children and 32 diagnosed with DSM-IV ADHD completed a signal-detection task in which correct discriminations between two stimuli were associated with different frequencies of reinforcement. The response alternative associated with the higher rate of reinforcement switched, without warning, after 30 rewards were delivered. The task continued until another 30 rewards were delivered. Both groups of children developed a response bias toward the initially more frequently reinforced alternative. This effect was larger in the control group. The response allocation of the two groups changed following the shift in reward availability. Over time the ADHD group developed a significant response bias toward the now more frequently reinforced alternative. In contrast, the bias of the control group stayed near zero after an initial decline following the contingency change. The overall shift in bias was similar for the two groups. The behavior of both groups of children was sensitive to the asymmetric reward distribution and to the change in reward availability. Subtle group differences in response patterns emerged, possibly reflecting differences in the time frame of reward effects and sensitivity to reward exposure.

Behavioral sensitivity of Japanese children with and without ADHD to changing reinforcer availability: an experimental study using signal detection methodology.

BACKGROUND: Most research on motivational processes in attention deficit hyperactivity disorder (ADHD) has been undertaken in Western Europe and North America. The extent to which these findings apply to other cultural groups is unclear. The current study evaluated the behavioral sensitivity of Japanese children with and without ADHD to changing reward availability. Forty-one school-aged children, 19 diagnosed with DSM-IV ADHD, completed a signal-detection task in which correct discriminations between two stimuli were associated with different reinforcement frequencies. The response alternative associated with the higher rate of reinforcement switched twice during the task without warning. FINDINGS: Both groups of children developed an initial bias toward the more frequently reinforced response alternative. When the reward contingencies switched the response allocation (bias) of the control group children followed suit. The response bias scores of the children with ADHD did not, suggesting impaired tracking of reward availability over time. CONCLUSIONS: Japanese children with ADHD adjust their behavioral responses to changing reinforcer availability less than their typically developing peers. This is not explained by poor attention to task or a lack of sensitivity to reward. The current results are consistent with altered sensitivity to changing reward contingencies identified in non-Japanese samples of children with ADHD. Irrespective of their country of origin, children with ADHD will likely benefit from behavioral expectations and reinforcement contingencies being made explicit together with high rates of reinforcement for appropriate behaviors.

Choosing among multiple alternatives: Relative and overall reinforcer rates.

Choice behavior among two alternatives has been widely researched, but fewer studies have examined the effect of multiple (more than two) alternatives on choice. Two experiments investigated whether changing the overall reinforcer rate affected preference among three and four concurrently scheduled alternatives. Experiment 1 trained six pigeons on concurrent schedules with three alternatives available simultaneously. These alternatives arranged reinforcers in a ratio of 9:3:1 with the configuration counterbalanced across pigeons. The overall rate of reinforcement was varied across conditions. Preference between the pair of keys arranging the 9:3 reinforcer ratio was less extreme than the pair arranging the 3:1 reinforcer ratio regardless of overall reinforcer rate. This difference was attributable to the richer alternative receiving fewer responses per reinforcer than the other alternatives. Experiment 2 trained pigeons on concurrent schedules with four alternatives available simultaneously. These alternatives arranged reinforcers in a ratio of 8:4:2:1, and the overall reinforcer rate was varied. Next, two of the alternatives were put into extinction and the random interval duration was changed from 60 s to 5 s. The ratio of absolute response rates was independent of interval length across all conditions. In both experiments, an analysis of sequences of visits following each reinforcer showed that the pigeons typically made their first response to the richer alternative irrespective of which alternative was just reinforced. Performance on these three- and four-alternative concurrent schedules is not easily extrapolated from corresponding research using two-alternative concurrent schedules.

Choice among two and three alternatives.

Although choice between two alternatives has been widely researched, fewer studies have examined choice across multiple (more than two) alternatives. Past models of choice behavior predict that the number of alternatives should not affect relative response allocation, but more recent research has found violations of this principle. Five pigeons were presented with three concurrently scheduled alternatives. Relative reinforcement rates across these alternatives were assigned 9:3:1. In some conditions three keys were available; in others, only two keys were available. The number of available alternatives did not affect relative response rates for pairs of alternatives; there were no significant differences in behavior between the two and three key conditions. For two birds in the three-alternative conditions and three birds in the two-alternative conditions, preference was more extreme for the pair of alternatives with the lower overall pairwise reinforcer rate (3:1) than the pair with higher overall reinforcer rate (9:3). However, when responding during the changeover was removed three birds showed the opposite pattern in the three-alternative conditions; preference was more extreme for the pair of alternatives with the higher overall reinforcer rate. These findings differ from past research and do not support established theories of choice behavior.

Evidence for increased behavioral control by punishment in children with attention-deficit hyperactivity disorder.

BACKGROUND: The behavioral sensitivity of children with ADHD to punishment has received limited theoretical and experimental attention. This study evaluated the effects of punishment on the response allocation of children with ADHD and typically developing children. METHOD: Two hundred and ten children, 145 diagnosed with ADHD, completed an operant task in which they chose between playing two simultaneously available games. Reward was arranged symmetrically across the games under concurrent variable interval schedules. Asymmetric punishment schedules were superimposed; responses on one game were punished four times as often as responses on the other. RESULTS: Both groups allocated more of their responses to the less frequently punished alternative. Response bias increased significantly in the ADHD group during later trials, resulting in missed reward trials and reduced earnings. CONCLUSIONS: Punishment exerted greater control over the response allocation of children with ADHD with increased time on task. Children with ADHD appear more sensitive to the cumulative effects of punishment than typically developing children.

Behavioral sensitivity to changing reinforcement contingencies in attention-deficit hyperactivity disorder.

BACKGROUND: Altered sensitivity to positive reinforcement has been hypothesized to contribute to the symptoms of attention-deficit hyperactivity disorder (ADHD). In this study, we evaluated the ability of children with and without ADHD to adapt their behavior to changing reinforcer availability. METHOD: Of one hundred sixty-seven children, 97 diagnosed with ADHD completed a signal-detection task in which correct discriminations between two stimuli were associated with different frequencies of reinforcement. The response alternative associated with the higher rate of reinforcement switched twice during the task without warning. For a subset of participants, this was followed by trials for which no reinforcement was delivered, irrespective of performance. RESULTS: Children in both groups developed an initial bias toward the more frequently reinforced response alternative. When the response alternative associated with the higher rate of reinforcement switched, the children's response allocation (bias) followed suit, but this effect was significantly smaller for children with ADHD. When reinforcement was discontinued, only children in the control group modified their response pattern. CONCLUSIONS: Children with ADHD adjust their behavioral responses to changing reinforcer availability less than typically developing children, when reinforcement is intermittent and the association between an action and its consequences is uncertain. This may explain the difficulty children with ADHD have adapting their behavior to new situations, with different reinforcement contingencies, in daily life.

Abnormal striatal BOLD responses to reward anticipation and reward delivery in ADHD.

Altered reward processing has been proposed to contribute to the symptoms of attention deficit hyperactivity disorder (ADHD). The neurobiological mechanism underlying this alteration remains unclear. We hypothesize that the transfer of dopamine release from reward to reward-predicting cues, as normally observed in animal studies, may be deficient in ADHD. Functional magnetic resonance imaging (fMRI) was used to investigate striatal responses to reward-predicting cues and reward delivery in a classical conditioning paradigm. Data from 14 high-functioning and stimulant-naïve young adults with elevated lifetime symptoms of ADHD (8 males, 6 females) and 15 well-matched controls (8 males, 7 females) were included in the analyses. During reward anticipation, increased blood-oxygen-level-dependent (BOLD) responses in the right ventral and left dorsal striatum were observed in controls, but not in the ADHD group. The opposite pattern was observed in response to reward delivery; the ADHD group demonstrated significantly greater BOLD responses in the ventral striatum bilaterally and the left dorsal striatum relative to controls. In the ADHD group, the number of current hyperactivity/impulsivity symptoms was inversely related to ventral striatal responses during reward anticipation and positively associated with responses to reward. The BOLD response patterns observed in the striatum are consistent with impaired predictive dopamine signaling in ADHD, which may explain altered reward-contingent behaviors and symptoms of ADHD.

The effect of opponent type on human performance in a three-alternative choice task.

Adult participants played computerised games of "Paper Scissors Rock". Participants in one group were told that they were playing against the computer, and those in the other group were told that they were playing against another participant in the adjacent room. The participant who won the most games would receive a $50 prize. For both groups however, the opponent's responses (paper, scissors, or rock) were generated by the computer, and the distribution of these responses was varied across four blocks of 126 trials. Results were analysed using the generalised matching law for the three possible pairs of alternatives (paper vs. scissors, paper vs. rock, and scissors vs. rock) across all participants in each group. Overall, significantly higher estimates of sensitivity to the distribution of opponent's responses were obtained from participants who were told their opponent was a computer compared to participants who were told their opponent was another participant. While adding to the existing literature showing that the generalised matching law is an adequate descriptor of human three-alternative choice behaviour, these findings show that external factors such as perceived opponent type can affect the efficacy of reinforcer contingencies on human behaviour. This suggests that generalising the results from tasks performed against a computer to real-life human-to-human interactions warrants some caution.

The dynamics of word retrieval in major depression.

OBJECTIVE: Impairment in the retrieval of specific episodes from autobiographical memory is commonly observed in major depression. However, it is unclear whether impairment in retrieval processes is a general characteristic of major depression or is confined to the recollection of personal memories. This study examined the time course of the retrieval of words from semantic memory. METHOD: A letter fluency test was administered to 65 inpatients with major depression and 50 healthy controls. A two-parameter model was fit to the decay curve representing the production of words over a 90-second period. One parameter, N, is an estimate of the total number of words that would be generated if the respondent was given unlimited time. The other, tau, is the average of the difference in time between the first word generated and each subsequent word. RESULTS: There was evidence of a deficit in the retrieval of words from long-term memory in depressed patients. The significant difference between groups suggested that even if given an extended period of time in which to respond to compensate for possible slowness, the depressed group would not retrieve as many words as the controls. The retrieval failure could not be attributed solely to cognitive slowing or the effects of antidepressant medication. CONCLUSIONS: The results extend findings of a deficit in the process of retrieving specific episodes from autobiographical memory and suggest that a generalised impairment in memory retrieval may be characteristic of major depression.

Stimulus disparity and punisher control of human signal-detection performance.

The present experiment examined the effects of varying stimulus disparity and relative punisher frequencies on signal detection by humans. Participants were placed into one of two groups. Group 3 participants were presented with 1:3 and 3:1 punisher frequency ratios, while Group 11 participants were presented with 1:11 and 11:1 punisher frequency ratios. For both groups, stimulus disparity was varied across three levels (low, medium, high) for each punisher ratio. In all conditions, correct responses were intermittently reinforced (1:1 reinforcer frequency ratio). Participants were mostly biased away from the more punished alternative, with more extreme response biases found for Group 11 participants compared to Group 3. For both groups, estimates of discriminability increased systematically across the three disparity levels and were unaffected by the punisher ratios. Likewise, estimates of response bias and sensitivity to the punisher ratios were unaffected by changes in discriminability, supporting the assumption of parameter invariance in the Davison and Tustin (1978) model of signal detection. Overall, the present experiment found no relation between stimulus control and punisher control, and provided further evidence for similar but opposite effects of punishers to reinforcers in signal-detection procedures.

Sensitivity to delay of reinforcement in two animal models of attention deficit hyperactivity disorder (ADHD).

An altered response to reinforcement has been proposed as a mechanism underlying many of the symptoms of attention deficit hyperactivity disorder (ADHD). We measured sensitivity to delay of reinforcement in two animal models of ADHD, the spontaneously hypertensive rat (SHR) and a newly proposed model, the genetically hypertensive (GH) rat. A task previously used to measure effects of delay of reinforcement in children with ADHD was adapted for use in the present experiment. The SHR and GH rats were compared to their respective genetic control strains, Wistar-Kyoto (WKY), and Wistar (WI). The experimental task required pressing one of two available levers each trial. One lever delivered an immediate reinforcement, and the other lever a delayed reinforcement. Both the SHR and GH strains allocated significantly more responses to the immediately reinforced lever than their genetic control strains. Individual instances of reinforcement differentially affected response allocation in the GH but not the SHR. These findings support the use of the SHR and GH rat to model altered response to reinforcement, and demonstrate the additional value of the GH strain to model the effects of individual instances of reinforcement in children with ADHD.

Effects of point-loss punishers on human signal-detection performance.

Three experiments using human participants varied the distribution of point-gain reinforcers or point-loss punishers in two-alternative signal-detection procedures. Experiment 1 varied the distribution of point-gain reinforcers for correct responses (Group A) and point-loss punishers for errors (Group B) across conditions. Response bias varied systematically as a function of the relative reinforcer or punisher frequencies. Experiment 2 arranged two conditions - one where an unequal ratio of reinforcement (5ratio1 or 1ratio5) was presented without punishment (R-only), and another where the same reinforcer ratio was presented with an equal distribution of point-loss punishers (R+P). Response bias was significantly greater in the R-only condition than the R+P condition, supporting a subtractive model of punishment. Experiment 3 varied the distribution of point-gain reinforcers for correct responses across four unequal reinforcer ratios (5ratio1, 2ratio1, 1ratio2, 1ratio5) both without (R-only) and with (R+P) an equal distribution of point-loss punishers for errors. Response bias varied systematically with changes in relative reinforcer frequency for both R-only and R+P conditions, with 5 out of 8 participants showing increases in sensitivity estimates from R-only to R+P conditions. Overall, the results indicated that punishers have similar but opposite effects to reinforcers in detection procedures and that combined reinforcer and punisher effects might be better modeled by a subtractive punishment model than an additive punishment model, consistent with research using concurrent-schedule choice procedures.

Reinforcer control by comparison-stimulus color and location in a delayed matching-to-sample task.

Six pigeons were trained in a delayed matching-to-sample task involving bright- and dim-yellow samples on a central key, a five-peck response requirement to either sample, a constant 1.5-s delay, and the presentation of comparison stimuli composed of red on the left key and green on the right key or vice versa. Green-key responses were occasionally reinforced following the dimmer-yellow sample, and red-key responses were occasionally reinforced following the brighter-yellow sample. Reinforcer delivery was controlled such that the distribution of reinforcers across both comparison-stimulus color and comparison-stimulus location could be varied systematically and independently across conditions. Matching accuracy was high throughout. The ratio of left to right side-key responses increased as the ratio of left to right reinforcers increased, the ratio of red to green responses increased as the ratio of red to green reinforcers increased, and there was no interaction between these variables. However, side-key biases were more sensitive to the distribution of reinforcers across key location than were comparison-color biases to the distribution of reinforcers across key color. An extension of Davison and Tustin's (1978) model of DMTS performance fit the data well, but the results were also consistent with an alternative theory of conditional discrimination performance (Jones, 2003) that calls for a conceptually distinct quantitative model.

Reprint of "Problems with spontaneously hypertensive rats (SHR) as a model of attention-deficit/hyperactivity disorder (AD/HD)".

Spontaneously hypertensive rats (SHR) have been promoted as animal models of attention-deficit hyperactivity disorder (AD/HD). Free-operant experiments have compared the effects of simple schedules, extinction, delayed reinforcers, and non-contingent reinforcement on the behaviour of SHR and Wistar Kyoto (WKY) controls. Differences between SHR and WKY performance have been paralleled with the overactivity, impulsiveness, and inattention that characterize AD/HD. Re-examination of selected studies suggests that differences in these strains' overall response rates can exaggerate other differences between SHR and WKY performance. Removing this confounding factor can reduce differences between these two strains' behaviours.

Problems with spontaneously hypertensive rats (SHR) as a model of attention-deficit/hyperactivity disorder (AD/HD).

Spontaneously hypertensive rats (SHR) have been promoted as animal models of attention-deficit hyperactivity disorder (AD/HD). Free-operant experiments have compared the effects of simple schedules, extinction, delayed reinforcers, and non-contingent reinforcement on the behaviour of SHR and Wistar Kyoto (WKY) controls. Differences between SHR and WKY performance have been paralleled with the overactivity, impulsiveness, and inattention that characterize AD/HD. Re-examination of selected studies suggests that differences in these strains' overall response rates can exaggerate other differences between SHR and WKY performance. Removing this confounding factor can reduce differences between these two strains' behaviours.

Speed of word retrieval in postconcussion syndrome.

Speed of information processing in persons with postconcussion syndrome (PCS) was examined using word fluency tasks. Twenty patients with PCS and twenty controls matched for age, gender, and occupation were given two word fluency tasks, and the speed of word generation was measured. Response latencies were analyzed to determine whether slowed retrieval or degradation of words in semantic memory was responsible for problems with word retrieval after traumatic brain injury. The PCS group recalled fewer words, had significantly longer interresponse times, and took significantly longer to generate their first word than the controls. There was no evidence that either structure loss or slowness in word retrieval from semantic memory could account for the word fluency deficits. Rather, the findings suggest that the primary cause of word retrieval difficulties in patients with PCS is a generalized slowness of cognitive processing.

Discriminability and sensitivity to reinforcer magnitude in a detection task.

Three pigeons discriminated between two sample stimuli (intensities of red light). The difficulty of the discrimination was varied over four levels. At each level, the relative reinforcer magnitude for the two correct responses was varied across conditions, and the reinforcer rates were equal. Within levels, discriminability between the sample stimuli did not change systematically as reinforcer magnitude varied. Across levels, the sensitivity of behavior to changes in the reinforcer-magnitude ratio decreased as the discriminability between the sample stimuli increased. Subsequent analysis showed that this relation was limited to performance following only one of the sample stimuli, the dim red light that remained constant across all conditions. Extant behavioral models of signal detection cannot easily accommodate these results.