Brent Alsop

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.

Transitive inference and awareness in humans.

Thirty-nine participants performed a transitive inference task. During training, they were shown six pairs of imaginary sports teams and learnt which team was better in each pair. The pairs of teams were adjacent members of a seven-term series of team rankings (A-G). The participants were not explicitly informed of this underlying stimulus hierarchy. Once a participant reached the training criterion, they were tested on the six training pairs and three novel pairs (BD, CE, and DF) that required the participant to make a transitive inference. The participants completed a post-experimental questionnaire that measured their awareness of the stimulus hierarchy. A more-aware group and a less-aware group were created from answers to the questionnaire. The less-aware group were significantly less accurate than the more-aware group on some training pairs and all three novel pairs, and the less-aware groups pattern of performance bore similarities to non-human animal performance.

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.

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.

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.

ON TWO EFFECTS OF SIGNALING THE CONSEQUENCES FOR REMEMBERING

Five pigeons were trained to perform a delayed matching-to-sample task in which red- and green-colored keys were presented as sample and choice stimuli, and the duration of a delay interval varied across trials. Experiment 1 investigated the effects on delayed-matching accuracy of signaling different durations of food access for the two correct responses (the differential-outcomes effect), and of signaling nondifferential but larger durations for both responses (the signaled-magnitudes effect). In Condition 1, a vertical bar on either sample signaled different rewards (or different outcomes, DOs) for correct red and correct green responses (0.5 and 3.5 sec, respectively), and a horizontal bar signaled equal durations of food access (or same outcomes, SOs) for these responses (1.5 sec). In Condition 2, the horizontal bar signaled equally large rewards for the two correct responses (3.5 sec), and the vertical bar signaled equally small rewards (0.5 sec). Delayed-matching accuracies were higher on DO trials than on SO trials, and they were higher on large-reward trials than on small-reward trials. However, analyses of discriminability estimates as a function of delay-interval duration revealed differences between the forgetting functions reflecting these two effects. Signaling DOs increased the initial level of the function and reduced its slope relative to signaling SOs, whereas signaling larger rewards increased the initial level of the function but did not affect its slope relative to signaling smaller rewards. Experiment 2 investigated whether the difference between the initial levels of DO and SO functions in Condition 1 resulted from overall longer food access on the former trials. However, varying the food-access times on SO trials across three conditions (0.5, 3.5, and 1.5 sec) failed to produce systematic effects consistent with this hypothesis. The results are discussed with respect to the mechanisms that could be responsible for the two effects.

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.

Receiver operating characteristics from nonhuman animals: Some implications and directions for research with humans

Reviews of signal detection have largely overlooked the research involving nonhuman animal subjects. Some of this research is presented and reanalyzed here. Plots of receiver operating characteristics show that human and nonhuman signal-detection performance is very similar. The studies emphasize the series of discriminations that comprise signal-detection tasks and illustrate the systematic effects of different methods of arranging payoffs or feedback, of the consistency of that feedback, and of the physical disparity between response alternatives. The data provide some support for recent theoretical accounts that favor a criterion location measure of isobias over the likelihood ratio, but they also suggest that more systematic work is required in this area. Overall, this research supports many contemporary views concerning signal detection, and it provides an alternative way of looking at some recurrent issues. It also suggests that extensions of signal detection to analyze data from other research paradigms require some caution, and it offers directions for complementary research with human subjects.

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.

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 childrens 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.