Epistasis effects of dopamine genes on interval timing and reward magnitude in humans

Impulsivity and risk-seeking as Bayesian inference under dopaminergic control

Bayesian models successfully account for several of dopamine (DA)'s effects on contextual calibration in interval timing and reward estimation. In these models, tonic levels of DA control the precision of stimulus encoding, which is weighed against contextual information when making decisions. When DA levels are high, the animal relies more heavily on the (highly precise) stimulus encoding, whereas when DA levels are low, the context affects decisions more strongly. Here, we extend this idea to intertemporal choice and probability discounting tasks. In intertemporal choice tasks, agents must choose between a small reward delivered soon and a large reward delivered later, whereas in probability discounting tasks, agents must choose between a small reward that is always delivered and a large reward that may be omitted with some probability. Beginning with the principle that animals will seek to maximize their reward rates, we show that the Bayesian model predicts a number of curious empirical findings in both tasks. First, the model predicts that higher DA levels should normally promote selection of the larger/later option, which is often taken to imply that DA decreases 'impulsivity,' and promote selection of the large/risky option, often taken to imply that DA increases 'risk-seeking.' However, if the temporal precision is sufficiently decreased, higher DA levels should have the opposite effect-promoting selection of the smaller/sooner option (higher impulsivity) and the small/safe option (lower risk-seeking). Second, high enough levels of DA can result in preference reversals. Third, selectively decreasing the temporal precision, without manipulating DA, should promote selection of the larger/later and large/risky options. Fourth, when a different post-reward delay is associated with each option, animals will not learn the option-delay contingencies, but this learning can be salvaged when the post-reward delays are made more salient. Finally, the Bayesian model predicts correlations among behavioral phenotypes: Animals that are better timers will also appear less impulsive.

Dopamine and the interdependency of time perception and reward

Time is a fundamental dimension of our perception of the world and is therefore of critical importance to the organization of human behavior. A corpus of work - including recent optogenetic evidence - implicates striatal dopamine as a crucial factor influencing the perception of time. Another stream of literature implicates dopamine in reward and motivation processes. However, these two domains of research have remained largely separated, despite neurobiological overlap and the apothegmatic notion that "time flies when you're having fun". This article constitutes a review of the literature linking time perception and reward, including neurobiological and behavioral studies. Together, these provide compelling support for the idea that time perception and reward processing interact via a common dopaminergic mechanism.

Rational inattention and tonic dopamine

Slow-timescale (tonic) changes in dopamine (DA) contribute to a wide variety of processes in reinforcement learning, interval timing, and other domains. Furthermore, changes in tonic DA exert distinct effects depending on when they occur (e.g., during learning vs. performance) and what task the subject is performing (e.g., operant vs. classical conditioning). Two influential theories of tonic DA-the average reward theory and the Bayesian theory in which DA controls precision-have each been successful at explaining a subset of empirical findings. But how the same DA signal performs two seemingly distinct functions without creating crosstalk is not well understood. Here we reconcile the two theories under the unifying framework of 'rational inattention,' which (1) conceptually links average reward and precision, (2) outlines how DA manipulations affect this relationship, and in so doing, (3) captures new empirical phenomena. In brief, rational inattention asserts that agents can increase their precision in a task (and thus improve their performance) by paying a cognitive cost. Crucially, whether this cost is worth paying depends on average reward availability, reported by DA. The monotonic relationship between average reward and precision means that the DA signal contains the information necessary to retrieve the precision. When this information is needed after the task is performed, as presumed by Bayesian inference, acute manipulations of DA will bias behavior in predictable ways. We show how this framework reconciles a remarkably large collection of experimental findings. In reinforcement learning, the rational inattention framework predicts that learning from positive and negative feedback should be enhanced in high and low DA states, respectively, and that DA should tip the exploration-exploitation balance toward exploitation. In interval timing, this framework predicts that DA should increase the speed of the internal clock and decrease the extent of interference by other temporal stimuli during temporal reproduction (the central tendency effect). Finally, rational inattention makes the new predictions that these effects should be critically dependent on the controllability of rewards, that post-reward delays in intertemporal choice tasks should be underestimated, and that average reward manipulations should affect the speed of the clock-thus capturing empirical findings that are unexplained by either theory alone. Our results suggest that a common computational repertoire may underlie the seemingly heterogeneous roles of DA.

The ventral hippocampus CA3 is critical in regulating timing uncertainty in temporal decision-making

Timing uncertainty is a critical component of temporal decision-making, as it determines the decision strategies that maximize reward rate. However, little is known about the biological substrates of timing uncertainty. In this study, we report that the CA3 subregion of the ventral hippocampus (vCA3), a relatively unexplored area in timing, is critical in regulating timing uncertainty that informs temporal decision making. Using a variant of the differential reinforcement of low rates of responding (DRL) task that incorporates differential levels of approach-avoidance conflict, rats were trained to wait a minimum of 6 s to earn a reward that was paired with varying durations of foot shock. Post-training chemogenetic inhibition of the vCA3 reduced timing uncertainty without affecting mean wait times, irrespective of the level of conflict experienced. Simulations based on the information-processing variant of scalar expectancy theory (SET) revealed that the vCA3 may be important in modulating decision threshold or switch closure latency variability.

Interval timing deficits and their neurobiological correlates in aging mice

Age-related neurobiological and cognitive alterations suggest that interval timing (as a related function) is also altered in aging, which can, in turn, disrupt timing-dependent functions. We investigated alterations in interval timing with aging and accompanying neurobiological changes. We tested 4-6, 10-12, and 18-20 month-old mice on the dual peak interval procedure. Results revealed a specific deficit in the termination of timed responses (stop-times). The decision processes contributed more to timing variability (vs. clock/memory process) in the aged mice. We observed age-dependent reductions in the number of dopaminergic neurons in the VTA and SNc, cholinergic neurons in the medial septum/diagonal band (MS/DB) complex, and density of dopaminergic axon terminals in the DLS/DMS. Negative correlations were found between the number of dopaminergic neurons in the VTA and stop times, and the number of cholinergic neurons in MS/DB complex and the acquisition of stop times. Our results point at age-dependent changes in the decisional components of interval timing and the role of dopaminergic and cholinergic functions in these behavioral alterations.

The hippocampus contributes to temporal duration memory in the context of event sequences: A cross-species perspective

Although a large body of research has implicated the hippocampus in the processing of memory for temporal duration, there is an exigent degree of inconsistency across studies that obfuscates the precise contributions of this structure. To shed light on this issue, the present review article surveys both historical and recent cross-species evidence emanating from a wide variety of experimental paradigms, identifying areas of convergence and divergence. We suggest that while factors such as time-scale (e.g. the length of durations involved) and the nature of memory processing (e.g. prospective vs. retrospective memory) are very helpful in the interpretation of existing data, an additional important consideration is the context in which the duration information is experienced and processed, with the hippocampus being preferentially involved in memory for durations that are embedded within a sequence of events. We consider the mechanisms that may underpin temporal duration memory and how the same mechanisms may contribute to memory for other aspects of event sequences such as temporal order.

The SLC6A3 3'-UTR VNTR and intron 8 VNTR polymorphisms association in the time estimation

OBJECTIVE

The present study investigated the association of 3'-UTR VNTR and intron 8 VNTR polymorphisms with a time estimation task performance.

MATERIALS AND METHODS

One hundred and eight men in a Brazilian Northeast population (18-32 years old) participated in the experiment. The 3'-UTR VNTR and intron 8 VNTR polymorphisms were associated alone and combined to absolute error (AE) and relative error (RE) in a time estimation task (target duration: 1 s, 4 s, 7 s and 9 s).

RESULTS

We found an association of the behavioral variable with intron 8 VNTR for the time intervals of 1 s and 9 s (p < 0.001) and polymorphisms combinatorial effect for 1 s (p ≤ 0.05).

CONCLUSION

The intron 8 VNTR polymorphism and the combinatorial effect can modulate the time estimate in the domain of supra seconds, and thus our study indicates a role of the dopamine transporter in the neurobiological areas related to the time intervals judgment.

Genetic influence alters the brain synchronism in perception and timing

Background

Studies at the molecular level aim to integrate genetic and neurobiological data to provide an increasingly detailed understanding of phenotypes related to the ability in time perception.

Main Text

This study suggests that the polymorphisms genetic SLC6A4 5-HTTLPR, 5HTR2A T102C, DRD2/ANKK1-Taq1A, SLC6A3 3’-UTR VNTR, COMT Val158Met, CLOCK genes and GABRB2 A/C as modification factor at neurochemical levels associated with several neurofunctional aspects, modifying the circadian rhythm and built-in cognitive functions in the timing. We conducted a literature review with 102 studies that met inclusion criteria to synthesize findings on genetic polymorphisms and their influence on the timing.

Conclusion

The findings suggest an association of genetic polymorphisms on behavioral aspects related in timing. However, order to confirm the paradigm of association in the timing as a function of the molecular level, still need to be addressed future research.

Neural Mechanisms Underlying Time Perception and Reward Anticipation

Findings suggest that the physiological mechanisms involved in the reward anticipation and time perception partially overlap. But the systematic investigation of a potential interaction between time and reward systems using neuroimaging is lacking. Eighteen healthy volunteers (all right-handed) participated in an event-related functional magnetic resonance imaging (fMRI) experiment that employs a visual paradigm that consists monetary reward to assess whether the functional neural representations of time perception and reward prospection are shared or distinct. Subjects performed a time perception task in which observers had to extrapolate the velocity of an occluded moving object in “reward” vs. “no-reward” sessions during fMRI scanning. There were also “control condition” trials in which participants judged about the color tone change of the stimuli. Time perception showed a fronto-parietal (more extensive in the right) cingulate and peristriate cortical as well as cerebellar activity. On the other hand, reward anticipation activated anterior insular cortex, nucleus accumbens, caudate nucleus, thalamus, cerebellum, postcentral gyrus, and peristriate cortex. Interaction between the time perception and the reward prospect showed dorsolateral, orbitofrontal, medial prefrontal and caudate nucleus activity. Our findings suggest that a prefrontal-striatal circuit might integrate reward and timing systems of the brain.

Neurochemical changes in basal ganglia affect time perception in parkinsonians

BACKGROUND

Parkinson's disease is described as resulting from dopaminergic cells progressive degeneration, specifically in the substantia nigra pars compacta that influence the voluntary movements control, decision making and time perception.

AIM

This review had a goal to update the relation between time perception and Parkinson's Disease.

METHODOLOGY

We used the PRISMA methodology for this investigation built guided for subjects dopaminergic dysfunction in the time judgment, pharmacological models with levodopa and new studies on the time perception in Parkinson's Disease. We researched on databases Scielo, Pubmed / Medline and ISI Web of Knowledge on August 2017 and repeated in September 2017 and February 2018 using terms and associations relevant for obtaining articles in English about the aspects neurobiology incorporated in time perception. No publication status or restriction of publication date was imposed, but we used as exclusion criteria: dissertations, book reviews, conferences or editorial work.

RESULTS/DISCUSSION

We have demonstrated that the time cognitive processes are underlying to performance in cognitive tasks and that many are the brain areas and functions involved and the modulators in the time perception performance.

CONCLUSIONS

The influence of dopaminergic on Parkinson's Disease is an important research tool in Neuroscience while allowing for the search for clarifications regarding behavioral phenotypes of Parkinson's disease patients and to study the areas of the brain that are involved in the dopaminergic circuit and their integration with the time perception mechanisms.

Dopamine, time perception, and future time perspective

RATIONALE

Impairment in time perception, a critical component of decision-making, represents a risk factor for psychiatric conditions including substance abuse. A therapeutic that ameliorates this impairment could be advantageous in the treatment of impulsivity and decision-making disorders.

OBJECTIVES

Here we hypothesize that the catechol-O-methyltransferase (COMT) inhibitor tolcapone, which increases dopamine tone in frontal cortex (Ceravolo et al Synapse 43:201-207, 2002), improves time perception, with predictive behavioral, genetic, and neurobiological components.

METHODS

Subjects (n = 66) completed a duration estimation task and other behavioral testing in each of two sessions after receiving a single oral dose of tolcapone (200 mg) or placebo in randomized, double-blind, counterbalanced, crossover fashion. Resting state fMRI data were obtained in a subset of subjects (n = 40). Subjects were also genotyped for the COMT (rs4680) polymorphism.

RESULTS

Time perception was significantly improved across four proximal time points ranging from 5 to 60 s (T(524) = 2.04, p = 0.042). The degree of this improvement positively correlated with subjective measures of stress, depression, and alcohol consumption and was most robust in carriers of the COMT Val158 allele. Using seed regions defined by a previous meta-analysis (Wiener et al Neuroimage 49:1728-1740, 2010), we found not only that a connection from right inferior frontal gyrus (RIFG) to right putamen decreases in strength on tolcapone versus placebo (p < 0.05, corrected), but also that the strength of this decrease correlates inversely with the increase in duration estimation on tolcapone versus placebo (r = - 0.37, p = 0.02).

CONCLUSIONS

Compressed time perception can be ameliorated by administration of tolcapone. Additional studies should be conducted to determine whether COMT inhibitors may be effective in treating decision-making disorders and addictive behaviors.

The dopaminergic system dynamic in the time perception: a review of the evidence

Dopaminergic system plays a key role in perception, which is an important executive function of the brain. Modulation in dopaminergic system forms an important biochemical underpinning of neural mechanisms of time perception in a very wide range, from milliseconds to seconds to longer daily rhythms. Distinct types of temporal experience are poorly understood, and the relationship between processing of different intervals by the brain has received little attention. A comprehensive understanding of interval timing functions should be sought within a wider context of temporal processing, involving genetic aspects, pharmacological models, cognitive aspects, motor control and the neurological diseases with impaired dopaminergic system. Particularly, an unexplored question is whether the role of dopamine in interval timing can be integrated with the role of dopamine in non-interval timing temporal components. In this review, we explore a wider perspective of dopaminergic system, involving genetic polymorphisms, pharmacological models, executive functions and neurological diseases on the time perception. We conclude that the dopaminergic system has great participation in impact on time perception and neurobiological basis of the executive functions and neurological diseases.

Deficits in temporal processing correlate with clinical progression in Huntington's disease

OBJECTIVES

Precise temporal performance is crucial for several complex tasks. Time estimation in the second-to-minutes range-known as interval timing-involves the interaction of the basal ganglia and the prefrontal cortex via dopaminergic-glutamatergic pathways. Patients with Huntington's disease (HD) present deficits in cognitive and motor functions that require fine control of temporal processing. The objective of the present work was to assess temporal cognition through a peak-interval time (PI) production task in patients with HD and its potential correlation with the Unified Huntington's Disease Rating Scale (UHDRS).

MATERIALS AND METHODS

Patients with molecular diagnosis of HD and controls matched by age, sex and educational level (n=18/group) were tested for interval timing in short- (3 seconds), medium- (6 seconds) and long (12 seconds)-duration stimuli.

RESULTS

Significant differences were observed in the PI task, with worse performance in HD compared to controls. Patients underestimated real time (left-shifted Peak location) for 6- and 12-second intervals (P<.05) and presented decreased temporal precision for all the intervals evaluated (P<.01). Importantly, a significant correlation was found between time performance and the UHDRS (P<.01). Patients' responses also deviated from the scalar property.

CONCLUSIONS

Our results contribute to support that timing functions are impaired in HD in correlation with clinical deterioration. Recordings of cognitive performance related to timing could be a potential useful tool to measure the neurodegenerative progression of movement disorder-related pathologies.

The effects of payoff manipulations on temporal bisection performance

There is growing evidence that alterations in reward rates modify timing behavior demonstrating the role of motivational factors in interval timing behavior. This study aimed to investigate the effects of manipulations of rewards and penalties on temporal bisection performance in humans. Participants were trained to classify experienced time intervals as short or long based on the reference durations. Two groups of participants were tested under three different bias conditions in which either the relative reward magnitude or penalty associated with correct or incorrect categorizations of short and long reference durations was manipulated. Participants adapted their choice behavior (i.e., psychometric functions shifted) based on these payoff manipulations in directions predicted by reward maximization. The signal detection theory-based analysis of the data revealed that payoff contingencies affected the response bias parameter (B″) without altering participants' sensitivity (A') to temporal distances. Finally, the response time (RT) analysis showed that short categorization RTs increased, whereas long categorization RTs decreased as a function of stimulus durations. However, overall RTs did not exhibit any modulation in response to payoff manipulations. Taken together, this study provides additional support for the effects of motivational variables on temporal decision-making.

Reward Contrast Effects on Impulsive Choice and Timing in Rats

Despite considerable interest in impulsive choice as a predictor of a variety of maladaptive behaviors, the mechanisms that drive choice behavior are still poorly understood. The present study sought to examine the influence of one understudied variable, reward magnitude contrast, on choice and timing behavior as changes in magnitude commonly occur within choice procedures. In addition, assessments of indirect effects on choice behavior through magnitude-timing interactions were assessed by measuring timing within the choice task. Rats were exposed to choice procedures composed of different pairs of magnitudes of rewards for either the smaller-sooner (SS) or larger-later (LL) option. In Phase 2, the magnitude of reward either increased or decreased by 1 pellet in different groups (LL increase = 1v1→1v2; SS decrease = 2v2 → 1v2; SS increase = 1v2 → 2v2), followed by a return to baseline in Phase 3. Choice behavior was affected by the initial magnitudes experienced in the task, demonstrating a strong anchor effect. The nature of the change in magnitude affected choice behavior as well. Timing behavior was also affected by the reward contrast manipulation albeit to a lesser degree and the timing and choice effects were correlated. The results suggest that models of choice behavior should incorporate reinforcement history, reward contrast elements, and magnitude-timing interactions, but that direct effects of reward contrast on choice should be given more weight than the indirect reward-timing interactions. A better understanding of the factors that contribute to choice behavior could supply key insights into this important individual differences variable.

The influence of genes on "positive valence systems" constructs: A systematic review

In 2009, the U.S. National Institute of Mental Health (NIMH) proposed an approach toward the deconstruction of psychiatric nosology under the research domain criteria (RDoC) framework. The overarching goal of RDoC is to identify robust, objective measures of behavior, emotion, cognition, and other domains that are more closely related to neurobiology than are diagnoses. A preliminary framework has been constructed, which has connected molecules, genes, brain circuits, behaviors, and other elements to dimensional psychiatric constructs. Although the RDoC framework has salience in emerging studies, foundational literature that pre-dated this framework requires synthesis and translation to the evolving objectives and nomenclature of RDoC. Toward this end, we review the candidate-gene association, linkage, and genome-wide studies that have implicated a variety of loci and genetic polymorphisms in selected Positive Valence Systems (PVS) constructs. Our goal is to review supporting evidence to currently listed genes implicated in this domain and novel candidates. We systematically searched and reviewed literature based on keywords listed under the June, 2011, edition of the PVS matrix on the RDoC website (http://www.nimh.nih.gov/research-priorities/rdoc/positive-valence-systems-workshop-proceedings.shtml), which were supplemented with de novo keywords pertinent to the scope of our review. Several candidate genes linked to the PVS framework were identified from candidate-gene association studies. We also identified novel candidates with loose association to PVS traits from genome-wide studies. There is strong evidence suggesting that PVS constructs, as currently conceptualized under the RDoC initiative, index genetically influenced traits; however, future research, including genetic epidemiological, and psychometric analyses, must be performed.

Reward alters the perception of time

Recent findings indicate that monetary rewards have a powerful effect on cognitive performance. In order to maximize overall gain, the prospect of earning reward biases visual attention to specific locations or stimulus features improving perceptual sensitivity and processing. The question we addressed in this study is whether the prospect of reward also affects the subjective perception of time. Here, participants performed a prospective timing task using temporal oddballs. The results show that temporal oddballs, displayed for varying durations, presented in a sequence of standard stimuli were perceived to last longer when they signaled a relatively high reward compared to when they signaled no or low reward. When instead of the oddball the standards signaled reward, the perception of the temporal oddball remained unaffected. We argue that by signaling reward, a stimulus becomes subjectively more salient thereby modulating its attentional deployment and distorting how it is perceived in time.

Analysis of Genetic and Non-Genetic Factors Influencing Timing and Time Perception

Performance on different psychophysical tasks measuring the sense of time indicates a large amount of individual variation in the accuracy and precision of timing in the hundredths of milliseconds-to-minutes range. Quantifying factors with an influence on timing is essential to isolating a biological (genetic) contribution to the perception and estimation of time. In the largest timing study to date, 647 participants completed a duration-discrimination task in the sub-second range and a time-production task in the supra-second range. We confirm the stability of a participant's time sense across multiple sessions and substantiate a modest sex difference on time production. Moreover, we demonstrate a strong correlation between performance on a standardized cognitive battery and performance in both duration-discrimination and time-production tasks; we further show that performance is uncorrelated with age after controlling for general intelligence. Additionally, we find an effect of ethnicity on time sense, with African Americans and possibly Hispanics in our cohort differing in accuracy and precision from other ethnic groups. Finally, a preliminary genome-wide association and exome chip study was performed on 148 of the participants, ruling out the possibility for a single common variant or groups of low-frequency coding variants within a single gene to explain more than ~18% of the variation in the sense of time.

Epistasis in the risk of human neuropsychiatric disease

Neuropsychiatric disease represents the ideal class of disease to assess the role of epistasis, as more genes are expressed in the brain than in any other tissue. In this chapter, two well-studied neuropsychiatric diseases are examined, Alzheimer's disease (AD) and schizophrenia, which have been shown to have multiple and, often, replicated interactions that associate with clinical endpoints or related phenotypes. In each case, a single gene is represented in a plurality of epistatic interactions, apolipoprotein E (APOE) for AD and catechol-O-methyltransferase for schizophrenia. Interestingly, of the two, only APOE has clear-cut and consistent evidence for a marginal association. Unraveling the underlying reasons is important in understanding both genetic etiology and architecture as well as how to use genetics to provide better personalized treatments.

Time perception: the bad news and the good

Time perception is fundamental and heavily researched, but the field faces a number of obstacles to theoretical progress. In this advanced review, we focus on three pieces of 'bad news' for time perception research: temporal perception is highly labile across changes in experimental context and task; there are pronounced individual differences not just in overall performance but in the use of different timing strategies and the effect of key variables; and laboratory studies typically bear little relation to timing in the 'real world'. We describe recent examples of these issues and in each case offer some 'good news' by showing how new research is addressing these challenges to provide rich insights into the neural and information-processing bases of timing and time perception. WIREs Cogn Sci 2014, 5:429-446. doi: 10.1002/wcs.1298 This article is categorized under: Psychology > Perception and Psychophysics Neuroscience > Cognition.

Involvement of dopamine signaling in the circadian modulation of interval timing

Duration discrimination within the seconds-to-minutes range, known as interval timing, involves the interaction of cortico-striatal circuits via dopaminergic-glutamatergic pathways. Besides interval timing, most (if not all) organisms exhibit circadian rhythms in physiological, metabolic and behavioral functions with periods close to 24 h. We have previously reported that both circadian disruption and desynchronization impaired interval timing in mice. In this work we studied the involvement of dopamine (DA) signaling in the interaction between circadian and interval timing. We report that daily injections of levodopa improved timing performance in the peak-interval procedure in C57BL/6 mice with circadian disruptions, suggesting that a daily increase of DA is necessary for an accurate performance in the timing task. Moreover, striatal DA levels measured by reverse-phase high-pressure liquid chromatography indicated a daily rhythm under light/dark conditions. This daily variation was affected by inducing circadian disruption under constant light (LL). We also demonstrated a daily oscillation in tyrosine hydroxylase levels, DA turnover (3,4-dihydroxyphenylacetic acid/DA levels), and both mRNA and protein levels of the circadian component Period2 (Per2) in the striatum and substantia nigra, two brain areas relevant for interval timing. None of these oscillations persisted under LL conditions. We suggest that the lack of DA rhythmicity in the striatum under LL - probably regulated by Per2 - could be responsible for impaired performance in the timing task. Our findings add further support to the notion that circadian and interval timing share some common processes, interacting at the level of the dopaminergic system.

Minutes, days and years: molecular interactions among different scales of biological timing

Biological clocks are genetically encoded oscillators that allow organisms to keep track of their environment. Among them, the circadian system is a highly conserved timing structure that regulates several physiological, metabolic and behavioural functions with periods close to 24 h. Time is also crucial for everyday activities that involve conscious time estimation. Timing behaviour in the second-to-minutes range, known as interval timing, involves the interaction of cortico-striatal circuits. In this review, we summarize current findings on the neurobiological basis of the circadian system, both at the genetic and behavioural level, and also focus on its interactions with interval timing and seasonal rhythms, in order to construct a multi-level biological clock.

Interval Timing, Dopamine, and Motivation

The dopamine clock hypothesis suggests that the dopamine level determines the speed of the hypothetical internal clock. However, dopaminergic function has also been implicated for motivation and thus the effect of dopaminergic manipulations on timing behavior might also be independently mediated by altered motivational state. Studies that investigated the effect of motivational manipulations on peak responding are reviewed in this paper. The majority of these studies show that a higher reward magnitude leads to a leftward shift, whereas reward devaluation leads to a rightward shift in the initiation of timed anticipatory behavior, typically in the absence of an effect on the timing of response termination. Similar behavioral effects are also present in a number of studies that investigated the effect of dopamine agonists and dopamine-related genetic factors on peak responding. These results can be readily accounted for by independent modulation of decision-thresholds for the initiation and termination of timed responding.

Dissociation of Neural Mechanisms for Intersensory Timing Deficits in Parkinson's Disease

This study investigated the ability of individuals with Parkinson's disease (PD) to synthesize temporal information across the senses, namely audition and vision. Auditory signals (A) are perceived as lasting longer than visual signals (V) when they are compared together, since attentsion is captured and sustained more easily than for visual information. We used the audiovisual illusion to probe for disturbances in brain networks that govern the resolution of time in two intersensory conditions that putatively differ in their attention demands. PD patients and controls judged the relative duration of successively presented pairs of unimodal (AA, VV) and crossmodal (VA, AV) signals whilst undergoing fMRI. There were four main findings. First, underestimation of time was exaggerated in PD when timing depended on controlled attention (AV), whereas subtle deficits were found when audition dominated and attention was more easily sustained (VA). Second, group differences in regional activation were observed only for the AV-unimodal comparison, where the PD group failed to modulate basal ganglia, anterior insula, and inferior cerebellum activity in accord with the timing condition. Third, the intersensory timing conditions were dissociated by patterns of abnormal functional connectivity. When intersensory timing emphasized controlled attention, patients showed weakened connectivity of the cortico-thalamus-basal ganglia (CTBG) circuit and the anterior insula with widespread cortical regions, yet enhanced cerebellar connectivity. When audition dominated intersensory timing, patients showed enhanced connectivity of CTBG elements, the anterior insula, and the cerebellum with the caudate tail and frontal cortex. Fourth, abnormal connectivity measures showed excellent sensitivity and specificity in accurately classifying subjects. The results demonstrate that intersensory timing deficits in PD were well characterized by context-dependent patterns of functional connectivity within a presumed core timing system (CTBG) and a ventral attention hub (anterior insula), and enhanced cerebellar connectivity irrespective of the hypothesized attention demands of timing.

Interactions of timing and prediction error learning

Timing and prediction error learning have historically been treated as independent processes, but growing evidence has indicated that they are not orthogonal. Timing emerges at the earliest time point when conditioned responses are observed, and temporal variables modulate prediction error learning in both simple conditioning and cue competition paradigms. In addition, prediction errors, through changes in reward magnitude or value alter timing of behavior. Thus, there appears to be a bi-directional interaction between timing and prediction error learning. Modern theories have attempted to integrate the two processes with mixed success. A neurocomputational approach to theory development is espoused, which draws on neurobiological evidence to guide and constrain computational model development. Heuristics for future model development are presented with the goal of sparking new approaches to theory development in the timing and prediction error fields.

Individual differences in the morphometry and activation of time perception networks are influenced by dopamine genotype

Individual participants vary greatly in their ability to estimate and discriminate intervals of time. This heterogeneity of performance may be caused by reliance on different time perception networks as well as individual differences in the activation of brain structures utilized for timing within those networks. To address these possibilities we utilized event-related functional magnetic resonance imaging (fMRI) while human participants (n=25) performed a temporal or color discrimination task. Additionally, based on our previous research, we genotyped participants for DRD2/ANKK1-Taq1a, a single-nucleotide polymorphism associated with a 30-40% reduction in striatal D2 density and associated with poorer timing performance. Similar to previous reports, a wide range of performance was found across our sample; crucially, better performance on the timing versus color task was associated with greater activation in prefrontal and sub-cortical regions previously associated with timing. Furthermore, better timing performance also correlated with increased volume of the right lateral cerebellum, as demonstrated by voxel-based morphometry. Our analysis also revealed that A1 carriers of the Taq1a polymorphism exhibited relatively worse performance on temporal, but not color discrimination, but greater activation in the striatum and right dorsolateral prefrontal cortex, as well as reduced volume in the cerebellar cluster. These results point to the neural bases for heterogeneous timing performance in humans, and suggest that differences in performance on a temporal discrimination task are, in part, attributable to the DRD2/ANKK1 genotype.

Reprint of: DAT genotype modulates striatal processing and long-term memory for items associated with reward and punishment

Previous studies have shown that appetitive motivation enhances episodic memory formation via a network including the substantia nigra/ventral tegmental area (SN/VTA), striatum and hippocampus. This functional magnetic resonance imaging (fMRI) study now contrasted the impact of aversive and appetitive motivation on episodic long-term memory. Cue pictures predicted monetary reward or punishment in alternating experimental blocks. One day later, episodic memory for the cue pictures was tested. We also investigated how the neural processing of appetitive and aversive motivation and episodic memory were modulated by dopaminergic mechanisms. To that end, participants were selected on the basis of their genotype for a variable number of tandem repeat polymorphism of the dopamine transporter (DAT) gene. The resulting groups were carefully matched for the 5-HTTLPR polymorphism of the serotonin transporter gene. Recognition memory for cues from both motivational categories was enhanced in participants homozygous for the 10-repeat allele of the DAT, the functional effects of which are not known yet, but not in heterozygous subjects. In comparison with heterozygous participants, 10-repeat homozygous participants also showed increased striatal activity for anticipation of motivational outcomes compared to neutral outcomes. In a subsequent memory analysis, encoding activity in striatum and hippocampus was found to be higher for later recognized items in 10-repeat homozygotes compared to 9/10-repeat heterozygotes. These findings suggest that processing of appetitive and aversive motivation in the human striatum involve the dopaminergic system and that dopamine plays a role in memory for both types of motivational information. In accordance with animal studies, these data support the idea that encoding of motivational events depends on dopaminergic processes in the hippocampus.

DAT genotype modulates striatal processing and long-term memory for items associated with reward and punishment

Previous studies have shown that appetitive motivation enhances episodic memory formation via a network including the substantia nigra/ventral tegmental area (SN/VTA), striatum and hippocampus. This functional magnetic resonance imaging (fMRI) study now contrasted the impact of aversive and appetitive motivation on episodic long-term memory. Cue pictures predicted monetary reward or punishment in alternating experimental blocks. One day later, episodic memory for the cue pictures was tested. We also investigated how the neural processing of appetitive and aversive motivation and episodic memory were modulated by dopaminergic mechanisms. To that end, participants were selected on the basis of their genotype for a variable number of tandem repeat polymorphism of the dopamine transporter (DAT) gene. The resulting groups were carefully matched for the 5-HTTLPR polymorphism of the serotonin transporter gene. Recognition memory for cues from both motivational categories was enhanced in participants homozygous for the 10-repeat allele of the DAT, the functional effects of which are not known yet, but not in heterozygous subjects. In comparison with heterozygous participants, 10-repeat homozygous participants also showed increased striatal activity for anticipation of motivational outcomes compared to neutral outcomes. In a subsequent memory analysis, encoding activity in striatum and hippocampus was found to be higher for later recognized items in 10-repeat homozygotes compared to 9/10-repeat heterozygotes. These findings suggest that processing of appetitive and aversive motivation in the human striatum involve the dopaminergic system and that dopamine plays a role in memory for both types of motivational information. In accordance with animal studies, these data support the idea that encoding of motivational events depends on dopaminergic processes in the hippocampus.

Count on dopamine: influences of COMT polymorphisms on numerical cognition

Catechol-O-methyltransferase (COMT) is an enzyme that is particularly important for the metabolism of dopamine. Functional polymorphisms of COMT have been implicated in working memory and numerical cognition. This is an exploratory study that aims at investigating associations between COMT polymorphisms, working memory, and numerical cognition. Elementary school children from 2th to 6th grades were divided into two groups according to their COMT val158met polymorphism [homozygous for valine allele (n = 61) vs. heterozygous plus methionine homozygous children or met+ group (n = 94)]. Both groups were matched for age and intelligence. Working memory was assessed through digit span and Corsi blocks. Symbolic numerical processing was assessed through transcoding and single-digit word problem tasks. Non-symbolic magnitude comparison and estimation tasks were used to assess number sense. Between-group differences were found in symbolic and non-symbolic numerical tasks, but not in working memory tasks. Children in the met+ group showed better performance in all numerical tasks while val homozygous children presented slower development of non-symbolic magnitude representations. These results suggest COMT-related dopaminergic modulation may be related not only to working memory, as found in previous studies, but also to the development of magnitude processing and magnitude representations.