Mesolimbic dopamine signals the value of work

Dopamine cell firing can encode errors in reward prediction, providing a learning signal to guide future behavior. Yet dopamine is also a key modulator of motivation, invigorating current behavior. Existing theories propose that fast (phasic) dopamine fluctuations support learning, whereas much slower (tonic) dopamine changes are involved in motivation. We examined dopamine release in the nucleus accumbens across multiple time scales, using complementary microdialysis and voltammetric methods during adaptive decision-making. We found that minute-by-minute dopamine levels covaried with reward rate and motivational vigor. Second-by-second dopamine release encoded an estimate of temporally discounted future reward (a value function). Changing dopamine immediately altered willingness to work and reinforced preceding action choices by encoding temporal-difference reward prediction errors. Our results indicate that dopamine conveys a single, rapidly evolving decision variable, the available reward for investment of effort, which is employed for both learning and motivational functions.

A 5-trial adjusting delay discounting task: accurate discount rates in less than one minute

Individuals who discount delayed rewards at a high rate are more likely to engage in substance abuse, overeating, or problem gambling. Such findings suggest the value of methods to obtain an accurate and fast measurement of discount rate that can be easily deployed in variety of settings. In the present study, we developed and evaluated the 5-trial adjusting delay task, a novel method of obtaining a discount rate in less than 1 min. We hypothesized that discount rates from the 5-trial adjusting delay task would be similar and would correlate with discount rates from a lengthier task we have used previously, and that 4 known effects relating to delay discounting would be replicable with this novel task. To test these hypotheses, the 5-trial adjusting delay task was administered to 111 college students 6 times to obtain discount rates for 6 different commodities, along with a lengthier adjusting amount discounting task. We found that discount rates were similar and correlated between the 5-trial adjusting delay task and the adjusting amount task. Each of the 4 known effects relating to delay discounting was replicated with the 5-trial adjusting delay task to varying degrees. First, discount rates were inversely correlated with amount. Second, discount rates between past and future outcomes were correlated. Third, discount rates were greater for consumable rewards than with money, although we did not control for amount in this comparison. Fourth, discount rates were lower when $0 amounts opposing the chosen time point were explicitly described. Results indicate that the 5-trial adjusting delay task is a viable, rapid method to assess discount rate.

Experimental reductions of delay discounting and impulsive choice: A systematic review and meta-analysis

Many behaviors posing significant risks to public health are characterized by repeated decisions to forego better long-term outcomes in the face of immediate temptations. Steeply discounting the value of delayed outcomes often underlies a pattern of impulsive choice. Steep delay discounting is correlated with addictions (e.g., substance abuse, obesity) and behaviors such as seatbelt use and risky sexual activity. As evidence accumulates suggesting steep delay discounting plays a causal role in these maladaptive behaviors, researchers have begun testing methods for reducing discounting. In this first systematic and comprehensive review of this literature, the findings of 92 articles employing different methodologies to reduce discounting are evaluated narratively and meta-analytically. Although most of the methods reviewed produced significant reductions in discounting, they varied in effect sizes. Most methods were ideal for influencing one-off choices (e.g., framing and priming manipulations), although other successful manipulations, such as episodic future thinking, could be incorporated into existing therapies designed to produce longer-lasting changes in decision-making. The largest and longest-lasting effects were produced by learning-based manipulations, although translational research is needed to determine the generality and clinical utility of these methods. Methodological shortcomings in the existing literature and suggestions for ameliorating these issues are discussed. This review reveals a variety of methods with translational potential, which, through continued refinement, may prove effective in reducing impulsive choice and its associated maladaptive decisions that negatively impact quality of life. (PsycINFO Database Record

Steep discounting of delayed monetary and food rewards in obesity: a meta-analysis

BACKGROUND

An increasing number of studies have investigated delay discounting (DD) in relation to obesity, but with mixed findings. This meta-analysis synthesized the literature on the relationship between monetary and food DD and obesity, with three objectives: (1) to characterize the relationship between DD and obesity in both case-control comparisons and continuous designs; (2) to examine potential moderators, including case-control v. continuous design, money v. food rewards, sample sex distribution, and sample age (18 years); and (3) to evaluate publication bias.

METHOD

From 134 candidate articles, 39 independent investigations yielded 29 case-control and 30 continuous comparisons (total n = 10 278). Random-effects meta-analysis was conducted using Cohen's d as the effect size. Publication bias was evaluated using fail-safe N, Begg-Mazumdar and Egger tests, meta-regression of publication year and effect size, and imputation of missing studies.

RESULTS

The primary analysis revealed a medium effect size across studies that was highly statistically significant (d = 0.43, p < 10-14). None of the moderators examined yielded statistically significant differences, although notably larger effect sizes were found for studies with case-control designs, food rewards and child/adolescent samples. Limited evidence of publication bias was present, although the Begg-Mazumdar test and meta-regression suggested a slightly diminishing effect size over time.

CONCLUSIONS

Steep DD of food and money appears to be a robust feature of obesity that is relatively consistent across the DD assessment methodologies and study designs examined. These findings are discussed in the context of research on DD in drug addiction, the neural bases of DD in obesity, and potential clinical applications.

Early life adversity and health-risk behaviors: proposed psychological and neural mechanisms

Early life adversity (ELA) is associated with poorer health in adulthood, an association explained, at least in part, by increased engagement in health-risk behaviors (HRBs). In this review, we make the case that ELA influences brain development in ways that increase the likelihood of engaging in HRBs. We argue that ELA alters neural circuitry underpinning cognitive control as well as emotional processing, including networks involved in processing threat and reward. These neural changes are associated psychologically and behaviorally with heightened emotional reactivity, blunted reward responsivity, poorer emotion regulation, and greater delay discounting. We then demonstrate that these adaptations to ELA are associated with an increased risk of smoking cigarettes, drinking alcohol, and eating high-fat, high-sugar foods. Furthermore, we explore how HRBs affect the brain in ways that reinforce addiction and further explain clustering of HRBs.

Temporal discounting in major depressive disorder

BACKGROUND

Major depressive disorder (MDD) is associated with abnormalities in financial reward processing. Previous research suggests that patients with MDD show reduced sensitivity to frequency of financial rewards. However, there is a lack of conclusive evidence from studies investigating the evaluation of financial rewards over time, an important aspect of reward processing that influences the way people plan long-term investments. Beck's cognitive model posits that patients with MDD hold a negative view of the future that may influence the amount of resources patients are willing to invest into their future selves.

METHOD

We administered a delay discounting task to 82 participants: 29 healthy controls, 29 unmedicated participants with fully remitted MDD (rMDD) and 24 participants with current MDD (11 on medication).

RESULTS

Patients with current MDD, relative to remitted patients and healthy subjects, discounted large-sized future rewards at a significantly higher rate and were insensitive to changes in reward size from medium to large. There was a main effect of clinical group on discounting rates for large-sized rewards, and discounting rates for large-sized rewards correlated with severity of depressive symptoms, particularly hopelessness.

CONCLUSIONS

Higher discounting of delayed rewards in MDD seems to be state dependent and may be a reflection of depressive symptoms, specifically hopelessness. Discounting distant rewards at a higher rate means that patients are more likely to choose immediate financial options. Such impairments related to long-term investment planning may be important for understanding value-based decision making in MDD, and contribute to ongoing functional impairment.

Can delay discounting deliver on the promise of RDoC?

The National Institute of Mental Health launched the Research Domain Criteria (RDoC) initiative to better understand dimensions of behavior and identify targets for treatment. Examining dimensions across psychiatric illnesses has proven challenging, as reliable behavioral paradigms that are known to engage specific neural circuits and translate across diagnostic populations are scarce. Delay discounting paradigms seem to be an exception: they are useful for understanding links between neural systems and behavior in healthy individuals, with potential for assessing how these mechanisms go awry in psychiatric illnesses. This article reviews relevant literature on delay discounting (or the rate at which the value of a reward decreases as the delay to receipt increases) in humans, including methods for examining it, its putative neural mechanisms, and its application in psychiatric research. There exist rigorous and reproducible paradigms to evaluate delay discounting, standard methods for calculating discount rate, and known neural systems probed by these paradigms. Abnormalities in discounting have been associated with psychopathology ranging from addiction (with steep discount rates indicating relative preference for immediate rewards) to anorexia nervosa (with shallow discount rates indicating preference for future rewards). The latest research suggests that delay discounting can be manipulated in the laboratory. Extensively studied in cognitive neuroscience, delay discounting assesses a dimension of behavior that is important for decision-making and is linked to neural substrates and to psychopathology. The question now is whether manipulating delay discounting can yield clinically significant changes in behavior that promote health. If so, then delay discounting could deliver on the RDoC promise.

Impulsivity in the self-harm and suicidal behavior of young people: A systematic review and meta-analysis

BACKGROUND

Impulsivity is considered a possible phenotype underlying the expression of self-harm and suicidal behaviors. Yet impulsivity is a not a unitary construct and there is evidence that different facets of impulsivity follow different neurodevelopmental trajectories and that some facets may be more strongly associated with such behaviors than others. Moreover, it is unclear whether impulsivity is a useful predictor of self-harm or suicidal behavior in young people, a population already considered to display heightened impulsive behavior.

METHODS

A systematic review and meta-analysis of studies published in Medline, PubMed, PsychInfo or Embase between 1970 and 2017 that used a neurocognitive measure to assess the independent variable of impulsivity and the dependent variable of self-harm and/or suicidal behavior among young people (mean age < 30 years old).

RESULTS

6183 titles were identified, 141 full texts were reviewed, and 18 studies were included, with 902 young people with a self-harm or suicidal behavior and 1591 controls without a history of these behaviors. Deficits in inhibitory control (13 studies, SMD 0.21, p-value = 0.002, 95% confidence interval (CI) (0.08-0.34), prediction interval (PI) = 0.06-0.35) and impulsive decision-making (14 studies, SMD 0.17, p-value = 0.008, 95% CI (0.045-0.3), PI = 0.03-0.31) were associated with self-harm or suicidal behavior. There were no significant differences between measures of different facets of impulsivity (ie. delay discounting, risky decision-making, cognitive or response inhibition) and self-harm or suicidal behavior.

CONCLUSION

Multiple facets of impulsivity are associated with suicidal behavior in young people. Future suicide research should be designed to capture impulsive states and investigate the impact on different subtypes of impulsivity.

Impulsivity in disorders of food and drug misuse

BACKGROUND

Evidence suggests some overlap between the pathological use of food and drugs, yet how impulsivity compares across these different clinical disorders remains unclear. Substance use disorders are commonly characterized by elevated impulsivity, and impulsivity subtypes may show commonalities and differences in various conditions. We hypothesized that obese subjects with binge-eating disorder (BED) and abstinent alcohol-dependent cohorts would have relatively more impulsive profiles compared to obese subjects without BED. We also predicted decision impulsivity impairment in obesity with and without BED.

METHOD

Thirty obese subjects with BED, 30 without BED and 30 abstinent alcohol-dependent subjects and age- and gender-matched controls were tested on delay discounting (preference for a smaller immediate reward over a larger delayed reward), reflection impulsivity (rapid decision making prior to evidence accumulation) and motor response inhibition (action cancellation of a prepotent response).

RESULTS

All three groups had greater delay discounting relative to healthy volunteers. Both obese subjects without BED and alcohol-dependent subjects had impaired motor response inhibition. Only obese subjects without BED had impaired integration of available information to optimize outcomes over later trials with a cost condition.

CONCLUSIONS

Delay discounting appears to be a common core impairment across disorders of food and drug intake. Unexpectedly, obese subjects without BED showed greater impulsivity than obese subjects with BED. We highlight the dissociability and heterogeneity of impulsivity subtypes and add to the understanding of neurocognitive profiles across disorders involving food and drugs. Our results have therapeutic implications suggesting that disorder-specific patterns of impulsivity could be targeted.

Temporal delay discounting in acutely ill and weight-recovered patients with anorexia nervosa

BACKGROUND

Patients with anorexia nervosa (AN) are characterized by a very low body weight but readily give up immediate rewards (food) for long-term goals (slim figure), which might indicate an unusual level of self-control. This everyday clinical observation may be quantifiable in the framework of the anticipation-discounting dilemma.

METHOD

Using a cross-sectional design, this study compared the capacity to delay reward in 34 patients suffering from acute AN (acAN), 33 weight-recovered AN patients (recAN) and 54 healthy controls. We also used a longitudinal study to reassess 21 acAN patients after short-term weight restoration. A validated intertemporal choice task and a hyperbolic model were used to estimate temporal discounting rates.

RESULTS

Confirming the validity of the task used, decreased delay discounting was associated with age and low self-reported impulsivity. However, no group differences in key measures of temporal discounting of monetary rewards were found.

CONCLUSIONS

Increased cognitive control, which has been suggested as a key characteristic of AN, does not seem to extend the capacity to wait for delayed monetary rewards. Differences between our study and the only previous study reporting decreased delay discounting in adult AN patients may be explained by the different age range and chronicity of acute patients, but the fact that weight recovery was not associated with changes in discount rates suggests that discounting behavior is not a trait marker in AN. Future studies using paradigms with disorder-specific stimuli may help to clarify the role of delay discounting in AN.

Subcortical Substrates of Explore-Exploit Decisions in Primates

The explore-exploit dilemma refers to the challenge of deciding when to forego immediate rewards and explore new opportunities that could lead to greater rewards in the future. While motivational neural circuits facilitate learning based on past choices and outcomes, it is unclear whether they also support computations relevant for deciding when to explore. We recorded neural activity in the amygdala and ventral striatum of rhesus macaques as they solved a task that required them to balance novelty-driven exploration with exploitation of what they had already learned. Using a partially observable Markov decision process (POMDP) model to quantify explore-exploit trade-offs, we identified that the ventral striatum and amygdala differ in how they represent the immediate value of exploitative choices and the future value of exploratory choices. These findings show that subcortical motivational circuits are important in guiding explore-exploit decisions.

Mesolimbic dopamine dynamically tracks, and is causally linked to, discrete aspects of value-based decision making

BACKGROUND

To make appropriate choices, organisms must weigh the costs and benefits of potential valuable outcomes, a process known to involve the nucleus accumbens (NAc) and its dopaminergic input. However, it is currently unknown if dopamine dynamically tracks alterations in expected reward value online as behavioral preferences change and if so, if it is causally linked to specific components of value such as reward magnitude and/or delay to reinforcement.

METHODS

Electrochemical methods were used to measure subsecond NAc dopamine release during a delay discounting task where magnitude was fixed but delay varied across blocks (n = 7 rats). Next, to assess whether this dopamine signaling was causally related to specific components of choice behavior, we employed selective optogenetic stimulation of dopamine terminals in the NAc using a modified delay discounting task in which both delay and magnitude varied independently (n = 23 rats).

RESULTS

Cues predictive of available choices evoked dopamine release that scaled with the rat's preferred choices and dynamically shifted as delay to reinforcement for the large reward increased. In the second experiment, dopamine signaling was causally related to features of decision making, as optogenetically enhanced dopamine release within the NAc during predictive cue presentation was sufficient to alter subsequent value-related choices. Importantly, this dopamine-mediated shift in choice was limited to delay-based, but not magnitude-based, decisions.

CONCLUSIONS

These findings indicate that NAc dopamine dynamically tracks delay discounting and establishes a causal role for this signaling in a subset of value-based associative strategies.

Relations of growth in effortful control to family income, cumulative risk, and adjustment in preschool-age children

The study examined growth in effortful control (executive control, delay ability) in relation to income, cumulative risk (aggregate of demographic and psychosocial risk factors), and adjustment in 306 preschool-age children (50 % girls, 50 % boys) from families representing a range of income (29 % at- or near-poverty; 28 % lower-income; 25 % middle-income; 18 % upper-income), with 4 assessments starting at 36-40 month. Income was directly related to levels of executive control and delay ability. Cumulative risk accounted for the effects of income on delay ability but not executive control. Higher initial executive control and slope of executive control and delay ability predicted academic readiness, whereas levels, but not growth, of executive control and delay ability predicted social competence and adjustment problems. Low income is a marker for lower effortful control, which demonstrates additive or mediating effects in the relation of income to children's preschool adjustment.

Behavioral trainings and manipulations to reduce delay discounting: A systematic review

In everyday decision-making, individuals make trade-offs between short-term and long-term benefits or costs. Depending on many factors, individuals may choose to wait for larger delayed reward, yet in other situations they may prefer the smaller, immediate reward. In addition to within-subject variation in the short-term versus long-term reward trade-off, there are also interindividual differences in delay discounting (DD), which have been shown to be quite stable. The extent to which individuals discount the value of delayed rewards turns out to be associated with important health and disorder-related outcomes: the more discounting, the more unhealthy or problematic choices. This has led to the hypothesis that DD can be conceptualized as trans-disease process. The current systematic review presents an overview of behavioral trainings and manipulations that have been developed to reduce DD in human participants aged 12 years or older. Manipulation studies mostly contain one session and measure DD directly after the manipulation. Training studies add a multiple session training component that is not per se related to DD, in between two DD task measurements. Ninety-eight studies (151 experiments) were identified that tested behavioral trainings and manipulations to decrease DD. Overall, results indicated that DD can be decreased, showing that DD is profoundly context dependent and changeable. Most promising avenues to pursue in future research seem to be acceptance-based/mindfulness-based trainings, and even more so manipulations involving a future orientation. Limitations and recommendations are discussed to identify the mechanistic processes that allow for changes in discount rate and behavior accordingly.

Discounting of delayed rewards: (A)theoretical interpretation of the Kirby questionnaire

111 Participants, recruited from Amazon's MTurk worker pool, completed Kirby's (2009) monetary choice questionnaire, which involves choosing between immediate, but smaller rewards and delayed, but larger rewards. Individual participants' responses were scored in three ways: first, by calculating the proportion of choices of the delayed rewards; second; using the scoring procedure described by Kirby et al. (1999) to estimate discounting rate (i.e., the value of the k-parameter in a hyperbolic discounting function); and third, using logistic regression to estimate discounting rate (Wileyto et al., 2004). Individuals' scores calculated using the proportion measure and the logarithms of their estimated k values were very strongly correlated (rs>.97). In addition, the proportions of choices of small, medium, and large amounts of the delayed rewards were strongly correlated (rs>.80). Taken together, these results suggest that the relative ease of calculating the proportion measure does not require sacrificing reliability. Moreover, the proportion measure is atheoretical and very easy to calculate whereas estimating an individual's discounting rate requires assuming a theoretical model that may not be appropriate. Significant differences in the proportion of delayed reward choices were observed between the small, medium, and large delayed reward amounts, with smaller rewards being discounted more steeply than larger ones, replicating previous findings of magnitude effects. These results provide further validation of the proportion of delayed reward choices on the Kirby questionnaire as a measure of individual and group differences in discounting.

Reflection impulsivity in binge drinking: behavioural and volumetric correlates

The degree to which an individual accumulates evidence prior to making a decision, also known as reflection impulsivity, can be affected in psychiatric disorders. Here, we study decisional impulsivity in binge drinkers, a group at elevated risk for developing alcohol use disorders, comparing two tasks assessing reflection impulsivity and a delay discounting task, hypothesizing impairments in both subtypes of impulsivity. We also assess volumetric correlates of reflection impulsivity focusing on regions previously implicated in functional magnetic resonance imaging studies. Sixty binge drinkers and healthy volunteers were tested using two different information-gathering paradigms: the beads task and the Information Sampling Task (IST). The beads task was analysed using a behavioural approach and a Bayesian model of decision making. Delay discounting was assessed using the Monetary Choice Questionnaire. Regression analyses of primary outcomes were conducted with voxel-based morphometry analyses. Binge drinkers sought less evidence prior to decision in the beads task compared with healthy volunteers in both the behavioural and computational modelling analysis. There were no group differences in the IST or delay discounting task. Greater impulsivity as indexed by lower evidence accumulation in the beads task was associated with smaller dorsolateral prefrontal cortex and inferior parietal volumes. In contrast, greater impulsivity as indexed by lower evidence accumulation in the IST was associated with greater dorsal cingulate and precuneus volumes. Binge drinking is characterized by impaired reflection impulsivity suggesting a deficit in deciding on the basis of future outcomes that are more difficult to represent. These findings emphasize the role of possible therapeutic interventions targeting decision-making deficits.

Subjective value representations during effort, probability and time discounting across adulthood

Every day, humans make countless decisions that require the integration of information about potential benefits (i.e. rewards) with other decision features (i.e. effort required, probability of an outcome or time delays). Here, we examine the overlap and dissociation of behavioral preferences and neural representations of subjective value in the context of three different decision features (physical effort, probability and time delays) in a healthy adult life span sample. While undergoing functional neuroimaging, participants (N = 75) made incentive compatible choices between a smaller monetary reward with lower physical effort, higher probability, or a shorter time delay versus a larger monetary reward with higher physical effort, lower probability, or a longer time delay. Behavioral preferences were estimated from observed choices, and subjective values were computed using individual hyperbolic discount functions. We found that discount rates were uncorrelated across tasks. Despite this apparent behavioral dissociation between preferences, we found overlapping subjective value-related activity in the medial prefrontal cortex across all three tasks. We found no consistent evidence for age differences in either preferences or the neural representations of subjective value across adulthood. These results suggest that while the tolerance of decision features is behaviorally dissociable, subjective value signals share a common representation across adulthood.

Vagal Tone and Children's Delay of Gratification: Differential Sensitivity in Resource-Poor and Resource-Rich Environments

Children from different socioeconomic backgrounds have differing abilities to delay gratification, and impoverished children have the greatest difficulties in doing so. In the present study, we examined the role of vagal tone in predicting the ability to delay gratification in both resource-rich and resource-poor environments. We derived hypotheses from evolutionary models of children's conditional adaptation to proximal rearing contexts. In Study 1, we tested whether elevated vagal tone was associated with shorter delay of gratification in impoverished children. In Study 2, we compared the relative role of vagal tone across two groups of children, one that had experienced greater impoverishment and one that was relatively middle-class. Results indicated that in resource-rich environments, higher vagal tone was associated with longer delay of gratification. In contrast, high vagal tone in children living in resource-poor environments was associated with reduced delay of gratification. We interpret the results with an eye to evolutionary-developmental models of the function of children's stress-response system and adaptive behavior across varying contexts of economic risk.

Viral over-expression of D1 dopamine receptors in the prefrontal cortex increase high-risk behaviors in adults: comparison with adolescents

RATIONALE

Adolescents are often described as "lacking brakes" resulting in an increase in several behaviors associated with risk for addiction. Prefrontal cortex dopamine and cortico-limbic interaction play an important role in addiction, and we have previously shown that the dopamine D1 receptor is elevated on prelimbic prefrontal output neurons in adolescent rats. We hypothesized that a constellation of risk-related behaviors is mediated by prefrontal output neuron expression of D1.

OBJECTIVES

We aimed to determine the role of the dopamine D1 receptor in behavioral and neural correlates of risk for addiction that are often observed in adolescents. Therefore, high-risk behaviors as well as subcortical D2 receptor expression were investigated in adult animals with experimentally elevated D1 on prefrontal glutamatergic neurons.

METHODS

A lentiviral vector that selectively expressed the D1 receptor within glutamate neurons was injected in the prelimbic prefrontal cortex of adult male rats. Place conditioning to cocaine, alcohol, and nicotine, as well as delay discounting, novelty preferences, anxiety, cocaine self-administration, and sucrose preferences were assessed.

RESULTS

Virally mediated D1 over-expression in adults leads to stronger drug-cue associations and greater consumption of sweet solutions, elevates bias towards immediate satisfaction rather than delaying gratification, decreases anxiety, and causes rats to work harder for and take more cocaine. Furthermore, elevated cortical D1 reduces D2 receptors in the accumbens (a putative risk marker).

CONCLUSIONS

Together, these data suggest a common mechanism for increased motivational drive to seek and consume substances with hedonic value, consistent with adolescent addictive processes.

Altered Neural Efficiency of Decision Making During Temporal Reward Discounting in Anorexia Nervosa

OBJECTIVE

The ability of individuals with anorexia nervosa (AN) to resist hunger and restrict caloric intake is often believed to reflect an unusual amount of self-control. However, the underlying neural substrate is poorly understood, especially in adolescent patients.

METHOD

Functional magnetic resonance imaging was used during an intertemporal choice task to probe the hemodynamic correlates of a common measurement of self-control-delayed (monetary) reward discounting-in a sample of acutely ill, predominately adolescent female patients with AN (n = 31) and age-matched healthy controls (n = 31).

RESULTS

Delayed discounting rates did not differ between the groups, but decision making was consistently faster in the AN group. Although no group differences in the neural correlates of reward valuation were evident, activation associated with decision making was decreased in the AN group, most notably in the lateral prefrontal and posterior parietal regions implicated in executive control. Follow-up analysis of difficult decisions showed decreased activation in the AN group in a region of the dorsal anterior cingulate cortex.

CONCLUSION

Decreased activation in frontoparietal regions involved in decision making, but faster and more consistent choice behavior, suggests that the altered efficiency of neural resource allocation might underlie an increased level of self-control in AN. This pattern of neural activation and behavior might reflect an ingrained "habit" to sustain high-level proactive (anticipatory) cognitive control in AN, which in turn might compromise reactive control mechanisms needed to adapt to changing cognitive demands, such as when difficult decisions must be made.

Evaluating non-medical prescription opioid demand using commodity purchase tasks: test-retest reliability and incremental validity

RATIONALE

Non-medical prescription opioid use and opioid use disorder (OUD) present a significant public health concern. Identifying behavioral mechanisms underlying OUD will assist in developing improved prevention and intervention approaches. Behavioral economic demand has been extensively evaluated as a measure of reinforcer valuation for alcohol and cigarettes, whereas prescription opioids have received comparatively little attention.

OBJECTIVES

Utilize a purchase task procedure to measure the incremental validity and test-retest reliability of opioid demand.

METHODS

Individuals reporting past year non-medical prescription opioid use were recruited using the crowdsourcing platform Amazon Mechanical Turk (mTurk). Participants completed an opioid purchase task as well as measures of cannabis demand, delay discounting, and self-reported pain. A 1-month follow-up was used to evaluate test-retest reliability.

RESULTS

More intense and inelastic opioid demand was associated with OUD and more intense cannabis demand was associated with cannabis use disorder. Multivariable models indicated that higher opioid intensity and steeper opioid delay discounting rates each significantly and uniquely predicted OUD. Increased opioid demand intensity, but not elasticity, was associated with higher self-reported pain, and no relationship was observed with perceived pain relief from opioids. Opioid demand showed acceptable-to-good test-retest reliability (e.g., intensity rxx = .75; elasticity rxx = .63). Temporal reliability was lower for cannabis demand (e.g., intensity rxx = .53; elasticity rxx = .58) and discounting rates (rxx = .42-.61).

CONCLUSIONS

Opioid demand was incrementally valid and test-retest reliable as measured by purchase tasks. These findings support behavioral economic demand as a clinically useful measure of drug valuation that is sensitive to individual difference variables.

Dissociable frontostriatal white matter connectivity underlies reward and motor impulsivity

Dysfunction of cognitive control often leads to impulsive decision-making in clinical and healthy populations. Some research suggests that a generalized cognitive control mechanism underlies the ability to modulate various types of impulsive behavior, while other evidence suggests different forms of impulsivity are dissociable, and rely on distinct neural circuitry. Past research consistently implicates several brain regions, such as the striatum and portions of the prefrontal cortex, in impulsive behavior. However the ventral and dorsal striatum are distinct in regards to function and connectivity. Nascent evidence points to the importance of frontostriatal white matter connectivity in impulsivity, yet it remains unclear whether particular tracts relate to different control behaviors. Here we used probabilistic tractography of diffusion imaging data to relate ventral and dorsal frontostriatal connectivity to reward and motor impulsivity measures. We found a double dissociation such that individual differences in white matter connectivity between the ventral striatum and the ventromedial prefrontal cortex and dorsolateral prefrontal cortex was associated with reward impulsivity, as measured by delay discounting, whereas connectivity between dorsal striatum and supplementary motor area was associated with motor impulsivity, but not vice versa. Our findings suggest that (a) structural connectivity can is associated with a large amount of behavioral variation; (b) different types of impulsivity are driven by dissociable frontostriatal neural circuitry.

Neural substrates of impulsive decision making modulated by modafinil in alcohol-dependent patients

BACKGROUND

Impulsive decision making is a hallmark of frequently occurring addiction disorders including alcohol dependence (AD). Therefore, ameliorating impulsive decision making is a promising target for the treatment of AD. Previous studies have shown that modafinil enhances cognitive control functions in various psychiatric disorders. However, the effects of modafinil on delay discounting and its underlying neural correlates have not been investigated as yet. The aim of the current study was to investigate the effects of modafinil on neural correlates of impulsive decision making in abstinent AD patients and healthy control (HC) subjects.

METHOD

A randomized, double-blind, placebo-controlled, within-subjects cross-over study using functional magnetic resonance imaging (fMRI) was conducted in 14 AD patients and 16 HC subjects. All subjects participated in two fMRI sessions in which they either received a single dose of placebo or 200 mg of modafinil 2 h before the session. During fMRI, subjects completed a delay-discounting task to measure impulsive decision making.

RESULTS

Modafinil improved impulsive decision making in AD pateints, which was accompanied by enhanced recruitment of frontoparietal regions and reduced activation of the ventromedial prefrontal cortex. Moreover, modafinil-induced enhancement of functional connectivity between the superior frontal gyrus and ventral striatum was specifically associated with improvement in impulsive decision making.

CONCLUSIONS

These findings indicate that modafinil can improve impulsive decision making in AD patients through an enhanced coupling of prefrontal control regions and brain regions coding the subjective value of rewards. Therefore, the current study supports the implementation of modafinil in future clinical trials for AD.

Sex-dependent associations between addiction-related behaviors and the microbiome in outbred rats

BACKGROUND

Multiple factors contribute to the etiology of addiction, including genetics, sex, and a number of addiction-related behavioral traits. One behavioral trait where individuals assign incentive salience to food stimuli ("sign-trackers", ST) are more impulsive compared to those that do not ("goal-trackers", GT), as well as more sensitive to drugs and drug stimuli. Furthermore, this GT/ST phenotype predicts differences in other behavioral measures. Recent studies have implicated the gut microbiota as a key regulator of brain and behavior, and have shown that many microbiota-associated changes occur in a sex-dependent manner. However, few studies have examined how the microbiome might influence addiction-related behaviors. To this end, we sought to determine if gut microbiome composition was correlated with addiction-related behaviors determined by the GT/ST phenotype.

METHODS

Outbred male (N=101) and female (N=101) heterogeneous stock rats underwent a series of behavioral tests measuring impulsivity, attention, reward-learning, incentive salience, and locomotor response. Cecal microbiome composition was estimated using 16S rRNA gene amplicon sequencing. Behavior and microbiome were characterized and correlated with behavioral phenotypes. Robust sex differences were observed in both behavior and microbiome; further analyses were conducted within sex using the pre-established goal/sign-tracking (GT/ST) phenotype and partial least squares differential analysis (PLS-DA) clustered behavioral phenotype.

RESULTS

Overall microbiome composition was not associated to the GT/ST phenotype. However, microbial alpha diversity was significantly decreased in female STs. On the other hand, a measure of impulsivity had many significant correlations to microbiome in both males and females. Several measures of impulsivity were correlated with the genus Barnesiella in females. Female STs had notable correlations between microbiome and attentional deficient. In both males and females, many measures were correlated with the bacterial families Ruminocococcaceae and Lachnospiraceae.

CONCLUSIONS

These data demonstrate correlations between several addiction-related behaviors and the microbiome specific to sex.

More Is Meaningful: The Magnitude Effect in Intertemporal Choice Depends on Self-Control

Impulsivity is a variable behavioral trait that depends on numerous factors. For example, increasing the absolute magnitude of available choice options promotes farsighted decisions. We argue that this magnitude effect arises in part from differential exertion of self-control as the perceived importance of the choice increases. First, we demonstrated that frontal executive-control areas were more engaged for more difficult decisions and that this effect was enhanced for high-magnitude rewards. Second, we showed that increased hunger, which is associated with lower self-control, reduced the magnitude effect. Third, we tested an intervention designed to increase self-control and showed that it reduced the magnitude effect. Taken together, our findings challenge existing theories about the magnitude effect and suggest that visceral and cognitive factors affecting choice may do so by influencing self-control.