Dissociable Motivational Deficits in Pre-manifest Huntington's Disease

Effort Expenditure Increases Risk-Taking for Improbable Rewards

Previous studies have found that exerting effort can lead people to engage in risk-taking behaviors. While effort can be either cognitive or physical, risk-taking can take place in either a risky context with known outcome probabilities or an ambiguous context with unknown outcome probabilities. The goal of the current research is to investigate how effort type and decision context influence risk-taking after effort exertion. Across three experiments, we find evidence that investing effort increases risk-taking at a short timescale. Importantly, this effect is particularly noticeable when the chance of winning is low, rather than when it is uncertain. Furthermore, the increase in risk-taking happens regardless of whether the effort is cognitive or physical. These findings suggest the existence of a cost-invariant but decision context-variant mechanism for the risk-taking after-effect of effort expenditure, which helps to bring the negative emotions caused by effort exertion back to a state of emotional homeostasis.

Dissociable neural after-effects of cognitive and physical effort expenditure during reward evaluation

The reward after-effect of effort expenditure refers to the phenomenon that previous effort investment changes the subjective value of rewards when obtained. However, the neural mechanisms underlying the after-effects of effort exertion are still not fully understood. We investigated the modulation of reward after-effects by effort type (cognitive vs. physical) through the lens of neural dynamics. Thirty-two participants performed a physically or cognitively demanding task during an effort phase and then played a simple gambling game during a subsequent reward phase to earn monetary rewards while their electroencephalogram (EEG) was recorded. We found that previous effort expenditure decreased electrocortical activity during feedback evaluation. Importantly, this effort effect occurred in a domain-general manner during the early stage (as indexed by the reward positivity) but in a domain-specific manner during the later and more elaborative stage (as indexed by the P3 and delta oscillation) of reward evaluation. Additionally, effort expenditure enhanced P3 sensitivity to feedback valence regardless of effort type. Our findings suggest that cognitive and physical effort, although bearing some surface resemblance to each other, may have dissociable neural influences on the reward after-effects.

Computational mechanisms underlying the dynamics of physical and cognitive fatigue

The willingness to exert effort for reward is essential but comes at the cost of fatigue. Theories suggest fatigue increases after both physical and cognitive exertion, subsequently reducing the motivation to exert effort. Yet a mechanistic understanding of how this happens on a moment-to-moment basis, and whether mechanisms are common to both mental and physical effort, is lacking. In two studies, participants reported momentary (trial-by-trial) ratings of fatigue during an effort-based decision-making task requiring either physical (grip-force) or cognitive (mental arithmetic) effort. Using a novel computational model, we show that fatigue fluctuates from trial-to-trial as a function of exerted effort and predicts subsequent choices. This mechanism was shared across the domains. Selective to the cognitive domain, committing errors also induced momentary increases in feelings of fatigue. These findings provide insight into the computations underlying the influence of effortful exertion on fatigue and motivation, in both physical and cognitive domains.

Amphetamines Improve the Motivation to Invest Effort in Attention-Deficit/Hyperactivity Disorder

Prevailing frameworks propose that a key feature of attention-deficit/hyperactivity disorder (ADHD) is lower motivation. An important component of motivation is the willingness to engage in cognitively or physically effortful behavior. However, the degree to which effort sensitivity is impaired in ADHD has rarely been tested, and the efficacy of stimulant medication in ameliorating any such impairments is unclear. Here, we tested 20 individuals with ADHD (11 males, 9 females) who were managed with amphetamine-based medication (dexamfetamine, lisdexamfetamine), and 24 controls (8 males, 16 females). Individuals with ADHD were tested over two counterbalanced sessions, ON and OFF their usual amphetamine-based medication. In each session, participants performed an effort-based decision-making task, in which they were required to choose how much cognitive or physical effort they were willing to engage in return for reward. Our results revealed three main findings. First, individuals with ADHD had lower motivation relative to controls to invest effort in both the cognitive and physical domains. Second, amphetamine increased motivation uniformly across both domains. Finally, the net effect of amphetamine treatment was to mostly restore motivation across both domains of effort relative to healthy controls. These data provide clear evidence for a heightened sensitivity to both cognitive and physical effort in ADHD, and reveal the efficacy of amphetamine-based drugs in restoring effort sensitivity to levels similar to controls. These findings confirm the existence of reduced motivational drive in ADHD, and more broadly provide direct causal evidence for a domain-general role of catecholamines in motivating effortful behavior.SIGNIFICANCE STATEMENT A core feature of attention-deficit/hyperactivity disorder (ADHD) is thought to be a heightened aversion to effort. Surprisingly, however, the degree to which effort sensitivity is impaired in ADHD has rarely been tested. More broadly, the relative efficacy of catecholamines in motivating the investment of cognitive and physical effort is unclear. We tested 20 individuals with ADHD ON and OFF amphetamines, and compared their behavior on an effort-based decision-making task to 24 controls. When tested OFF medication, the ADHD group was less cognitively and physically motivated than controls. However, amphetamines led to a comparable increase in motivation across both domains. This demonstrates the efficacy of catecholamines in facilitating domain-general effort, and highlights the broader potential of such drugs to treat disorders of motivation.

Associations Between Cognitive and Physical Effort-Based Decision Making in People With Schizophrenia and Healthy Control Subjects

BACKGROUND

Effort can take a variety of forms including physical (e.g., button pressing) and cognitive (e.g., working memory tasks). Few studies have examined whether individual differences in willingness to expend effort are similar or different across modalities.

METHODS

We recruited 30 individuals with schizophrenia and 44 healthy control subjects to complete 2 effort-cost decision-making tasks: the Effort Expenditure for Rewards Task (physical effort) and the cognitive effort discounting task (cognitive effort).

RESULTS

Willingness to expend cognitive and physical effort was positively associated for both individuals with schizophrenia and control subjects. Further, we found that individual differences in motivation and pleasure dimension of negative symptoms modulated the association between physical and cognitive effort. Specifically, participants with lower motivation and pleasure scores, irrespective of group status, showed stronger associations between task measures of cognitive and physical effort-cost decision making.

CONCLUSIONS

These results suggest a generalized deficit across effort modalities in individuals with schizophrenia. Further, reductions in motivation and pleasure may impact effort-cost decision making in a domain-general manner.

"Ten euros now" temporal discounting in Huntington disease

BACKGROUND

When making decisions, one often faces a trade-off between immediate and long-term rewards. In these situations, people may prefer immediate over later rewards, even if immediate rewards are smaller than later ones; a phenomenon known as temporal discounting. In this study, we, for the first time, assessed temporal discounting in three populations: participants with manifest Huntington disease (HD), participants with premanifest HD, and control participants.

METHODS

Using the temporal discounting task, we invited participants to choose between small immediate amount of money vs. delayed, but larger amount of money (e.g., "Which do you prefer: you get 10 euros right now or 50 euros in a month?"). We also measured inhibition in order to test if it impacts discounting performance.

RESULTS

Analysis demonstrated higher temporal discounting (i.e., a preference for the immediate rewards) in participants with manifest HD compared to those with premanifest HD or control participants, but no significant differences were observed in participants with premanifest HD and control participants. Analysis also demonstrated significant correlations between temporal discounting and scores on an inhibition test in participants with manifest HD, but not in those with premanifest HD or in control participants.

DISCUSSION

We suggest that, when making decisions, patients with manifest HD may have difficulties with suppressing the temptation of smaller, but immediate, rewards.

Frequency and Pathophysiology of Apathy in Huntington Disease: A Systematic Review and Meta-Analysis

OBJECTIVE

Apathy is a common behavioral symptom of Huntington disease (HD). This systematic review describes current evidence on the pathophysiology, assessment, and frequency of apathy in HD.

METHODS

This systematic review was conducted in accordance with PRISMA guidelines. Using a comprehensive search strategy, the investigators searched the MEDLINE, Embase, and PsycINFO databases. All studies that evaluated apathy in HD patients with a valid scale and reported apathy frequency or scores were included. Apathy scores were analyzed by mean or standardized mean differences in accordance with Cochrane guidelines.

RESULTS

A total of 1,085 records were screened and 80 studies were ultimately included. The Problem Behaviors Assessment-Short was the most frequently used apathy assessment tool. Apathy frequency generally ranged from 10%-33% in premanifest HD to 24%-76% in manifest HD. A meta-analysis of 5,311 records of patients with premanifest HD showed significantly higher apathy scores, with a standardized mean difference of 0.41 (CI=0.29-0.52; p<0.001). A comparison of 1,247 patients showed significantly higher apathy scores in manifest than premanifest HD, with a mean difference of 1.87 (CI=1.48-2.26; p<0.001). There was evidence of involvement of various cortical and subcortical brain regions in HD patients with apathy.

CONCLUSIONS

Apathy was more frequent among individuals with premanifest HD compared with those in a control group and among individuals with manifest HD compared with those with premanifest HD. Considering the complexity and unique pattern of development in neurodegenerative disease, further studies are required to explore the pathophysiology of apathy in HD.

Disordered Decision Making: A Cognitive Framework for Apathy and Impulsivity in Huntington's Disease

A caregiver's all‐too‐familiar narrative ‐ “He doesn't think through what he does, but mostly he does nothing.” Apathy and impulsivity, debilitating and poorly understood, commonly co‐occur in Huntington's disease (HD). HD is a neurodegenerative disease with manifestations bridging clinical neurology and psychiatry. In addition to movement and cognitive symptoms, neurobehavioral disturbances, particularly apathy and impulsivity, are prevalent features of HD, occurring early in the disease course, often worsening with disease progression, and substantially reducing quality of life. Treatments remain limited, in part because of limited mechanistic understanding of these behavioral disturbances. However, emerging work within the field of decision‐making neuroscience and beyond points to common neurobiological mechanisms underpinning these seemingly disparate problems. These insights bridge the gap between underlying disease pathology and clinical phenotype, offering new treatment strategies, novel behavioral and physiological biomarkers of HD, and deeper understanding of human behavior. In this review, we apply the neurobiological framework of cost‐benefit decision making to the problems of apathy and impulsivity in HD. Through this decision‐making lens, we develop a mechanistic model that elucidates the occurrence of these behavioral disturbances and points to potential treatment strategies and crucial research priorities. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson Movement Disorder Society.

Sleep Restriction Reduces Cognitive but Not Physical Motivation

PURPOSE

Motivation is an important driver of behaviour, and several frameworks distinguish the willingness of individuals to invest cognitive versus physical effort to achieve a goal. One outstanding question is whether sleep loss lowers motivation within specific domains of effort, or has a global effect on motivation across multiple domains. Here, we investigated the effects of sleep restriction on the motivation to invest cognitive or physical effort in return for reward.

MATERIALS AND METHODS

24 healthy young adults (11 females) completed an effort-based decision-making task over two laboratory sessions - once while sleep restricted (three consecutive nights with a three-hour sleep opportunity), and the other while fully rested (nine-hour sleep opportunity on each night). In an initial reinforcement phase, participants were trained to ceiling performance across six levels of effort on separate cognitively and physically demanding tasks. Then, in the critical decision-making phase, participants revealed their preference for how much cognitive or physical effort they would be willing to invest for reward.

RESULTS

Sleep restriction reduced the willingness to exert cognitive effort, but spared motivation in the physical domain. Furthermore, the reduction in cognitive motivation appeared to be a primary motivational deficit, which could not be attributed to differences in reward-likelihood of different levels of effort or the temporal structure of the task.

CONCLUSION

The results suggest that sleep restriction has a selective effect on cognitive over physical motivation, which has significant implications for real-world settings in which individuals must maintain high levels of cognitive motivation in the face of chronic sleep loss.

Neurocomputational mechanisms underlying the subjective value of information

Humans have a striking desire to actively seek new information, even when it is devoid of any instrumental utility. However, the mechanisms that drive individuals' subjective preference for information remain unclear. Here, we used fMRI to examine the processing of subjective information value, by having participants decide how much effort they were willing to trade-off for non-instrumental information. We showed that choices were best described by a model that accounted for: (1) the variability in individuals' estimates of uncertainty, (2) their desire to reduce that uncertainty, and (3) their subjective preference for positively valenced information. Model-based analyses revealed the anterior cingulate as a key node that encodes the subjective value of information across multiple stages of decision-making - including when information was prospectively valued, and when the outcome was definitively delivered. These findings emphasise the multidimensionality of information value, and reveal the neurocomputational mechanisms underlying the variability in individuals' desire to physically pursue informative outcomes.

Heightened effort discounting is a common feature of both apathy and fatigue

Apathy and fatigue have distinct aetiologies, yet can manifest in phenotypically similar ways. In particular, each can give rise to diminished goal-directed behaviour, which is often cited as a key characteristic of both traits. An important issue therefore is whether currently available approaches are capable of distinguishing between them. Here, we examined the relationship between commonly administered inventories of apathy and fatigue, and a measure of goal-directed activity that assesses the motivation to engage in effortful behaviour. 103 healthy adults completed self-report inventories on apathy (the Dimensional Apathy Scale), and fatigue (the Multidimensional Fatigue Inventory, and/or Modified Fatigue Impact Scale). In addition, all participants performed an effort discounting task, in which they made choices about their willingness to engage in physically effortful activity. Importantly, self-report ratings of apathy and fatigue were strongly correlated, suggesting that these inventories were insensitive to the fundamental differences between the two traits. Furthermore, greater effort discounting was strongly associated with higher ratings across all inventories, suggesting that a common feature of both traits is a lower motivation to engage in effortful behaviour. These results have significant implications for the assessment of both apathy and fatigue, particularly in clinical groups in which they commonly co-exist.

Choosing increases the value of non-instrumental information

Curiosity pervades all aspects of human behaviour and decision-making. Recent research indicates that the value of information is determined by its propensity to reduce uncertainty, and the hedonic value of the outcomes it predicts. Previous findings also indicate a preference for options that are freely chosen, compared to equivalently valued alternatives that are externally assigned. Here, we asked whether the value of information also varies as a function of self- or externally-imposed choices. Participants rated their preference for information that followed either a self-chosen decision, or an externally imposed condition. Our results showed that choosing a lottery significantly increased the subjective value of information about the outcome. Computational modelling indicated that this change in information-seeking behaviour was not due to changes in the subjective probability of winning, but instead reflected an independent effect of choosing on the value of resolving uncertainty. These results demonstrate that agency over a prospect is an important source of information value.