What Is Mental Effort: A Clinical Perspective

Although mental effort is a frequently used term, it is poorly de ﬁ ned and understood. Consequently, its usage is frequently loose and potentially misleading. In neuroscience research, the term is used to mean both the cognitive work that is done to meet task demands and the subjective experience of performing that work. We argue that con ﬂ ating these two meanings hampers progress in understanding cognitive impairments in neuropsychiatric conditions because cognitive work and the subjective experience of it have distinct underlying mechanisms. We suggest that the most coherent and clinically useful perspective on mental effort is that it is a subjective experience. This makes a clear distinction between cognitive impairments that arise from changes in the cognitive apparatus, as in dementia and brain injury, and those that arise from subjective dif ﬁ culties in carrying out the cognitive work, as in attention-de ﬁ cit/hyperactivity disorder, depression, and other motivational disorders. We review recent advances in neuroscience research that suggests that the experience of effort has emerged to control task switches so as to minimize costs relative to bene ﬁ ts. We consider how these advances can contribute to our understanding of the experience of increased effort perception in clinical populations. This more speci ﬁ c framing of mental effort will offer a deeper understanding of the mechanisms of cognitive impairments in differing clinical groups and will ultimately facilitate better therapeutic interventions.

The term mental effort is frequently used in cognitive and clinical neuroscience, but its usage varies, and its precise meaning is often unclear (1,2).The term is used in two main ways (3), with the two usages sometimes treated interchangeably.First, mental effort refers to the cognitive work that is done to meet task demands (4)(5)(6)(7)(8)(9).Second, it refers to a subjective experience that is attached to performing the work but is dissociable from the work itself (10,11).
Because both cognitive work and the experience that accompanies it are closely linked to cognitive performance, it is crucial to better understand their underlying mechanisms.Importantly, in clinical populations that show impairments in cognitive performance, disambiguating mental effort will help us understand whether a person's cognitive impairment is driven by specific changes either in the cognitive apparatus that carries out the cognitive work or in the subjective experience that accompanies it.For example, dementia, neurodevelopmental conditions, and brain injury are associated with impairments in the former.By contrast, patients across many neuropsychiatric conditions experience mental tasks as so effortful that they are unable to sustain or complete them (12)(13)(14).In this article, we argue that conflating these two meanings hampers attempts to understand the mechanisms of cognitive impairments in neuropsychiatric conditions.
The two phenomena of cognitive work and mental effort are likely causally related, and some conditions may be associated with deficits in both.For example, after a stroke, damage to mental processing capacity is often associated with increased perception of effort (15).However, as we discuss below, mental work and effort are dissociable, and their impairments have distinct underlying pathophysiology, which indicate the need for different therapeutic interventions.A clearer understanding of cognitive work, mental effort, and their relationship to cognitive impairments is clinically vital.
Given this perspective, we first define mental effort and then review recent advances in neuroscience research, examining why we experience mental effort and how the brain signals effort.Finally, we relate these emergent ideas and insights to the altered experiences of mental effort in the clinical domain.

(RE)DEFINING MENTAL EFFORT
Mental effort is sometimes used synonymously with executive function, cognitive effort (16), and attention (17).In neuroscience literature, two primary formulations of these terms frequently appear, often within a single research paper.One refers to the cognitive activity or the work that is done, often with regard to its intensity, duration, and output (6); another refers to the subjective feeling that this work is difficult or perhaps aversive (10).
Varied usage is also notable in research on physical effort (18).Here, the conflation of terms may feel justified because there is a credible assumption that the work exerted is more directly related to the accompanying experience of effort.However, this has proven to be untrue in terms of physical activity, and it is also untrue in the cognitive domain, where the dual usage has even greater potential to cause confusion because the nature and markers of cognitive work are less clear (see What is Mental Work? in the Supplement).
In both the physical and cognitive domains, a particular set of tasks may be experienced as more or less effortful depending on several factors that are independent from the actual work required: 1) self-motivational determinants (19,20), 2) affective and emotional determinants (21,22), 3) an individual's own sense of competence or self-efficacy (23,24), 4) an individual's levels of fatigue (25-27), 5) the subjective attitude to the task or to its potential outcome (28), 6) the anticipation of how worthwhile the activity is (29,30), 7) the amount of time available for the task (31,32), 8) the ongoing feedback provided about task performance (33), and 9) one's beliefs about the outcome of their actions (34)(35)(36).Therefore, a given level of cognitive work may be associated with varying subjective experiences of effort, which suggests that these are two separate constructs (Figure 1).
Importantly, cognitive work and the subjective experience that accompanies it can also be dissociable in clinical populations.In many neuropsychiatric conditions, cognitive performance can be affected by disturbances in cognitive processing or work itself as is often observed in patients with brain changes due to neurodegenerative or neurodevelopmental conditions.However, cognitive impairments may also arise from disturbances in how patients value and experience this work as seen in cases of apathy.
We argue that it is crucial to separate these constructs in basic and clinical research to better understand their underlying mechanisms and contribution to reduced cognitive performance (i.e., cognitive impairment) in clinical populations (key definitions in Table 1).Next, we review recent advances in the neuroscientific literature on why and how the experience of mental effort emerges.

WHY DO WE EXPERIENCE MENTAL EFFORT?
Mental effort is a characteristic subjective experience that accompanies certain types of cognitive tasks.Something that seems to be common to mentally effortful actions is that they involve the suppression of distractors, particularly habitual responses (5).The need to suppress distractors may occur within a task, such as the Stroop task, or between competing tasks.The experience of mental effort is predominantly aversive (37) and, particularly when prolonged, limits a person's willingness or ability to engage in the task at hand.This raises the question of what purpose the experience of effort may serve.
A number of ideas have been put forward to account for why we experience mental effort (6,38,39).Their common premise is that mental effort exists to signal that prolongation of the activity in question will have, directly or indirectly, deleterious consequences or costs, and therefore a behavioral change, either stopping or switching [which is effortful itself (40)], should be considered.The theories mainly differ in the way in which mental effort is considered to be costly: 1) it may be costly insofar as it requires resources that are valuable and depletable (41)(42)(43)(44); 2) it may generate by-products that are potentially detrimental (43,45,46); 3) it may compromise an individual's capacity to monitor and respond to their A clinically informed framework for mental effort.Our clinically informed framework of mental effort makes a clear distinction between mental effort and cognitive work.While cognitive work is a costly activity that is performed by the cognitive apparatus and that mediates between the currently engaged cognitive action (Action i ) and its outcome (mechanism 1), mental effort is the subjective experience that accompanies that work (mechanism 2).The experience of effort is further influenced by the action outcome and how an individual perceives this outcome (mechanism 3), as well as by the utility of alternative actions not currently engaged in, i.e., opportunity cost (mechanism 4).We discuss several common mental health disorders associated with cognitive impairments and their possible underlying mechanisms within the scheme (mechanisms 1-4).environment in a rapid and flexible way (47); and 4) it may be computationally or behaviorally costly in that it deprives the individual of an opportunity to engage in another cognitive activity that may have potentially more valuable outcomes (opportunity cost) (48)(49)(50).The latter account reflects a growing literature comprising studies of mental effort within value-based decision making (38,51).This proposes that the decision to engage in a mentally effortful action, and if so which action to choose, depends on the action's utility (i.e., how much an individual values a particular action, taking into account costs and benefits).This line of literature examines how people allocate cognitive effort, or according to our proposed terminology, cognitive work, as a function of different experimental variables or disease states.An example of framing cognitive work allocation in decision theory is the expected value of control theory.This theory specifically suggests that the amount of cognitive work that we choose to expend depends on the expected value of the outcome relative to the cost associated with that work (5,52).Another suggestion is that effort reflects an opportunity cost (49,53,54) [see discussion on Open Peer Commentary (48)], with the experience of effort increasing as the utility of competing cognitive actions increases or as the utility of the current action decreases (48).According to these decisionmaking accounts, the experience of mental effort plays a normative role in minimizing some computational costs while maximizing reward.Importantly, all accounts invoke the adaptive advantages of effort as a signal that is sufficiently aversive to impose limitations to engagement in a given task.
A useful analogy to the adaptive role of effort is the experience of pain.Both are subjective experiences that vary between people and depend on many bodily and psychological factors.Moreover, both pain and effort are typically aversive experiences but can sometimes be experienced as nonaversive or even valuable experiences (55).This has been termed the "effort paradox" [reviewed in (56)], because individuals sometimes choose to perform the more effortful task over doing nothing (57) or may abandon valuable goals if they require less effort (58).Importantly, both pain and effort can be seen as adaptive responses that serve a protective function.The subjective sense of effort can be seen as a signal that arises from potential harm, similar to the idea that pain is a subjective experience that has evolved to signal tissue damage.
The harm that the experience of effort may mitigate has been considered to be resource depletion for many years (5,59,60).That is, the experience of effort may have evolved to converse resources.However, resource conservation accounts face challenges.For example, apathy-a clinical state of reduced goal-directed behavior that typically involves inflated effort perception (61)-is associated with action perseveration (62,63), suggesting that persistence can sometimes be less effortful than changing or desisting.Similarly, apathy and impulsivity (which involves excessive effortful action switching) are correlated in healthy individuals and in clinical populations (64)(65)(66).Finally, the psychology literature on flow suggests that even seemingly effortful and potentially costly actions can, under certain circumstances, be experienced as effortless (67,68).Together with the effort paradox phenomenon, these instances not only support our contention that we must separate cognitive work from effort but also suggest that the experience of effort is not always associated with resource conservation.
Instead, the work reviewed above suggests that the experience of effort can be seen as a signal that regulates behavior to maximize its anticipated utility (39,48,52,69).This leads to the prediction that when the anticipated utility of a mental action drops or when the anticipated utility of an alternative action rises, then initiating that action or persisting with it, if it is already underway, will feel effortful.However, stopping such an action will feel effortless.Therefore, effort can facilitate task switching, task persistence, or task cessation depending on anticipated utility, which in turn varies between and within people depending on contextual salience.This has important implications for neuropsychiatric conditions (see Mental Effort Disturbance in Neuropsychiatric Conditions), as well as for understanding day-to-day behavior.For example, an individual who engages in drug-seeking behavior may struggle to carry out the basics of daily life but may expend a great deal of mental and physical energy in gaining access to substances.
In summary, recent research suggests that the experience of effort has evolved as an (usually aversive) experience ( 70) that controls behavior by facilitating switches, persistence, or inhibition according to the currently anticipated utility.In the next section, we consider how the brain may signal the experience of mental effort.

HOW DOES THE BRAIN SIGNAL MENTAL EFFORT?
Unlike physical effort, where the experience could be signaled by both peripheral and central brain processes (11), it seems likely that mental effort is driven by signals from the central nervous system, perhaps through direct readout of cognitive work (5), anticipation of this work (71), or consequences of this work such as downstream changes in resources (72) and byproducts (43,45).The former would imply that cognitive work itself signals the subjective experience of effort.For physical effort, it would mean that muscle contraction would signal effort [sometimes termed the peripheralist account of effort (11)].By contrast, a centralist account would call for signals linked to (or even caused by) cognitive work that the brain interprets as effort.
These direct and indirect accounts invite a theoretical discussion about whether it is truly the case that the experience of effort is caused by cognitive work.This representationalist or effort first (11,71,73) assumption is intuitively compelling, drawing on the notion that our subjective experience represents-directly or indirectly-the mental work being done, perhaps by monitoring the activity itself or by receiving some signal related to resource depletion or by-product accumulation.However, there is also a feeling first perspective that does not view effort as representing an activity or its downstream effects (11,71,73).Instead, effort is defined purely in subjective terms, and activities are deemed effortful insofar as they are accompanied by these subjective experiences.These accounts may not be mutually exclusive because even if the representations involved do not represent the actual activity or its downstream physiological changes, they may represent the anticipation of these phenomena.What may these physiological signals be?At the molecular level, the basic signals for effort and their routes to awareness are not well characterized, but several candidate biological processes have been proposed to link mental work with mental effort.The thing that is common to these proposed signals is that they are limited in some way: limited because they depend on active metabolism, as in the case of blood glucose (74,75) or astrocyte glycogen (76), or because they require active clearance of toxic by-products, as in the case of beta amyloid (45).There is evidence for and against the idea that something is being depleted (60,72,77,78).
Instead of actual depletion of resources, it is possible that effort, like most biological processes whose goal is to maintain energy balance and homeostasis, is more useful as an anticipatory rather than a reactive signal.That is, rather than arising because of cost or depletion, it is more valuable and efficient to signal that the ongoing activity will likely have a future impact.Predictive, rather than error-based, regulation is more effective, and it follows that if effort is a signal of cost as discussed above, it functions more effectively if it arises from the cause of that cost rather than the cost itself (79).Signals that are important to the anticipation and expectation of signal change, such as the neuromodulator dopamine, alter perceived effort (80)(81)(82)(83) [but also see Walton and Bouret (84)].
Across the brain, a few key areas have been shown to play a central role in signaling effort, and among them is the anterior cingulate cortex (ACC).It has been suggested that the ACC signals parameters that are key to the expected value of control (5,6,52), such as tracking opportunity costs (80), and is a hub region for the decision of whether and how much cognitive work to exert (52,85).The ACC is also thought to modulate signals related to action costs from the striatum as a function of their contextual salience (e.g., motivational factors and expected reward), thereby influencing the perception of effort (86).Therefore, the learned action-cost associations may originate in the striatum, but contextual salience and feedbackrelated prediction error signals that originate in higher regions, such as the ACC and other medial prefrontal cortex regions (87,88), can modulate the learned value to facilitate behavioral changes that maximize utility.The mechanism by which the ACC achieves this could be through noradrenergic transmission via its interactions with the locus coeruleus, which modulates the expected costs of action (89).
Overall, recent research has cast doubt on resource depletion theories and suggests a greater role for the anticipation of cognitive activity in the perception of effort, which may be mediated by corticostriatal loops centered on the ACC (28,71,88,90,91).The link between effort signaling at the molecular/cellular level and brain regions associated with the experience of effort remains to be investigated.In the final section, we relate the accounts of mental effort reviewed here to the potential mechanisms of cognitive impairments observed in key neuropsychiatric conditions.

MENTAL EFFORT DISTURBANCE IN NEUROPSYCHIATRIC CONDITIONS
We suggest that a more precise and comprehensive description of clinical disturbances requires making a clear distinction between the subjective experience and the cognitive work.This encourages us to reframe impaired cognitive performance in terms of dissociable deficits.For example, suboptimal task performance may arise from an impairment in cognitive processing that sets a fundamental limit on potential performance.This may be accompanied by an increase in the experience of effort, as seen in stroke survivors, among whom mental effort has been linked to increased fatigue due to changes in the cost of cognitive processing (mental work) (15,92,93).Importantly, however, there are instances in which impairment in cognitive processing may not be accompanied by an increase in the experience of effort.Similarly, cognitive performance may or may not be impaired in the context of a disturbed sense of mental effort.This distinction calls for a closer examination of the phenomenology of patient experience while measuring cognitive performance.
Our clinical perspective on mental effort emphasizes that abnormal cognitive performance across a wide range of neuropsychiatric conditions is due to an abnormal experience of mental effort or to its anticipation.Together with the recent insights from the neuroscience literature reviewed above, this perspective can be used to frame cognitive impairments in psychiatry in terms of distinct alterations in the experience of mental effort.Many of these insights have come from using decision-making paradigms, which examine people's choice as a function of cognitive work for rewards (i.e., an action's anticipated utility).We illustrate this perspective by considering several common mental health disorders associated with cognitive impairments.

Depression
Depression is characterized by loss of interest or pleasure in activities, which leads to difficulties in concentration and reduced cognitive performance (94).Studies using effortreward decision-making paradigms have revealed reduced sensitivity to rewards in depression (13,95,96), while sensitivity to the work required may be preserved (97).A generally degraded reward landscape may attenuate anticipated utility and promote a sense of greater effort, with potential avoidant disengagement from the task due to a greater opportunity cost of inactivity (98).This leads to the prediction that cognitive deficits in depression are caused by inflated mental effort, due to the lower subjective value of reward (Figure 1, mechanism 3).Treatments for depression, such as behavioral activation, may reshape the reward landscape, enabling relearning of action-outcome values (99).This process may take time, which would explain the typical lag seen in antidepressant treatments.

Generalized Anxiety Disorder
Similar to depression, this condition is characterized by difficulties in concentration and high susceptibility to fatiguability (94).The processing efficiency theory suggests that anxiety impairs cognitive performance because worrisome thoughts interfere with attention to task-relevant information, thus reducing the cognitive resources available for task-processing activities.As a consequence, performance is impaired.(100).Viewed in terms of our perspective, increased effort perception in anxiety (101) and poor cognitive performance are due to the consistently high value of worrying thoughts and their What Is Mental Effort: A Clinical Perspective Biological Psychiatry June 1, 2024; 95:1030-1037 www.sobp.org/journal1033 associated mental actions.That is, the subjective value of successful completion of everyday life tasks may be reduced relative to the array of intrusive thoughts or worries.The value of these worries, and hence their high opportunity costs while engaged in other cognitive tasks, will make patients experience everyday tasks as effortful.This predicts that cognitive performance in tasks related to the worries will be perceived as less effortful and will be associated with better performance than other unrelated cognitive tasks (Figure 1, mechanism 4).

Attention-Deficit/Hyperactivity Disorder
Cognitive impairments typically arise from difficulties in sustaining attention (94), which can be alleviated by medications, such as stimulants.Examining attention-deficit/hyperactivity disorder as a state of motivational deficits (102) and an enhanced sense of mental effort (103)(104)(105) leads to the prediction that these symptoms of inattention and hyperactivity result from a general disturbance in the calculation of opportunity costs (Figure 1, mechanism 4) (106).Specifically, alternative actions would become salient distractors that result from erroneously inflated opportunity cost, possibly mediated by disturbed dopamine transmission (80,107).Phenomenologically, this would lead to an enhanced experience of effort when persisting with a task and a constant urge to switch tasks (106).

Schizophrenia
Apathy, a major negative symptom in schizophrenia, has been explained by a reduced tendency to exert work for rewards due to higher costs of cognitive work rather than a blunted value of reward (12,(108)(109)(110).Patients also experience cognitive deficits, which are highly related to, but separable from, negative symptoms (111).Both negative and cognitive symptoms are strong predictors of long-term clinical outcomes in patients (112), showing the importance of understanding these symptoms and their relationships.Neuroscience research suggests that cognitive deficits are at least partially accounted for by motivational deficits (113) and the increased cost of cognitive work (114).Moreover, the relationship between negative and cognitive symptoms is moderated by defeatist beliefs-that is, one's own judgment of their ability to complete a goal-directed task (115).This predicts that reduced willingness to exert mental work, whether because of enhanced cost of mental processing (Figure 1, mechanism 2) (12) or a defeatist belief and an exaggerated anticipation of failure (Figure 1, mechanism 3) (36,115), contributes significantly to cognitive deficits in schizophrenia.Importantly, however, cognitive deficits in schizophrenia can also be explained by impairments in the cognitive apparatus itself (Figure 1, mechanism 1); neuronal loss, either due to neurodevelopmental or neurodegenerative causes (116), has a major role in the reduced cognitive performance.Thus, the decline in cognitive performance in schizophrenia can result from both changes in mental effort and impairment in cognitive processing.

IMPLICATIONS AND FUTURE DIRECTIONS
We have argued for making a clear distinction between cognitive work and the subjective experience that accompanies it in mental effort.We have used the terms work and effort to make this distinction (see Table 1), but we recognize that it may be challenging to adhere to these terms given the colloquial use of effort.Although we think work and effort are clearer, other terms, such as effort and perception of effort can be used (117) as long as these are clearly defined and used.The distinction between work and the subjective experience of effort has implications for future research.Our perspective calls for better operationalization of cognitive work and effort when assessing cognitive performance.Toward this end, a closer scrutiny of the subjective experience of mental effort is required in the characterization of cognitive deficits.A better understanding of the patient experience when performing cognitive tasks will shed light on their motivation, engagement, self-appraisal, and defeatists beliefs to separate their contributions to cognitive impairments.
Attempts have been made to titrate task difficulty levels, so as to match performance across individuals, when examining effort-reward decision-making behavior (9) or when comparing brain responses between patients and control participants (118).Although potentially useful, such attempts may be insufficient unless we find some way to objectively record cognitive work rather than relying on performance as a surrogate for ability.For example, when measuring physical effort, experimental stimuli are typically standardized according to a person's maximal voluntary contraction (18).By contrast, there is no reliable measure of a person's maximal cognitive performance on a task.To address this, we suggest that repeated testing over time (119) can provide some clue to a person's maximal performance and its contributors, but this is currently only investigated in the context of test-retest reliability (120).Relatedly, examining performance evolution within a task can help dissociate cognitive work, effort, and performance.For example, learning classically leads to higher performance associated with less effort for a given amount of work.
To separate cognitive work, performance, and effort, we suggest combining patient phenomenology with more objective neurophysiological measures of cognitive work, such as neuroimaging or pupillometry (121), over time.Such a research framework can help tease apart the specific causes of changes in cognitive performance (see Studying Work, Performance, and Effort in the Supplement).

CONCLUSIONS
In this article, we have presented a clinically informed framework for mental effort.We reviewed recent neuroscience research, looking at what cognitive work may be, what purpose mental effort serves, and how it is signaled in the brain.This research emphasizes that mental effort may act to control behavior by guiding task persistence, inhibition, and switches so as to maximize anticipated utility.
Our clinical framework makes an explicit distinction between two main types of cognitive impairments: one type caused by neurodevelopmental, neurodegenerative, or other acquired brain injuries that compromise the cognitive apparatus carrying out the cognitive work and another type caused by enhanced experience of mental effort, which influences the ability of a person to exert and persist in mental work to successfully complete the task at hand.This distinction is important both for informing treatment and elucidating candidate mechanisms for the effect of mental effort on cognitive performance in key mental health disorders.The lack of agreement about and understanding of what mental effort is has negative and far-reaching consequences in clinical populations.We hope this perspective will help to frame basic and clinical research in a way that will increase our understanding of cognitive deficits in patients with neuropsychiatric illness.

Figure 1 .
Figure 1.A clinically informed framework for NW was supported by an Israel Science Foundation Personal Research (Grant No. 1603/22) and a National Institute of Health and Care Research (NIHR) Academic Clinical Fellowship (No. ACF-2019-14-013).PCF was supported by the Bernard Wolfe Health Neuroscience Fund and a Wellcome Trust Investigator Award (No. 206368/Z/17/Z).All research at the Department of Psychiatry in the University of Cambridge is supported by the NIHR Cambridge Biomedical Research Centre (Grant No. NIHR203312) and the NIHR Applied Research Collaboration East of England.The views expressed are those of the author(s) and not necessarily those of the NIHR or the Department of Health and Social Care.

Table 1 .
Definitions of Main Terms Used for Mental Effort Mental/Cognitive WorkThe action performed by the cognitive apparatus to achieve a goal.The contraction of the cognitive muscle.Mental/Cognitive Effort The subjective experience that accompanies cognitive work.Mental/Cognitive Cost An ill-defined term, derived from neuroeconomics, meaning some kind of currency that is paid to perform cognitive work.This currency is sometimes considered as energy expenditure or glucose use but currently remains unknown.Mental/Cognitive Performance The outcome of the cognitive work or action.This is often considered in terms of success/failure, accuracy, and reaction time in cognitive tasks.It is reduced in many neuropsychiatric conditions.What Is Mental Effort: A Clinical Perspective Biological Psychiatry June 1, 2024; 95:1030-1037 www.sobp.org/journal1031 What Is Mental Effort: A Clinical Perspective 1034Biological Psychiatry June 1, 2024; 95:1030-1037 www.sobp.org/journal