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Clinical Psychology Review

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Review

New directions in behavioral activation: Using findings from basic scienceand translational neuroscience to inform the exploration of potentialmechanisms of changeCourtney N. ForbesDepartment of Psychology, University of Toledo, Mail Stop 948, 2801 West Bancroft Street, Toledo, OH 43606, USA

H I G H L I G H T S

• Understanding mechanisms of change can facilitate improvements in BA treatments.

• BA treatments may work by targeting (low) reward responsiveness directly.

• Basic science findings can inform hypotheses about potential mechanisms of change.

A R T I C L E I N F O

Keywords:Behavioral activationReward responsivenessDepressionMechanisms of changeTranslational neuroscience

A B S T R A C T

Interest in behavioral activation treatments for depression has increased over the past two decades. Behavioralactivation treatments have been shown to be effective in treating depression across a variety of populations andsettings. However, little is known about the mechanisms of change that may bring about symptom improvementin behavioral activation treatments. Recent developments in the theoretical and empirical literature on beha-vioral activation treatments have coincided with advances in basic science and translational neuroscience re-garding the mechanisms underlying individual differences in responsiveness to reward. Attenuated reward re-sponsiveness has been associated with depression and related clinical outcomes at the self-report, behavioral,and neural levels of analysis. Given that behavioral activation treatments are focused on increasing individuals'contact and engagement with sustainable sources of reward in their environment, it is plausible that behavioralactivation treatments bring about improvements in depression symptoms by targeting (low) reward respon-siveness directly. This paper integrates findings from the clinical research literature on behavioral activationtreatments with insights drawn from basic science and translational neuroscience in order to propose hypothesesabout potential mechanisms of change in behavioral activation. Conceptual issues and recommendations forfuture research on behavioral activation treatments are discussed.

1. Introduction

The past two decades have seen a resurgence of interest in treat-ments that are rooted in behavioral and contextual theories of psy-chopathology. Behavioral and contextual theories emphasize the in-fluence of idiographic contextual factors, such as the environmentalantecedents and consequences of behavior, on the development andmaintenance of psychological disorders (Ferster & Skinner, 1957;Jacobson, 1994; Skinner, 1953). In particular, there has been a renewedinterest in behavioral activation treatments for depression, which focusprimarily on increasing individuals' engagement with “rewarding” ac-tivities; that is, sustainable sources of positive reinforcement in theirenvironment. In the late 1990s, a component analysis study (Jacobson

et al., 1996) of cognitive-behavioral therapy for depression (CBT; Beck,Rush, Shaw, & Emery, 1979) demonstrated that the behavioral activa-tion (BA) component of CBT was as efficacious as the full CBT packagein improving depression symptoms and preventing depression relapsefor up to two years following the initial treatment (Gortner, Gollan,Dobson, & Jacobson, 1998). Results of the component analysis studyinspired the development of several novel BA treatments (e.g., Lejuez,Hopko, Acierno, Daughters, & Pagoto, 2011; Lejuez, Hopko, & Hopko,2001; Martell, Addis, & Jacobson, 2001) based on this parsimoniousapproach. These BA treatments have been shown to be effective fortreating depression across a variety of populations and settings.

The renewed interest in the development and dissemination of BAtreatments over the past several decades has occurred concurrently

https://doi.org/10.1016/j.cpr.2020.101860Received 25 December 2019; Received in revised form 3 March 2020; Accepted 1 May 2020

E-mail address: courtney.forbes@rockets.utoledo.edu.

Clinical Psychology Review 79 (2020) 101860

Available online 05 May 20200272-7358/ © 2020 Elsevier Ltd. All rights reserved.

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with advances in basic science and translational neuroscience researchon biobehavioral mechanisms underlying psychological disorders. Onemechanism of particular relevance to depression and other disorderscharacterized by attenuated motivation and positive affect is respon-siveness to rewarding stimuli, which is thought to be driven by dopa-minergic activity in the brain's mesolimbic pathway (Nestler &Carlezon, 2006). The neural reward system is thought to govern goal-directed behavior related to seeking out primary rewards, such as foodand sex; as well as secondary rewards, such as monetary gain and po-sitive social feedback (Arias-Carrión, Stamelou, Murillo-Rodríguez,Menéndez-González, & Pöppel, 2010). Thus, the reward system isthought to play a critical role in human motivation as well as hedonicenjoyment of rewards (Berridge & Robinson, 2003). Abnormalities inthe functioning of this system, which governs behavioral responsivenessto positive stimuli (i.e., reward functioning), have been associated withsymptoms of major depressive disorder (MDD) including reductions inmotivation, activity level, and the ability to experience pleasure(Treadway & Zald, 2011).

The activity of the reward system may be particularly relevant to BAtreatments for depression, given that the primary emphasis of BAtreatments is on increasing patients' activity level and engagement withrewarding activities. Robust associations have been observed betweendepression and attenuated reward functioning (Pizzagalli et al., 2009;Smoski et al., 2009; Treadway, Bossaller, Shelton, & Zald, 2012). Byincreasing patients' contact with rewarding stimuli, BA treatments maybring about improvements in depression symptoms by targeting (low)reward functioning directly. However, research on mechanisms ofchange in BA treatments has been limited. Despite their conceptualoverlap, the literature on the efficacy of BA treatments has, for the mostpart, developed independently of basic science and translational neu-roscience research on biobehavioral processes involved in rewardfunctioning.

The purpose of this paper is to integrate the clinical literature on BAtreatments with research on biobehavioral processes involved in rewardfunctioning in order to use insights from basic science and translationalneuroscience research to inform hypotheses about potential mechan-isms of change in BA treatments. The development of novel hypothesesinformed by basic science may pave the way for future research focusedon identification of the mechanisms in BA treatments that drivesymptom change. Increased knowledge of these mechanisms of changemay, in turn, facilitate the development and refinement of future BAtreatments.

The need for greater understanding of mechanisms of change inreward-focused treatments is highlighted by recent findings thatwidely-used treatments for depression, including CBT and anti-depressant medications, may have limited efficacy in treating low po-sitive affect and anhedonia. In a recent analysis of two randomizedcontrolled trials of CBT, antidepressant medications, and a combinationof both treatments (Dunn et al., 2019), there was less improvement inpositive (vs. negative) affect from pre- to post-treatment across trialsand treatment conditions. Moreover, positive affect was more likely toremain below general population levels following either course oftreatment. These findings suggest that new treatment approaches areneeded to target low positive affect and anhedonia. Elucidating themechanisms driving symptom change in reward-focused treatmentsmay shed light on new strategies for addressing these difficult-to-treatsymptoms (McMakin et al., 2012; Uher et al., 2012).

Nagy et al. (2020) presented a review of the theoretical bases of BAtreatments, as well as evidence supporting the relevance of rewardfunctioning to clinical outcomes in BA. Based on this review and withthe intention of translating research on reward functioning into clinicalpractice, Nagy et al. (2020) proposed a novel, transdiagnostic treatmentfor anhedonia (Behavioral Activation Treatment for Anhedonia; BATA).The present review extends Nagy et al.'s (2020) work by highlightingconcrete pathways through which BA treatment strategies – includingthose proposed in BATA – may bring about symptom improvement.

Furthermore, this review builds upon Nagy et al.'s (2020) work byconceptualizing relations between basic science findings on rewardfunctioning and BA treatment strategies within the framework of spe-cific Positive Valence Systems (PVS) sub-domains.

2. Early models of reward functioning in depression: Gray,Ferster, and Lewinsohn

Individual differences in reward-seeking behavior and responsive-ness to reward have long been recognized. The ReinforcementSensitivity Theory of Personality (RST; Gray, 1972, 1990) proposes thatbehavior is influenced by two primary motivational systems: the be-havioral activation system (BAS) and the behavioral inhibition system(BIS). The BAS is thought to govern reward sensitivity (i.e., interest inand responsiveness to reward) and approach motivation. Atypically lowfunctioning of the BAS and other personality constructs related to po-sitive emotionality (e.g., positive affect, extraversion) has been asso-ciated with depression symptom severity cross-sectionally and long-itudinally across a variety of populations, and has been shown topredict the maintenance of depression symptoms over time (Kasch,Rottenberg, Arnow, & Gotlib, 2002; McFarland, Shankman, Tenke,Bruder, & Klein, 2006; see Khazanov & Ruscio, 2016, for a meta-ana-lysis). In addition to highlighting associations between reward func-tioning and depression, Gray's work has influenced the development ofcontemporary conceptualizations of approach motivation in relation tothe PVS domain (Olino, McMakin, & Forbes, 2018), and is reflected inPVS sub-domains including Reward Anticipation, Initial Responsive-ness, and Effort Valuation.

Theoretical understanding of the role of approach motivation indepression was advanced by a seminal behavioral theory proposed byCharles Ferster (1973, 1974), a colleague of B.F. Skinner's who appliedthe principles of radical behaviorism (Skinner, 1953, 1957) to a func-tional analysis of behaviors associated with depression. Specifically,Ferster proposed that behaviors associated with depression are drivenby both positive reinforcement, or the presentation of a positive sti-mulus following a behavior; and negative reinforcement, or the removalof an aversive stimulus following a behavior. Ferster observed that in-dividuals with depression often exhibit a reduction in overall activitylevel, which may be influenced by the absence of positive reinforce-ment for adaptive behaviors in work, social relationships, and other lifedomains. Ferster also suggested that depression is associated with anincrease in the frequency of avoidance and escape behaviors, whichfunction to decrease contact with aversive stimuli such as distressingsituations and/or undesired emotions. Furthermore, Ferster posited thatan increase in the frequency of avoidance behaviors may interfere withan individual's ability to access sources of positive reinforcement intheir environment. For example, if an individual chooses not to attend aconcert with friends because they anticipate that the event will be loud,tiring, and uncomfortable, they might lose the opportunity to experi-ence positive consequences, such as enjoyment of time with friends.Similarly, if an individual chooses not to take on a challenging projectat work out of a concern that it will require too much effort, they mightmiss out on the opportunity to experience a sense of mastery uponcompleting the project.

Consistent with Hernstein's (1961, 1970) Matching Law, whichposits that the frequency of a particular behavior is directly propor-tional to the relative value of reinforcement obtained for that behavior,Ferster suggested that the frequency of “depressed” behaviors (e.g.,staying in bed, turning down social invitations) and “non-depressed”behaviors (e.g., participating in hobbies, engaging with work, seekingout social activities) are directly proportional to the relative value ofreinforcement obtained for each type of behavior. The relative value ofreinforcement (i.e., its accessibility, duration, and immediacy) for a“depressed” behavior, such as staying in bed, may be increased when itis easily accessible and results in an immediate reduction in distress.With a higher relative value of reinforcement, the frequency of the

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behavior of staying in bed is likely to increase. Likewise, the relativevalue of reinforcement for a “non-depressed” behavior, such as in-itiating conversations with a romantic partner, may decrease if there isa lack of positive reinforcement for the behavior (e.g., if the partnerdoes not engage in conversation once it is initiated). With a lower re-lative value of reinforcement, the behavior is likely to decrease in fre-quency. With this in mind, Ferster and others (e.g., McDowell, 1982)suggested that depression persists when there is a low relative value ofreinforcement for “non-depressed” behaviors and a relatively highvalue of reinforcement for “depressed” behaviors.

Around the same time, Peter Lewinsohn and colleagues (e.g.,Lewinsohn & Graf, 1973) proposed a behavioral explanation of theonset and maintenance of depression. Their theory suggested that de-pression results from a low rate of response-contingent positive re-inforcement (RCPR) for “non-depressed” behaviors related to work,relationships, and other life domains. Lewinsohn and colleagues hy-pothesized that the onset of depression may result from chronically lowlevels of RCPR; or may follow an event that deprives an individual of animportant source of reinforcement, such as the death of a spouse, loss ofa job, or loss of physical ability due to an illness or injury. For example,an individual who has recently lost a spouse may experience a decreasein RCPR for behaviors such as cooking dinner and doing householdchores, because the positive consequence of spending time with theirspouse associated with these activities has been removed. Lewinsohnand colleagues suggested that the frequency of “non-depressed” beha-viors would decrease as a function of reduced or nonexistant positivereinforcement for these behaviors. The severity of depression wasthought to be proportional to (low) levels of RCPR. By extension, in-creasing RCPR was thought to bring about a reduction in depressionsymptoms.

Based on this model, Lewinsohn and colleagues developed thePleasant Events Schedule (PES; MacPhillamy & Lewinsohn, 1971), a listof 320 activities that were thought to be positively reinforcing, whichbecame the basis of a novel treatment for depression (Lewinsohn &Graf, 1973). At the outset of this treatment, patients were instructed torate the pleasantness of each PES item on a 3-point scale, and then toidentify the 160 activities rated as most pleasant. Patients were sub-sequently instructed to engage in activities they identified as pleasur-able, and to maintain a record of their activities and mood each day.The utility of the PES received empirical support; for example, in astudy of 90 undergraduates including currently depressed participants,psychiatric controls, and nonpsychiatric controls, less engagement inpleasant activites was significantly associated with low mood across allthree groups over a 30-day period; and depressed participants werefound to engage in significantly fewer pleasant activities overall com-pared to psychiatric and nonpsychiatric controls (Lewinsohn & Graf,1973).

Several key concepts from Ferster and Lewinsohn's work laid thefoundations for the development of novel BA treatments several dec-ades later. Both Ferster and Lewinsohn conceptualized depression asdriven, in part, by low levels of RCPR. Both scholars suggested that alow rate of RCPR for “non-depressed” behaviors is likely to decrease thefrequency of those behaviors; and a high rate of negative reinforcementfor “depressed” behaviors (e.g., avoidance and escape behaviors) wouldlikely increase the frequency of these behaviors. Furthermore, increasesin avoidance and escape behaviors were thought to narrow individuals'opportunities to engage with environmental sources of RCPR. Thecombination of decreases in “non-depressed” behaviors and increases in“depressed” behaviors were thought to reduce contact with sources ofRCPR; and this in turn was thought to bring about symptom worsening.

3. Current empirically supported BA treatments

Over time, the pleasant event scheduling intervention (Lewinsohn &Graf, 1973) shifted away from a purely behavioral approach as strate-gies such as cognitive restructuring, relaxation training, assertiveness

training, and problem-solving skills were incorporated into the treat-ment (Brown & Lewinsohn, 1984; Lewinsohn, Sullivan, & Grosscup,1980). Approaches drawn from Lewinsohn and colleagues' behavioraltreatment were also incorporated into cognitive-behavioral treatments,such as CBT for depression (Beck et al., 1979), which gained popularityin the 1970s and 1980s. In the late 1990s, however, results of a com-ponent analysis study of CBT for depression (Jacobson et al., 1996)demonstrated that the more parsimonious BA component of CBT was asefficacious in improving depression symptoms as the full CBT package,which also includes the identification of maladaptive core beliefs,challenging of automatic thoughts, cognitive restructuring, and trainingin social skills, problem-solving skills, and relaxation techniques.

The finding that BA alone could improve symptoms as much asmore complex and time-intensive CBT treatments led to a renewedinterest in BA as a stand-alone intervention. Two BA treatments de-veloped in the early 2000's have received extensive empirical support:Behavioral Activation (Martell et al., 2001) and Brief Behavioral Acti-vation Treatment for Depression (BATD; Lejuez et al., 2001, 2011).Given that the term “Behavioral Activation” may refer to the broadcategory of behavioral activation treatments as well as Martell et al.'s(2001) treatment, the acronym “BA” will be used to refer to the broadcategory of BA treatments, and the term “Behavioral Activation” will beused to refer to Martell et al.'s (2001) treatment in particular.

3.1. Behavioral activation

A primary goal of Behavioral Activation (Martell et al., 2001) is toincrease patients' engagement in rewarding activities, with a particularfocus on disrupting patterns of avoidance and escape behavior thatlimit opportunities to engage with environmental sources of positivereinforcement. As in Ferster (1973, 1974) and Lewhinsohn's(Lewinsohn & Graf, 1973) models, “depressed” behaviors are thought tofunction as avoidance and escape strategies. Therefore, there is anemphasis on increasing patients' awareness of the consequences ofavoidance-oriented coping behaviors. The acronym TRAP (Trigger,Response, Avoidance Pattern) is used to illustrate a pattern of avoid-ance behavior that may contribute to the maintenance of depression.For example, a person might receive negative feedback on a project atwork (Trigger), which precipitates feelings of frustration and shame(Response). Consequently, they might choose to call in sick to work andstay in bed the next day (Avoidance Pattern). Although the behavior ofcalling in sick to work and staying in bed is likely to alleviate distress inthe short-term, it may also lead to work-related problems. Moreover,this behavior limits opportunities for positive reinforcement, such asaccomplishing a different task at work.

Once ineffective coping strategies are identified, the next step is todisrupt patterns of avoidance behavior through the use of alternativecoping behaviors. The acronym TRAC (Trigger, Response, AlternativeCoping) illustrates this process. Extending the example above, the in-dividual who received negative feedback on a project (Trigger) andexperienced a negative emotional response (Response) might reflect onthe negative feedback and make a plan for improving their performanceon future projects (Alternative Coping). The use of alternative copingstrategies is thought to interrupt the feedback loop of avoidance be-havior and worsening depression by identifying opportunities to ad-dress and modify environmental precipitants of avoidance. A reductionin avoidance coping, in turn, is thought to bring about increased op-portunities for engagement with environmental sources of reward.

Behavioral Activation prioritizes the identification of activities thatpatients find enjoyable. The therapist and patient work together togenerate a list of positively reinforcing activities, and then prioritize theactivities and break them down into manageable components. Thetherapist and patient then develop concrete, specific plans for engagingin those activities. Patients are instructed to engage in activities duringthe week and monitor any mood changes that occur as a result. Through“graded task assignment,” patients gradually increase their level of

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activity over the course of treatment. Over time, reductions in avoid-ance behavior and increases in approach-oriented coping strategies arethought to facilitate engagement with sources of reward and sub-sequent improvement in depression symptoms (Martell et al., 2001).

3.2. Brief behavioral activation treatment for depression

Similar to Behavioral Activation, Brief Behavioral ActivationTreatment for Depression (BATD; Lejuez et al., 2001, 2011) emphasizesincreasing patients' engagement with sources of RCPR in their en-vironment. The theoretical foundations of BATD are informed byHernstein's (1961, 1970) Matching Law and Ferster's (1973) behavioraltheory of depression, which suggest that the frequency of “depressed”and “non-depressed” behaviors exhibited by an individual will be di-rectly proportional to the relative value of reinforcement obtained forthose behaviors. Thus, the treatment employs strategies for increasingthe relative value of reinforcement for “non-depressed” behaviors, withthe intention of increasing the frequency of these behaviors; and de-creasing relative value of reinforcement for “depressed” behaviors, withthe goal of decreasing the frequency of these behaviors (Lejuez et al.,2001, 2011). Given that BATD is focused almost exclusively on mod-ifying behavior, the treatment is straightforward and transportable,which has facilitated its application across different populations andsettings (Lejuez et al., 2011).

BATD is typically administered over 8–15 sessions in a highlystructured format, with specific activities assigned to be completedduring and in-between weekly individual therapy sessions. At the outsetof treatment, patients use self-monitoring strategies to establish abaseline activity level. The therapist and patient then work together toidentify behavioral goals related to the patient's personal values in lifedomains including relationships, education, employment, hobbies, andphysical health. Longer-terms goals are broken down into specific ac-tion steps, and behaviors are then ranked according to their perceiveddifficulty in order to create a hierarchy from least to most challengingbehaviors. During individual therapy sessions, concrete plans are de-veloped for activities to be completed during the week. Patients usemonitoring forms to track their activities throughout the week.Monitoring forms are reviewed in subsequent sessions, and barriers thatmay have interfered with the completion of assigned activities arediscussed. Next, activities are planned for the following week, and thetherapist and patient problem-solve potential barriers to engagement.The amount and/or difficulty of assigned activities gradually increaseseach week, with the ultimate goal of building sustainable patterns ofengagement with rewarding activities that will persist after the end oftreatment (Lejuez et al., 2001, 2011). A key assumption of BATD is thatnew skills are developed and practiced most effectivly in the patient'senvironment while they are engaging in rewarding activities. There-fore, rather than teaching coping skills prior to increasing a patient'sactivity level, skill and performance deficits related to specific beha-viors are problem-solved during weekly sessions (Hopko, Lejuez,Ruggiero, & Eifert, 2003).

4. Effectiveness of BA treatments across varied populations andsettings

There is abundant support for the efficacy of BA treatments (Lejuezet al., 2001, 2011; see meta-analyses by Cuijpers, Van Straten, &Warmerdam, 2007; Ekers et al., 2014; and Mazzuchelli, Kane, & Rees,2009). BA treatments have been found to improve depression symptoms ina variety of populations, including veterans (Wagner, Jakupcak, Kowalski,Bittinger, & Golshan, 2019), cancer patients (Hopko, Magidson, & Lejuez,2011; Hopko, Robertson, & Carvalho, 2009), Latinx (Kanter et al., 2015)and African-American (Jacob, Keeley, Ritschel, & Craighead, 2013) po-pulations, children and adolescents (see Martin & Oliver, 2019, for a re-view), pregnant women (Dimidjian et al., 2017), psychiatric inpatients(Hopko, Lejuez, Lepage, Hopko, & McNeil, 2003), and drug users in

residential substance abuse treatment (Daughters et al., 2008). BA treat-ments have also been adapted to different treatment modalities, includinggroup therapy (Chu et al., 2016; Hershenberg, Smith, Goodson, & Thase,2018), internet-delivered treatments (see Huguet et al., 2018, for a re-view), one-session interventions (Tull, Berghoff, Bardeen, Schoenleber, &Konkle-Parker, 2018), and delivery by primary care paraprofessionals(Ekers, Dawson, & Bailey, 2013; Ekers, Richards, McMillan, Bland, &Gilbody, 2011). Strategies from BA have also been incorporated into otherempirically supported treatments, such as exposure therapy for PTSD(Acierno et al., 2016).

Notably, Jacobson et al.'s (1996) finding that the BA component ofCBT was comparable to the full CBT package in improving depressionsymptoms has been extended in several recent studies. A recent re-analysis of Jacobson et al.'s (1996) outcome data found that there wereno significant differences in treatment outcomes between the BA-onlygroup and full-CBT group when only individuals with severe depression(Hamilton Rating Scale for Depression score ≥ 20) were included inanalyses (Lorenzo-Luaces & Dobson, 2019). These findings are con-sistent with Jacobson et al.'s (1996) results; however, they failed toreplicate Dimidjian et al.'s (2006) finding that BA is more efficaciousthan CBT for severe depression. By contrast, other studies have de-monstrated that BA alone facilitates greater symptom improvemementthan CBT. For example, a recent 12-week clinical trial of CBT foradolescent females with depression (Webb et al., 2019) found thatpatients' engagement in the BA component of CBT was a significantpredictor of session-by-session symptom changes, while patients' self-rated use of cognitive skills did not significantly predict symptom im-provement. These somewhat conflicting findings highlight the need fora more nuanced understanding of the processes underlying symptomchange in BA treatments.

5. What's next? Mechanisms of change in behavioral activation

Despite robust empirical support for the efficacy of BA treatments,there has been a surprising lack of research on specific mechanisms ofchange that may bring about symptom improvement. Research onmechanisms of change is critical for the development and refinement ofpsychological treatments (Kazdin, 2005), given that identifying thesemechanisms can help researchers and clinicians determine whichcomponents of a treatment are causally related to symptom improve-ment. A small number of studies have examined factors that may drivesymptom change in BA treatments. One participant-level study of fourdepressed adolescents who demonstrated remission following BAtreatment (Gaynor & Harris, 2008) found that increases in activity levelover the course of treatment were followed by improvements in de-pression symptoms. Furthermore, in an open-label trial of BA adaptedfor Spanish-speaking Latinx individuals (N = 10; Collado, Castillo,Maero, Lejuez, & MacPherson, 2014), improvements in depressionsymptoms were found to occur concurrently with increases in activitylevel. In a session-by-session analysis of 21 Latinx individuals receivingthe intervention described above (Santos et al., 2017), increases inactivity level preceded or co-occurred with changes in depression for amajority of participants. Consistent with these preliminary findings,results from a trial of BATD among 23 depressed cancer patients (Ryba,Lejuez, & Hopko, 2014) demonstrated that greater engagement in as-signed activities accounted for substantial improvements in depressionsymptoms over the course of treatment. In the same study, all patientswho compled 100% of assigned activities achieved remission of a de-pressive episode by the end of the 8-week treatment. These studiesprovide preliminary indications that increases in activity level may leadto symptom improvement in BA treatments. However, relations be-tween activity level and symptom change would need to be observed inlarger and more diverse samples in order to further validate thesefindings. Given the small number of studies on mechanisms of change inBA, there is a need for further examination of mechanisms that couldinform the development and refinement of future BA interventions.

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6. Reward functioning

As noted above, a potential mechanism of change in BA that war-rants attention is improvement in reward functioning. Given that theprimary goal of BA treatments is to increase individuals' engagementwith rewarding stimuli in their environment, BA may bring aboutsymptom improvement by increasing individuals' engagement in andresponsiveness to rewarding stimuli. Reward functioning is a multi-faceted construct that captures individuals' tendencies to seek out, an-ticipate, and respond to rewarding stimuli; as well as the willingness toexpend effort to obtain rewards and the ability to modify behavior inresponse to shifting environmental reward contingencies (Gard, Gard,Kring, & John, 2006; Pizzagalli, Jahn, & O'Shea, 2005; Treadway,Buckholtz, Schwartzman, Lambert, & Zald, 2009; Whitton, Treadway, &Pizzagalli, 2015). In recent decades, research on neural and behavioralprocesses related to reward functioning has shed new light on the roleof this mechanism in the etiology and maintenance of depression(Treadway & Zald, 2011). In particular, research on reward functioninghas been advanced by the identification of two relevant neural circuits.The first, a ventral (limbic) neurocircuit that includes the amygdala,insula, ventral striatum, and ventral regions of the anterior cingulatecortex and orbitofrontal cortex, is involved in assigning emotionalsalience to stimuli and generating affective responses. The second, adorsal (cognitive) neurocircuit that includes the hippocampus, dorsalregions of the anterior cingulate cortex, and the parietal cortex, is in-volved in selective attention, planning, and effortful regulation ofemotions (Nusslock & Alloy, 2017; Phillips, Drevets, Rauch, & Lane,2003a; Phillips, Drevets, Rauch, & Lane, 2003b). Attenuated activationin these circuits has been linked to depression-relevant outcomes. Re-cent findings point to attenuated fronto-striatal responding to reward asan endophenotypic marker that signals risk for MDD among youth witha family history of depression (Gotlib et al., 2010; Olino et al., 2014),distinguishes individuals with depression from nondepressed controls(Epstein et al., 2006), and predicts increases in depression symptomsover time (Morgan, Olino, McMakin, Ryan, & Forbes, 2013).

Recognizing the importance of reward functioning as a broad do-main underlying psychological functioning at multiple levels of analysis(e.g., neural, behavioral, self-report), the National Institute of MentalHealth (NIMH) included a PVS domain in its Research Domain Criteriaframework (RDoC; Insel et al., 2010). The PVS domain encompassesseveral sub-domains of reward functioning (NIMH, 2018; PVSProceedings, 2011): 1) Reward anticipation, which reflects anticipationof reward; 2) Initial responsiveness, which reflects in-the-moment re-sponseiveness to reward; 3) Reward satiation, which reflects longer-term responseiveness to reward; 4) Reward learning, which refers to theability to adapt reward-seeking behavior in response to changing con-tingencies; 5) Reward probability, which reflects expectations about theprobability of obtaining a reward; 6) Delay, which reflects the ability tointegrate information about the time interval prior to the delivery of areward into reward-seeking behavior; and 7) Effort valuation, whichreflects willingness to expend effort to obtain a reward.

To illustrate the relevance of each PVS category to reward attain-ment, imagine a situation in which a person could potentially receive apromotion based on their performance on an important project at work.The person might estimate that the likelihood of receiving a promotionis high if the project is successful (Reward Probability). They mightdecide that it is worthwhile to spend extra time working on the projectduring the evenings and over the weekend, based on the assumptionthat doing so will increase the probability of receiving the promotion(Effort Valuation). Moreover, they may be willing to maintain this ad-ditional effort for several months if they perceive the promotion to behighly desirable (Delay). Following successful completion of the pro-ject, the person might look forward in anticipation to receiving thepromotion (Reward Anticipation). When they receive good news aboutthe promotion, they might experience a positive emotional response inthe moment (Initial Responsiveness) that persists over the next several

days (Reward Satiation). Finally, the person might learn from this ex-perience that hard work is likely to lead to a promotion, and might thenapply similar strategies in the pursuit of other work-related rewards inthe future (Reward Learning).

Basic science findings related to several sub-domains of rewardfunctioning will be reviewed below. In particular, Reward Anticipation,Initial Responsiveness, Effort Valuation, and Reward Learning will bediscussed in relation to potential mechanisms of change in BA treat-ments. Recent findings on distinctions among sub-types of rewardfunctioning that may be relevant to BA treatments will also be con-sidered.

7. Potential reward-related mechanisms of change in behavioralactivation

To date, one study has examined changes in neural reward func-tioning over the course of BA treatment (Dichter et al., 2009). Twenty-seven adults with and without MDD completed a behavioral Wheel ofFortune task, which parses the anticipation, selection, and feedbackphases of reward, while undergoing functional magnetic resonanceimaging (fMRI). Participants with MDD subsequently received 8–14weekly individual sessions of BATD. Relative to controls, depressedparticipants who received BATD demonstrated increased functionalresponding in the dorsal striatum during anticipation of reward, theparacingulate gyrus during selection of reward, and the paracingulateand orbitofrontal gyri while receiving feedback about the amount ofreward obtained. Findings from this preliminary study raise the possi-bility that BA treatments may increase the activation of neural circuitsrelevant to reward functioning. However, there is a need for furtherresearch on the influence of BA treatments on this mechanism, as wellas whether increased neural reward functioning leads to subsequentsymptom improvement.

7.1. Anticipatory versus consummatory reward functioning

One line of research has focused on temporal components of rewardfunctioning. Researchers have distinguished between reward anticipa-tion, or the desire for and pleasant anticipation of a reward, and con-sumption, or enjoyment of a reward in-the-moment (Klein, 1984;Knutson, Adams, Fong, & Hommer, 2001). Although this researchpredates the PVS domain, these constructs would align with RewardAnticipation and Initial Responsiveness. The distinction between re-ward anticipation and consumption was informed by preclinical re-search conducted in animals, which showed that “wanting” and “liking”are associated with different brain structures and patterns of neuro-chemical activity (Berridge & Robinson, 2003). “Wanting” is associatedwith dopamine systems involved in assigning salience to rewards, while“liking” is associated with opioid, endocannabinoid, and GABA neuro-transmitter systems involved in pleasurable reactions to reward.

Several studies have used the Temporal Experience of Pleasure Scale, aself-report measure of anticipatory and consummatory components ofreward functioning (Gard et al., 2006), to examine temporal compo-nents of reward functioning. Results have identified attenuation in self-reported reward anticipation among individuals with depression(Chentsova-Dutton & Hanley, 2010) and schizophrenia (Gard, Kring,Gard, Horan, & Green, 2007). There were not significant differences inconsummatory reward functioning, however, between clinical popula-tions and nonclinical controls. Relatedly, associations between antici-pation of reward and personality traits associated with depression (i.e.,low approach motivation and positive emotionality) have been found tobe stronger than associations between reward consumption and thesame measures (Ho, Cooper, Hall, & Smillie, 2015), suggesting that lowreward anticipation may be particularly relevant to depression.

Temporal components of reward functioning have also been ex-amined at the behavioral level of analysis. In one study, currently de-pressed, previously depressed, and never-depressed undergraduates

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completed a puzzle-solving task for monetary reward (McFarland &Klein, 2009). Consistent with findings from self-report studies, de-pressed participants evidenced the lowest levels of reward anticipationduring the puzzle-solving task. In another study, adults with a range ofdepression symptoms completed a task in which they were required toexpend effort to view enjoyable cartoons (Sherdell, Waugh, & Gotlib,2012). Results indicated that participants with low reward anticipationwere significantly less willing to expend effort in order to view thecartoons. There were no differences, however, between depressed andnon-depressed participants in their in-vivo enjoyment of the cartoons.Finally, among nonclinical university students, low reward anticipationwas associated with weaker affective responses to a positive emotioninduction and less willingness to expend effort on a behavioral task(Geaney, Treadway, & Smillie, 2015). These effects were not observedfor reward consumption. These results are consistent with findings fromself-report studies suggesting that reward anticipation may be espe-cially relevant to depression, while reward consumption may be lessdisrupted among depressed individuals.

Several recent neuroimaging studies have contributed to thegrowing base of knowledge regarding associations between depressionand temporal components of reward functioning. In a recent meta-analysis of fMRI studies examining reward processing abnormalities indepression, reduced striatal activation during reward anticipation wasshown to distinguish individuals with depression from non-depressedcontrols (Keren et al., 2018). Moreover, in a large study (N= 1576) ofcommunity adolescents (Stringaris et al., 2015), low ventral striatalactivation during reward anticipation in a laboratory-based monetaryreward task (Knutson et al., 2001) differentiated individuals with sub-threshold and clinical depression from non-depressed controls, pre-dicted the transition to subthreshold or clinical depression at two-yearfollow-up, and demonstrated stronger associations with the symptom ofanhedonia relative to low mood.

In summary, observations from multiple levels of analysis suggestthat the processes underlying reward anticipation and consumption arerelated to depression, and are to some extent separable. This distinctionis relevant to BA treatments, as it may shed light on antecedent barriersand reinforcing contingencies that influence individuals' choices toengage in potentially rewarding activities. In particular, findings from anumber of studies indicate that reward anticipation tends to be moreimpaired than reward consumption among individuals with depression.Therefore, attenuated reward anticipation may serve as an antecedentbarrier to engagement in meaningful and enjoyable activities. Patientswith low reward anticipation may feel as though there is “nothing goodto look forward to,” or that there would be “no point” to an activity. Forexample, a patient who is thinking about getting involved in vo-lunteering for a community organization might experience thoughtssuch as “It probably won't be much fun,” or “I can't imagine I'll getanything positive out of it.” Unfortunately, if the individual chooses notto start volunteering, they might miss out on opportunities to accessother sources of reward that could come about as a result (e.g., positiveinteractions with new people, a sense of purpose).

Given research demonstrating that reward consumption may remainrelatively intact among individuals with depression, it is possible thatthe same individual may indeed enjoy volunteering if they choose to doso, regardless of their low anticipation of positive consequences. Thefocus of BA treatments on increasing approach-oriented behavior, re-gardless of internal barriers to action (e.g., hopelessness, low motiva-tion, low energy), may be particularly useful for patients presentingwith low reward anticipation. Strategies used in BA treatments to in-crease approach-oriented behavior may promote engagement in situa-tions where patients are likely to experience in-vivo enjoyment, eventhough they do not anticipate that those situations will be pleasurableor meaningful. In other words, a BA therapist might take a “just do it”approach and instruct the patient to engage in assigned activities evenif they do not feel motivated. Moreover, monitoring forms used to trackactivities and mood could be used to help the patient make connections

between activities and resulting mood changes (see discussion ofReward Learning, below). Furthermore, strategies to increase mindfulawareness may help patients connect with natural contingencies asso-ciated with engaging in rewarding activities (Jacobson et al., 1996). Inother words, mindfulness may increase the positive emotional con-sequences of engaging in rewarding activities by drawing an in-dividuals' focus toward enjoyable aspects of an activity, and perhapsaway from perseverative thoughts that might otherwise limit engage-ment.

7.2. Willingness to expend effort for rewards

Another key component of reward functioning is effort valuation, orthe willingness to expend effort in order to obtain a reward. Extendingpreclinical research on effort-based decision-making in rodents (Correa,Carlson, Wisniecki, & Salamone, 2002; Salamone, Correa, Farrar, &Mingote, 2007), Treadway et al. (2009) developed the Effort Ex-penditure for Rewards Task (EEfRT) in order to assess effort-baseddecision-making in a laboratory setting. During the task, participantschoose between performing high-effort or low-effort tasks to obtainvarying amounts of monetary reward. However, monetary reward is notprovided for all tasks. Instead, participants are informed during eachtrial of the probability that the monetary reward will be provided fol-lowing task completion. Thus, individuals' choices on the task take intoaccount the influence of the amount of effort required for the task, thevalue of the potential monetary reward, and the probability that thereward will be provided upon completion of the task. In the initialvalidation sample (Treadway et al., 2009), which included adultsscreened to ensure a range of trait anhedonia scores, trait anhedoniawas significantly and negatively associated with the proportion of high-effort tasks selected for medium probability trials. Moreover, depres-sion symptoms were significantly and negatively associated with theproportion of high-effort tasks selected for high probability trials. Inother words, individuals with higher levels of anhedonia and otherdepression symptoms tended to choose fewer high-effort tasks in trialswhere there was a medium or high probability of obtaining a monetaryreward.

Subsequent studies using the EEfRT task have confirmed associa-tions between depression and attenuated willingness to expend effortfor rewards. In a sample of adults with and without MDD (Treadwayet al., 2012), participants with MDD selected a significantly lowerproportion of high-effort tasks compared to participants without MDD.Moreover, in the MDD group, the selection of low-effort tasks wassignificantly associated with the length of the current depressive epi-sode, suggesting that the willingness to expend effort for reward may besensitive to the duration of depression symptoms. Attenuated will-ingness to expend effort for reward has also been observed among in-dividuals with subsyndromal depression (Yang et al., 2014).

Recently, the EEfRT task has been adapted for use in the fMRI en-vironment (Arulpragasam, Cooper, Nuutinen, & Treadway, 2018). In thevalidation study for this task, the ventromedial prefrontal cortex was foundto be active during encoding of the expected subjective value for a trial,which involves the integration of information regarding the amount ofpossible reward and the amount of effort required to obtain it. This studydid not examine associations between depression symptoms and ven-tromedial prefrontal cortex activity during expected subjective value en-coding; however, given previous findings linking depression and atte-nuated willingness to expend effort for reward (Treadway et al., 2009,Treadway et al., 2012; Yang et al., 2014), this task could be used in futurestudies to investigate the influence of depression on expected subjectivevalue encoding. Though results are preliminary, the fMRI-adapted EEfRTparadigm holds promise for identifying neural substrates of effort-baseddecision-making in depressed individuals. Future studies using this taskwould also benefit from the assessment of neural activitvation in responseto reward attainment, in addition to effort valuation and expected sub-jective value encoding.

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Results from these studies suggest that attenuated effort valuation isassociated with depression-relevant outcomes. Given that a primarygoal of BA treatments is increasing patients' engagement in rewardingactivities, a patient's willingness to expend effort in pursuit of rewardsis likely to play a critical role in homework compliance and motivationfor treatment. Specifically, the perception that engaging in a potentiallyrewarding activity is “not worth the effort” would present an ante-cedent behavior to engaging in that behavior. Paradoxically, depres-sion-relevant symptoms such as low motivation, low energy, difficultiesin concentration and decision-making, and hopelessness may be sig-nificant antecedent barriers to engaging in rewarding activities, eventhough these activities would likely result in mood improvement and adecrease in depression symptoms (Lejuez et al., 2001, 2011; Martellet al., 2001).

Strategies drawn from BA treatments may be particularly beneficialfor patients presenting with low effort valuation. As noted above, theexplicit focus of BA on modifying behavior regardless of undesired in-ternal experiences could be used to encourage patients to increase theiractivity level, even while having thoughts that doing so would not beworth the effort. Moreover, the identification of personal values thatone associates with living a meaningful life (i.e., values clarification)can be used to highlight the aspects of meaningful activities that maymake them feel “worth the effort.” Connecting specific behaviors tovalues (Lejuez et al., 2001, 2011) may provide an additional source ofpositive reinforcement for engaging in activities; in other words, a pa-tient may experience a reinforcing sense of satisfaction while takingaction consistent with their values. A focus on values may also assistpatients in making connections between present behaviors and desiredlong-term consequences.

Breaking down behaviors into concrete, manageable steps may alsobe beneficial for patients with low effort valuation, because this wouldlikely reduce the perceived amount of effort associated with a givenactivity. This can be accomplished through strategies such as gradedtask assignment (Martell et al., 2001) and the development of activityheirarchies (Lejuez et al., 2001, 2011), which are used to graduallyshape behavior toward desired goals and establish longer-term patternsof behavior. For example, a patient in BA treatment might set a goal ofbeginning a new exercise routine. Although the patient wants to in-crease their exercise behavior, they may perceive that the potentialbenefits of exercising may not be worth the amount of effort involved ininitiating the new routine. The patient and therapist might work to-gether to identify personal values associated with physical exercise,such as maintaining health and setting a good example for one's chil-dren. This could provide an additional source of reinforcement for ex-ercise behavior (i.e., reinforcement associated with behaving in linewith one's values).

Extending the example above, the patient and therapist couldidentify and schedule maneageable weekly goals related to exercisebehavior that do not require large amounts of effort expenditure.Weekly homework assignments might include purchasing exercise at-tire, researching local gyms, or taking a 10-min walk each day afterwork. Through shaping behavior in small increments, the patient andtherapist could gradually increase these behaviors until a regular ex-ercise routine is established. Furthermore, in the context of a positiveand supportive therapeutic relationship, positive reinforcement pro-vided by the therapist for completing weekly assignments may alsofacilitate continued engagement in those behaviors.

7.3. Alterations in reward learning

Reward learning, or the ability to modulate behavior in response toshifting environmental reward contingencies, may also provide insightinto mechanisms of change in BA treatments. Attenuated rewardlearning is thought to contribute to abnormalities in reward-relateddecision-making and goal-directed behavior among individuals withdepression (Eshel & Roiser, 2010; Pizzagalli et al., 2005). Reward

learning has primarily been assessed at the behavioral level of analysisusing laboratory-based tasks. For example, the Probabilistic RewardTask (PRT; Pizzagalli et al., 2005) provides participants with the op-portunity to obtain monetary reward for responding correctly on asignal-detection task. The signal-detection task requires participants toidentify the correct length (short or long) of a smile that was previouslypresented on a drawing of a face. In order to elicit a response bias,correct identification of either the short or long smile is reinforced withmonetary reward three times more frequently than correct identifica-tion of the other smile. Thus, response bias on the task is oper-ationalized as the extent to which participants develop a systematic biastoward the stimulus that is reinforced more frequently over the courseof the task. Performance on the task is thought to reflect the extent towhich decision-making behavior is influenced by reinforcement history.

In one study using the PRT task (Vrieze et al., 2013) to assess rewardlearning in psychiatric inpatients with MDD and nonclinical controls,depressed participants demonstrated a lesser response bias toward thestimulus that was more frequently reinforced over the course of thetask, compared to control participants. Within the MDD group, in-dividuals with low reward functioning demonstrated significiantly lessof a response bias over the course of the task, compared to depressedindividuals with higher reward functioning. Moreover, individuals whodemonstrated poorer reward learning on the task were more likely tohave a persisting MDD diagnosis after 8 weeks of pharmacologicaltreatment (OR = 7.84). These findings suggest that attenuated rewardlearning is associated with depression and low overall reward func-tioning, and may hold promise as a predictor of response to anti-depressant treatment.

Attenauted reward learning has also been observed among in-dividuals with remitted depression. In two independent studies, adultswith remitted depression and nonclinical controls completed the PRT(Pechtel, Dutra, Goetz, & Pizzagalli, 2013). Relative to nonclinicalcontrols, individuals with remitted depression evidenced less of a re-sponse bias toward more frequently rewarded stimuli over the course ofthe task, suggesting that attenuation in reward learning may persistbeyond the remission of a depressive episode. A history of depressionpredicted attenuated reward learning on the task over and above re-sidual depression symptoms. While it is not possible to determinewhether attenuated reward learning reflects a trait-like characteristic, aclinical feature of remitted depression, or a combination of both, giventhat reward learing prior to the onset of a depressive eposide was notassessed, the persistence of attenuated reward learning beyond the re-mission of a depressive episode certainly merits further consideration.

The Iowa Gambling Task (IGT; Bechara, Damasio, Damasio, &Anderson, 1994) has also been used as a behavioral measure of rewardlearning. During the IGT, participants are instructed to maximize profiton a $2000 “loan” by selecting cards from four decks (decks A, B, C, andD). Two of the decks (A and B) yield larger rewards on individual trialsbut also occasional large losses, resulting in a net loss over time, and areconsidered “disadvantageous.” Conversely, the other two decks (C andD) yield smaller rewards on individual trials and smaller occasionallosses, resulting in a net gain over time, and are considered “advanta-geous.” Reward learning on the task is operationalized as the extent towhich individuals are able to incorporate information about monetaryreward contingencies associated with each deck into future decisionsabout card selection, as reflected by a shift toward selection of cardsfrom the advantageous decks over time (Must, Horvath, Nemeth, &Janka, 2013).

Impaired reward learning on the IGT (i.e., a lesser shift toward se-lection of cards from the advantageous decks over time) has been as-sociated with depression-relevant outcomes. In several studies com-paring IGT performance among depressed outpatients and nonclinicalcontrols (Cella, Dymond, & Cooper, 2010; Moniz, Jesus, Gonçalves,Pacheco, & Viseu, 2016; Must et al., 2006), those with depression de-monstrated less of a shift toward “advantageous” decks over the courseof the task. Cella et al. (2010) also found a significant relation between

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severity of depression symptoms and poorer reward learning on theIGT. Moreover, a lesser tendency to adapt behavior in response to re-inforcing contingencies has been observed among first-degree relativesof suicide completers (Hoehne, Richard-Devantoy, Ding, Turecki, &Jollant, 2015), and has been shown to distinguish depressed individualswith a recent (past 72 h) suicide attempt from nonclinical controls(Gorlyn, Keilp, Oquendo, Burke, & Mann, 2013).

There is a focus in BA treatments on building sustainable patterns ofbehavior that facilitate regular engagement in rewarding activities.Impairment in the ability to adapt behavior in response to reinforcingcontingencies may interfere with patients' ability to “learn” from posi-tive reinforcement associated with past activities and incorporate thisinformation into subsequent behavioral choices. For example, a patientin BA treatment might choose to increase their engagement with so-cially rewarding activities by joining a local book club. During the firstbook club meeting, the patient may experience a sense of enjoymentfrom positive interactions with new people. However, a patient withattenuated reward learning may have difficulty in using this positivereinforcement as a guide for future behavior. The patient might ex-perience thoughts such as “I had a little bit of fun at the first meeting,though I don't think I will enjoy the next one.” BA treatments ofteninvolve monitoring the impact of activities on mood (Lejuez et al.,2001, 2011). This may assist patients in “learning” from positive re-inforcement by highlighting associations between rewarding activitiesand positive mood changes. Over time, patients may be able to in-tegrate information about reward contingencies associated with activ-ities into the development of sustainable patterns of engagement withenvironmental rewards.

Reward learning may be particularly relevant to modifying beha-vioral patterns of avoidance coping and increasing patients' use of al-ternative coping strategies (i.e., TRAP and TRAC; Martell et al., 2001).For example, a patient might want to replace the strategy of avoidingtime with their romantic partner in effort to prevent conflict with anapproach-oriented strategy, such as discussing conflicts openly andworking collaboratively toward conflict resolution. The patient mayexperience a positive response from their partner as a result of utilizingapproach-oriented strategies. However, a patient with attenuated re-ward learning may have a difficult time incorporating these positiveconsequences into decisions about conflicts that may occur in the fu-ture. This may, in turn, result in a return to earlier avoidance strategies.As in the example of mood monitoring described above, the patient andtherapist could work together to closely monitor the consequences ofapproach-oriented strategies. This may assist the patient in drawingconnections between approach-oriented coping and positive con-sequences, which could inform the patient's decisions to use approach-oriented strategies in the future.

7.4. Distinctions among reward types

A growing literature on differential associations between sub-typesof rewards and clinical outcomes may also inform the development andrefinement of BA treatments. Several broad categories of rewards havebeen identified, including physical rewards, such as food, sex, andphysical touch; social rewards, such as positive social feedback andenjoyable social interactions; mastery-oriented rewards, such asmeeting a goal or mastering a skill; and recreational rewards, such asspending time outdoors or participating in a hobby (Chapman,Chapman, & Raulin, 1976; Forbes & Dahl, 2012; Johnson, Fulford, &Carver, 2012; Ryba & Hopko, 2012; Zhang, Harris, Split, Troiani, &Olson, 2016). For example, several fMRI studies have examined dif-ferences in responding to physical and social rewards in youth at highfamilial risk for depression. One study comparing youth with a familyhistory of depression to those with no family history of mental illness(Monk et al., 2008) found less nucleus accumbens activation in re-sponse to viewing happy faces among high-risk participants. Relatedly,Olino, Silk, Osterritter, and Forbes (2015) found that high-risk youth

demonstrated less activation in the ventral striatum and anterior cin-gulate cortex in response to positive social feedback during a “chat-room” task, compared to low-risk youth. Another study (McCabe,Woffindale, Harmer, & Cowen, 2012) found that high-risk youth de-monstrated less activation in the orbitofrontal cortex to the taste ofchocolate and pictures of chocolate (i.e., a physical reward), comparedto low-risk youth. These results suggest that neural responsiveness toboth social and physical rewards may be impaired among youth at highrisk for depression; however, future research is needed to examinewhether neural responses differ across reward types in this population.

A small number of studies at the self-report level of analysis havedemonstrated differences in reward functioning across domains. In onestudy, social reward responsiveness on the Dimensional AnhedoniaRating Scale (Rizvi et al., 2015) distinguished between depressed andnondepressed individuals better than responsiveness to other rewardtypes (i.e., physical and recreational rewards). More recently,Khazanov, Ruscio, and Forbes (2019) developed the Positive ValenceSystems Scale (PVSS), whose factor-analytically derived subscales as-sess responsiveness to rewards in the domains of food, touch, the out-doors, positive feedback, hobbies, social interactions, and goals. In thevalidation study for the PVSS, responsiveness to social rewards (andhobbies) significantly distinguished depressed individuals from non-clinical controls, consistent with other studies demonstrating largedifferences between depressed and nondepressed individuals in thedomain of social rewards (Olino et al., 2015; Rizvi et al., 2015).

These findings are preliminary, and the relations between rewardsub-types and depression require additional exploration within diversesamples and across multiple levels of analysis. Nonetheless, resultsconfirming that certain sub-types of reward are more relevant to de-pression than others may highlight the utility of emphasizing rewardtypes that are most relevant in BA treatments. For example, findingsthat low social reward functioning is a significant predictor of depres-sion severity, or that social rewards are significantly lower among de-pressed (vs. nondepressed) individuals, would suggest that BA treat-ments should emphasize social engagement over and above other typesof reward.

Consideration of distinct sub-types of reward may also facilitatepersonalization of BA treatments to meet the needs of individual pa-tients. For example, a patient who is employed as a surgeon may havemany opportunities to experience positive reinforcement from mastery-oriented tasks, but may feel isolated as a result of having limited timefor hobbies and social relationships. For this patient, BA treatmentmight focus on identifying feasible opportunities for recreation andsocial engagement within a busy schedule. The focus of BA treatmentmight be different, however, for a patient who is a full-time parent andenjoys fulfilling relationships with their partner and children, but hasfew opportunities to engage in activities that generate feelings of ac-complishment. Given that this patient has sufficient social rewards,treatment could focus on identifying mastery-oriented activities such aslearning a new skill or taking on a leadership position in a local orga-nization. Of course, emphasis on one or more sub-types of reward mayshift over the course of BA treatment in response to progress andchanges in patients' needs.

8. Additional considerations for future study

A number of hypotheses have been proposed regarding the ways inwhich BA treatments may address facets of (low) reward functioningthat are characteristic of depression; particularly reward anticipation,effort valuation, reward learning, and responsiveness to specific sub-types of rewards. The next step in testing these hypotheses would be toestablish whether changes in reward functioning do indeed operate as amechanism of change in BA. Two novel treatments designed to speci-fically target low reward functioning may speak to this question:Positive Affect Treatment (PAT; Craske et al., 2019) and BATA (Nagyet al., 2020). Data on the efficacy of BATA is not currently available.

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However, promising preliminary data from a trial of PAT indicate thatindividuals receiving the treatment experienced significant increases inpositive affect, and decreases in depression symptoms, from pre-treat-ment to 6-month follow-up. Additional trials of these interventions mayshed light on whether improvements in reward functioning have acausal influence on symptom change. Moreover, larger-scale trials areneeded to examine whether BA treatments have a greater impact onanhedonia and low positive emotionality relative to other depressiontreatments, such as CBT and antidepressant medication.

In addition, given the diagnostic heterogeneity of depression(Drysdale et al., 2017; Monroe & Anderson, 2015), future research onmechanisms of change in BA would benefit from examining the influ-ence of BA on reward functioning across different subsets of depressedparticipants. Recently, a precision medicine approach has been used topredict which treatments for depression may be most efficacious forindividual patients (Cohen & DeRubeis, 2018; Huibers et al., 2015). Tothis end, viewing BA treatments through the lens of compensatory versuscapitalization models may facilitate increased understanding of whichindividuals and/or groups may benefit most from the intervention. Thepresent review has conceptualized BA in terms of a compensatorymodel, in which treatment strategies are designed to address deficits inreward functioning. An implicit assumption of this model is that in-dividuals with greater deficits in reward functioning may benefit mostfrom BA treatments. However, as noted previously, findings on the ef-ficacy of BA for individuals with severe depression have been mixed,with some studies demonstrating that BA is comparable to anti-depressant treatment and more efficacious than CBT in this population(Dimidjian et al., 2006) and others finding no significant differences inBA treatment response compared to CBT, antidepressant, or combinedCBT-antidepressant treatment (Lorenzo-Luaces & Dobson, 2019) forseverely depressed individuals.

An alternative approach would be a capitalization model, whichemphasizes enhancing the strengths (vs. deficits) that individuals bringto treatment. A capitalization approach would imply that individualswith higher pre-treatment reward functioning may benefit most from atreatment that leverages their pre-existing interest in and motivation topursue rewards. Consistent with this approach, a study of depressedadults receiving a 16-week CBT treatment found better outcomesamong individuals whose treatment was personalized to capitalize ontheir relative strengths, compared to those whose treatment was per-sonalized to address their relative deficits (Cheavens, Strunk, Lazarus, &Goldstein, 2012). Future research would benefit from the application ofthis model in order to understand which individuals and groups mayachieve the greatest responses to BA treatments (and, conversely, thosewho may be better-suited for other types of treatment).

The efficacy of BA treatments for adolescents also warrants furtherexamination, particularly given that adolescence is a critical vulner-ability period for the development of depression (Andersen & Teicher,2008; Bertha & Balázs, 2013). As noted above, attenuated neural re-ward functioning has been observed among adolescents at familial riskfor depression (Gotlib et al., 2010; McCabe et al., 2012; Monk et al.,2008; Olino et al., 2014), as well as currently depressed adolescents(Stringaris et al., 2015). Though the PVS domain was developed basedon research on reward functioning in adults, there has been increasinginterest in developmental considerations related to PVS processes(Olino, 2016).

The PVS sub-domains of Initial Responsiveness and Effort Valuationhave been identified as particularly relevant to adolescence, as evi-denced by findings from self-report and behavioral studies that ap-proach motivation and the pursuit of immediate (vs. delayed) rewardsis heightened during this developmental period (Anokhin, Golosheykin,& Mulligan, 2015; Cauffman et al., 2010; Lee et al., 2013). Respon-siveness to social rewards is also thought to be particularly relevant,given the importance of affiliative processes in adolescent development(Forbes, 2009). Viewed through the lens of a capitalization model, theseresults suggest that adolescents may be well-suited for BA treatments,

given that BA can leverage the heightened sensitivity to reward in thisdevelopmental period. Furthermore, interest in and motivation topursue social rewards may be a particularly important target in BAtreatments for adolescents given the salience of these rewards for thisage group. Indeed, BA treatments have demonstrated efficacy intreating depression among adolescents (Gaynor & Harris, 2008; Webbet al., 2019; see Martin & Oliver, 2019, for a review). BA has beenidentified as developmentally appropriate for adolescents, given thatthe treatment focuses primarily on behavioral change and does notrequire the full maturation of cognitive processes required for cognitivetherapy (Dimidjian & McCauley, 2016). However, given that measuresof reward functioning designed for adults may have limited validity inadolescent populations (Olino, 2016), there is a need for further ex-amination of BA processes using assessments that are sensitive to thedevelopmental context of adolescence.

Another promising direction for future research would be to identifyand examine cognitive processes that may drive symptom change in BAtreatments. Rumination may be a particularly relevant mechanism,given that entanglement in ruminative thoughts may lead to isolationand inactivity (e.g., an individual who spends a great deal of time athome ruminating may be less likely to seek out opportunities to engagein meaningful and rewarding activities). Rumination may also interferewith an individual's ability to connect with meaningful and rewardingaspects of activities, if they are more focused on ruminative thoughtsthan their experiences in the present moment. Indeed, decreases inrumination have been observed over the course of BA treatment. In astudy of internet-delivered exposure and BA for complicated grief andgrief rumination, BA resulted in reductions in grief rumination atposttreatment (after 6–8 weeks) that were maintained at 3-months(Eisma et al., 2015). Decreases in rumination over the course of BAtreatment have also been observed in a 16-week open trial of BA amongindividuals with atypical depression (Weinstock, Munroe, & Miller,2011), as well as in an 8-week group BA treatment for university stu-dents with depression and anxiety (Zemestani, Davoudi, Mehrabizadeh,& Zargar, 2014). Although decreases in rumination might be thought toresult from increases in control over cognitive processes, adjunctivecognitive control training along with standard BA treatment has notbeen associated with significant changes in treatment outcomes(Moshier & Otto, 2017). An alternative explanation would be that in-creased activity and behavioral engagement decrease ruminationsimply by providing opportunities for activities that engage individuals'cognitive processes. More research is needed, however, to understandthe relations between BA, rumination, and treatment outcomes.

The role of mindfulness (i.e., full attentional engagement with thepresent moment), which can be conceptualized as an opponent processto rumination, also warrants additional study. Martell et al.'s (2001)Behavioral Activation treatment and subsequent iterations of thistreatment include mindfulness skills training, based upon the assump-tion that increasing individuals' mindful engagement with enjoyableand meaningful aspects of their experiences will increase the salience ofreinforcement (i.e., positive emotional responses) associated with as-signed activities. Given that mindfulness may oppose maladaptivecognitive processes, such as worry and rumination, future research onBA treatments would benefit from investigating the relation of changesin mindfulness to treatment outcomes.

The influence of homework completion on BA treatment outcomesalso warrants consideration. Several preliminary studies (Collado et al.,2014; Ryba et al., 2014; Santos et al., 2017) have found that greaterengagement in assigned homework activities precedes or co-occurs withimprovement in depression symptoms. And yet, motivational deficitsassociated with attenuated reward functioning may present a sig-nificant antecedent barrier to homework completion. Indeed, low mo-tivation (Dimidjian & Hollon, 2011) and homework non-completion(Hopko et al., 2011) have been identified as significant barriers totreatment response. Balán, Lejuez, Hoffer, and Blanco (2016) en-couraged the integration of motivational interviewing strategies into

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BA treatments in order to build motivation for homework completion;for example, reminding patients of the personal relevance of activities,using functional analysis to address factors motivating noncompliance,and collaborating with the patient to problem-solve barriers to home-work completion. Additional strategies might include using graduatedactivity assignment (i.e., starting with easier activities and workingtoward more challenging ones over time), building activities into pa-tients' existing routines, and matching activities to relevant positive-valence deficits. More research is needed to elucidate the influence ofmotivation on homework completion and BA treatment outcomes, aswell as the development and refinement of strategies that can be usedby therapists to improve homework compliance among patients withlow motivation.

Finally, as noted above, most experimental studies of reward func-tioning have used tasks that assess responding to monetary reward.While monetary reward tasks are certainly informative, findings fromstudies using these tasks may be limited in their generalizability toother types of rewards. Given that individuals typically encounter awide range of reward types in their daily lives, future research shouldutilize stimuli that maximize ecological validity by assessing in-dividuals' responses to different types of rewards. For example, in ad-dition to the social reward tasks described above, the IGT has recentlybeen adapted to assess responses to social, rather than monetary, re-wards (Case & Olino, 2020). The development of similar behavioraltasks to asses a variety of reward stimuli could provide insight intomechanisms of change that are associated with different types of re-warding activities in the context of BA.

9. Summary and conclusions

Behavioral theories of depression (e.g., Ferster, 1973; Lewinsohn &Graf, 1973), which emphasize the influence of environmental rewardcontingencies on the onset and maintenance of depression symptomsand reated behaviors, form the theoretical basis for modern BA treat-ments. Both Behavioral Activation (Martell et al., 2001) and Brief Be-havioral Activation Treatment for Depression (Lejuez et al., 2001,2011) focus on increasing engagement with environmental sources ofpositive reinforcement in order to bring about improvement in de-pression symptoms. Given that BA treatments focus specifically on in-creasing engagement in rewarding activities, understanding the bio-behavioral processes underlying reward functioning may provideinsight into mechanisms of change in BA. Specifically, findings relatedto neural and behavioral processes involved in sub-domains of rewardfunctioning, including reward anticipation, effort valuation, rewardlearning, and responsiveness to sub-types of reward, can highlightprocesses through which BA treatments may bring about improvementin reward functioning and depression symptoms (see Table 1).

Understanding mechanisms of change in BA can facilitate the devel-opment of more targeted and streamlined treatment approaches to bestmeet the unique needs of individual patients. Although research onbiobehavioral mechanisms underlying reward functioning in depressedindividuals has occurred in large part independently of research on BAtreatments for depression, integrating these disparate literatures will bean important next step for the development and refinement of BAtreatments that efficiently target underlying biobehavioral processes indepression and other reward-related clinical problems.

Role of funding sources

There were no funding sources for this study.

Contributors

The author conceptualized the review paper, conducted literaturereviews, and wrote the manuscript.

Declaration of Competing Interest

The author has no conflicts of interests to declare.

Acknowledgements

The author would like to thank Dr. Matthew Tull, Dr. Kim Gratz,and Dr. Jason Levine for their formative feedback on earlier versions ofthis manuscript.

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Table 1Domains of reward functioning corresponding to strategies in behavioral activation treatments.

Domain Behavioral activation treatment strategies

Reward anticipation Behavior-first approach; encourage patients to engage in activities even if they do not feel motivated or do not expect positive consequences.Increase accountability for engaging in activities (even in the absence of reward anticipation) by reviewing assigned homework in weekly sessions.

Initial responsiveness Encourage patients to be mindful and present while engaging in rewarding activities in order to maximize the positive emotional consequences associatedwith those activities.

Reward satiation Use monitoring forms to help patients identify activities that bring about sustained positive changes in mood.Reward learning Use monitoring forms to help patients learn to make connections between activities and positive mood changes that occur as a result.

Encourage patients to incorporate information about positive consequences of past behavior into future behavioral choices.Reward probability Address feelings of hopelessness and low reward anticipation by discussing the probability that engaging in rewarding activities will bring about positive

consequences.Delay Discuss short-term versus long-term consequences of behaviors (e.g., avoidance and withdrawal behaviors may bring about a short-term reduction in

distress, while some adaptive reward-seeking behaviors may have longer-term positive consequences).Effort valuation Use values clarification to provide additional sources of reinforcement for effortful behaviors (i.e., reinforcement from taking action in line with one's

values).Use graded task assignment to break down effortful tasks into manageable sub-tasks.

Reward subtypes Select activities from reward types that are most relevant to individual patients.

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Courtney N. Forbes is a doctoral candidate in clinical psychology at the University ofToledo. Her research interests include transdiagnostic biobehavioral processes in mood,anxiety, and trauma-related disorders, as well as integration of findings from basic re-search into the development and dissemination of evidence-based treatments.

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