Neurobiology of habit formation

Maturation of striatal dopamine supports the development of habitual behavior through adolescence

Developmental trajectories during the transition from adolescence to adulthood contribute to the establishment of stable, adult forms of operation. Understanding the neural mechanisms underlying this transition is crucial for identifying variability in normal development and the onset of psychiatric disorders, which typically emerge during this time. Habitual behaviors can serve as a model for understanding brain mechanisms underlying the stabilization of adult behavior, while also conferring risk for psychopathologies. Dopaminergic (DA) processes in the basal ganglia are thought to facilitate the formation of habits; however, developmental trajectories of habits and the brain systems supporting them have not been characterized in vivo in developing humans. The current study examined trajectories of habitual behavior from adolescence to adulthood and sought to understand how the maturing striatal DA system may act as a potential mechanism in the process of habit formation. We used data from two longitudinal studies (combined n = 217, 10 - 32 years of age, 1-3 visits each, 320 total sessions) to characterize normative developmental trajectories of basal ganglia tissue iron concentration (a proxy for DA-related neurophysiology) and goal-direct and habitual control behaviors in a two-stage decision-making task. Tissue iron concentrations across the basal ganglia and habitual responding during the two-stage sequential decision-making task both increased with age (all p < 0.001). Importantly, habitual responding was associated with tissue iron concentrations in the putamen (F = 4.34, p = 0.014), such that increases in habitual responding were supported by increases in putamen tissue iron concentration during childhood through late adolescence. Exploratory analyses of further subdivisions of anatomical regions found that this association was specific to the posterior putamen. These results provide novel evidence in humans that habitual behavior continues to mature into adulthood and may be supported by increased specialization of reward systems.

Dorsomedial Striatum (DMS) CB1R Signaling Promotes Pavlovian Devaluation Sensitivity in Male Long Evans Rats and Reduces DMS Inhibitory Synaptic Transmission in Both Sexes

Cannabinoid receptor-1 (CB1R) signaling in the dorsal striatum regulates the shift from flexible to habitual behavior in instrumental outcome devaluation. Based on prior work establishing individual-, sex-, and experience-dependent differences in pavlovian behaviors, we predicted a role for dorsomedial striatum (DMS) CB1R signaling in driving rigid responding in pavlovian autoshaping and outcome devaluation. We trained male and female Long Evans rats in pavlovian lever autoshaping (PLA). We gave intra-DMS infusions of the CB1R inverse agonist, rimonabant, before satiety-induced outcome devaluation test sessions, where we sated rats on training pellets or home cage chow and tested them in brief nonreinforced PLA sessions. Overall, inhibition of DMS CB1R signaling prevented pavlovian outcome devaluation but did not affect behavior in reinforced PLA sessions. Males were sensitive to devaluation while females were not, and DMS CB1R blockade impaired devaluation sensitivity in males. Because these results suggest DMS CB1R signaling supports flexible responding, we investigated how DMS CB1R signaling impacts local inhibitory synaptic transmission in male and female Long Evans rats. We recorded spontaneous inhibitory postsynaptic currents (sIPSC) from DMS neurons at baseline and after application of a CB1R agonist, WIN 55,212-2. We found that male rats showed decreased sIPSC frequency compared with females and that CB1R activation reduced DMS inhibitory transmission independent of sex. Altogether our results demonstrate that DMS CB1Rs regulate pavlovian devaluation sensitivity and DMS inhibitory synaptic transmission and suggest that basal sex differences in inhibitory synaptic transmission may underly sex differences in DMS function and behavioral flexibility.

Leveraging cognitive neuroscience for making and breaking real-world habits

Habits are the behavioral output of two brain systems. A stimulus-response (S-R) system that encourages us to efficiently repeat well-practiced actions in familiar settings, and a goal-directed system concerned with flexibility, prospection, and planning. Getting the balance between these systems right is crucial: an imbalance may leave people vulnerable to action slips, impulsive behaviors, and even compulsive behaviors. In this review we examine how recent advances in our understanding of these competing brain mechanisms can be harnessed to increase the control over both making and breaking habits. We discuss applications in everyday life, as well as validated and emergent interventions for clinical populations affected by the balance between these systems. As research in this area accelerates, we anticipate a rapid influx of new insights into intentional behavioral change and clinical interventions, including new opportunities for personalization of these interventions based on the neurobiology, environmental context, and personal preferences of an individual.

Dorsomedial Striatum CB1R signaling is required for Pavlovian outcome devaluation in male Long Evans rats and reduces inhibitory synaptic transmission in both sexes

Cannabinoid-1 receptor (CB1R) signaling in the dorsal striatum regulates the shift from flexible to habitual behavior in instrumental outcome devaluation. Based on prior work establishing individual, sex, and experience-dependent differences in Pavlovian behaviors, we predicted a role for dorsomedial striatum CB1R signaling in driving rigid responding in Pavlovian autoshaping and outcome devaluation. We trained male and female Long Evans rats in Pavlovian Lever Autoshaping (PLA). We gave intra-dorsomedial striatum (DMS) infusions of the CB1R inverse agonist, rimonabant, before satiety-induced outcome devaluation test sessions, where we sated rats on training pellets or home cage chow and tested them in brief nonreinforced Pavlovian Lever Autoshaping sessions. Overall, inhibition of DMS CB1R signaling prevented Pavlovian outcome devaluation but did not affect behavior in reinforced PLA sessions. Males were sensitive to devaluation while females were not and DMS CB1R inhibition impaired devaluation sensitivity in males. We then investigated how DMS CB1R signaling impacts local inhibitory synaptic transmission in male and female Long Evans rats. We recorded spontaneous inhibitory postsynaptic currents (sIPSC) from DMS neurons at baseline and before and after application of a CB1R agonist, WIN 55,212-2. We found that male rats showed decreased sIPSC frequency compared to females, and that CB1R activation reduced DMS inhibitory transmission independent of sex. Altogether our results demonstrate that DMS CB1Rs regulate Pavlovian devaluation sensitivity and inhibitory synaptic transmission and suggest that basal sex differences in inhibitory synaptic transmission may underly sex differences in DMS function and behavioral flexibility.

Chronic Stress Alters Synaptic Inhibition/Excitation Balance of Pyramidal Neurons But Not PV Interneurons in the Infralimbic and Prelimbic Cortices of C57BL/6J Mice

The medial prefrontal cortex (mPFC) plays a pivotal role in regulating working memory, executive function, and self-regulatory behaviors. Dysfunction in the mPFC circuits is a characteristic feature of several neuropsychiatric disorders including schizophrenia, depression, and post-traumatic stress disorder. Chronic stress (CS) is widely recognized as a major triggering factor for the onset of these disorders. Although evidence suggests synaptic dysfunction in mPFC circuits following CS exposure, it remains unclear how different neuronal populations in the infralimbic (IL) and prelimbic (PL) cortices are affected in terms of synaptic inhibition/excitation balance (I/E ratio). Here, using neuroproteomic analysis and whole-cell patch-clamp recordings in pyramidal neurons (PNs) and parvalbumin (PV) interneurons within the PL and IL cortices, we examined the synaptic changes after 21 d of chronic unpredictable stress, in male mice. Our results reveal distinct impacts of CS on PL and IL PNs, resulting in an increased I/E ratio in both subregions but through different mechanisms: CS increases inhibitory synaptic drive in the PL while decreasing excitatory synaptic drive in the IL. Notably, the I/E ratio and excitatory and inhibitory synaptic drive of PV interneurons remained unaffected in both PL and IL circuits following CS exposure. These findings offer novel mechanistic insights into the influence of CS on mPFC circuits and support the hypothesis of stress-induced mPFC hypofunction.

Bad habits-good goals? Meta-analysis and translation of the habit construct to alcoholism

Excessive alcohol consumption remains a global public health crisis, with millions suffering from alcohol use disorder (AUD, or simply "alcoholism"), leading to significantly reduced life expectancy. This review examines the interplay between habitual and goal-directed behaviors and the associated neurobiological changes induced by chronic alcohol exposure. Contrary to a strict habit-goal dichotomy, our meta-analysis of the published animal experiments combined with a review of human studies reveals a nuanced transition between these behavioral control systems, emphasizing the need for refined terminology to capture the probabilistic nature of decision biases in individuals with a history of chronic alcohol exposure. Furthermore, we distinguish habitual responding from compulsivity, viewing them as separate entities with diverse roles throughout the stages of the addiction cycle. By addressing species-specific differences and translational challenges in habit research, we provide insights to enhance future investigations and inform strategies for combatting AUD.

A role for the dorsolateral striatum in prospective action control

The dorsolateral striatum (DLS) is important for performing actions persistently, even when it becomes suboptimal, reflecting a function that is reflexive and habitual. However, there are also ways in which persistent behaviors can result from a more prospective, planning mode of behavior. To help tease apart these possibilities for DLS function, we trained animals to perform a lever press for reward and then inhibited the DLS in key test phases: as the task shifted from a 1-press to a 3-press rule (upshift), as the task was maintained, as the task shifted back to the one-press rule (downshift), and when rewards came independent of pressing. During DLS inhibition, animals always favored their initially learned strategy to press just once, particularly so during the free-reward period. DLS inhibition surprisingly changed performance speed bidirectionally depending on the task shifts. DLS inhibition thus encouraged habitual behavior, suggesting it could normally help adapt to changing conditions.

Dopaminergic reinforcement in the motor system: Implications for Parkinson's disease and deep brain stimulation

Millions of people suffer from dopamine-related disorders spanning disturbances in movement, cognition and emotion. These changes are often attributed to changes in striatal dopamine function. Thus, understanding how dopamine signalling in the striatum and basal ganglia shapes human behaviour is fundamental to advancing the treatment of affected patients. Dopaminergic neurons innervate large-scale brain networks, and accordingly, many different roles for dopamine signals have been proposed, such as invigoration of movement and tracking of reward contingencies. The canonical circuit architecture of cortico-striatal loops sparks the question, of whether dopamine signals in the basal ganglia serve an overarching computational principle. Such a holistic understanding of dopamine functioning could provide new insights into symptom generation in psychiatry to neurology. Here, we review the perspective that dopamine could bidirectionally control neural population dynamics, increasing or decreasing their strength and likelihood to reoccur in the future, a process previously termed neural reinforcement. We outline how the basal ganglia pathways could drive strengthening and weakening of circuit dynamics and discuss the implication of this hypothesis on the understanding of motor signs of Parkinson's disease (PD), the most frequent dopaminergic disorder. We propose that loss of dopamine in PD may lead to a pathological brain state where repetition of neural activity leads to weakening and instability, possibly explanatory for the fact that movement in PD deteriorates with repetition. Finally, we speculate on how therapeutic interventions such as deep brain stimulation may be able to reinstate reinforcement signals and thereby improve treatment strategies for PD in the future.

Mapping the neuroanatomical abnormalities in a phenotype of male compulsive rats

Compulsivity is considered a transdiagnostic dimension in obsessive-compulsive and related disorders, characterized by heterogeneous cognitive and behavioral phenotypes associated with abnormalities in cortico-striatal-thalamic-cortical circuitry. The present study investigated the structural morphology of white and gray matter in rats selected for low- (LD) and high- (HD) compulsive drinking behavior on a schedule-induced polydipsia (SIP) task. Regional brain morphology was assessed using ex-vivo high-resolution magnetic resonance imaging (MRI). Voxel-based morphometry of segmented MRI images revealed larger white matter volumes in anterior commissure and corpus callosum of HD rats compared with LD rats. HD rats also showed significantly larger regional volumes of dorsolateral orbitofrontal cortex, striatum, amygdala, hippocampus, midbrain, sub-thalamic nucleus, and cerebellum. By contrast, the medial prefrontal cortex was significantly smaller in HD rats compared with LD rats with no significant group differences in whole brain, ventricular, or cerebrospinal fluid volumes. These findings show that limbic cortico-basal ganglia structures implicated in impulse control disorders are distinct in rats that are vulnerable to develop compulsive behavior. Such abnormalities may be relevant to the etiology of compulsive disorders in humans.

The Willpower Paradox: Possible and Impossible Conceptions of Self-Control

Self-control denotes the ability to override current desires to render behavior consistent with long-term goals. A key assumption is that self-control is required when short-term desires are transiently stronger (more preferred) than long-term goals and people would yield to temptation without exerting self-control. We argue that this widely shared conception of self-control raises a fundamental yet rarely discussed conceptual paradox: How is it possible that a person most strongly desires to perform a behavior (e.g., eat chocolate) and at the same time desires to recruit self-control to prevent themselves from doing it? A detailed analysis reveals that three common assumptions about self-control cannot be true simultaneously. To avoid the paradox, any coherent theory of self-control must abandon either the assumption (a) that recruitment of self-control is an intentional process, or (b) that humans are unitary agents, or (c) that self-control consists in overriding the currently strongest desire. We propose a taxonomy of different kinds of self-control processes that helps organize current theories according to which of these assumptions they abandon. We conclude by outlining unresolved questions and future research perspectives raised by different conceptions of self-control and discuss implications for the question of whether self-control can be considered rational.

Chronic stress promotes basal ganglia disinhibition by increasing the excitatory drive of direct-pathway neurons

Chronic stress (CS) is a well-recognized triggering factor in obsessive-compulsive disorder (OCD) and Tourette's syndrome (TS), two neuropsychiatric disorders characterized by the presence of stereotypic motor symptoms. Planning and execution of motor actions are controlled by the dorsal striatum, a brain region that promotes or suppresses motor movement by activating striatal neurons from the direct- or indirect-pathway, respectively. Despite the dorsal striatum being affected in motor disorders and by CS exposure, how CS affects the two opposing pathways is not fully understood. Here, we report that CS in mice selectively potentiates the direct-pathway, while sparing the indirect-pathway. Specifically, we show that CS both increases excitation and reduces inhibition over direct-pathway neurons in the dorsomedial striatum (DMS). Furthermore, inhibitory interneurons located in the DMS also display reduced excitatory drive after chronic stress, thus amplifying striatal disinhibition. Altogether, we propose a model where both increased excitatory drive and decreased inhibitory drive in the striatum causes disinhibition of basal ganglia's motor direct pathway - a mechanism that might explain the emergence of motor stereotypies and tic disorders under stress.

Revealing abrupt transitions from goal-directed to habitual behavior

A fundamental tenet of animal behavior is that decision-making involves multiple 'controllers.' Initially, behavior is goal-directed, driven by desired outcomes, shifting later to habitual control, where cues trigger actions independent of motivational state. Clark Hull's question from 1943 still resonates today: "Is this transition abrupt, or is it gradual and progressive?"1 Despite a century-long belief in gradual transitions, this question remains unanswered2,3 as current methods cannot disambiguate goal-directed versus habitual control in real-time. Here, we introduce a novel 'volitional engagement' approach, motivating animals by palatability rather than biological need. Offering less palatable water in the home cage4,5 reduced motivation to 'work' for plain water in an auditory discrimination task when compared to water-restricted animals. Using quantitative behavior and computational modeling6, we found that palatability-driven animals learned to discriminate as quickly as water-restricted animals but exhibited state-like fluctuations when responding to the reward-predicting cue-reflecting goal-directed behavior. These fluctuations spontaneously and abruptly ceased after thousands of trials, with animals now always responding to the reward-predicting cue. In line with habitual control, post-transition behavior displayed motor automaticity, decreased error sensitivity (assessed via pupillary responses), and insensitivity to outcome devaluation. Bilateral lesions of the habit-related dorsolateral striatum7 blocked transitions to habitual behavior. Thus, 'volitional engagement' reveals spontaneous and abrupt transitions from goal-directed to habitual behavior, suggesting the involvement of a higher-level process that arbitrates between the two.

Distinct cortico-striatal compartments drive competition between adaptive and automatized behavior

Cortical and basal ganglia circuits play a crucial role in the formation of goal-directed and habitual behaviors. In this study, we investigate the cortico-striatal circuitry involved in learning and the role of this circuitry in the emergence of inflexible behaviors such as those observed in addiction. Specifically, we develop a computational model of cortico-striatal interactions that performs concurrent goal-directed and habit learning. The model accomplishes this by distinguishing learning processes in the dorsomedial striatum (DMS) that rely on reward prediction error signals as distinct from the dorsolateral striatum (DLS) where learning is supported by salience signals. These striatal subregions each operate on unique cortical input: the DMS receives input from the prefrontal cortex (PFC) which represents outcomes, and the DLS receives input from the premotor cortex which determines action selection. Following an initial learning of a two-alternative forced choice task, we subjected the model to reversal learning, reward devaluation, and learning a punished outcome. Behavior driven by stimulus-response associations in the DLS resisted goal-directed learning of new reward feedback rules despite devaluation or punishment, indicating the expression of habit. We repeated these simulations after the impairment of executive control, which was implemented as poor outcome representation in the PFC. The degraded executive control reduced the efficacy of goal-directed learning, and stimulus-response associations in the DLS were even more resistant to the learning of new reward feedback rules. In summary, this model describes how circuits of the dorsal striatum are dynamically engaged to control behavior and how the impairment of executive control by the PFC enhances inflexible behavior.

Adolescent social isolation shifts the balance of decision-making strategy from goal-directed action to habitual response in adulthood via suppressing the excitatory neurotransmission onto the direct pathway of the dorsomedial striatum

Adverse experience, such as social isolation, during adolescence is one of the major causes of neuropsychiatric disorders that extend from adolescence into adulthood, such as substance addiction, obsessive-compulsive disorder, and eating disorders leading to obesity. A common behavioral feature of these neuropsychiatric disorders is a shift in the balance of decision-making strategy from goal-directed action to habitual response. This study has verified that adolescent social isolation directly shifts the balance of decision-making strategy from goal-directed action to habitual response, and that it cannot be reversed by simple regrouping. This study has further revealed that adolescent social isolation induces a suppression in the excitatory neurotransmission onto the direct-pathway medium spiny neurons of the dorsomedial striatum (DMS), and that chemogenetically compensating this suppression effect shifts the balance of decision-making strategy from habitual response back to goal-directed action. These findings suggest that the plasticity in the DMS causes the shift in the balance of decision-making strategy, which would potentially help to develop a general therapy to treat the various neuropsychiatric disorders caused by adolescent social isolation. Such a study is especially necessary under the circumstances that social distancing and lockdown have caused during times of world-wide, society-wide pandemic.

Task history dictates how the dorsolateral striatum controls action strategy and vigor

The dorsolateral striatum (DLS) is linked to the learning and honing of action routines. However, the DLS is also important for performing behaviors that have been successful in the past. The learning function can be thought of as prospective, helping to plan ongoing actions to be efficient and often optimal. The performance function is more retrospective, helping the animal continue to behave in a way that had worked previously. How the DLS manages this all is curious. What happens when a learned behavior becomes sub-optimal due to environment changes. In this case, the prospective function of the DLS would cause animals to (adaptively) learn and plan more optimal actions. In contrast, the retrospective function would cause animals to (maladaptively) favor the old behavior. Here we find that, during a change in learned task rules, DLS inhibition causes animals to adjust less rapidly to the new task (and to behave less vigorously) in a 'maladaptive' way. Yet, when the task is changed back to the initially learned rules, DLS inhibition instead causes a rapid and vigorous adjustment of behavior in an 'adaptive' way. These results show that inhibiting the DLS biases behavior towards initially acquired strategies, implying a more retrospective outlook in action selection when the DLS is offline. Thus, an active DLS could encourage planning and learning action routines more prospectively. Moreover, the DLS control over behavior can appear to be either advantageous/flexible or disadvantageous/inflexible depending on task context, and its control over vigor can change depending on task context.

Significant Statement

Basal ganglia networks aid behavioral learning (a prospective planning function) but also favor the use of old behaviors (a retrospective performance function), making it unclear what happens when learned behaviors become suboptimal. Here we inhibit the dorsolateral striatum (DLS) as animals encounter a change in task rules, and again when they shift back to those learned task rules. DLS inhibition reduces adjustment to new task rules (and reduces behavioral vigor), but it increases adjustment back to the initially learned task rules later (and increases vigor). Thus, in both cases, DLS inhibition favored the use of the initially learned behavioral strategy, which could appear either maladaptive or adaptive. We suggest that the DLS might promote a prospective orientation of action control.

The influence of the subcortex and brain stem on overeating: How advances in functional neuroimaging can be applied to expand neurobiological models to beyond the cortex

Functional neuroimaging has become a widely used tool in obesity and eating disorder research to explore the alterations in neurobiology that underlie overeating and binge eating behaviors. Current and traditional neurobiological models underscore the importance of impairments in brain systems supporting reward, cognitive control, attention, and emotion regulation as primary drivers for overeating. Due to the technical limitations of standard field strength functional magnetic resonance imaging (fMRI) scanners, human neuroimaging research to date has focused largely on cortical and basal ganglia effects on appetitive behaviors. The present review draws on animal and human research to highlight how neural signaling encoding energy regulation, reward-learning, and habit formation converge on hypothalamic, brainstem, thalamic, and striatal regions to contribute to overeating in humans. We also consider the role of regions such as the mediodorsal thalamus, ventral striatum, lateral hypothalamus and locus coeruleus in supporting habit formation, inhibitory control of food craving, and attentional biases. Through these discussions, we present proposals on how the neurobiology underlying these processes could be examined using functional neuroimaging and highlight how ultra-high field 7-Tesla (7 T) fMRI may be leveraged to elucidate the potential functional alterations in subcortical networks. Focus is given to how interactions of these regions with peripheral endocannabinoids and neuropeptides, such as orexin, could be explored. Technical and methodological aspects regarding the use of ultra-high field 7 T fMRI to study eating behaviors are also reviewed.

Striatal dopamine signals are region specific and temporally stable across action-sequence habit formation

Habits are automatic, inflexible behaviors that develop slowly with repeated performance. Striatal dopamine signaling instantiates this habit-formation process, presumably region specifically and via ventral-to-dorsal and medial-to-lateral signal shifts. Here, we quantify dopamine release in regions implicated in these presumed shifts (ventromedial striatum [VMS], dorsomedial striatum [DMS], and dorsolateral striatum [DLS]) in rats performing an action-sequence task and characterize habit development throughout a 10-week training. Surprisingly, all regions exhibited stable dopamine dynamics throughout habit development. VMS and DLS signals did not differ between habitual and non-habitual animals, but DMS dopamine release increased during action-sequence initiation and decreased during action-sequence completion in habitual rats, whereas non-habitual rats showed opposite effects. Consistently, optogenetic stimulation of DMS dopamine release accelerated habit formation. Thus, we demonstrate that dopamine signals do not shift regionally during habit formation and that dopamine in DMS, but not VMS or DLS, determines habit bias, attributing "habit functions" to a region previously associated exclusively with non-habitual behavior.

Regulation of Voluntary Physical Activity Behavior: A Review of Evidence Involving Dopaminergic Pathways in the Brain

Physical activity leads to well-established health benefits. Current efforts to enhance physical activity have targeted mainly socioeconomic factors. However, despite these efforts, only a small number of adults engage in regular physical activity to the point of meeting current recommendations. Evidence collected in rodent models and humans establish a strong central nervous system component that regulates physical activity behavior. In particular, dopaminergic pathways in the central nervous system are among the best-characterized biological mechanisms to date with respect to regulating reward, motivation, and habit formation, which are critical for establishing regular physical activity. Herein, we discuss evidence for a role of brain dopamine in the regulation of voluntary physical activity behavior based on selective breeding and pharmacological studies in rodents, as well as genetic studies in both rodents and humans. While these studies establish a role of dopamine and associated mechanisms in the brain in the regulation of voluntary physical activity behavior, there is clearly need for more research on the underlying biology involved in motivation for physical activity and the formation of a physical activity habit. Such knowledge at the basic science level may ultimately be translated into better strategies to enhance physical activity levels within the society.

Pyk2 Stabilizes Striatal Medium Spiny Neuron Structure and Striatal-Dependent Action

In day-to-day life, we often choose between pursuing familiar behaviors that have been rewarded in the past or adjusting behaviors when new strategies might be more fruitful. The dorsomedial striatum (DMS) is indispensable for flexibly arbitrating between old and new behavioral strategies. The way in which DMS neurons host stable connections necessary for sustained flexibility is still being defined. An entry point to addressing this question may be the structural scaffolds on DMS neurons that house synaptic connections. We find that the non-receptor tyrosine kinase Proline-rich tyrosine kinase 2 (Pyk2) stabilizes both dendrites and spines on striatal medium spiny neurons, such that Pyk2 loss causes dendrite arbor and spine loss. Viral-mediated Pyk2 silencing in the DMS obstructs the ability of mice to arbitrate between rewarded and non-rewarded behaviors. Meanwhile, the overexpression of Pyk2 or the closely related focal adhesion kinase (FAK) enhances this ability. Finally, experiments using combinatorial viral vector strategies suggest that flexible, Pyk2-dependent action involves inputs from the medial prefrontal cortex (mPFC), but not the ventrolateral orbitofrontal cortex (OFC). Thus, Pyk2 stabilizes the striatal medium spiny neuron structure, likely providing substrates for inputs, and supports the capacity of mice to arbitrate between novel and familiar behaviors, including via interactions with the medial-prefrontal cortex.

Attitudes, Habits, and Behavior Change

Efforts to guide peoples' behavior toward environmental sustainability, good health, or new products have emphasized informational and attitude change strategies. There is evidence that changing attitudes leads to changes in behavior, yet this approach takes insufficient account of the nature and operation of habits, which form boundary conditions for attitude-directed interventions. Integration of research on attitudes and habits might enable investigators to identify when and how behavior change strategies will be most effective. How might attitudinally driven behavior change be consolidated into lasting habits? How do habits protect the individual against the vicissitudes of attitudes and temptations and promote goal achievement? How might attitudinal approaches aiming to change habits be improved by capitalizing on habit discontinuities and strategic planning? When and how might changing or creating habit architecture shape habits directly? A systematic approach to these questions might help move behavior change efforts from attitude change strategies to habit change strategies. Expected final online publication date for the Annual Review of Psychology, Volume 73 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Habits and Goals in Human Behavior: Separate but Interacting Systems

People automatically repeat behaviors that were frequently rewarded in the past in a given context. Such repetition is commonly attributed to habit, or associations in memory between a context and a response. Once habits form, contexts directly activate the response in mind. An opposing view is that habitual behaviors depend on goals. However, we show that this view is challenged by the goal independence of habits across the fields of social and health psychology, behavioral neuroscience, animal learning, and computational modeling. It also is challenged by direct tests revealing that habits do not depend on implicit goals. Furthermore, we show that two features of habit memory-rapid activation of specific responses and resistance to change-explain the different conditions under which people act on habit versus persuing goals. Finally, we tested these features with a novel secondary analysis of action-slip data. We found that habitual responses are activated regardless of goals, but they can be performed in concert with goal pursuit.

Depression in the Direct Pathway of the Dorsomedial Striatum Permits the Formation of Habitual Action

In order to achieve optimal outcomes in an ever-changing environment, humans and animals generally manage their action control via either goal-directed action or habitual action. These two action strategies are thought to be encoded in distinct parallel circuits in the dorsal striatum, specifically, the posterior dorsomedial striatum (DMS) and the dorsolateral striatum (DLS), respectively. The striatum is primarily composed of two subtypes of medium spiny neurons (MSNs): the direct-pathway striatonigral and the indirect-pathway striatopallidal MSNs. MSN-subtype-specific synaptic plasticity in the DMS and the DLS has been revealed to underlie goal-directed action and habitual action, respectively. However, whether any MSN-subtype-specific synaptic plasticity in the DMS is associated with habitual action, and if so, whether the synaptic plasticity affects the formation of habitual action, are not known. This study demonstrates that postsynaptic depression in the excitatory synapses of the direct-pathway striatonigral MSNs in the DMS is formed after habit learning. Moreover, chemogenetically rescuing this depression compromises the acquisition, but not the expression, of habitual action. These findings reveal that an MSN-subtype-specific synaptic plasticity in the DMS affects habitual action and suggest that plasticity in the DMS as well as in the DLS contributes to the formation of habitual action.

Handedness Development: A Model for Investigating the Development of Hemispheric Specialization and Interhemispheric Coordination

The author presents his perspective on the character of science, development, and handedness and relates these to his investigations of the early development of handedness. After presenting some ideas on what hemispheric specialization of function might mean for neural processing and how handedness should be assessed, the neuroscience of control of the arms/hands and interhemispheric communication and coordination are examined for how developmental processes can affect these mechanisms. The author’s work on the development of early handedness is reviewed and placed within a context of cascading events in which different forms of handedness emerge from earlier forms but not in a deterministic manner. This approach supports a continuous rather than categorical distribution of handedness and accounts for the predominance of right-handedness while maintaining a minority of left-handedness. Finally, the relation of the development of handedness to the development of several language and cognitive skills is examined.

Spatially restricted inhibition of cholinergic interneurons in the dorsolateral striatum encourages behavioral exploration

When pursuing desirable outcomes, one must make the decision between exploring possible actions to obtain those outcomes and exploiting known strategies to maximize efficiency. The dorsolateral striatum (DLS) has been extensively studied with respect to how actions can develop into habits and has also been implicated as an area involved in governing exploitative behavior. Surprisingly, prior work has shown that DLS cholinergic interneurons (ChIs) are not involved in the canonical habit formation function ascribed to the DLS but are instead modulators of behavioral flexibility after initial learning. To further probe this, we evaluated the role of DLS ChIs in behavioral exploration during a brief instrumental training experiment. Through designer receptors exclusively activated by designer drugs (DREADDs) in ChAT-Cre rats, ChIs in the DLS were inhibited during specific phases of the experiment: instrumental training, free-reward delivery, at both times, or never. Without ChI activity during instrumental training, animals biased their responding toward an "optimal" strategy while continuing to work efficiently. This effect was observed again when contingencies were removed as animals with ChIs offline during that phase, regardless of ChI inhibition previously, decreased responding more than animals with ChIs intact. These findings build upon a growing body of literature implicating ChIs in the striatum as gate-keepers of behavioral flexibility and exploration.

Involvement of the rodent prelimbic and medial orbitofrontal cortices in goal-directed action: A brief review

Goal-directed action refers to selecting behaviors based on the expectation that they will be reinforced with desirable outcomes. It is typically conceptualized as opposing habit-based behaviors, which are instead supported by stimulus-response associations and insensitive to consequences. The prelimbic prefrontal cortex (PL) is positioned along the medial wall of the rodent prefrontal cortex. It is indispensable for action-outcome-driven (goal-directed) behavior, consolidating action-outcome relationships and linking contextual information with instrumental behavior. In this brief review, we will discuss the growing list of molecular factors involved in PL function. Ventral to the PL is the medial orbitofrontal cortex (mOFC). We will also summarize emerging evidence from rodents (complementing existing literature describing humans) that it too is involved in action-outcome conditioning. We describe experiments using procedures that quantify responding based on reward value, the likelihood of reinforcement, or effort requirements, touching also on experiments assessing food consumption more generally. We synthesize these findings with the argument that the mOFC is essential to goal-directed action when outcome value information is not immediately observable and must be recalled and inferred.

Dorsal Striatal Circuits for Habits, Compulsions and Addictions

Here, we review the neural circuit bases of habits, compulsions, and addictions, behaviors which are all characterized by relatively automatic action performance. We discuss relevant studies, primarily from the rodent literature, and describe how major headway has been made in identifying the brain regions and neural cell types whose activity is modulated during the acquisition and performance of these automated behaviors. The dorsal striatum and cortical inputs to this structure have emerged as key players in the wider basal ganglia circuitry encoding behavioral automaticity, and changes in the activity of different neuronal cell-types in these brain regions have been shown to co-occur with the formation of automatic behaviors. We highlight how disordered functioning of these neural circuits can result in neuropsychiatric disorders, such as obsessive-compulsive disorder (OCD) and drug addiction. Finally, we discuss how the next phase of research in the field may benefit from integration of approaches for access to cells based on their genetic makeup, activity, connectivity and precise anatomical location.

Self-administration of the synthetic cathinone MDPV enhances reward function via a nicotinic receptor dependent mechanism

Methylenedioxypyrovalerone (MDPV) is an addictive synthetic drug with severe side effects. Previous studies have shown that MDPV has positive reinforcing properties. However, little is known about the effect of MDPV self-administration on the state of the brain reward system and the neuronal mechanisms by which MDPV mediates its effects. The goal of the present studies was to determine the effect of MDPV self-administration on reward function and the role of cholinergic neurotransmission in the reinforcing effects of MDPV. To study the effect of MDPV self-administration on the brain reward system, rats were prepared with intravenous catheters and intracranial self-stimulation electrodes (ICSS). For 10 days, the reward thresholds were assessed immediately before (23 h post prior session) and after 1 h of MDPV self-administration. The reward thresholds were decreased immediately after MDPV self-administration, which is indicative of a potentiation of brain reward function. The reward thresholds 23 h after MDPV intake gradually increased over time, which is indicative of anhedonia. Pretreatment with the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine decreased the self-administration of MDPV and completely prevented the decrease in reward thresholds. A control study with palatable chocolate pellets showed that responding for a natural reinforcer does not affect the state of the brain reward system. Furthermore, mecamylamine did not affect responding for food pellets. In conclusion, the self-administration of MDPV potentiates reward function and nAChR blockade prevents the reward enhancing effects of MDPV self-administration. Preventing the MDPV-induced increase in cholinergic neurotransmission might be a safe approach to diminish MDPV abuse.