Making habits measurable beyond what they are not: A focus on associative dual-process models

Stimulus conditions that promote habitual control

Two experiments in rats examined how training where a stimulus signaled when to respond for reward, conditions that should favour S-R learning, might lead to habitual control of behaviour. Experiment 1 investigated how animals trained with a stimulus preceding lever insertion would impact learning relative to a group that was self-paced and could control lever insertion with a second, distinct response. Rats were then tested for sensitivity to outcome devaluation to distinguish between goal-directed and habitual control. We found that free-operant, self-paced conditions promoted goal-directed control while signaling trials with a stimulus promoted habitual control evidenced as insensitivity to outcome devaluation. Experiment 2 assessed whether the stimulus-outcome association is important for driving habitual responding when training occurs with a traditional discriminative stimulus. A comparison group was trained under free-operant conditions and experienced the same stimulus presented alongside the earned reward. Following devaluation, animals trained under discriminated-operant conditions had reduced goal-directed control, but only after extended training. The free-operant group remained goal-directed, even after extended training, and their performance was not altered by stimulus presentations, suggesting effects of a stimulus-outcome association were unlikely to account for the deficit in the discriminative stimulus group. These results extend understanding of how stimuli present during instrumental training promote the development of habitual control.

Hippocampal neuroinflammation induced by lipopolysaccharide causes sex-specific disruptions in action selection, food approach memories, and neuronal activation

Hippocampal neuroinflammation is present in multiple diseases and disorders that impact motivated behaviour in a sex-specific manner, but whether neuroinflammation alone is sufficient to disrupt this behaviour is unknown. We investigated this question here using mice. First, the application of an endotoxin to primary cultures containing only hippocampal neurons did not affect their activation. However, when the same endotoxin was applied to mixed neuronal/glial cultures it did increase neuronal activation, providing initial indications of how it might be able to effect behavioural change. We next showed neuroinflammatory effects on behaviour directly, demonstrating that intra-hippocampal administration of the same endotoxin increased locomotor activity and accelerated goal-directed learning in both male and female mice. In contrast, lipopolysaccharide-induced hippocampal neuroinflammation caused sex-specific disruptions to the acquisition of instrumental actions and to Pavlovian food-approach memories. Finally, we showed that LPS-induced hippocampal neuroinflammation had a sexually dimorphic effect on neuronal activation: increasing it in females and decreasing it in males.

Manipulations of the context-response relationship reduce the expression of response habits

Habitual instrumental behaviour is believed to rely on stimulus-response (S-R) associations. However, the method most commonly used to identify habitual behaviour, outcome devaluation, provides only indirect evidence of S-R control. Therefore, it is important to have a better understanding of the S-R association believed to underlie habitual responding. Under free-operant conditions, the context itself likely serves as at least part of the relevant stimuli in the association, and so modifications to the predictive power of the context should alter the expression of habits. The following experiments investigated how changes to the relationship between the training context and performance of the response, either by changing the context during testing or by exposing animals to the context alone, without the response lever present, impacted behavioural control during a devaluation test. We found evidence that the training context is important for the expression of habits; testing animals in a different context than where they were trained resulted in increased goal-directed control (Experiment 1). Furthermore, context alone exposure also increased goal-directed control with animals that received context alone exposure showing stronger devaluation effects, whether the context alone exposure happened on the last day of training (Experiment 2) or throughout training (Experiment 3). These findings are consistent with prior reports that the training context is important for the expression of habits and extends these findings by using sensory-specific satiety as a means for devaluation and by using context alone exposure to alter behavioural control.

The consequences of AI training on human decision-making

AI is now an integral part of everyday decision-making, assisting us in both routine and high-stakes choices. These AI models often learn from human behavior, assuming this training data is unbiased. However, we report five studies that show that people change their behavior to instill desired routines into AI, indicating this assumption is invalid. To show this behavioral shift, we recruited participants to play the ultimatum game, where they were asked to decide whether to accept proposals of monetary splits made by either other human participants or AI. Some participants were informed their choices would be used to train an AI proposer, while others did not receive this information. Across five experiments, we found that people modified their behavior to train AI to make fair proposals, regardless of whether they could directly benefit from the AI training. After completing this task once, participants were invited to complete this task again but were told their responses would not be used for AI training. People who had previously trained AI persisted with this behavioral shift, indicating that the new behavioral routine had become habitual. This work demonstrates that using human behavior as training data has more consequences than previously thought since it can engender AI to perpetuate human biases and cause people to form habits that deviate from how they would normally act. Therefore, this work underscores a problem for AI algorithms that aim to learn unbiased representations of human preferences.

Cortico-striatal white-matter connectivity underlies the ability to exert goal-directed control

The balance between goal-directed and habitual control has been proposed to determine the flexibility of instrumental behaviour, in both humans and animals. This view is supported by neuroscientific studies that have implicated dissociable neural pathways in the ability to flexibly adjust behaviour when outcome values change. A previous Diffusion Tensor Imaging study provided preliminary evidence that flexible instrumental performance depends on the strength of parallel cortico-striatal white-matter pathways previously implicated in goal-directed and habitual control. Specifically, estimated white-matter strength between caudate and ventromedial prefrontal cortex correlated positively with behavioural flexibility, and posterior putamen-premotor cortex connectivity correlated negatively, in line with the notion that these pathways compete for control. However, the sample size of the original study was limited, and so far, there have been no attempts to replicate these findings. In the present study, we aimed to conceptually replicate these findings by testing a large sample of 205 young adults to relate cortico-striatal connectivity to performance on the slips-of-action task. In short, we found only positive neural correlates of goal-directed performance, including striatal connectivity (caudate and anterior putamen) with the dorsolateral prefrontal cortex. However, we failed to provide converging evidence for the existence of a neural habit system that puts limits on the capacity for flexible, goal-directed action. We discuss the implications of our findings for dual-process theories of instrumental action.

Value-Driven Adaptations of Mesolimbic Dopamine Release Are Governed by Both Model-Based and Model-Free Mechanisms

The magnitude of dopamine signals elicited by rewarding events and their predictors is updated when reward value changes. It is actively debated how readily these dopamine signals adapt and whether adaptation aligns with model-free or model-based reinforcement-learning principles. To investigate this, we trained male rats in a pavlovian-conditioning paradigm and measured dopamine release in the nucleus accumbens core in response to food reward (unconditioned stimulus) and reward-predictive conditioned stimuli (CS), both before and after reward devaluation, induced via either sensory-specific or nonspecific satiety. We demonstrate that (1) such devaluation reduces CS-induced dopamine release rapidly, without additional pairing of CS with devalued reward and irrespective of whether the devaluation was sensory-specific or nonspecific. In contrast, (2) reward devaluation did not decrease food reward-induced dopamine release. Surprisingly, (3) postdevaluation reconditioning, by additional pairing of CS with devalued reward, rapidly reinstated CS-induced dopamine signals to predevaluation levels. Taken together, we identify distinct, divergent adaptations in dopamine-signal magnitude when reward value is decreased: CS dopamine diminishes but reinstates fast, whereas reward dopamine is resistant to change. This implies that, respective to abovementioned findings, (1) CS dopamine may be governed by a model-based mechanism and (2) reward dopamine by a model-free one, where (3) the latter may contribute to swift reinstatement of the former. However, changes in CS dopamine were not selective for sensory specificity of reward devaluation, which is inconsistent with model-based processes. Thus, mesolimbic dopamine signaling incorporates both model-free and model-based mechanisms and is not exclusively governed by either.

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.

Action sequence learning, habits, and automaticity in obsessive-compulsive disorder

This study investigates the goal/habit imbalance theory of compulsion in obsessive-compulsive disorder (OCD), which postulates enhanced habit formation, increased automaticity, and impaired goal/habit arbitration. It directly tests these hypotheses using newly developed behavioral tasks. First, OCD patients and healthy participants were trained daily for a month using a smartphone app to perform chunked action sequences. Despite similar procedural learning and attainment of habitual performance (measured by an objective automaticity criterion) by both groups, OCD patients self-reported higher subjective habitual tendencies via a recently developed questionnaire. Subsequently, in a re-evaluation task assessing choices between established automatic and novel goal-directed actions, both groups were sensitive to re-evaluation based on monetary feedback. However, OCD patients, especially those with higher compulsive symptoms and habitual tendencies, showed a clear preference for trained/habitual sequences when choices were based on physical effort, possibly due to their higher attributed intrinsic value. These patients also used the habit-training app more extensively and reported symptom relief post-study. The tendency to attribute higher intrinsic value to familiar actions may be a potential mechanism leading to compulsions and an important addition to the goal/habit imbalance hypothesis in OCD. We also highlight the potential of smartphone app training as a habit reversal therapeutic tool.

The mediodorsal thalamus in executive control

Executive control, the ability to organize thoughts and action plans in real time, is a defining feature of higher cognition. Classical theories have emphasized cortical contributions to this process, but recent studies have reinvigorated interest in the role of the thalamus. Although it is well established that local thalamic damage diminishes cognitive capacity, such observations have been difficult to inform functional models. Recent progress in experimental techniques is beginning to enrich our understanding of the anatomical, physiological, and computational substrates underlying thalamic engagement in executive control. In this review, we discuss this progress and particularly focus on the mediodorsal thalamus, which regulates the activity within and across frontal cortical areas. We end with a synthesis that highlights frontal thalamocortical interactions in cognitive computations and discusses its functional implications in normal and pathological conditions.

Acute stress facilitates habitual behavior in female rats

Instrumental behavior can reflect the influence of goal-directed and habitual systems. Contemporary research suggests that stress may facilitate control by the habitual system under conditions where the behavior would otherwise reflect control by the goal-directed system. However, it is unclear how stress modulates the influence of these systems on instrumental responding to achieve this effect, particularly in females. Here, we examine whether a mild psychogenic stressor experienced before acquisition training (Experiment 1), or prior to the test of expression (Experiment 2) would influence goal-directed and habitual control of instrumental responding in female rats. In both experiments, rats acquired an instrumental nose-poke response for a sucrose reward. This was followed by a reinforcer devaluation phase in which half the rats in Stressed and Non-Stressed conditions received pairings of the sucrose pellet with illness induced by lithium chloride until they rejected the pellet when offered. The remaining rats received a control treatment consisting of pellets and illness on separate days (Unpaired). Control by goal-directed and habitual systems was evaluated in a subsequent nonreinforced test of nose poking. The results of Experiment 1 indicated that the Non-Stressed Paired group reduced nose-poking compared to the Unpaired controls, identifying the response as goal directed, whereas the Stressed Paired and Unpaired groups made a similar number of nose pokes identifying the response as habitual despite a similar amount of training. Results from Experiment 2 indicated habitual control of nose-poke responding was present when stress was experienced just prior to the test. Collectively, these data suggest that stress may facilitate habitual control by altering the relative influence of goal-directed and habitual processes underpinning instrumental behavior. These results may be clinically relevant for understanding the contributions of stress to dysregulated instrumental behavior in compulsive pathologies.

Making and breaking habits: Revisiting the definitions and behavioral factors that influence habits in animals

Habits have garnered significant interest in studies of associative learning and maladaptive behavior. However, habit research has faced scrutiny and challenges related to the definitions and methods. Differences in the conceptualizations of habits between animal and human studies create difficulties for translational research. Here, we review the definitions and commonly used methods for studying habits in animals and humans and discuss potential alternative ways to assess habits, such as automaticity. To better understand habits, we then focus on the behavioral factors that have been shown to make or break habits in animals, as well as potential mechanisms underlying the influence of these factors. We discuss the evidence that habitual and goal-directed systems learn in parallel and that they seem to interact in competitive and cooperative manners. Finally, we draw parallels between habitual responding and compulsive drug seeking in animals to delineate the similarities and differences in these behaviors.

Stimulus control of habits: Evidence for both stimulus specificity and devaluation insensitivity in a dual-response task

Goal-directed and habitual actions are clearly defined by their associative relations. Whereas goal-directed control can be confirmed via tests of outcome devaluation and contingency-degradation sensitivity, a comparable criterion for positively detecting habits has not been established. To confirm habitual responding, a test of control by the stimulus-response association is required while also ruling out goal-directed control. Here we describe an approach to developing such a test in rats using two discriminative stimuli that set the occasion for two different responses that then earn the same outcome. Performance was insensitive to outcome devaluation and showed stimulus-response specificity, indicative of stimulus-controlled behavior. The reliance of stimulus-response associations was further supported by a lack of sensitivity during the single extinction test session used here. These results demonstrate that two concurrently trained responses can come under habitual control when they share a common outcome. By reducing the ability of one stimulus to signal its corresponding response-outcome association, we found evidence for goal-directed control that can be dissociated from habits. Overall, these experiments provide evidence that tests assessing specific stimulus-response associations can be used to investigate habits.

Sex and individual differences in the effect of chronic low-dose ethanol on behavioral strategy selection

BACKGROUND

The development of an alcohol use disorder (AUD) involves impaired behavioral control and flexibility. Behavioral inflexibility includes an inability to shift behavior in response to changes in behavioral outcomes. Low levels of ethanol drinking may promote the formation of inflexible, habitual reward seeking, but this may depend on the timing of ethanol exposure in relation to learning. The goal of this study was to determine whether a history of low-dose ethanol exposure promoted contingency-insensitive sucrose seeking and altered behavioral strategy selection.

METHODS

Male and female C57BL/6J mice were trained to perform a response (lever press) for sucrose on two different reinforcement schedules: one that is thought to promote inflexible responding (random interval) and one that maintains flexible responding (variable ratio [VR]). Following instrumental training each day, mice were exposed to saline or low-dose ethanol (0.5 g/kg; i.p.) either proximal (1 h after) or distal (4 h after) to learning. Mice were then tested for sensitivity to changes in contingency in a contingency degradation test.

RESULTS

A history of low-dose ethanol exposure shifted behavioral strategy selection, as measured by reward tracking behavior, but this depended on sex and reinforcement schedule history. Both male and female mice used different strategies depending on the reinforcement schedule, but only males exhibited ethanol-induced shifts in strategy selection. A history of low-dose ethanol exposure did not impact contingency sensitivity in males but promoted insensitivity in females specifically on the VR lever.

CONCLUSIONS

Female mice show distinct behavioral effects of repeated, low-dose ethanol exposure as compared to males, with sex differences in the use of reward tracking strategies to guide behavior. Future studies should investigate sex differences in the neural consequences of chronic low-dose ethanol exposure that may underlie behavioral changes.

Persistence of extensively trained avoidance is not elevated in anxiety disorders in an outcome devaluation paradigm

BACKGROUND

A habitual avoidance component may enforce the persistence of maladaptive avoidance behavior in anxiety disorders. Whether habitual avoidance is acquired more strongly in anxiety disorders is unclear.

METHODS

Individuals with current social anxiety disorder, panic disorder and/or agoraphobia (n = 62) and healthy individuals (n = 62) completed a devaluation paradigm with extensive avoidance training, followed by the devaluation of the aversive outcome. In the subsequent test phase, habitual response tendencies were inferred from compatibility effects. Neutral control trials were added to assess general approach learning in the absence of previous extensive avoidance training.

RESULTS

The compatibility effects indicating habitual control did not differ between patients with anxiety disorders and healthy controls. Patients showed lower overall approach accuracy, but this effect was unrelated to the compatibility effects.

CONCLUSIONS

In this study, anxiety disorders were characterized by reduced approach but not stronger habitual avoidance. These results do not indicate a direct association between anxiety disorders and the acquisition of pervasive habitual avoidance in this devaluation paradigm.

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.