Dissociable contributions of dorsal and ventral striatal regions on a rodent cost/benefit decision-making task requiring cognitive effort

Insight into differing decision-making strategies that underlie cognitively effort-based decision making using computational modeling in rats

RATIONALE

The rat cognitive effort task (rCET), a rodent model of cognitive rather than physical effort, requires animals to choose between an easy or hard visuospatial discrimination, with a correct hard choice more highly rewarded. Like in humans, there is stable individual variation in choice behavior. In previous reports, animals were divided into two groups-workers and slackers-based on their mean preference for the harder option. Although these groups differed in their response to pharmacological challenges, the rationale for using this criterion for grouping was not robust.

METHODS

We collated experimental data from multiple cohorts of male and female rats performing the rCET and used a model-based framework combining drift diffusion modeling with cluster analysis to identify the decision-making processes underlying variation in choice behavior.

RESULTS

We verified that workers and slackers are statistically different groups but also found distinct intra-group profiles. These subgroups exhibited dissociable performance during the attentional phase, linked to distinct decision-making profiles during choice. Reanalysis of previous pharmacology data using this model-based framework showed that serotonergic drug effects were explained by changes in decision boundaries and non-decision times, while scopolamine's effects were driven by changes in decision starting points and rates of evidence accumulation.

CONCLUSIONS

Modeling revealed the decision-making processes that are associated with cognitive effort costs, and how these differ across individuals. Reanalysis of drug data provided insight into the mechanisms through which different neurotransmitter systems impact cognitively effortful attention and decision-making processes, with relevance to multiple psychiatric disorders.

Dorsomedial striatal contributions to different forms of risk/reward decision making

Optimal decision making involving reward uncertainty is integral to adaptive goal-directed behavior. In some instances, these decisions are guided by internal representations of reward history, whereas in other situations, external cues inform a decision maker about how likely certain actions are to yield reward. Different regions of the frontal lobe form distributed networks with striatal and amygdalar regions that facilitate different types of risk/reward decision making. The dorsal medial striatum (DMS) is one key output region of the prefrontal cortex, yet there have been few preclinical studies investigating the involvement of the DMS in different forms of risk/reward decision making. The present study addressed this issue, wherein separate groups of male rats were trained on one of two tasks where they chose between a small/certain or a large/risky reward. In a probabilistic discounting task, reward probabilities changed systematically over blocks of trials (100-6.25% or 6.25-100%), requiring rats to use internal representations of reward history to guide choice. Cue-guided decision-making was assessed with a "Blackjack" task, where different auditory cues indicated the odds associated with the large/risky option (50 or 12.5%). Inactivation of the DMS with GABA agonists impaired adjustments in choice biases during probabilistic discounting, resulting in either increases or decreases in risky choice as the probabilities associated with the large/risky reward decreased or increased over a session. In comparison, DMS inactivation increased risky choices on poor-odds trials on the Blackjack task, which was associated with a reduced impact that non-rewarded choices had on subsequent choices. DMS inactivation also impaired performance of an auditory conditional discrimination. These findings highlight a previously uncharacterized role for the DMS in facilitating flexible action selection during multiple forms of risk/reward decision making.

Dissociable Motivational Deficits in Pre-manifest Huntington's Disease

Motivation is characterized by a willingness to overcome both cognitive and physical effort costs. Impairments in motivation are common in striatal disorders, such as Huntington’s disease (HD), but whether these impairments are isolated to particular domains of behavior is controversial. We ask whether HD differentially affects the willingness of individuals to overcome cognitive versus physical effort. We tested 20 individuals with pre-manifest HD and compared their behavior to 20 controls. Across separate trials, participants made choices about how much cognitive or physical effort they were willing to invest for reward. Our key results were that individuals with pre-manifest HD were less willing than controls to invest cognitive effort but were no different in their overall preference for physical effort. These results cannot be explained by group differences in neuropsychological or psychiatric profiles. This dissociation of cognitive- and physical-effort-based decisions provides important evidence for separable, domain-specific mechanisms of motivation.

We examine cognitive and physical effort discounting in pre-manifest HD

Individuals with pre-manifest HD are less cognitively motivated than controls

There are no differences in physical motivation between the two groups

This dissociation is not confounded by neuropsychological or psychiatric factors

Dissociable mechanisms underpinning effort-cost decision-making across the psychosis spectrum

Recent theoretical models propose that abnormal effort-cost decision-making (ECDM) likely has divergent underpinnings across mood and psychotic disorders. However, whether this same model applies to individuals across the psychosis spectrum, including individuals with affective psychosis, remains unclear. This study aimed to empirically test whether two component processes - working memory and reward learning - contribute to ECDM impairment across the psychosis spectrum. ECDM was assessed using the Effort Expenditure for Rewards Task in individuals with psychotic disorders (n = 190) and healthy controls (n = 52). Working memory was assessed using a Digit Sequencing Task and reward learning was assessed using a Probabilistic Reward Task. Relative to the control group, the psychosis group showed reduced willingness to expend effort for higher probability, higher value rewards. This effect was most pronounced in individuals with schizophrenia and schizoaffective disorder relative to individuals with psychotic bipolar disorder. Across the whole sample, better working memory but not reward learning predicted greater willingness to expend effort for higher probability rewards. However, the link between working memory and ECDM differed as a function of patient symptom profile. Specifically, working memory was only predictive of ECDM for individuals with less severe negative symptoms and minimal depressive symptoms. For individuals with more severe negative symptoms, poorer ECDM was instead predicted by deficits in reward learning. Although these findings reiterate the important link between working memory and ECDM in individuals with psychotic disorders, they also show that this link varies in accordance with the presence of prominent negative and depressive symptoms.

Evaluation of cognitive effort in rats is not critically dependent on ventrolateral orbitofrontal cortex

Organisms must frequently evaluate the amount of effort to invest in pursuing future rewards. Despite explicit awareness of the potential benefits of cognitive work, individuals vary in their willingness to attempt cognitively demanding tasks, regardless of intellectual ability. Such differences may suggest that the degree to which cognitive effort degrades perceived outcome value is a subjective, rather than objective, process, similar to risk and delay discounting. Although numerous studies suggest the orbitofrontal cortex (OFC) is important for allowing subjective value estimates to be updated and/or used in cost/benefit decision-making, the causal role of the OFC in valuations of mental effort has received scant investigation. We therefore trained 24 female Long-Evans rats on the rodent cognitive effort task (rCET) and assessed performance following temporary bilateral inactivation of the ventrolateral OFC (vlOFC). In the rCET, rats decide at trial outset whether to perform an easy or hard attentional challenge, namely to localize a brief visual stimulus to one of five possible locations. The difficulty of the challenge is determined by the stimulus duration (1.0 vs. 0.2s for easy vs. hard trials respectively), and success on hard trials results in double the sugar pellet rewards. Somewhat surprisingly, inactivations of the vlOFC did not affect rats' willingness or ability to exert cognitive effort for larger rewards, despite increasing omissions and motor impulsivity on-task. When considered with previous work, it appears the vlOFC plays a minimal role in cognitive effort allocation specifically, and in valuations of effort more generally.

Investigating serotonergic contributions to cognitive effort allocation, attention, and impulsive action in female rats

BACKGROUND

Individuals must frequently evaluate whether it is worth allocating cognitive effort for desired outcomes. Motivational deficits are a common feature of psychiatric illness such as major depression. Selective serotonin reuptake inhibitors are commonly used to treat this disorder, yet some data suggest these compounds are ineffective at treating amotivation, and may even exacerbate it.

AIMS

Here we used the rodent Cognitive Effort Task (rCET) to assess serotonergic (5-hydroxytryptamine, 5-HT) contributions to decision-making with cognitive effort costs.

METHODS

The rCET is a modified version of the 5-choice serial reaction time task, a well-validated test of visuospatial attention and impulse control. At the start of each rCET trial, rats chose one of two levers, which set the difficulty of an attentional challenge, namely the localization of a visual stimulus illuminated for 0.2 or 1 s on hard versus easy trials. Successful completion of hard trials was rewarded with double the sugar pellets. Twenty-four female Long-Evans rats were trained on the rCET and systemically administered the 5-HT_1A agonist 8-OH-DPAT, the 5-HT_2A antagonist M100907, the 5-HT_2C agonist Ro-60-0175, as well as the 5-HT_2C antagonist SB 242, 084.

RESULTS

5-HT_2A antagonism dose-dependently reduced premature responding, while 5-HT_2C antagonism had the opposite effect. 8-OH-DPAT impaired accuracy of target detection at higher doses, while Ro-60-0175 dose-dependently improved accuracy on difficult trials. However, none of the drugs affected the rats' choice of the harder option.

CONCLUSION

When considered with existing work evaluating decision-making with physical effort costs, it appears that serotonergic signalling plays a minor role in guiding effort allocation.

Anabolic-androgenic steroids and cognitive effort discounting in male rats

Anabolic-androgenic steroids (AAS) are drugs of abuse that impair behavior and cognition. In a rodent model of AAS abuse, testosterone-treated male rats expend more physical effort, by repeatedly pressing a lever for a large reward in an operant discounting task. However, since modern society prioritizes cognitive over physical effort, it is important to determine if AAS limit cognitive effort. Here we tested the effects of AAS on a novel cognitive-effort discounting task. Each operant chamber had 3 nose-pokes, opposite 2 levers and a pellet dispenser. Rats pressed a lever to illuminate 1 nose-poke; they responded in the illuminated nose-poke to receive sugar pellets. For the 'easy' lever, the light remained on for 1 s, and a correct response earned 1 pellet. For the 'hard' lever, the light duration decreased from 1 s to 0.1 s across 5 blocks of trials, and a correct response earned 4 pellets. As the duration of the nose-poke light decreased, all rats decreased their choice of the hard lever in a modest discounting curve. Task accuracy also decreased significantly across the 5 blocks of trials. However, there was no effect of testosterone on choice of the hard lever or task accuracy. Antagonism of dopamine D1 or D2 receptors had no effect on lever choice or task accuracy. However, serotonin depletion significantly decreased preference for the hard lever, and impaired task accuracy. Thus, physical effort discounting depends on dopamine activity, while cognitive effort discounting task is sensitive to serotonin. AAS impair physical effort discounting, but not cognitive effort discounting.

Deciphering Decision Making: Variation in Animal Models of Effort- and Uncertainty-Based Choice Reveals Distinct Neural Circuitries Underlying Core Cognitive Processes

Maladaptive decision-making is increasingly recognized to play a significant role in numerous psychiatric disorders, such that therapeutics capable of ameliorating core impairments in judgment may be beneficial in a range of patient populations. The field of "decision neuroscience" is therefore in its ascendancy, with researchers from diverse fields bringing their expertise to bear on this complex and fascinating problem. In addition to the advances in neuroimaging and computational neuroscience that contribute enormously to this area, an increase in the complexity and sophistication of behavioral paradigms designed for nonhuman laboratory animals has also had a significant impact on researchers' ability to test the causal nature of hypotheses pertaining to the neural circuitry underlying the choice process. Multiple such decision-making assays have been developed to investigate the neural and neurochemical bases of different types of cost/benefit decisions. However, what may seem like relatively trivial variation in behavioral methodologies can actually result in recruitment of distinct cognitive mechanisms, and alter the neurobiological processes that regulate choice. Here we focus on two areas of particular interest, namely, decisions that involve an assessment of uncertainty or effort, and compare some of the most prominent behavioral paradigms that have been used to investigate these processes in laboratory rodents. We illustrate how an appreciation of the diversity in the nature of these tasks can lead to important insights into the circumstances under which different neural regions make critical contributions to decision making.