Role of the medial and lateral caudate-putamen in mediating an auditory conditional response association

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.

Dorsolateral striatum implicated in the acquisition, but not expression, of immediate response learning in rodent submerged T-maze

Animals can use multiple strategies when learning about, and navigating within, their environment. Typically, in the frequently-studied food-rewarded T-maze, rats initially adopt a flexible, hippocampal-dependent place strategy. However, as learning progresses, rats switch to an automatic, striatal-dependent response strategy (Packard & McGaugh, 1996). Interestingly, in a similar but aversively motivating water-submerged T-maze, rats exhibit the opposite behavioral pattern, initially adopting a response strategy but switching to a place strategy with extended training (Asem & Holland, 2013). Here, we examined the effects of transient lidocaine inactivation of the dorsolateral striatum (DLS) on rats' acquisition and expression of place and response strategies in the submerged T-maze. DLS inactivation prior to probe tests had no effect on rats' initial expression of a response strategy nor on their transition to the use of a place strategy with further training. Nevertheless, in a second experiment using the same rats, identical inactivation parameters significantly affected performance in an appetitively motivating positive control task, which required a response strategy. Furthermore, in a third experiment, DLS inactivation prior to early learning trials interfered with the acquisition of the response strategy in the submerged T-maze. These differences in DLS inactivation effects across appetitive and aversive tasks support the view that task motivation plays crucial roles in guiding learning, memory, and behavior. Additionally, differences in DLS inactivation effects between tests of acquisition and expression suggest that the DLS is required during early acquisition but not expression of the response learning strategy.

Contralateral disconnection of the rat prelimbic cortex and dorsomedial striatum impairs cue-guided behavioral switching

Switches in reward outcomes or reward-predictive cues are two fundamental ways in which information is used to flexibly shift response patterns. The rat prelimbic cortex and dorsomedial striatum support behavioral flexibility based on a change in outcomes. The present experiments investigated whether these two brain regions are necessary for conditional discrimination performance in which a switch in reward-predictive cues occurs every three to six trials. The GABA agonists baclofen and muscimol infused into the prelimbic cortex significantly impaired performance leading rats to adopt an inappropriate turn strategy. The NMDA receptor antagonist D-AP5 infused into the dorsomedial striatum or prelimbic cortex and dorsomedial striatum contralateral disconnection impaired performance due to a rat failing to switch a response choice for an entire trial block in about two out of 13 test blocks. In an additional study, contralateral disconnection did not affect nonswitch discrimination performance. The results suggest that the prelimbic cortex and dorsomedial striatum are necessary to support cue-guided behavioral switching. The prelimbic cortex may be critical for generating alternative response patterns while the dorsomedial striatum supports the selection of an appropriate response when cue information must be used to flexibly switch response patterns.

Dissociable roles of the dorsal striatum and dorsal hippocampus in conditional discrimination and spatial alternation T-maze tasks

The roles of the dorsal hippocampus (DH) and dorsal striatum (DS) in the learning and retention of conditional discrimination (CD) rules is a subject of debate. Although previous studies have examined the relationship between the DH and DS and the performance of CD tasks in operant chambers, the relative contributions of these two brain regions to the retention of CD rules requiring an association between a cue and a spatial location have not been characterized. We designed an experiment to assess the roles of the DH and DS in the retention of a visuospatial CD task by transiently inactivating either structure with muscimol in separate groups of rats and measuring performance on a previously learned CD task. The performance of two other groups of rats on a previously learned delayed spatial alternation (DA) task was also measured following inactivation of either DS or DH, which allowed us to control for any possibly confounding effects of spatial cues present in the testing room, length of the intertrial interval period on the performance of the CD task, and muscimol on sensorimotor or motivational processing. Muscimol inactivation of dorsal striatum, but not dorsal hippocampus, impaired CD performance, while inactivation of dorsal hippocampus, but not dorsal striatum impaired DA performance. These results demonstrate a double dissociation between the roles of the DH and DS in these two tasks, and provide a systematic characterization of the relationship between these two brain areas and CD performance.

Integrating cortico-limbic-basal ganglia architectures for learning model-based and model-free navigation strategies

Behavior in spatial navigation is often organized into map-based (place-driven) vs. map-free (cue-driven) strategies; behavior in operant conditioning research is often organized into goal-directed vs. habitual strategies. Here we attempt to unify the two. We review one powerful theory for distinct forms of learning during instrumental conditioning, namely model-based (maintaining a representation of the world) and model-free (reacting to immediate stimuli) learning algorithms. We extend these lines of argument to propose an alternative taxonomy for spatial navigation, showing how various previously identified strategies can be distinguished as “model-based” or “model-free” depending on the usage of information and not on the type of information (e.g., cue vs. place). We argue that identifying “model-free” learning with dorsolateral striatum and “model-based” learning with dorsomedial striatum could reconcile numerous conflicting results in the spatial navigation literature. From this perspective, we further propose that the ventral striatum plays key roles in the model-building process. We propose that the core of the ventral striatum is positioned to learn the probability of action selection for every transition between states of the world. We further review suggestions that the ventral striatal core and shell are positioned to act as “critics” contributing to the computation of a reward prediction error for model-free and model-based systems, respectively.

Parallel associative processing in the dorsal striatum: segregation of stimulus-response and cognitive control subregions

Although evidence suggests that the dorsal striatum contributes to multiple learning and memory functions, there nevertheless remains considerable disagreement on the specific associative roles of different neuroanatomical subregions. We review evidence indicating that the dorsolateral striatum (DLS) is a substrate for stimulus-response habit formation - incremental strengthening of simple S-R bonds - via input from sensorimotor neocortex while the dorsomedial striatum (DMS) contributes to behavioral flexibility - the cognitive control of behavior - via prefrontal and limbic circuits engaged in relational and spatial information processing. The parallel circuits through dorsal striatum interact with incentive/affective motivational processing in the ventral striatum and portions of the prefrontal cortex leading to overt responding under specific testing conditions. Converging evidence obtained through a detailed task analysis and neurobehavioral assessment is beginning to illuminate striatal subregional interactions and relations to the rest of the mammalian brain.

Developmental effects of reward on sustained attention networks

Adolescence is typified by significant maturation in higher-level attention functions coupled with less developed control over motivation, and enhanced sensitivity to novelty and reward. This study used event-related functional magnetic resonance imaging (fMRI) in seventy male and female participants aged between 10 and 43 years to identify age-related linear changes in cognitive sustained attention systems and the impact of reward on these systems, using a sustained attention task with and without a rewarded condition. For the non-rewarded sustained attention contrast, increasing age was associated with activation increases in typical regions of sustained attention including right inferior frontal, superior temporo-parietal and cerebellar cortices. Age-related activation decreases were observed within more posterior regions including posterior cingulate, insula and posterior cerebellar cortices, presumably mediating visual-spatial saliency detection. The effect of reward on sustained attention networks was associated with increased activation with age in regions associated with both executive attention control and reward processing, including dorsolateral, inferior and ventromedial prefrontal cortices (PFC), striatum, and temporo-parietal regions, suggestive of greater integration and executive control of motivation and cognition with maturity. Activation in paralimbic posterior cingulate and inferior temporal brain regions of visual-spatial saliency processing was progressively reduced in activation with increasing development. Thus, with increasing development between adolescence and adulthood, reward appears to enhance maturing cognitive sustained attention and executive reward-processing networks, whilst reducing paralimbic regions of saliency detection. These findings may be the neural underpinnings for the progressive maturation of motivational control over risk taking behaviours between adolescence and adulthood.

The role of dopamine in the dorsomedial striatum in general and outcome-selective Pavlovian-instrumental transfer

Pavlovian stimuli predictive of appetitive outcomes can influence the selection and initiation of instrumental behaviour. For instance, Pavlovian stimuli can act to enhance those actions with which they share an outcome, but not others with which they do not share an outcome, a phenomenon termed outcome-selective Pavlovian-instrumental transfer (PIT). Furthermore, Pavlovian stimuli can invigorate an action by inducing a general appetitive arousal that elevates instrumental responding, a phenomenon termed general PIT. The dorsomedial striatum has been implicated in outcome-selective, but not general PIT. However, the role of dopamine (DA) signals in this subregion in mediating PIT is unknown. Here we examined in rats the effects of a 6-hydroxydopamine-induced DA depletion of the anterior (aDMS) or posterior (pDMS) subregion of the dorsomedial striatum on outcome-selective and general PIT as well as on instrumental performance on a FR-5 schedule (five lever presses earned one pellet). Results demonstrate that aDMS and pDMS DA depletions compromised the rate of responding on a FR-5 schedule, suggesting that DA signals in the dorsomedial striatum are necessary to maintain high rates of instrumental responding. By contrast, aDMS and pDMS DA depletions did not affect general PIT, suggesting that DA signals in the dorsomedial striatum do not mediate general activating effects of reward-predictive stimuli to invigorate instrumental responding. Furthermore, aDMS DA depletions did not impair outcome-selective PIT, while pDMS DA depletions had no or only minor effects. Thus, DA signals in the DMS may not be involved in mediating the specific cueing effects of reward-predictive stimuli.

Selective activation of striatal fast-spiking interneurons during choice execution

Basal ganglia circuits are essential for the organization and execution of voluntary actions. Within the striatum, fast-spiking interneurons (FSIs) are thought to tightly regulate the activity of medium-spiny projection neurons (MSNs) through feed-forward inhibition, yet few studies have investigated the functional contributions of FSIs in behaving animals. We recorded presumed MSNs and FSIs together with motor cortex and globus pallidus (GP) neurons, in rats performing a simple choice task. MSN activity was widely distributed across the task sequence, especially near reward receipt. By contrast, FSIs showed a coordinated pulse of increased activity as chosen actions were initiated, in conjunction with a sharp decrease in GP activity. Both MSNs and FSIs were direction selective, but neighboring MSNs and FSIs showed opposite selectivity. Our findings suggest that individual FSIs participate in local striatal information processing, but more global disinhibition of FSIs by GP is important for initiating chosen actions while suppressing unwanted alternatives.

Differential dynamics of activity changes in dorsolateral and dorsomedial striatal loops during learning

The basal ganglia are implicated in a remarkable range of functions influencing emotion and cognition as well as motor behavior. Current models of basal ganglia function hypothesize that parallel limbic, associative, and motor cortico-basal ganglia loops contribute to this diverse set of functions, but little is yet known about how these loops operate and how their activities evolve during learning. To address these issues, we recorded simultaneously in sensorimotor and associative regions of the striatum as rats learned different versions of a conditional T-maze task. We found highly contrasting patterns of activity in these regions during task performance and found that these different patterns of structured activity developed concurrently, but with sharply different dynamics. Based on the region-specific dynamics of these patterns across learning, we suggest a working model whereby dorsomedial associative loops can modulate the access of dorsolateral sensorimotor loops to the control of action.

The dorsomedial striatum reflects response bias during learning

Previous studies have established that neurons in the dorsomedial striatum track the behavioral significance of external stimuli, are sensitive to contingencies between actions and outcomes, and show rapid flexibility in representing task-related information. Here, we describe how neural activity in the dorsomedial striatum changes during the initial acquisition of a Go/NoGo task and during an initial reversal of stimulus-response contingencies. Rats made nosepoke responses over delay periods and then received one of two acoustic stimuli. Liquid rewards were delivered after one stimulus (S+) if the rats made a Go response (entering a reward port on the opposite wall of the chamber). If a Go response was made to other stimulus (S-), rats experienced a timeout. On 10% of trials, no stimulus was presented. These trials were used to assess response bias, the animals' tendency to collect reward independent of the stimulus. Response bias increased during the reversal, corresponding to the animals' uncertainty about the stimulus-response contingencies. Most task-modulated neurons fired during the response at the end of the delay period. The fraction of response-modulated neurons was correlated with response bias and neural activity was sensitive to the behavioral response made on the previous trial. During initial task acquisition and initial reversal learning, there was a remarkable change in the percentages of neurons that fired in relation to the task events, especially during withdrawal from the nosepoke aperture. These results suggest that changes in task-related activity in the dorsomedial striatum during learning are driven by the animal's bias to collect rewards.

Dynamic encoding of action selection by the medial striatum

Successful foragers respond flexibly to environmental stimuli. Behavioral flexibility depends on a number of brain areas that send convergent projections to the medial striatum, such as the medial prefrontal cortex, orbital frontal cortex, and amygdala. Here, we tested the hypothesis that neurons in the medial striatum are involved in flexible action selection, by representing changes in stimulus-reward contingencies. Using a novel Go/No-go reaction-time task, we changed the reward value of individual stimuli within single experimental sessions. We simultaneously recorded neuronal activity in the medial and ventral parts of the striatum of rats. The rats modified their actions in the task after the changes in stimulus-reward contingencies. This was preceded by dynamic modulations of spike activity in the medial, but not the ventral, striatum. Our results suggest that the medial striatum biases animals to collect rewards to potentially valuable stimuli and can rapidly influence flexible behavior.

Some highlights of research on the effects of caudate nucleus lesions over the past 200 years

This review describes experiments on the effects of caudate nucleus lesions on behavior in monkeys, cats and rats. Early work on monkeys and cats focused on the relationship of the caudate to the cortex in motor control, leading to the idea that the caudate serves to inhibit behaviors initiated by the cortex. However, investigation of this hypothesis with systematic behavioral testing in all three species did not support this idea; rather, these studies provided evidence that caudate lesions affect memory functions. Two main types of memory tasks were affected. One type involved reinforced stimulus-response (S-R) associations, the other involved spatial information, response-reinforcer contingencies, or working memory. Recent evidence, mainly from rats, suggests that the dorsolateral part of the caudoputamen is central to the processing and consolidation of memory for reinforced S-R associations, and that the more medial and anterior parts of the same structure are part of a neural circuit that (in some cases) also includes the hippocampus, and mediates relational information and certain forms of working memory. The possibility that the spatial distribution of the patch and matrix compartments within the caudoputamen underlies these regional differences is discussed.

A unified framework for addiction: vulnerabilities in the decision process

The understanding of decision-making systems has come together in recent years to form a unified theory of decision-making in the mammalian brain as arising from multiple, interacting systems (a planning system, a habit system, and a situation-recognition system). This unified decision-making system has multiple potential access points through which it can be driven to make maladaptive choices, particularly choices that entail seeking of certain drugs or behaviors. We identify 10 key vulnerabilities in the system: (1) moving away from homeostasis, (2) changing allostatic set points, (3) euphorigenic "reward-like" signals, (4) overvaluation in the planning system, (5) incorrect search of situation-action-outcome relationships, (6) misclassification of situations, (7) overvaluation in the habit system, (8) a mismatch in the balance of the two decision systems, (9) over-fast discounting processes, and (10) changed learning rates. These vulnerabilities provide a taxonomy of potential problems with decision-making systems. Although each vulnerability can drive an agent to return to the addictive choice, each vulnerability also implies a characteristic symptomology. Different drugs, different behaviors, and different individuals are likely to access different vulnerabilities. This has implications for an individual's susceptibility to addiction and the transition to addiction, for the potential for relapse, and for the potential for treatment.

Inactivation of the lateral but not medial dorsal striatum eliminates the excitatory impact of Pavlovian stimuli on instrumental responding

Conditioned stimuli are important guides for behavioral actions. This experiment determined the role of the dorsal striatum in conditioned-stimulus modulation of instrumental responding using the pavlovian-instrumental transfer (PIT) paradigm. Rats received pavlovian training wherein two different auditory stimuli predicted the delivery of two food rewards. Next, rats were trained to perform two instrumental actions earning the same two rewards. Finally, the impact of pavlovian stimuli on instrumental performance was assessed in extinction: the stimuli were periodically presented while rats were free to perform the lever-press response. Before testing, medial or lateral dorsal striatum was infused with saline or baclofen/muscimol, to temporarily inactivate the region. Under saline, outcome-selective PIT was observed: presentation of a stimulus paired with the same outcome as the instrumental action elevated responding, whereas presentation of a stimulus paired with a different outcome did not. Inactivation of the dorsolateral striatum dramatically reduced this effect. Inactivation of the dorsomedial striatum left intact the ability of reward-related stimuli to invigorate responding; however, the selectivity of the stimulus effect was lost (i.e., both stimuli excited responding). These results indicate that subregions of the dorsal striatum play distinct roles in stimulus modulation of instrumental performance with the lateral region being vital for reward-related stimuli to excite responding and the medial region being involved in the integration of stimulus-reward associations with specific response-outcome associations to produce selective responding. These findings identify new roles for the dorsal striatum in mediating the incentive effects of reward-predictive stimuli on behavioral actions made to obtain reward.

Integrating hippocampus and striatum in decision-making

Learning and memory and navigation literatures emphasize interactions between multiple memory systems: a flexible, planning-based system and a rigid, cached-value system. This has profound implications for decision-making. Recent conceptualizations of flexible decision-making employ prospection and projection arising from a network involving the hippocampus. Recent recordings from rodent hippocampus in decision-making situations have found transient forward-shifted representations. Evaluation of that prediction and subsequent action-selection probably occurs downstream (e.g. in orbitofrontal cortex, in ventral and dorsomedial striatum). Classically, striatum has been identified as a crucial component of the less-flexible, incremental system. Current evidence, however, suggests that striatum is involved in both flexible and stimulus-response decision-making, with dorsolateral striatum involved in stimulus-response strategies and ventral and dorsomedial striatum involved in goal-directed strategies.

Single unit and population responses during inhibitory gating of striatal activity in freely moving rats

The striatum is thought to be an essential region for integrating diverse information in the brain. Rapid inhibitory gating (IG) of sensory input is most likely an early factor necessary for appropriate integration to be completed. Gating is currently evaluated in clinical settings and is dramatically altered in a variety of psychiatric illnesses. Basic neuroscience research using animals has revealed specific neural sites involved in IG including the hippocampus, thalamus, brainstem, amygdala and medial prefrontal cortex. The present study investigated local IG in the basal ganglia structure of the striatum using chronic recording microwires. We obtained both single unit activations and local field potentials (LFPs) in awake behaving rats from each wire during the standard two-tone paradigm. Single units responded with different types of activations including a phasic and sustained excitation, an inhibitory response and a combination response that contained both excitatory and inhibitory components. IG was observed in all the response types; however, non-gating was observed in a large proportion of responses as well. Positive wave field potentials at 50-60 ms post-stimulus (P60) showed consistent gating across the wire arrays. No significant correlations were found between single unit and LFP measures of gating during the initial baseline session. Gating was strengthened (Tamp/Camp ratios approaching 0) following acute stress (saline injection) at both the single unit and LFP level due to the reduction in the response to the second tone. Alterations in sensory responding reflected by changes in the neural response to the initial tone were primarily observed following long-term internal state deviation (food deprivation) and during general locomotion. Overall, our results support local IG by single neurons in striatum but also suggest that rapid inhibition is not the dominant activation profile observed in other brain regions.

The role of the medial caudate nucleus, but not the hippocampus, in a matching-to sample task for a motor response

A delayed-match-to-sample task was used to assess memory for motor responses in rats with control, hippocampus, or medial caudate nucleus (MCN) lesions. All testing was conducted on a cheeseboard maze in complete darkness using an infrared camera. A start box was positioned in the centre of the maze facing a randomly determined direction on each trial. On the sample phase, a phosphorescent object was randomly positioned to cover a baited food well in one of five equally spaced positions around the circumference of the maze forming a 180-degree arc 60 cm from the box. The rat had to displace the object to receive food and return to the start box. The box was then rotated to face a different direction. An identical baited phosphorescent object was placed in the same position relative to the start box. A second identical object was positioned to cover a different unbaited well. On the choice phase, the rat must remember the motor response made on the sample phase and make the same motor response on the choice phase to receive a reward. Hippocampus lesioned and control rats improved as a function of increased angle separation used to separate the correct object from the foil (45, 90, 135, and 180 degrees) and matched the performance of controls. However, rats with MCN lesions were impaired across all separations. Results suggest that the MCN, but not the hippocampus, supports working memory and/or a process aimed at reducing interference for motor response selection based on vector angle information.

The contribution of NMDA receptors in the dorsolateral striatum to egocentric response learning

The present study examined the effects of the N-methyl-D-aspartate (NMDA) competitive antagonist, 2-amino-5-phosphonopentanoic acid (AP-5), injected into the dorsolateral striatum on the acquisition and reversal learning of a response discrimination. Male Long-Evans rats were tested across 2 consecutive days in a modified cross-maze. An infusion of either saline or AP-5 (5 or 25 nM) occurred 5 min prior to testing. In acquisition rats learned to turn left or right. In reversal learning rats learned to turn in the opposite direction. An AP-5 infusion at 25 nmol, but not 5 nmol, impaired response acquisition. Neither AP-5 dose impaired response reversal learning. The results suggest that NMDA receptors in the dorsolateral striatum are critical for the initial learning of an egocentric response discrimination.

Lesions of the dorsolateral or dorsomedial striatum impair performance of a previously acquired simple discrimination task

Previous evidence has suggested a specific role for the dorsal striatum, especially the dorsolateral region of the dorsal striatum, in stimulus-response learning. In a previous study, we found an impairment in animals with dorsolateral striatal lesions on a simple discrimination task (CS+/CS-), thought to require the involvement of both stimulus-reward and stimulus-response learning. It is possible that the generally poor performance of dorsolateral lesioned animals on this experiment precluded adequate exposure to stimulus-reward pairings necessary for solving this task, and, thus, had little to do with stimulus-response learning. To test this hypothesis, the performance of animals with dorsolateral and dorsomedial striatal lesions was assessed on a previously acquired simple discrimination task. To independently assess the effects of each lesion on the performance of stimulus-reward learning, dorsolateral and dorsomedial lesioned animals were assessed on a previously acquired conditioned place preference task (CPP). In agreement with our earlier experiment, and the stimulus-response interpretation of dorsolateral striatal function, animals with dorsolateral striatal lesions were found to be impaired during post-lesion performance of the simple discrimination task, but not CPP learning. Additionally, dorsomedial lesioned animals were found to be impaired in performance of the simple discrimination task, but not on the CPP task. Possible explanations for the differences between the role of the dorsomedial striatum in acquisition and expression of the simple discrimination task are proposed.

Lesions of the dorsolateral striatum impair the acquisition of a simplified stimulus-response dependent conditional discrimination task

The dorsal striatum has long been thought to be important for some types of learning and memory, especially stimulus-response learning. Recently, we demonstrated that selective lesions of the dorsolateral striatum, but not dorsomedial striatum in rats, retarded the acquisition of two instrumental discrimination tasks thought to require stimulus-response learning. However, since these studies investigated the effects of dorsal striatal lesions on task acquisition, which can be confounded by differences in level of reinforcement and motor impairment caused by the lesion, the interpretation of these findings was somewhat problematic. The present experiment was designed to address these issues by assessing the effects of lesions of the dorsolateral striatum on a simplified version of the conditional discrimination task, in which the importance of reinforcement and motor factors was minimized. Animals with lesions of the dorsolateral striatum showed marked impairments in learning this task, a finding that is in agreement with the notion that the dorsolateral striatum is necessary for stimulus-response learning.

A study on the role of the dorsal striatum and the nucleus accumbens in allocentric and egocentric spatial memory consolidation

There is now accumulating evidence that the striatal complex in its two major components, the dorsal striatum and the nucleus accumbens, contributes to spatial memory. However, the possibility that different striatal subregions might modulate specific aspects of spatial navigation has not been completely elucidated. Therefore, in this study, two different learning procedures were used to determine whether the two striatal components could be distinguished on the basis of their involvement in spatial learning using different frames of reference: allocentric and egocentric. The task used involved the detection of a spatial change in the configuration of four objects placed in an arena, after the mice had had the opportunity to experience the objects in a constant position for three previous sessions. In the first part of the study we investigated whether changes in the place where the animals were introduced into the arena during habituation and testing could induce a preferential use of an egocentric or an allocentric frame of reference. In the second part of the study we performed focal injections of the N-methyl-d-aspartate (NMDA) receptors' antagonist, AP-5, within the two subregions immediately after training. The results indicate that using the two behavioral procedures, the animals rely on an egocentric and an allocentric spatial frame of reference. Furthermore, they demonstrate that AP-5 (37.5, 75, and 150 ng/side) injections into the dorsal striatum selectively impaired consolidation of spatial information in the egocentric but not in the allocentric procedure. Intra-accumbens AP-5 administration, instead, impaired animals trained using both procedures.

Lesion to the nigrostriatal dopamine system disrupts stimulus-response habit formation

Acquisition and performance of instrumental actions are assumed to require both action-outcome and stimulus-response (S-R) habit processes. Over the course of extended training, control over instrumental performance shifts from goal-directed action-outcome associations to S-R associations that progressively gain domination over behavior. Lesions of the lateral part of the dorsal striatum disrupt this process, and rats with lesions to the lateral striatum showed selective sensitivity to devaluation of the instrumental outcome (Yin et al., 2004), indicating that this area is necessary for habit formation. The present experiment further explored the basis of this dysfunction by examining the ability of rats subjected to bilateral 6-hydroxydopamine lesions of the nigrostriatal dopaminergic pathway to develop behavioral autonomy with overtraining. Rats were given extended training on two cued instrumental tasks associating a stimulus (a tone or a light) with an instrumental action (lever press or chain pull) and a food reward (pellets or sucrose). Both tasks were run daily in separate sessions. Overtraining was followed by a test of goal sensitivity by satiety-specific devaluation of the reward. In control animals, one action (lever press) was insensitive to reward devaluation, indicating that it became a habit, whereas the second action (chain pull) was still sensitive to goal devaluation. This result provides evidence that the development of habit learning may depend on the characteristics of the response. In dopamine-depleted rats, lever press and chain pull remained sensitive to reward devaluation, evidencing a role of striatal dopamine transmission in habit formation.

An analysis of independence and interactions of brain substrates that subserve multiple attributes, memory systems, and underlying processes

It is proposed that memory is organized into event-based, knowledge-based, and rule-based memory systems. Furthermore, each system is composed of the same set of multiple attributes and characterized by a set of process oriented operating characteristics that are mapped onto multiple neural regions and interconnected neural circuits. Based on this theoretical model of memory, it is possible to investigate the independence and interaction among brain regions between any two systems for any of the proposed attributes or processes. This applies also to the investigation of independence and interactions between any two attributes within a system and between processes associated with a system for any of the proposed attributes. In this article, research evidence is presented to suggest that there are both dissociations and interactions between the hippocampus and caudate nucleus in mediating spatial and response attributes within the event-based memory system, between the hippocampus and the parietal cortex in subserving the spatial attribute within the event-based and knowledge-based memory systems, and between the hippocampus and the prefrontal cortex in subserving the spatial attribute within the event-based and rule-based memory systems.

Contributions of striatal subregions to place and response learning

The involvement of different subregions of the striatum in place and response learning was examined using a T-maze. Rats were given NMDA lesions of the dorsolateral striatum (DLS), anterior dorsomedial striatum (ADMS), posterior dorsomedial striatum (PDMS), or sham surgery. They were then trained to retrieve food from the west arm of the maze, starting from the south arm, by turning left at the choice point. After 7 d of training, with four trials a day, a probe test was given in which the starting arm is inserted as the north arm, at the opposite side of the maze. A left turn would indicate a "response" strategy; a right turn, a "place" strategy. The rats were then trained for 7 more days, followed by a second probe test. Unlike rats in the other groups, most of the rats in the PDMS group turned left, using the response strategy on both probe tests. These results suggest that the PDMS plays a role in spatially guided behavior.

Dorsal striatum and stimulus-response learning: lesions of the dorsolateral, but not dorsomedial, striatum impair acquisition of a stimulus-response-based instrumental discrimination task, while sparing conditioned place preference learning

While some evidence suggests that the dorsal striatum is important for stimulus-response learning, disagreement exists about the relative contribution of the dorsolateral and dorsomedial striatum to this form of learning. In the present experiment, the effects of lesions of the dorsolateral and dorsomedial striatum were investigated on two tasks that differentially require the development of stimulus-response learning. The first task utilized an operant conditional discrimination task, which is likely to rely heavily upon stimulus-response learning. The second task looked conditioned place preference learning, a task that is unlikely to require the development of stimulus-response associations. Animals with lesions of the dorsolateral striatum were impaired on the operant conditional discrimination task, but retained the ability to learn the conditioned place preference task. In contrast, animals with lesions of the dorsomedial striatum were not found to be impaired on either task used in the present experiment. These results suggest that the dorsolateral striatum is necessary for the successful acquisition of tasks that place a demand upon stimulus-response learning, while the dorsomedial striatum is not involved in this type of learning.

Dorsal striatum and stimulus-response learning: lesions of the dorsolateral, but not dorsomedial, striatum impair acquisition of a simple discrimination task

In the present experiment, the effects of neurotoxic lesions (quinolinic acid) of the dorsolateral or dorsomedial striatum were investigated on a simple instrumental discrimination task (CS+/CS-). Rats with lesions of the dorsolateral striatum were found to be impaired in the acquisition of this task, as compared to rats with either dorsomedial striatal or sham lesions. Furthermore, dorsolateral striatal lesioned animals had significantly lower levels of responding across the course of discrimination training, as assessed both by overall rate of response during CS+ presentations and number of CS+ trials without a response, despite having shown levels of responding during variable interval training that did not differ from that of sham lesioned animals. In contrast, animals with lesions of the dorsomedial striatum did not show an impairment in acquisition of the present task, but had slightly higher rates of responding during CS- presentations. It is argued that the poor acquisition and low response rates observed in animals with dorsolateral striatal lesions reflect a failure in stimulus-response learning, while the performance of animals with dorsomedial striatal lesions may have been the result of an increase in overall activity rate.

A new rat model of the human serial reaction time task: contrasting effects of caudate and hippocampal lesions

There is often little correspondence between human and animal examples of nondeclarative memory. The serial reaction time task (SRT) is a sequence learning example of human nondeclarative memory that may be suitable for development as an animal model. The SRT is believed to be impaired by basal ganglia, not limbic system damage, but there is uncertainty whether limbic system pathology does in fact leave the SRT unimpaired. We therefore developed a new rat model that closely approximated the human SRT, using intracranial self-stimulation to promote rapid continuous responding to four adjacent nose pokes in a single test session. Intact rats that experienced repeated sequences demonstrated robust interference effects when switched to a random sequence of cued responses (at 4-, 8-, and 12-sequence lengths), unlike intact controls that experienced the random sequences only. The interference effect in the human task is a key measure for nondeclarative sequence learning. Rats with dorsal caudate lesions that experienced massed sequence repetitions showed an interference effect at the four-sequence length only. By contrast, rats with dorsal hippocampal lesions showed an interference effect at all sequence lengths. This new rat SRT model clarifies the basal ganglia-limbic system dichotomy suggested by human work.

Involvement of the dorsomedial striatum in behavioral flexibility: role of muscarinic cholinergic receptors

The present experiments determined whether temporary inactivation or blockade of muscarinic cholinergic receptors in the dorsomedial striatum affects acquisition or reversal learning of a response discrimination. Testing occurred in a modified cross-maze across two consecutive sessions. In the acquisition phase, a rat learned to make a turn to the left or to the right for 10 consecutive correct choices. In the reversal learning phase, a rat learned to turn in the opposite direction as required during acquisition for 10 consecutive correct choices. Experiment 1 investigated the effects of the local anesthetic, 2% bupivacaine, infused into the dorsomedial striatum on acquisition and reversal learning. Experiment 2 examined the effects of the muscarinic cholinergic antagonist, scopolamine injected into the dorsomedial striatum on acquisition and reversal learning. Bupivacaine infusions did not impair acquisition, but did impair reversal learning of the response discrimination. Analysis of the errors indicated that the deficit was not due to perseveration of the previously learned strategy, but to an inability to learn the new strategy. Bilateral injections of scopolamine, 1 or 8 microg/side, did not affect acquisition. Infusions of scopolamine at 8 microg, but not 1 microg, produced a reversal learning deficit. The scopolamine-induced deficit resulted from an inability to learn the new strategy. The results suggest that the dorsomedial striatum is important for behavioral flexibility and that activation of muscarinic cholinergic receptors in this region may facilitate the learning of situationally adaptive response patterns.

Learning and memory functions of the Basal Ganglia

Although the mammalian basal ganglia have long been implicated in motor behavior, it is generally recognized that the behavioral functions of this subcortical group of structures are not exclusively motoric in nature. Extensive evidence now indicates a role for the basal ganglia, in particular the dorsal striatum, in learning and memory. One prominent hypothesis is that this brain region mediates a form of learning in which stimulus-response (S-R) associations or habits are incrementally acquired. Support for this hypothesis is provided by numerous neurobehavioral studies in different mammalian species, including rats, monkeys, and humans. In rats and monkeys, localized brain lesion and pharmacological approaches have been used to examine the role of the basal ganglia in S-R learning. In humans, study of patients with neurodegenerative diseases that compromise the basal ganglia, as well as research using brain neuroimaging techniques, also provide evidence of a role for the basal ganglia in habit learning. Several of these studies have dissociated the role of the basal ganglia in S-R learning from those of a cognitive or declarative medial temporal lobe memory system that includes the hippocampus as a primary component. Evidence suggests that during learning, basal ganglia and medial temporal lobe memory systems are activated simultaneously and that in some learning situations competitive interference exists between these two systems.

A double dissociation within striatum between serial reaction time and radial maze delayed nonmatching performance in rats

Lesions involving the intralaminar thalamic nuclei have been associated with impairments in working memory and intentional motor function in human clinical cases and animal models of amnesia. The intralaminar nuclei have afferent and efferent connections related to striatum. To test whether disruption of striatal function can account for impairments produced by intralaminar lesions, we investigated the effects of striatal lesions on two tasks known to be impaired by intralaminar damage in the rat: radial maze delayed nonmatching (DNM), a measure of spatial working memory, and self-paced serial reaction time (SRT), a measure of intentional response speed. We compared the effects of lesions in four sites: the medial and lateral caudate putamen, nucleus accumbens, and olfactory tubercle. We found that lesions of the medial, accumbens, or tubercle sites impaired DNM performance, and that lesions of the lateral caudate putamen increased choice response time for the SRT task. There was a double dissociation between the effects of the ventral and the lateral lesions on these two tasks. For both tasks, the effects of striatal lesions were qualitatively similar and at least as large as intralaminar lesions in previous studies. These results provide evidence that striatal dysfunction can account for the DNM and SRT impairments produced by intralaminar lesions. The dissociation of functional impairments suggests that lateral sensorimotor areas of caudate putamen are important for responding based on external sensory stimuli and limbic-related areas in ventral striatum are important for responding based on information held in working memory.

Role of the dorsomedial striatum in behavioral flexibility for response and visual cue discrimination learning

These experiments examined the effects of dorsomedial striatal inactivation on the acquisition of a response and visual cue discrimination task, as well as a shift from a response to a visual cue discrimination, and vice versa. In Experiment 1, rats were tested on the response discrimination task followed by the visual cue discrimination task. In Experiment 2, the testing order was reversed. Infusions of 2% tetracaine did not impair acquisition of the response or visual cue discrimination but impaired performance when shifting from a response to a visual cue discrimination, and vice versa. Analysis of the errors revealed that the deficit was not due to perseveration of the previously learned strategy, but to an inability to maintain the new strategy. These results contrast with findings indicating that prelimbic inactivation impairs behavioral flexibility due to perseveration of a previously learned strategy. Thus, specific circuits in the prefrontal cortex and striatum may interact to enable behavioral flexibility, but each region may contribute to distinct processes that facilitate strategy switching.

Role of the dorsomedial striatum in behavioral flexibility for response and visual cue discrimination learning

These experiments examined the effects of dorsomedial striatal inactivation on the acquisition of a response and visual cue discrimination task, as well as a shift from a response to a visual cue discrimination, and vice versa. In Experiment 1, rats were tested on the response discrimination task followed by the visual cue discrimination task. In Experiment 2, the testing order was reversed. Infusions of 2% tetracaine did not impair acquisition of the response or visual cue discrimination but impaired performance when shifting from a response to a visual cue discrimination, and vice versa. Analysis of the errors revealed that the deficit was not due to perseveration of the previously learned strategy, but to an inability to maintain the new strategy. These results contrast with findings indicating that prelimbic inactivation impairs behavioral flexibility due to perseveration of a previously learned strategy. Thus, specific circuits in the prefrontal cortex and striatum may interact to enable behavioral flexibility, but each region may contribute to distinct processes that facilitate strategy switching.