Procedural memory in recent-onset Parkinson's disease

Thalamocortical contribution to flexible learning in neural systems

Animal brains evolved to optimize behavior in dynamic environments, flexibly selecting actions that maximize future rewards in different contexts. A large body of experimental work indicates that such optimization changes the wiring of neural circuits, appropriately mapping environmental input onto behavioral outputs. A major unsolved scientific question is how optimal wiring adjustments, which must target the connections responsible for rewards, can be accomplished when the relation between sensory inputs, action taken, and environmental context with rewards is ambiguous. The credit assignment problem can be categorized into context-independent structural credit assignment and context-dependent continual learning. In this perspective, we survey prior approaches to these two problems and advance the notion that the brain's specialized neural architectures provide efficient solutions. Within this framework, the thalamus with its cortical and basal ganglia interactions serves as a systems-level solution to credit assignment. Specifically, we propose that thalamocortical interaction is the locus of meta-learning where the thalamus provides cortical control functions that parametrize the cortical activity association space. By selecting among these control functions, the basal ganglia hierarchically guide thalamocortical plasticity across two timescales to enable meta-learning. The faster timescale establishes contextual associations to enable behavioral flexibility, while the slower one enables generalization to new contexts.

Consolidation of a Learned Skill Correlates with Dopamine SPECT Uptake in Early Parkinson's Disease

BACKGROUND AND PURPOSE

Basal ganglia play a pivotal role in procedural memory. However, the correlation between skill learning and striatal ¹²³I-ioflupane uptake in Parkinson's disease (PD) has not been reported previously. Our objective was to determine whether visuomotor skill learning is associated with striatal ¹²³I-ioflupane uptake in early PD.

METHODS

We designed a case-control study to assess learning and consolidation of a visuomotor learning task (mirrored drawing of star-shaped figures) performed on two consecutive days by early-PD patients (disease duration <2 years) and age-matched healthy subjects. Outcomes were the error rate and time per trial, as well as performance indices to assess the relative improvement on the first day (learning) and the retention on the second day (consolidation). For PD patients, we evaluated the correlation of skill learning with semiquantitative ¹²³I-ioflupane uptake.

RESULTS

We included 9 PD patients and 10 control subjects with the same baseline characteristics (age, male/female ratio, educational level, Mini Mental State Examination score, and Hospital Anxiety and Depression Scale score, all p>0.18) other than the score on part III of the Movement Disorders Society Unified Parkinson's Disease Rating Scale, which was higher in the PD patients (mean±SD: 15.0±10.4 vs. 1.3±1.1, p<0.001). The learning indices were the same in the two groups (p>0.5), whereas PD patients showed a lower consolidation index for the time per trial (p=0.009). Moreover, this performance was correlated with uptake in the right caudate nucleus (Spearman's rho=0.82, p=0.007) and the right striatum (Spearman's rho=0.67, p=0.049), including when multiple linear regression adjusting for the levodopa equivalent daily dose was performed (p=0.005 for the caudate nucleus and p=0.024 for the striatum).

CONCLUSIONS

This study provides evidence of a correlation between procedural memory impairment and striatal dopaminergic dysfunction in early PD.

Automaticity and multiple memory systems

A large number of criteria have been proposed for determining when a behavior has become automatic. Almost all of these were developed before the widespread acceptance of multiple memory systems. Consequently, popular frameworks for studying automaticity often neglect qualitative differences in how different memory systems guide initial learning. Unfortunately, evidence suggests that automaticity criteria derived from these frameworks consistently misclassify certain sets of initial behaviors as automatic. Specifically, criteria derived from cognitive science mislabel much behavior still under the control of procedural memory as automatic, and criteria derived from animal learning mislabel some behaviors under the control of declarative memory as automatic. Even so, neither set of criteria make the opposite error-that is, both sets correctly identify any automatic behavior as automatic. In fact, evidence suggests that although there are multiple memory systems and therefore multiple routes to automaticity, there might nevertheless be only one common representation for automatic behaviors. A number of possible cognitive and cognitive neuroscience models of this single automaticity system are reviewed. WIREs Cogn Sci 2012, 3:363-376. doi: 10.1002/wcs.1172 For further resources related to this article, please visit the WIREs website.

Impaired acquisition of skilled behavior in rotarod task by moderate depletion of striatal dopamine in a pre-symptomatic stage model of Parkinson's disease

In view of recent findings that suggest that the nigrostriatal dopamine (DA) system plays a role in motor control and the acquisition of habits and skills, we hypothesized that the striatum-based function underlying the acquisition of skilled behaviors might be more vulnerable to dopamine depletion than the motor control. To test this hypothesis, we investigated whether impaired acquisition of skilled behaviors occurs in a pre-symptomatic stage model of Parkinson's disease (PD). By using the microdialysis method and the 6-OHDA-technique to destroy dopamine neurons, we confirmed that rats with unilateral partial lesions of the nigral dopamine cells by 6-OHDA are suitable for a pre-symptomatic stage model of Parkinson's disease. The rats in this model exhibited moderate disruption of striatal dopamine release function and relatively intact motor functions. In a rotarod test, the impaired acquisition of skilled behavior occurred in rats with bilateral partial lesions of the nigral dopamine cells by 6-OHDA. These rats displayed intact general motor functions, such as locomotor activity, adjusting steps, equilibrium function and muscle strength. Based on these results, we concluded that the striatum-based function underlying the acquisition of skilled behaviors or sensorimotor learning may be more vulnerable to dopamine depletion than the motor control.

Dynamic Dopamine Modulation in the Basal Ganglia: A Neurocomputational Account of Cognitive Deficits in Medicated and Nonmedicated Parkinsonism

Dopamine (DA) depletion in the basal ganglia (BG) of Parkinson's patients gives rise to both frontal-like and implicit learning impairments. Dopaminergic medication alleviates some cognitive deficits but impairs those that depend on intact areas of the BG, apparently due to DA “overdose.” These findings are difficult to accommodate with verbal theories of BG/DA function, owing to complexity of system dynamics: DA dynamically modulates function in the BG, which is itself a modulatory system. This article presents a neural network model that instantiates key biological properties and provides insight into the underlying role of DA in the BG during learning and execution of cognitive tasks. Specifically, the BG modulates the execution of “actions” (e.g., motor responses and working memory updating) being considered in different parts of the frontal cortex. Phasic changes in DA, which occur during error feedback, dynamically modulate the BG threshold for facilitating/suppressing a cortical command in response to particular stimuli. Reduced dynamic range of DA explains Parkinson and DA overdose deficits with a single underlying dysfunction, despite overall differences in raw DA levels. Simulated Parkinsonism and medication effects provide a theoretical basis for behavioral data in probabilistic classification and reversal tasks. The model also provides novel testable predictions for neuropsychological and pharmacological studies, and motivates further investigation of BG/DA interactions with the prefrontal cortex in working memory.

Procedural learning in perceptual categorization

In two experiments, observers learned two types of category structures: those in which perfect accuracy could be achieved via some explicit rule-based strategy and those in which perfect accuracy required integrating information from separate perceptual dimensions at some predecisional stage. At the end of training, some observers were required to switch their hands on the response keys, whereas the assignment of categories to response keys was switched for other observers. With the rule-based category structures, neither change in response instructions interfered with categorization accuracy. However, with the information-integration structures, switching response key assignments interfered with categorization performance, but switching hands did not. These results are consistent with the hypothesis that abstract category labels are learned in rule-based categorization, whereas response positions are learned in information-integration categorization. The association to response positions also supports the hypothesis of a procedural-learning-based component to information integration categorization.

Psychomotor and cognitive effects of piribedil, a dopamine agonist, in young healthy volunteers

Piribedil is a dopamine agonist acting on D2 and D3 central nervous system dopamine receptors. This drug has been administered to 12 young healthy male volunteers (age 22 +/- 2 years) according to a single center randomized, double-blind, two ways cross-over, placebo controlled trial, including a washout period of one week. Placebo and piribedil were administered by a single intravenous infusion over 2 h (3 mg). Psychomotor performance and cognitive functions were assessed through a standardized and computerized psychometric tests battery and a continuous electroencephalogram (EEG) mapping. Piribedil improved simple reaction time (P=0.02), immediate (P=0.045 and 0.004), and delayed free recall (P=0.05), dual coding test (P=0.02) and increased theta and fast beta waves on the EEG (P < 0.05 and 0.001, respectively). No deleterious effect was observed on the tests exploring attention and concentration via the other procedures. It is concluded that a single intravenous perfusion of piribedil 3 mg improves alertness and the information processing speed within the central nervous system, in healthy volunteers.