Selective impairments in implicit learning in Parkinson's disease

Retrograde procedural memory is impaired in people with Parkinson's disease with freezing of gait

Background

Freezing of gait (FOG), is associated with impairment of different cognitive functions. Previous studies hypothesized that FOG may be due to a loss of automaticity.

Research question

To explore whether FOG is associated with impairment in cognitive functions, focusing on retrograde procedural memory, the memory responsible for the automatic, implicit stored procedures that have been acquired in earlier life stages.

Methods

In this cross-sectional, case-control study, 288 people with typical Parkinson's disease (PD) from the Luxembourg Parkinson's Study were assigned to Freezers (FOG+) and non-Freezers (FOG-) based on the MDS-UPDRS 2.13 (self-reported FOG episodes) and 3.11 (FOG evaluated by clinicians during gait assessment). Both groups were matched on age, sex and disease duration. Global cognition (MoCA), retrograde procedural memory and visuo-constructive abilities (CUPRO), psychomotor speed and mental flexibility (TMT) were assessed. Furthermore, we repeated our analyses by additionally controlling for depression (BDI-I).

Results

Besides lower global cognition (MoCA; p = 0.007) and mental flexibility (TMT-B and Delta-TMT; p < 0.001), FOG+ showed a lower performance in retrograde procedural memory (CUPRO-IS1; p < 0.001) compared to FOG-. After controlling additionally for depression, our main outcome variable CUPRO-IS1 remained significantly lower in FOG+ (p = 0.010).

Conclusion

Our findings demonstrated that besides lower global cognition and mental flexibility scores, FOG+ showed lower performance in retrograde procedural memory compared to matched FOG-control patients, even when accounting for factors such as age, sex, disease duration or depression.

Significance

In the context of limited treatment options, especially for non-invasive therapeutic approaches, these insights on procedural memory and FOG may lead to new hypotheses on FOG etiology and consequently the development of new treatment options.

Waiting impulsivity in progressive supranuclear palsy-Richardson's syndrome

Background

Waiting impulsivity in progressive supranuclear palsy-Richardson's syndrome (PSP-RS) is difficult to assess, and its regulation is known to involve nucleus accumbens (NAc) subregions. We investigated waiting impulsivity using the "jumping the gun" (JTG) sign, which is defined as premature initiation of clapping before the start signal in the three-clap test and compared clinical features of PSP-RS patients with and without the sign and analyzed neural connectivity and microstructural changes in NAc subregions.

Materials and methods

A positive JTG sign was defined as the participant starting to clap before the start sign in the three-clap test. We classified participants into the JTG positive (JTG +) and JTG negative (JTG-) groups and compared their clinical features, microstructural changes, and connectivity between NAc subregions using diffusion tension imaging. The NAc was parcellated into core and shell subregions using data-driven connectivity-based methods.

Results

Seventy-seven patients with PSP-RS were recruited, and the JTG + group had worse frontal lobe battery (FAB) scores, more frequent falls, and more occurrence of the applause sign than the JTG- group. A logistic regression analysis revealed that FAB scores were associated with a positive JTG sign. The mean fiber density between the right NAc core and right medial orbitofrontal gyrus was higher in the JTG + group than the JTG- group.

Discussion

We show that the JTG sign is a surrogate marker of waiting impulsivity in PSP-RS patients. Our findings enrich the current literature by deepening our understanding of waiting impulsivity in PSP patients and introducing a novel method for its evaluation.

Nomen est omen: Serial reaction time task is not a motor but a visuomotor learning task

The serial reaction time task is a widely used task in behavioural and cognitive neuroscience to assess human and animal learning. Many publications refer to this task as a 'motor learning task', but it is also a perceptual learning task. We emphasize here that the incorrect use of the term 'motor learning' misleads researchers and medical doctors by emphasizing the motor cortex's exclusive role. It has the potential to lead to the misinterpretation of neuroscientific, neuroimaging and clinical studies. The domino effect has the potential to generate more flawed hypotheses and theories.

What neurological diseases tell us about procedural perceptual-motor learning? A systematic review of the literature

INTRODUCTION

Procedural perceptual-motor learning of sequences (PPMLS) provides perceptual-motor skills in many activities of daily living. Based on behavioral and neuroimaging results, theoretical models of PPMLS postulate that the cortico-striatal loop, the cortico-cerebellar loop and the hippocampus are specifically involved in the early stage of PPMLS while the cortico-striatal loop would be specifically involved in the late stage of PPMLS. Hence, current models predict that the early stage of PPMLS should be impaired in Parkinson's disease (PD: lesion of the cortico-striatal loop), in cerebellar disease (CD: lesion of the cortico-cerebellar loop) and in Alzheimer's disease (AD: lesion of the hippocampus), whereas the late stage of PPMLS should be specifically impaired in PD.

OBJECTIVE

The aim of the study is (1) to draw a complete picture of experimental results on PPMLS in PD, CD and AD (2) to understand heterogeneity of results as regard to participant and task characteristics.

METHOD

This review is based on the guideline proposed by the PRISMA statement.

RESULTS

Our review reveals (1) that the experimental results clarify the theoretical models and (2) that the impairment of PPMLS depends on both the personal characteristics of the participants and the characteristics of the task to-be-learnt rather than on the disease itself.

CONCLUSION

Our results highlight that these characteristics should be more carefully considered to understand the heterogeneity of results across studies on PPMLS and the effects of rehabilitation programs.

The Effects of Subthalamic Nucleus Deep Brain Stimulation and Retention Delay on Memory-Guided Reaching Performance in People with Parkinson's Disease

BACKGROUND

Subthalamic nucleus deep brain stimulation (STN-DBS) improves intensive aspects of movement (velocity) in people with Parkinson's disease (PD) but impairs the more cognitively demanding coordinative aspects of movement (error). We extended these findings by evaluating STN-DBS induced changes in intensive and coordinative aspects of movement during a memory-guided reaching task with varying retention delays.

OBJECTIVE

We evaluated the effect of STN-DBS on motor control during a memory-guided reaching task with short and long retention delays in participants with PD and compared performance to healthy controls (HC).

METHODS

Eleven participants with PD completed the motor section of the Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS III) and performed a memory-guided reaching task under four different STN-DBS conditions (DBS-OFF, DBS-RIGHT, DBS-LEFT, and DBS-BOTH) and two retention delays (0.5 s and 5 s). An additional 13 HC completed the memory-guided reaching task.

RESULTS

Unilateral and bilateral STN-DBS improved the MDS-UPDRS III scores. In the memory-guided reaching task, both unilateral and bilateral STN-DBS increased the intensive aspects of movement (amplitude and velocity) in the direction toward HC but impaired coordinative aspects of movement (error) away from the HC. Furthermore, movement time was decreased but reaction time was unaffected by STN-DBS. Shorter retention delays increased amplitude and velocity, decreased movement times, and decreased error, but increased reaction times in the participants with PD. There were no interactions between STN-DBS condition and retention delay.

CONCLUSION

STN-DBS may affect cognitive-motor functioning by altering activity throughout cortico-basal ganglia networks and the oscillatory activity subserving them.

Retrograde Procedural Memory in Parkinson’s Disease: A Cross-Sectional, Case-Control Study

Background:

The analysis of the procedural memory is particularly relevant in neurodegenerative disorders like Parkinson’s disease, due to the central role of the basal ganglia in procedural memory. It has been shown that anterograde procedural memory, the ability to learn a new skill, is impaired in Parkinson’s disease. However, retrograde procedural memory, the long-term retention and execution of skills learned in earlier life stages, has not yet been systematically investigated in Parkinson’s disease.

Objective:

This study aims to investigate retrograde procedural memory in people with Parkinson’s disease. We hypothesized that retrograde procedural memory is impaired in people with Parkinson’s disease compared to an age- and gender-matched control group.

Methods:

First, we developed the CUPRO evaluation system, an extended evaluation system based on the Cube Copying Test, to distinguish the cube copying procedure, representing functioning of retrograde procedural memory, and the final result, representing the visuo-constructive abilities. Development of the evaluation system included tests of discriminant validity.

Results:

Comparing people with typical Parkinson’s disease (n = 201) with age- and gender-matched control subjects (n = 201), we identified cube copying performance to be significantly impaired in people with Parkinson’s disease (p = 0.008). No significant correlation was observed between retrograde procedural memory and disease duration.

Conclusion:

We demonstrated lower cube copying performance in people with Parkinson’s disease compared to control subjects, which suggests an impaired functioning of retrograde procedural memory in Parkinson’s disease.

Altered neural oscillations during complex sequential movements in patients with Parkinson's disease

The sequelae of Parkinson's disease (PD) includes both motor- and cognitive-related symptoms. Although traditionally considered a subcortical disease, there is increasing evidence that PD has a major impact on cortical function as well. Prior studies have reported alterations in cortical neural function in patients with PD during movement, but to date such studies have not examined whether the complexity of multicomponent movements modulate these alterations. In this study, 23 patients with PD (medication "off" state) and 27 matched healthy controls performed simple and complex finger tapping sequences during magnetoencephalography (MEG), and the resulting MEG data were imaged to identify the cortical oscillatory dynamics serving motor performance. The patients with PD were significantly slower than controls at executing the sequences overall, and both groups took longer to complete the complex sequences than the simple. In terms of neural differences, patients also exhibited weaker beta complexity-related effects in the right medial frontal gyrus and weaker complexity-related alpha activity in the right posterior and inferior parietal lobules, suggesting impaired motor sequence execution. Characterizing the cortical pathophysiology of PD could inform current and future therapeutic interventions that address both motor and cognitive symptoms.

Motor Sequence Learning Deficits in Idiopathic Parkinson's Disease Are Associated With Increased Substantia Nigra Activity

Previous studies have shown that persons with Parkinson’s disease (pwPD) share specific deficits in learning new sequential movements, but the neural substrates of this impairment remain unclear. In addition, the degree to which striatal dopaminergic denervation in PD affects the cortico-striato-thalamo-cerebellar motor learning network remains unknown. We aimed to answer these questions using fMRI in 16 pwPD and 16 healthy age-matched control subjects while they performed an implicit motor sequence learning task. While learning was absent in both pwPD and controls assessed with reaction time differences between sequential and random trials, larger error-rates during the latter suggest that at least some of the complex sequence was encoded. Moreover, we found that while healthy controls could improve general task performance indexed by decreased reaction times across both sequence and random blocks, pwPD could not, suggesting disease-specific deficits in learning of stimulus-response associations. Using fMRI, we found that this effect in pwPD was correlated with decreased activity in the hippocampus over time. Importantly, activity in the substantia nigra (SN) and adjacent bilateral midbrain was specifically increased during sequence learning in pwPD compared to healthy controls, and significantly correlated with sequence-specific learning deficits. As increased SN activity was also associated (on trend) with higher doses of dopaminergic medication as well as disease duration, the results suggest that learning deficits in PD are associated with disease progression, indexing an increased drive to recruit dopaminergic neurons in the SN, however, unsuccessfully. Finally, there were no differences between pwPD and controls in task modulation of the cortico-striato-thalamo-cerebellar network. However, a restricted nigral-striatal model showed that negative modulation of SN to putamen connection was larger in pwPD compared to controls during random trials, while no differences between the groups were found during sequence learning. We speculate that learning-specific SN recruitment leads to a relative increase in SN- > putamen connectivity, which returns to a pathological reduced state when no learning takes place.

Fluency adaptation in speakers with Parkinson disease: a motor learning perspective

PURPOSE

Fluency adaptation is characterised by a reduction in stuttering-like behaviours over successive readings of the same speech material and is an effect that is typically observed in developmental stuttering. Prominent theories suggest that short-term motor learning associated with practice explain, in part, fluency adaptation. The current investigation examined the fluency adaptation effect in a group of speakers with Parkinson disease (PD) who exhibited stuttering-like disfluencies.

METHOD

Individuals with PD (n = 21) and neurologically healthy controls (n = 19) read a passage five times. Per cent syllables stuttered was measured and calculated for each reading passage.

RESULT

Participants in the PD group exhibited significantly more stuttering-like disfluencies than control speakers. Twelve individuals in the PD group exhibited at least three per cent syllable stuttered on at least one reading. Statistical trends revealed that the subgroup of individuals with PD who stuttered exhibited a significant reduction in stuttering moments over the five successive readings.

CONCLUSION

A significant fluency adaptation effect was observed for the group of speakers with PD who exhibited stuttering-like disfluencies. Results of the current study are discussed within the framework of the motor learning hypothesis of fluency adaptation.

Preservation of explicit learning of visuomotor sequences during Parkinson's disease progression

While motor learning approaches are effective in rehabilitating Parkinson’s disease (PD) patients, many studies reported deficits in sequential motor learning in these patients. We hypothesised that preserved explicit learning of visuomotor sequences in PD patients contributed to the effectiveness of motor learning approaches. However, there are very few studies analysing explicit learning of visuomotor sequences during the progression of PD. We investigated this phenomenon in 23 patients with moderate to severe PD (Hoehn–Yahr stages II-IV) and 17 age-matched controls using sequential button-press tasks (2 × 5 task). We found (1) no significant differences in numbers of errors in the 2 × 5 task among control and PD groups. (2) There was a significant difference in response times while exploring correct sequences (ERT) among control and PD groups; ERTs in stage-IV patients tended to be longer than those of control and stage-II groups. (3) All four groups significantly improved their performance (i.e., reduced ERTs in the 2 × 5 task) with sequence repetition, although stage-III:IV patients were slower. Thus, even patients with severe PD can learn visual sequences and can translate them into visuomotor sequences (explicit visuomotor sequence learning), albeit slower than controls, providing evidence for effective motor learning approaches during rehabilitation of patients with advanced PD.

Speech Motor Sequence Learning: Acquisition and Retention in Parkinson Disease and Normal Aging

PURPOSE

The aim of the current investigation was to examine speech motor sequence learning in neurologically healthy younger adults, neurologically healthy older adults, and individuals with Parkinson disease (PD) over a 2-day period.

METHOD

A sequential nonword repetition task was used to examine learning over 2 days. Participants practiced a sequence of 6 monosyllabic nonwords that was retested following nighttime sleep. The speed and accuracy of the nonword sequence were measured, and learning was inferred by examining performance within and between sessions.

RESULTS

Though all groups exhibited comparable improvements of the nonword sequence performance during the initial session, between-session retention of the nonword sequence differed between groups. Younger adult controls exhibited offline gains, characterized by an increase in the speed and accuracy of nonword sequence performance across sessions, whereas older adults exhibited stable between-session performance. Individuals with PD exhibited offline losses, marked by an increase in sequence duration between sessions.

CONCLUSIONS

The current results demonstrate that both PD and normal aging affect retention of speech motor learning. Furthermore, these data suggest that basal ganglia dysfunction associated with PD may affect the later stages of speech motor learning. Findings from the current investigation are discussed in relation to studies examining consolidation of nonspeech motor learning.

The many facets of motor learning and their relevance for Parkinson's disease

The final goal of motor learning, a complex process that includes both implicit and explicit (or declarative) components, is the optimization and automatization of motor skills. Motor learning involves different neural networks and neurotransmitters systems depending on the type of task and on the stage of learning. After the first phase of acquisition, a motor skill goes through consolidation (i.e., becoming resistant to interference) and retention, processes in which sleep and long-term potentiation seem to play important roles. The studies of motor learning in Parkinson's disease have yielded controversial results that likely stem from the use of different experimental paradigms. When a task's characteristics, instructions, context, learning phase and type of measures are taken into consideration, it is apparent that, in general, only learning that relies on attentional resources and cognitive strategies is affected by PD, in agreement with the finding of a fronto-striatal deficit in this disease. Levodopa administration does not seem to reverse the learning deficits in PD, while deep brain stimulation of either globus pallidus or subthalamic nucleus appears to be beneficial. Finally and most importantly, patients with PD often show a decrease in retention of newly learned skill, a problem that is present even in the early stages of the disease. A thorough dissection and understanding of the processes involved in motor learning is warranted to provide solid bases for effective medical, surgical and rehabilitative approaches in PD.

Implicit Motor Sequence Learning in Individuals with Parkinson Disease: A Meta-Analysis

BACKGROUND

Deficits in implicit motor sequence learning (IMSL) in individuals with Parkinson disease (PD) compared to age matched healthy controls (HC) are unclear.

OBJECTIVE

The purpose of this paper is to present results of a systematic review with a meta-analysis examining the hypothesis that IMSL is impaired in individuals with PD when compared to HC.

METHODS

Fifteen articles met our final criteria and assessed 299 individuals with PD and 244 HC. Raw mean and standard deviation data for the final block of repeated and final block of random practice trials were obtained to calculate sequence-specific learning (SSL) for individuals with PD and HC. Forest plots were used to depict the comparison of the groups by assessing standardized mean difference with random effect size.

RESULTS

A significant and moderate effect size, 0.83 was found suggesting that individuals with PD demonstrated impaired SSL of motor sequences compared to HC.

CONCLUSIONS

Individuals with PD demonstrate a deficit compared with HC in their ability to implicitly learn motor tasks. Existing research lacks detail on the factors which may alter IMSL, either negatively or positively, such as the design features of current IMSL paradigms utilized and disease-specific characteristics. Successful motor rehabilitation of functional tasks in persons with PD is highly dependent on IMSL; therefore, an improved knowledge of the influence of these additional variables is critical.

Motor Sequence Learning and Consolidation in Unilateral De Novo Patients with Parkinson's Disease

Previous research investigating motor sequence learning (MSL) and consolidation in patients with Parkinson’s disease (PD) has predominantly included heterogeneous participant samples with early and advanced disease stages; thus, little is known about the onset of potential behavioral impairments. We employed a multisession MSL paradigm to investigate whether behavioral deficits in learning and consolidation appear immediately after or prior to the detection of clinical symptoms in the tested (left) hand. Specifically, our patient sample was limited to recently diagnosed patients with pure unilateral PD. The left hand symptomatic (LH-S) patients provided an assessment of performance following the onset of clinical symptoms in the tested hand. Conversely, right hand affected (left hand asymptomatic, LH-A) patients served to investigate whether MSL impairments appear before symptoms in the tested hand. LH-S patients demonstrated impaired learning during the initial training session and both LH-S and LH-A patients demonstrated decreased performance compared to controls during the next-day retest. Critically, the impairments in later learning stages in the LH-A patients were evident even before the appearance of traditional clinical symptoms in the tested hand. Results may be explained by the progression of disease-related alterations in relevant corticostriatal networks.

TJ, Lo SE, Ghosh PT, Howard Jr. JH, Howard DV. Implicit sequence learning in people with Parkinson's disease

Implicit sequence learning involves learning about dependencies in sequences of events without intent to learn or awareness of what has been learned. Sequence learning is related to striatal dopamine levels, striatal activation, and integrity of white matter connections. People with Parkinson's disease (PD) have degeneration of dopamine-producing neurons, leading to dopamine deficiency and therefore striatal deficits, and they have difficulties with sequencing, including complex language comprehension and postural stability. Most research on implicit sequence learning in PD has used motor-based tasks. However, because PD presents with motor deficits, it is difficult to assess whether learning itself is impaired in these tasks. The present study used an implicit sequence learning task with a reduced motor component, the Triplets Learning Task (TLT). People with PD and age- and education-matched healthy older adults completed three sessions (each consisting of 10 blocks of 50 trials) of the TLT. Results revealed that the PD group was able to learn the sequence, however, when learning was examined using a Half Blocks analysis (Nemeth et al., 2013), which compared learning in the 1st 25/50 trials of all blocks to that in the 2nd 25/50 trials, the PD group showed significantly less learning than Controls in the 2nd Half Blocks, but not in the 1st. Nemeth et al. (2013) hypothesized that the 1st Half Blocks involve recall and reactivation of the sequence learned, thus reflecting hippocampal-dependent learning, while the 2nd Half Blocks involve proceduralized behavior of learned sequences, reflecting striatal-based learning. The present results suggest that the PD group had intact hippocampal-dependent implicit sequence learning, but impaired striatal-dependent learning. Thus, sequencing deficits in PD are likely due to striatal impairments, but other brain systems, such as the hippocampus, may be able to partially compensate for striatal decline to improve performance.

Encoding of sequence boundaries in the subthalamic nucleus of patients with Parkinson's disease

Sequential behaviour is widespread not only in humans but also in animals, ranging in different degrees of complexity from locomotion to birdsong or music performance. The capacity to learn new motor sequences relies on the integrity of basal ganglia-cortical loops. In Parkinson's disease the execution of habitual action sequences as well as the acquisition of novel sequences is impaired partly due to a deficiency in being able to generate internal cues to trigger movement sequences. In addition, patients suffering from Parkinson's disease have difficulty initiating or terminating a self-paced sequence of actions. Direct recordings from the basal ganglia in these patients show an increased level of beta (14-30 Hz) band oscillatory activity associated with impairment in movement initiation. In this framework, the current study aims to evaluate in patients with Parkinson's disease the neuronal activity in the subthalamic nucleus related to the encoding of sequence boundaries during the explicit learning of sensorimotor sequences. We recorded local field potential activity from the subthalamic nucleus of 12 patients who underwent deep brain stimulation for the treatment of advanced Parkinson's disease, while the patients in their usual medicated state practiced sequences of finger movements on a digital piano with corresponding auditory feedback. Our results demonstrate that variability in performance during an early phase of sequence acquisition correlates across patients with changes in the pattern of subthalamic beta-band oscillations; specifically, an anticipatory suppression of beta-band activity at sequence boundaries is linked to better performance. By contrast, a more compromised performance is related to attenuation of beta-band activity before within-sequence elements. Moreover, multivariate pattern classification analysis reveals that differential information about boundaries and within-sequence elements can be decoded at least 100 ms before the keystroke from the amplitude of oscillations of subthalamic nucleus activity across different frequency bands, not just from the beta-band. Additional analysis was performed to assess the strength of how much the putative signal encoding class of ordinal position (boundaries, within-sequence elements) is reflected in each frequency band. This analysis demonstrates that suppression of power in the beta-band contains the most class-related information, whereas enhancement of gamma band (31-100 Hz) activity is the second main contributor to the encoding. Our findings support the hypothesis that subthalamic nucleus-mediated gating of salient boundary elements during sequence encoding may be a prerequisite for the adequate acquisition of action sequences and the transition to habitual behaviour.

The effect of haptic cues on motor and perceptual based implicit sequence learning

We introduced haptic cues to the serial reaction time (SRT) sequence learning task alongside the standard visual cues to assess the relative contributions of visual and haptic stimuli to the formation of motor and perceptual memories. We used motorized keys to deliver brief pulse-like displacements to the resting fingers, expecting that the proximity and similarity of these cues to the subsequent response motor actions (finger-activated key-presses) would strengthen the motor memory trace in particular. We adopted the experimental protocol developed by Willingham (1999) to explore whether haptic cues contribute differently than visual cues to the balance of motor and perceptual learning. We found that sequence learning occurs with haptic stimuli as well as with visual stimuli and we found that irrespective of the stimuli (visual or haptic) the SRT task leads to a greater amount of motor learning than perceptual learning.

The effects of practice on the concurrent performance of a speech and postural task in persons with Parkinson disease and healthy controls

Purpose. Persons with Parkinson disease (PD) demonstrate deficits in motor learning as well as bidirectional interference (the performance of one task concurrently interferes with the performance of another task) during dual-task performance. Few studies have examined the practice dosages necessary for behavioral change in rehabilitation relevant tasks. Therefore, to compare the effects of age and PD on motor learning during dual-task performance, this pilot study examined persons with PD as well as neurologically healthy participants during concurrent performance of postural and speaking tasks. Methods. Seven persons with PD and 7 healthy age-matched and 10 healthy young control subjects were tested in a motion capture facility. Task performances were performed concurrently and recorded during 3 time periods (acquisition (beginning and ending), 48-hour retention, and 1-week retention). Postural control and speech articulatory acoustic variables were measured. Results. Healthy young participants consistently performed better than other groups on all measured postural and speech variables. Healthy young participants showed decreased variability at retention, while persons with PD and healthy age-matched controls were unable to consistently improve their performance as a result of practice. No changes were noted in the speech variables. Conclusion. The lack of consistent changes in motor performance in any of the tasks, except in the healthy young group, suggests a decreased efficiency of motor learning in the age-matched and PD groups and argues for increased practice dosages during balance training.

Correlations between brain activity and components of motor learning in middle-aged adults: an fMRI study

Implicit learning may be shown by improvements in motor performance, which occur unconsciously with practice and are typically restricted to the task that was practiced. The purpose of this study was to examine behaviorally relevant brain activation associated with change in motor behavior during sequence-specific motor learning of a perceptuomotor continuous tracking (CT) task in middle-aged adults. To gain further insight into the neural structures associated with change in motor behavior, overall improvement in tracking (root mean square error; RMSE) was decomposed into two components—temporal precision and spatial accuracy. We hypothesized that individual differences in CT task performance would be evident in unique networks of brain activation that supported overall tracking behavior as well-temporal and spatial tracking accuracy. A group of middle-aged healthy individuals performed the CT task, which contains repeated and random segments for seven days. Functional magnetic resonance imaging (fMRI) data was collected on the first and seventh day while the participants performed the task. Subjects did not gain explicit awareness of the sequence. To assess behaviorally-relevant changes in the blood oxygenation level-dependent (BOLD) response associated with individual sequence-specific tracking performance, separate statistical images were created for each participant and weighted by the difference score between repeated and random performance for days 1 and 7. Given the similarity of performance for random and repeated sequences during early practice, there were no unique networks evident at day 1. On Day 7 the resultant group statistical fMRI image demonstrated a positive correlation between RMSE difference score and bilateral cerebellar activation (lobule VI). In addition, individuals who showed greater sequence-specific temporal precision demonstrated increased activation in the precentral gyrus, middle occipital gyrus, and putamen of the right hemisphere and the thalamus, cuneus, and cerebellum of the left hemisphere. Activation of this neural network further confirms its involvement in timing of movements as it has been previously associated with task performance when individuals are instructed to emphasize speed over accuracy. In the present study, behavioral performance was associated with neural correlates of individual variation in motor learning that characterized the ability to implicitly learn a sequence-specific CT task.

Diminished activation of motor working-memory networks in Parkinson's disease

Parkinson's disease (PD) is characterized by typical extrapyramidal motor features and increasingly recognized non-motor symptoms such as working memory (WM) deficits. Using functional magnetic resonance imaging (fMRI), we investigated differences in neuronal activation during a motor WM task in 23 non-demented PD patients and 23 age- and gender-matched healthy controls. Participants had to memorize and retype variably long visuo-spatial stimulus sequences after short or long delays (immediate or delayed serial recall). PD patients showed deficient WM performance compared to controls, which was accompanied by reduced encoding-related activation in WM-related regions. Mirroring slower motor initiation and execution, reduced activation in motor structures such as the basal ganglia and superior parietal cortex was detected for both immediate and delayed recall. Increased activation in limbic, parietal and cerebellar regions was found during delayed recall only. Increased load-related activation for delayed recall was found in the posterior midline and the cerebellum. Overall, our results demonstrate that impairment of WM in PD is primarily associated with a widespread reduction of task-relevant activation, whereas additional parietal, limbic and cerebellar regions become more activated relative to matched controls. While the reduced WM-related activity mirrors the deficient WM performance, the additional recruitment may point to either dysfunctional compensatory strategies or detrimental crosstalk from “default-mode” regions, contributing to the observed impairment.

Partial dopamine depletion in MPTP-treated mice differentially altered motor skill learning and action control

Recent findings suggest that the neurotransmitter dopamine (DA) system plays a role in motor control and the acquisition of habits and skills. However, isolating DA-mediated motor learning from motor performance remains challenging as most studies include often severely DA-depleted mice. Using the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), we investigated the effect of various degrees of DA-depletion in mice on three tests of motor behaviors: the accelerating rotarod, wire suspension and pole tests. Three protocols were performed to decrease DA synthesis to various extents: 4 injections (i.p.) of 9 mg/kg in 1 day; 4 injections (i.p.) of 15 mg/kg in 1 day; or 5 injections (s.c.) of 30 mg/kg in 5 days. Severity of DA-depletion was assessed by the evaluation of tyrosine hydroxylase (TH) and dopamine transporter levels in the striatum using the Western blot technique. Mice were gathered into four different groups according their TH levels: mild, moderate, marked and severe. In these mice, the general motor abilities such as coordination, motion speed and muscular strength were relatively intact whereas impaired acquisition of skilled behavior occurred in mice with marked and severe reduction in TH levels. Marked and severely DA-depleted mice exhibited lower scores within the first trials of the first training day as well as a much slower progression in the following days on the accelerating rotarod. Based on these results, we conclude that the learning of a skilled behavior is more vulnerable to DA depletion than the DA-mediated control of motor activity.

Parallel contributions of cerebellar, striatal and M1 mechanisms to motor sequence learning

When learning a new motor sequence, we must execute the correct order of movements while simultaneously optimizing sensorimotor parameters such as trajectory, timing, velocity and force. Neurophysiological studies in animals and humans have identified the major brain regions involved in sequence learning, including the motor cortex (M1), basal ganglia (BG) and cerebellum. Current models link these regions to different stages of learning (early vs. late) or different components of performance (spatial vs. sensorimotor). At the same time, research in motor control has given rise to the concept that internal models at different levels of the motor system may contribute to learning. The goal of this review is to develop a new framework for motor sequence learning that combines stage and component models within the context of internal models. To do this, we review behavioral and neuroimaging studies in humans and neurophysiological studies in animals. Based on this evidence, we present a model proposing that sequence learning is underwritten by parallel, interacting processes, including internal model formation and sequence representation, that are instantiated in specific cerebellar, BG or M1 mechanisms depending on task demands and the stage of learning. The striatal system learns predictive stimulus-response associations and is critical for motor chunking. The role of the cerebellum is to acquire the optimal internal model for sequence performance in a particular context, and to contribute to error correction and control of on-going movement. M1 acts to store the representation of a learned sequence, likely as part of a distributed network including the parietal lobe and premotor cortex.

Preservation of function in Parkinson's disease: what's learning got to do with it?

Dopamine denervation gives rise to abnormal corticostriatal plasticity; however, its role in the symptoms and progression of Parkinson's disease (PD) has not been articulated or incorporated into current clinical models. The 'integrative selective gain' framework proposed here integrates dopaminergic mechanisms known to modulate basal ganglia throughput into a single conceptual framework: (1) synaptic weights, the neural instantiation of accumulated experience and skill modulated by dopamine-dependent plasticity and (2) system gain, the operating parameters of the basal ganglia, modulated by dopamine's on-line effects on cell excitability, glutamatergic transmission and the balance between facilitatory and inhibitory pathways. Within this framework and based on recent work, a hypothesis is presented that prior synaptic weights and established skills can facilitate motor performance and preserve function despite diminished dopamine; however, dopamine denervation induces aberrant corticostriatal plasticity that degrades established synaptic weights and replaces them with inappropriate, inhibitory learning that inverts the function of the basal ganglia resulting in 'anti-optimization' of motor performance. Consequently, mitigating aberrant corticostriatal plasticity represents an important therapeutic objective, as reflected in the long-duration response to levodopa, reinterpreted here as the correction of aberrant learning. It is proposed that viewing aberrant corticostriatal plasticity and learning as a provisional endophenotype of PD would facilitate investigation of this hypothesis.

Basal ganglia-dependent processes in recalling learned visual-motor adaptations

Humans learn and remember motor skills to permit adaptation to a changing environment. During adaptation, the brain develops new sensory-motor relationships that become stored in an internal model (IM) that may be retained for extended periods. How the brain learns new IMs and transforms them into long-term memory remains incompletely understood since prior work has mostly focused on the learning process. A current model suggests that basal ganglia, cerebellum, and their neocortical targets actively participate in forming new IMs but that a cerebellar cortical network would mediate automatization. However, a recent study (Marinelli et al. 2009) reported that patients with Parkinson's disease (PD), who have basal ganglia dysfunction, had similar adaptation rates as controls but demonstrated no savings at recall tests (24 and 48 h). Here, we assessed whether a longer training session, a feature known to increase long-term retention of IM in healthy individuals, could allow PD patients to demonstrate savings. We recruited PD patients and age-matched healthy adults and used a visual-motor adaptation paradigm similar to the study by Marinelli et al. (2009), doubling the number of training trials and assessed recall after a short and a 24-h delay. We hypothesized that a longer training session would allow PD patients to develop an enhanced representation of the IM as demonstrated by savings at the recall tests. Our results showed that PD patients had similar adaptation rates as controls but did not demonstrate savings at both recall tests. We interpret these results as evidence that fronto-striatal networks have involvement in the early to late phase of motor memory formation, but not during initial learning.

Motor sequence learning performance in Parkinson's disease patients depends on the stage of disease

It is still unclear, whether patients with Parkinson's disease (PD) are impaired in the incidental learning of different motor sequences in short succession, although such a deficit might greatly impact their daily life. The aim of this study was thus to clarify the relation between disease parameters of PD and incidental motor learning of two different sequences in short succession. Results revealed that the PD patients were able to acquire two sequences in short succession but needed more time than healthy subjects. However, both the severity of axial manifestations, as assessed on a subsection of the Unified Parkinson's Disease Rating Scale III (UPDRS III) and the Hoehn and Yahr score, and the levodopa-equivalent dose (LED) were negatively correlated with the sequence learning performance. These findings indicate that, although PD patients are able to learn two sequences in short succession, they need more time and their overall sequence learning performance is strongly correlated with the stage of disease.

Effects of dopamine medication on sequence learning with stochastic feedback in Parkinson's disease

A growing body of evidence suggests that the midbrain dopamine system plays a key role in reinforcement learning and disruption of the midbrain dopamine system in Parkinson's disease (PD) may lead to deficits on tasks that require learning from feedback. We examined how changes in dopamine levels ("ON" and "OFF" their dopamine medication) affect sequence learning from stochastic positive and negative feedback using Bayesian reinforcement learning models. We found deficits in sequence learning in patients with PD when they were "ON" and "OFF" medication relative to healthy controls, but smaller differences between patients "OFF" and "ON". The deficits were mainly due to decreased learning from positive feedback, although across all participant groups learning was more strongly associated with positive than negative feedback in our task. The learning in our task is likely mediated by the relatively depleted dorsal striatum and not the relatively intact ventral striatum. Therefore, the changes we see in our task may be due to a strong loss of phasic dopamine signals in the dorsal striatum in PD.

Movement chunking during sequence learning is a dopamine-dependant process: a study conducted in Parkinson's disease

Chunking of single movements into integrated sequences has been described during motor learning, and we have recently demonstrated that this process involves a dopamine-dependant mechanism in animal (Levesque et al. in Exp Brain Res 182:499-508, 2007; Tremblay et al. in Behav Brain Res 198:231-239, 2009). However, there is no such evidence in human. The aim of the present study was to assess this question in Parkinson's disease (PD), a neurological condition known for its dopamine depletion in the striatum. Eleven PD patients were tested under their usual levodopa medication (ON state), and following a 12-h levodopa withdrawal (OFF state). Patients were compared with 12 healthy participants on a motor learning sequencing task, requiring pressing fourteen buttons in the correct order, which was determined by visual stimuli presented on a computer screen. Learning was assessed from three blocks of 20 trials administered successively. Chunks of movements were intrinsically created by each participant during this learning period. Then, the sequence was shuffled according to the participant's own chunks, generating two new sequences, with either preserved or broken chunks. Those new motor sequences had to be performed separately in a fourth and fifth blocks of 20 trials. Results showed that execution time improved in every group during the learning period (from blocks 1 to 3). However, while motor chunking occurred in healthy controls and ON-PD patients, it did not in OFF-PD patients. In the shuffling conditions, a significant difference was seen between the preserved and the broken chunks conditions for both healthy participants and ON-PD patients, but not for OFF-PD patients. These results suggest that movement chunking during motor sequence learning is a dopamine-dependent process in human.

Improvement of postural adjustment steps in hemiparkinsonian rats chronically treated with caffeine is mediated by concurrent blockade of A1 and A2A adenosine receptors

Chronic treatment with the non-selective adenosine receptor antagonist caffeine produces full recovery of the contralateral adjusting steps in hemiparkinsonian rats. In order to disclose which adenosine receptor subtype mediates this effect, a group of hemiparkinsonian rats (n=9) was treated with caffeine (5.15 mumol/kg/day), or equimolar doses of selective A1 (DPCPX) or A2A (ZM 241385) adenosine receptor antagonists, administered in a counterbalanced order over periods of 3 weeks, interspersed with equivalent washout intervals. Treatment with ZM 241385 caused full recovery (102+/-6%) of the contralateral forepaw stepping, while the maximal effect of DPCPX was only 73+/-7% of that produced by caffeine. The maximal effect of caffeine and ZM 241385 remained stable throughout the treatment period. The response to DPCPX showed more fluctuations, but tolerance did not develop. Stepping improvement was significantly faster with DPCPX than with ZM 241385, while caffeine had intermediate values. Stepping decrease after treatment interruption was faster with ZM 241385 than with caffeine, while DPCPX had intermediate values. In other experiments with the same rats, addition of the A2AR agonist CGS 21680 (5.15 mumol/kg) or the A1R agonist CCPA (2.71 mumol/kg) during the second week of caffeine treatment reversed the improvement of contralateral stepping by 59+/-4% and 30+/-3%, respectively. The combined treatment with CGS 21680 and CCPA caused complete reversal of the contralateral stepping recovery afforded by caffeine, which was more than additive (114+/-5%) compared with the sum of the maximal inhibition produced by either agonist administered alone (89+/-4%). In all cases, after interrupting the adenosine agonists, the effect of caffeine was fully restored. None of the aforementioned treatments induced significant changes in the stepping of the ipsilateral forepaw. Collectively, these results suggest that the improvement of postural adjustments induced by chronic treatment with low doses of caffeine in hemiparkinsonian rats is mediated by concurrent blockade of A1 and A2A adenosine receptors, with a larger involvement of the latter.

Effect of dopaminergic medications on the time course of explicit motor sequence learning in Parkinson's disease

The capacity to learn new motor sequences is fundamental to adaptive motor behavior. The early phase of motor sequence learning relies on the ventral and anterior striatal circuitry, whereas the late phase relies on the dorsal and posterior striatal circuitry. Early Parkinson's disease (PD) is mainly characterized by dopaminergic denervation of the dorsal and posterior striatum while sparing anterior and ventral regions. Dopaminergic medication improves dorsal and posterior striatum function by compensating for the loss of dopamine. However, previous work has shown that dopaminergic medication interferes with the ventral and anterior striatum function by overdosing this relatively intact structure in early-state PD. Here we test whether these effects are also observed over the time course of motor sequence learning. Fourteen PD patients ON and OFF dopaminergic medications and 11 healthy age-matched control participants performed an explicit motor sequence learning task. When sequence learning was compared across different learning phases in patients ON and OFF medication, a significant impairment associated with medication was observed in the early relative to later phases of learning. The rate of learning in the early phase measured trial by trial in patients ON medication was significantly slower than that in controls and when patients were OFF medication. No significant impairment was found in the later learning phases. These results demonstrate that dopaminergic medications may selectively impair early-phase motor sequence learning. These results extend and generalize the dopamine overdose effects previously reported for (antero)ventral striatum-mediated cognitive tasks to motor sequence learning.

Lesions of the premotor and supplementary motor areas fail to prevent implicit learning in the operant serial implicit learning task

An implicit learning deficit in people with Huntington's or Parkinson's diseases has implicated the striatum as being of importance for non-declarative learning. We have sought to identify the neurological substrate of this function using a Serial Implicit Learning Task (SILT), an operant task that requires the animal to produce 2-phase (S1 and S2) sequential nose pokes to receive a reward in the nine-hole box apparatus. Differences in performance on the speed and accuracy of responding to stimuli occurring in predictable locations over those to unpredictable locations provide an index of implicit learning, within the context of generalised performance of a skilled motor habit. Previous studies with striatal lesions demonstrated clear functional deficits on the SILT that implicated a generalised impairment in the speed and accuracy of skilled motor performance, whereas the specific implicit learning component of the task remained intact. Since imaging studies in man have identified the premotor and supplementary motor area (SMA) of the cortex as being of importance in implicit learning, we here explore the effects of similar lesions in animals on performance of the SILT. Premotor and SMA lesions produced a generalised impairment in both the accuracy and reaction time measures of SILT performance, whereas - like striatal lesions - they remained able to utilise the benefit of predictable information. A similar profile of impairments was apparent both in animals pretrained on the task prior to lesion, and in animals trained under acquisition post-lesion. The presented results suggest that the premotor and SMA are not essential for implicit learning, but are important in the performance of sequenced motor tasks.

Motor sequence learning and movement disorders

PURPOSE OF REVIEW

New insights into the psychophysiological determinants of performance changes and brain plasticity associated with motor sequence learning have recently been gained through behavioral and imaging studies in healthy individuals. In addition, using a variety of motor sequential paradigms in groups of patients affected by a movement disorder, major advances have been achieved in our understanding of the pathophysiological mechanisms underlying Parkinson's and Huntington's diseases, as well as primary forms of dystonia.

RECENT FINDINGS

This review begins by describing the latest findings in normal participants with regards to the dynamic alterations in neural networks observed across the different phases of motor sequence learning. It then focuses on the hotly debated issue of motor memory consolidation, highlighting the results of novel studies that investigated the role of both day and night sleep, the neural substrates and the developmental evolution mediating this process. Finally, this paper addresses current work looking at motor sequence learning in movement disorders that helps to better comprehend the functional contribution of basal ganglia structures to this type of memory, to assess the impact of such diseases on related patterns of brain activation, as well as to identify the neuronal compensatory mechanisms educed by these basal ganglia disorders.

SUMMARY

Such advances have major implications, not only for optimizing ways to learn new skilled behaviors in real-life situations, but also for guiding therapeutic approaches in patients with movement disorders.

Striatal lesions in the mouse disrupt acquisition and retention, but not implicit learning, in the SILT procedural motor learning task

People with Huntington's disease (HD) have been found to have an implicit learning deficit whereby they are typically unable to detect repeated sequences embedded within randomly presented stimuli. The operant serial implicit learning task (SILT) was designed to probe animal models of HD for implicit learning deficits using the 9-hole box apparatus. The present study used mice to determine whether "early" striatal lesions would prevent SILT acquisition and to confirm previous findings that post-training "late lesions" would impair the retention of task performance. The SILT is a two-phase task whereby an initial stimulus light (S1) presentation was presented in one of five possible locations. A correct nose-poke response to the S1 resulted in this light being extinguished and a second, apparently random light presentation (S2). A correct nose-poke to S2 resulted in a reward. Within the apparently random stimulus light presentations, a predictable S1/S2 combination was embedded. Both lesion groups ("early" pre-acquisition and "late" post-acquisition lesions) demonstrated increased reaction times to S1, with the late-lesion group also recording reduced task accuracy when compared with the sham control group. The early-lesion group also demonstrated increased response latencies for the S2 stimuli during task acquisition, this was also true for task retention in the late-lesion group. No difference between the control group and early-lesion group was found for the S2 response accuracy during the acquisition period. After the lesioning of the late-lesion group, both lesion groups demonstrated reduced accuracy to the S2 stimuli as the control group improved their performance throughout the test period, while the accuracy of both lesion groups remained stable at a lower performance level. All three experimental groups were able to utilize the embedded predictable information. The present data suggest that the striatum is important for the acquisition and retention of motor learning tasks, but does not play a role in the learning of implicit information.