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

Intensity-dependent effects of tDCS on motor learning are related to dopamine

Non-invasive brain stimulation techniques, such as transcranial direct current stimulation (tDCS), are popular methods for inducing neuroplastic changes to alter cognition and behaviour. One challenge for the field is to optimise stimulation protocols to maximise benefits. For this to happen, we need a better understanding of how stimulation modulates cortical functioning/behaviour. To date, there is increasing evidence for a dose-response relationship between tDCS and brain excitability, however how this relates to behaviour is not well understood. Even less is known about the neurochemical mechanisms which may drive the dose-response relationship between stimulation intensities and behaviour. Here, we examine the effect of three different tDCS stimulation intensities (1 mA, 2 mA, 4 mA anodal motor cortex tDCS) administered during the explicit learning of motor sequences. Further, to assess the role of dopamine in the dose-response relationship between tDCS intensities and behaviour, we examined how pharmacologically increasing dopamine availability, via 100 mg of levodopa, modulated the effect of stimulation on learning. In the absence of levodopa, we found that 4 mA tDCS improved and 1 mA tDCS impaired acquisition of motor sequences relative to sham stimulation. Conversely, levodopa reversed the beneficial effect of 4 mA tDCS. This effect of levodopa was no longer evident at the 48-h follow-up, consistent with previous work characterising the persistence of neuroplastic changes in the motor cortex resulting from combining levodopa with tDCS. These results provide the first direct evidence for a role of dopamine in the intensity-dependent effects of tDCS on behaviour.

Dopamine Increases Accuracy and Lengthens Deliberation Time in Explicit Motor Skill Learning

Although animal research implicates a central role for dopamine in motor skill learning, a direct causal link has yet to be established in neurotypical humans. Here, we tested if a pharmacological manipulation of dopamine alters motor learning, using a paradigm which engaged explicit, goal-directed strategies. Participants (27 females; 11 males; aged 18-29 years) first consumed either 100 mg of levodopa (n = 19), a dopamine precursor that increases dopamine availability, or placebo (n = 19). Then, during training, participants learnt the explicit strategy of aiming away from presented targets by instructed angles of varying sizes. Targets jumped mid-movement by the instructed aiming angle. Task success was thus contingent upon aiming accuracy and not speed. The effect of the dopamine manipulations on skill learning was assessed during training and after an overnight follow-up. Increasing dopamine availability at training improved aiming accuracy and lengthened reaction times, particularly for larger, more difficult aiming angles, both at training and, importantly, at follow-up, despite prominent session-by-session performance improvements in both accuracy and speed. Exogenous dopamine thus seems to result in a learnt, persistent propensity to better adhere to task goals. Results support the proposal that dopamine is important in engagement of instrumental motivation to optimize adherence to task goals, particularly when learning to execute goal-directed strategies in motor skill learning.

Aberrant neural processing of event boundaries in persons with Parkinson's disease

The perception of everyday events implies the segmentation into discrete sub-events (i.e. event segmentation). This process is relevant for the prediction of upcoming events and for the recall of recent activities. It is thought to involve dopaminergic networks which are strongly compromised in Parkinson's disease (PD). Indeed, deficits of event segmentation have been previously shown in PD, but underlying neuronal mechanisms remain unknown. We therefore investigated 22 persons with PD and 22 age-matched healthy controls, who performed an event segmentation task with simultaneous electroencephalography (EEG). Both groups had to indicate by button press the beginning of sub-events within three movies showing persons performing everyday activities. The segmentation performance of persons with PD deviated significantly from that of controls. Neurophysiologically, persons with PD expressed reduced theta (4-7 Hz) activity around identified event boundaries compared to healthy controls. Together, these results point to disturbed event processing in PD. According to functions attributed to EEG activities in particular frequency ranges, the PD-related theta reduction could reflect impaired matching of perceptual input with stored event representations and decreased updating processes of event information in working memory and, thus, event boundary identification.

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.

Impaired perceptual phonetic plasticity in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative condition primarily associated with its motor consequences. Although much of the focus within the speech domain has focused on PD's consequences for production, people with PD have been shown to differ in the perception of emotional prosody, loudness, and speech rate from age-matched controls. The current study targeted the effect of PD on perceptual phonetic plasticity, defined as the ability to learn and adjust to novel phonetic input, both in second language and native language contexts. People with PD were compared to age-matched controls (and, for three of the studies, a younger control population) in tasks of explicit non-native speech learning and adaptation to variation in native speech (compressed rate, accent, and the use of timing information within a sentence to parse ambiguities). The participants with PD showed significantly worse performance on the task of compressed rate and used the duration of an ambiguous fricative to segment speech to a lesser degree than age-matched controls, indicating impaired speech perceptual abilities. Exploratory comparisons also showed people with PD who were on medication performed significantly worse than their peers off medication on those two tasks and the task of explicit non-native learning.

Parkinson's disease: Alterations of motor plasticity and motor learning

This chapter reviews the alterations in motor learning and motor cortical plasticity in Parkinson's disease (PD), the most common movement disorder. Impairments in motor learning, which is a hallmark of basal ganglia disorders, influence the performance of motor learning-related behavioral tasks and have clinical implications for the management of disturbance in gait and posture, and for rehabilitative management of PD. Although plasticity is classically induced and assessed in sliced preparation in animal models, in this review we have concentrated on the results from non-invasive brain stimulation techniques such as transcranial magnetic stimulation (TMS), transcranial alternating current stimulation (tACS) and transcranial direct current stimulation (tDCS) in patients with PD, in addition to a few animal electrophysiologic studies. The chapter summarizes the results from different cortical and subcortical plasticity investigations. Plasticity induction protocols reveal deficient plasticity in PD and these plasticity measures are modulated by medications and deep brain stimulation. There is considerable variability in these measures that are related to inter-individual variations, different disease characteristics and methodological considerations. Nevertheless, these pathophysiologic studies expand our knowledge of cortical excitability, plasticity and the effects of different treatments in PD. These tools of modulating plasticity and motor learning improve our understanding of PD pathophysiology and help to develop new treatments for this disabling condition.

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.

Null effects of levodopa on reward- and error-based motor adaptation, savings, and anterograde interference

Dopamine signaling is thought to mediate reward-based learning. We tested for a role of dopamine in motor adaptation by administering the dopamine precursor levodopa to healthy participants in two experiments involving reaching movements. Levodopa has been shown to impair reward-based learning in cognitive tasks. Thus, we hypothesized that levodopa would selectively impair aspects of motor adaptation that depend on the reinforcement of rewarding actions. In the first experiment, participants performed two separate tasks in which adaptation was driven either by visual error-based feedback of the hand position or binary reward feedback. We used EEG to measure event-related potentials evoked by task feedback. We hypothesized that levodopa would specifically diminish adaptation and the neural responses to feedback in the reward learning task. However, levodopa did not affect motor adaptation in either task nor did it diminish event-related potentials elicited by reward outcomes. In the second experiment, participants learned to compensate for mechanical force field perturbations applied to the hand during reaching. Previous exposure to a particular force field can result in savings during subsequent adaptation to the same force field or interference during adaptation to an opposite force field. We hypothesized that levodopa would diminish savings and anterograde interference, as previous work suggests that these phenomena result from a reinforcement learning process. However, we found no reliable effects of levodopa. These results suggest that reward-based motor adaptation, savings, and interference may not depend on the same dopaminergic mechanisms that have been shown to be disrupted by levodopa during various cognitive tasks.NEW & NOTEWORTHY Motor adaptation relies on multiple processes including reinforcement of successful actions. Cognitive reinforcement learning is impaired by levodopa-induced disruption of dopamine function. We administered levodopa to healthy adults who participated in multiple motor adaptation tasks. We found no effects of levodopa on any component of motor adaptation. This suggests that motor adaptation may not depend on the same dopaminergic mechanisms as cognitive forms or reinforcement learning that have been shown to be impaired by levodopa.

Using high-definition transcranial direct current stimulation to investigate the role of the dorsolateral prefrontal cortex in explicit sequence learning

Though we have a general understanding of the brain areas involved in motor sequence learning, there is more to discover about the neural mechanisms underlying skill acquisition. Skill acquisition may be subserved, in part, by interactions between the cerebellum and prefrontal cortex through a cerebello-thalamo-prefrontal network. In prior work, we investigated this network by targeting the cerebellum; here, we explored the consequence of stimulating the dorsolateral prefrontal cortex using high-definition transcranial direct current stimulation (HD-tDCS) before administering an explicit motor sequence learning paradigm. Using a mixed within- and between- subjects design, we employed anodal (n = 24) and cathodal (n = 25) HD-tDCS (relative to sham) to temporarily alter brain function and examine effects on skill acquisition. The results indicate that both anodal and cathodal prefrontal stimulation impedes motor sequence learning, relative to sham. These findings suggest an overall negative influence of active prefrontal stimulation on the acquisition of a sequential pattern of finger movements. Collectively, this provides novel insight on the role of the dorsolateral prefrontal cortex in initial skill acquisition, when cognitive processes such as working memory are used. Exploring methods that may improve motor learning is important in developing therapeutic strategies for motor-related diseases and rehabilitation.

Neurotransmitters, Cell Types, and Circuit Mechanisms of Motor Skill Learning and Clinical Applications

Animals acquire motor skills to better survive and adapt to a changing environment. The ability to learn novel motor actions without disturbing learned ones is essential to maintaining a broad motor repertoire. During motor learning, the brain makes a series of adjustments to build novel sensory–motor relationships that are stored within specific circuits for long-term retention. The neural mechanism of learning novel motor actions and transforming them into long-term memory still remains unclear. Here we review the latest findings with regard to the contributions of various brain subregions, cell types, and neurotransmitters to motor learning. Aiming to seek therapeutic strategies to restore the motor memory in relative neurodegenerative disorders, we also briefly describe the common experimental tests and manipulations for motor memory in rodents.

The effect of aging, Parkinson's disease, and exogenous dopamine on the neural response associated with auditory regularity processing

Processing regular patterns in auditory scenes is important for navigating complex environments. Electroencephalography studies find enhancement of sustained brain activity, correlating with the emergence of a regular pattern in sounds. How aging, aging-related diseases such as Parkinson's disease (PD), and treatment of PD with dopaminergic therapy affect this fundamental function remain unknown. We addressed this knowledge gap. Healthy younger and older adults and patients with PD listened to sounds that contained or were devoid of regular patterns. Healthy older adults and patients with PD were tested twice-off and on dopaminergic medication, in counterbalanced order. Regularity-evoked, sustained electroencephalography activity was reduced in older, compared with younger adults. Patients with PD and older controls evidenced comparable attenuation of the sustained response. Dopaminergic therapy further weakened the sustained response in both older controls and patients with PD. These findings suggest that fundamental regularity processing is impacted by aging but not specifically by PD. The finding that dopaminergic therapy attenuates rather than improves the sustained response coheres with the dopamine overdose response and is in line with previous findings that regularity processing implicates brain regions receiving dopamine from the ventral tegmental area that is relatively spared in PD and normal aging.

Dopamine replacement improves motor learning of an upper extremity task in people with Parkinson disease

BACKGROUND

Dopamine replacement medication has positive effects on existing motor skills for people with Parkinson disease (PD), but may have detrimental effects on the learning of motor skills necessary for effective rehabilitation according to the dopamine overdose hypothesis.

OBJECTIVES

This study aimed to determine whether dopamine replacement medication (i.e. levodopa) affects: learning of a novel upper extremity task, decrements in skill following withdrawal of practice, the rate of learning, and the transfer of movement skill to untrained upper extremity tasks compared to training "off" medication, in people with PD.

METHODS

Participants with mild-moderate PD (Hoehn and Yahr stage 2) were randomized to train "on" (n = 12) or "off" (n = 11) levodopa medication. Participants practiced 10 blocks of five trials of a functional motor task with their non-dominant upper extremity over three consecutive days (acquisition period), followed by a single block of five trials two and nine days later. Participants were also assessed "on" levodopa with two transfer tasks (the nine-hole peg test and a functional dexterity task) prior to any practice and nine days after the end of the acquisition period.

RESULTS

Participants who practiced "on" levodopa medication learned the upper extremity task to a greater extent that those who practiced "off" medication, as determined by retained performance two days after practice. Skill decrement and skill transfer were not significantly different between groups. Rate of learning was unable to be modelled in this sample.

CONCLUSIONS

Levodopa medication improved the learning of an upper extremity task in people with mild-moderate PD.

Retention of touchscreen skills is compromised in Parkinson's disease

Fine motor skill impairments likely have a severe impact on the use of touchscreens in Parkinson's disease (PD). Although recent work showed positive effects of intensive writing training, many questions remained regarding the consolidation of motor learning in PD. The current study examined the effects of PD on practicing the manipulation of touchscreen technology and whether this can lead to 24h-retention and transfer. We developed the Swipe-Slide Pattern (SSP)-task, similar to handling a touchscreen unlock-trace. On day 1, 11 patients and 10 healthy, age-matched controls underwent two consecutive runs of early and late learning (9 × 36 s SSP and 36 s rest). This was followed by a retention test after 24 h, including the assessment of transfer. Movement time (MT, s), Euclidean distance (ED) and a performance index (PI = MT/ED) were compared across the learning phases (early, late, retention and transfer) for both groups. Additionally, a learning, retention and transfer index were compared between groups and correlated to clinical characteristics. Both groups significantly improved in MT and PI across practice. However, while healthy adults showed further improvements after a 24h-retention period, patients presented with impaired retention indices. This was correlated with disease duration, disease severity and performance on a daily life mobile phone task. Finally, transfer to a similar, but untrained pattern was comparable between both groups. Overall, short-term practice of the SSP-task results in improvements for PD patients, albeit with impaired retention. Future work should investigate whether prolonged touchscreen skill training can be retained in motor memory in PD.

Effects of cerebellar transcranial direct current stimulation on the cognitive stage of sequence learning

Though the cerebellum has been previously implicated in explicit sequence learning, the exact role of this structure in the acquisition of motor skills is not completely clear. The cerebellum contributes to both motor and nonmotor behavior. Thus, this structure not only may contribute to the motoric aspects of sequence learning but may also play a role in the cognitive components of these learning paradigms. Therefore, we investigated the consequence of both disrupting and facilitating cerebellar function using high-definition transcranial direct current stimulation (tDCS) before the completion of an explicit motor sequence learning paradigm. Using a mixed within- and between-subjects design, we employed cathodal (n = 21) and anodal (n = 23) tDCS (relative to sham), targeting the lateral posterior cerebellum, to temporarily modulate function and investigate the resulting effects on the acquisition of a sequential pattern of finger movements. Results indicate that cathodal stimulation has a positive influence on learning while anodal stimulation has the opposite effect, relative to sham. Though the cerebellum is presumed to be primarily involved in motor function and movement coordination, our results support a cognitive contribution that may come into play during the initial stages of learning. Using tDCS targeting the right posterior cerebellum, which communicates with the prefrontal cortex via closed-loop circuits, we found polarity-specific effects of cathodal and anodal stimulation on sequence learning. Thus, our results substantiate the role of the cerebellum in the cognitive aspect of motor learning and provide important new insights into the polarity-specific effects of tDCS in this area.NEW & NOTEWORTHY The cerebellum contributes to motor and cognitive processes. Investigating the cognitive contributions of the cerebellum in explicit sequence learning stands to provide new insights into this learning domain, and cerebellar function more generally. Using high-definition transcranial direct current stimulation, we demonstrated polarity-specific effects of stimulation on explicit sequence learning. We speculate that this is due to facilitation of working memory processes. This provides new evidence supporting a role for the cerebellum in the cognitive aspects of sequence learning.

Towards understanding neural network signatures of motor skill learning in Parkinson's disease and healthy aging

In the past decade, neurorehabilitation has been shown to be an effective therapeutic supplement for patients with Parkinson's disease (PD). However, patients still experience severe problems with the consolidation of learned motor skills. Knowledge on the neural correlates underlying this process is thus essential to optimize rehabilitation for PD. This review investigates the existing studies on neural network connectivity changes in relation to motor learning in healthy aging and PD and critically evaluates the imaging methods used from a methodological point of view. The results indicate that despite neurodegeneration there is still potential to modify connectivity within and between motor and cognitive networks in response to motor training, although these alterations largely bypass the most affected regions in PD. However, so far training-related changes are inferred and possible relationships are not substantiated by brain-behavior correlations. Furthermore, the studies included suffer from many methodological drawbacks. This review also highlights the potential for using neural network measures as predictors for the response to rehabilitation, mainly based on work in young healthy adults. We speculate that future approaches, including graph theory and multimodal neuroimaging, may be more sensitive than brain activation patterns and model-based connectivity maps to capture the effects of motor learning. Overall, this review suggests that methodological developments in neuroimaging will eventually provide more detailed knowledge on how neural networks are modified by training, thereby paving the way for optimized neurorehabilitation for patients.

Dopamine Replacement Medication Does Not Influence Implicit Learning of a Stepping Task in People With Parkinson's Disease

INTRODUCTION

Treatment of Parkinson's disease (PD) with exogenous dopamine (ie, levodopa) may positively affect motor symptoms, but may negatively affect other functions such as the learning of motor skills necessary for rehabilitation. This study aimed to determine whether levodopa medication affects general and sequence-specific learning of a stepping task and the transfer of movement skill to untrained balance tasks in people with PD.

METHODS

Participants with PD were randomized to practice "on" (n = 14) or "off" (n = 13) levodopa medication. Participants practiced 6 blocks of 6 trials of 24 steps of a stepping task over an acquisition period of 3 consecutive days, followed by single retention blocks of 6 trials 2 and 9 days later. Participants were also assessed on untrained balance (ie, transfer) tasks "on" levodopa before practice and following late retention.

RESULTS

There were no between-group differences in general learning, sequence-specific learning, or transfer of skill to untrained balance tasks ( P > .05). Both groups demonstrated general and sequence-specific learning ( P < .001) and trends for improvement in untrained tasks ( P < .001 to P = .26) following practice. Detailed analysis of early acquisition revealed no difference between medication groups.

CONCLUSION

People with PD improved performance on the stepping task with practice. The between-group effect sizes were small, suggesting that levodopa medication status ("on" versus "off") during practice did not significantly affect general or sequence-specific learning of the task or components of early acquisition. The practice dose required to optimally result in functional improvements in untrained balance tasks, including reductions in falls, remains to be determined.

HIV-1 Tat regulation of dopamine transmission and microglial reactivity is brain region specific

The transactivator of transcription protein, HIV-1 Tat, is linked to neuroAIDS, where degeneration of dopamine neurons occurs. Using a mouse model expressing GFAP-driven Tat protein under doxycycline (Dox) regulation, we investigated microglial-neuronal interactions in the rostral substantia nigra pars compacta (SNc). Immunohistochemistry for microglia and tyrosine hydroxylase (TH) showed that the ratio of microglia to dopamine neurons is smaller in the SNc than in the ventral tegmental area (VTA) and that this difference is maintained following 7-day Dox exposure in wild type animals. Administration of Dox to wild types had no effect on microglial densities. In addressing the sensitivity of neurons to potentially adverse effects of HIV-1 Tat, we found that HIV-1 Tat exposure in vivo selectively decreased TH immunoreactivity in the SNc but not in the VTA, while levels of TH mRNA in the SNc remained unchanged. HIV-1 Tat induction in vivo did not alter the total number of neurons in these brain regions. Application of Tat (5 ng) into dopamine neurons with whole-cell patch pipette decreased spontaneous firing activity. Tat induction also produced a decline in microglial cell numbers, but no microglial activation. Thus, disappearance of dopaminergic phenotype is due to a loss of TH immunoreactivity rather than to neuronal death, which would have triggered microglial activation. We conclude that adverse effects of HIV-1 Tat produce a hypodopamine state by decreasing TH immunoreactivity and firing activity of dopamine neurons. Reduced microglial numbers after Tat exposure in vivo suggest impaired microglial functions and altered bidirectional interactions between dopamine neurons and microglia.

Dopamine-related dissociation of cortical and subcortical brain activations in cognitively unimpaired Parkinson's disease patients OFF and ON medications

Background:

Despite dopaminergic depletion that is severe enough to cause the motor symptoms of Parkinson’s disease (PD), m any patients remain cognitively unimpaired. Little is known about brain mechanism s underlying such preserved cognitive abilities and their alteration by dopaminergic medications.

Objectives:

We investigated brain activations underlying dopamine-related differences in cognitive function using a unique experimental design with PD patients off and on dopaminergic medications. We tested the dopamine overdose hypothesis, which posits that the excess of exogenous dopamine in the frontal cortical regions can impair cognition.

Methods:

We used a two-choice forced response Choice Reaction Time (CRT) task to probe cognitive processes underlying response selection and execution. Functional magnetic resonance imaging data Were acquired from 16 cognitively unimpaired (Level-II) PD participants and 15 Well-matched healthy controls (HC). We compared task performance (i.e. reaction time and accuracy) and brain activation of PD participants off dopaminergic medications (PD_OFF) in comparison with HC, and PD_OFF participants with those on dopaminergic medications (PD_ON).

Results:

PD_OFF and PD_ON groups did not differ from each other, or from the HC group, in reaction time or accuracy. Compared to HC, PD_OFF activated the bilateral putamen less, and this w as compensated by higher activation of the anterior insula. No such differences Were observed in the PD_ON group, Compared to HC. Compared to both HC and PD_OFF, PD_ON participants showed dopamine-related hyperactivation in the frontal cortical regions and hypoactivation in the amygdala.

Conclusion:

Our data provide further evidence that PD_OFF and PD_ON participants engage different cortical and subcortical systems to achieve similar levels of cognitive performance as HC. Crucially, our findings demonstrate dopamine-related dissociation in brain activation between cortical and subcortical regions, and provide novel support for the dopamine overdose hypothesis.

Pharmacological Dopamine Manipulation Does Not Alter Reward-Based Improvements in Memory Retention during a Visuomotor Adaptation Task

Motor adaptation tasks investigate our ability to adjust motor behaviors to an ever-changing and unpredictable world. Previous work has shown that punishment-based feedback delivered during a visuomotor adaptation task enhances error-reduction, whereas reward increases memory retention. While the neural underpinnings of the influence of punishment on the adaptation phase remain unclear, reward has been hypothesized to increase retention through dopaminergic mechanisms. We directly tested this hypothesis through pharmacological manipulation of the dopaminergic system. A total of 96 young healthy human participants were tested in a placebo-controlled double-blind between-subjects design in which they adapted to a 40° visuomotor rotation under reward or punishment conditions. We confirmed previous evidence that reward enhances retention, but the dopamine (DA) precursor levodopa (LD) or the DA antagonist haloperidol failed to influence performance. We reason that such a negative result could be due to experimental limitations or it may suggest that the effect of reward on motor memory retention is not driven by dopaminergic processes. This provides further insight regarding the role of motivational feedback in optimizing motor learning, and the basis for further decomposing the effect of reward on the subprocesses known to underlie motor adaptation paradigms.

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.

Impaired sequential and partially compensated probabilistic skill learning in Parkinson's disease

The striatal dopaminergic dysfunction in Parkinson's disease (PD) has been associated with deficits in skill learning in numerous studies, but some of the findings remain controversial. Our aim was to explore the generality of the learning deficit using two widely reported skill learning tasks in the same group of Parkinson's patients. Thirty-four patients with PD (mean age: 62.83 years, SD: 7.67) were compared to age-matched healthy adults. Two tasks were employed: the Serial Reaction Time Task (SRT), testing the learning of motor sequences, and the Weather Prediction (WP) task, testing non-sequential probabilistic category learning. On the SRT task, patients with PD showed no significant evidence for sequence learning. These results support and also extend previous findings, suggesting that motor skill learning is vulnerable in PD. On the WP task, the PD group showed the same amount of learning as controls, but they exploited qualitatively different strategies in predicting the target categories. While controls typically combined probabilities from multiple predicting cues, patients with PD instead focused on individual cues. We also found moderate to high correlations between the different measures of skill learning. These findings support our hypothesis that skill learning is generally impaired in PD, and can in some cases be compensated by relying on alternative learning strategies.

Reduced striatal dopamine release during motor skill acquisition in Parkinson's disease

Background

Striatal dopamine is functionally important for the acquisition of motor skills. However, it remains controversial as to whether intrinsic processing of motor learning is impaired in patients with Parkinson’s disease (PD), and if yes, whether the impairment is associated with altered striatal dopamine release. Additionally, most neuro-imaging studies of patients with PD have focused on motor sequence learning. In contrast, skill acquisition, specifically, the reconstruction of muscle control of isolated movements, has barely been studied.

Method

In this study, we used a repetitive skill training task to measure the peak acceleration of left thumb movement during a process to achieve fine tuning of motor skill. Using ¹¹C-raclopride (RAC) positron emission tomography, we investigated changes in striatal dopamine levels in two conditions of a skill acquisition task: initial skill training (Day 1) and acquired condition (Day 2) with eight patients with PD and age-matched healthy subjects (HS).

Result

In HS, the mean acceleration of each session improved through repeated training sessions on Day 1. However, in patients with PD, the training-associated increase was less than that for HS, and this suggests that repetitive skill training does not result in the effective improvement of motor performance. The regions of interest (ROI) analysis revealed that the RAC-binding potential (BP) was significantly reduced in the right putamen on Day 1 compared with Day 2 in HS. In patients with PD, BP within the right putamen was unchanged. Further, we found that patients with PD had increased dopamine levels within the right ventral striatum (VST) and right caudate (CAU) on Day 2, which was greater than that in HS. These results suggest the impaired activation of the putamen during skill acquisition in patients with PD and compensated hyperactivation of the VST and CAU for the reduced dopamine release within the dorsal putamen (DPU).

Conclusion

Our findings suggest that patients with PD had insufficiency in the process to improve motor skills. Different patterns of striatal dopamine release are relevant to the impairment of these motor functions in patients with PD, at the early stage of the disease.

Independent effects of age and levodopa on reversal learning in healthy volunteers

The dopamine overdose hypothesis has provided an important theoretical framework for understanding cognition in Parkinson's disease. It posits that effects of dopaminergic therapy on cognition in Parkinson's disease depend on baseline dopamine levels in brain regions that support different functions. Although functions performed by more severely dopamine-depleted brain regions improve with medication, those associated with less dopamine deficient areas are actually worsened. It is presumed that medication-related worsening of cognition owes to dopamine overdose. We investigated whether age-related changes in baseline dopamine levels would modulate effects of dopaminergic therapy on reward learning in healthy volunteers. In a double-blind, crossover design, healthy younger and older adults completed a probabilistic reversal learning task after treatment with 100/25 mg of levodopa/carbidopa versus placebo. Older adults learned more poorly than younger adults at baseline, being more likely to shift responses after misleading punishment. Levodopa worsened stimulus-reward learning relative to placebo to the same extent in both groups, irrespective of differences in baseline performance and expected dopamine levels. When order effects were eliminated, levodopa induced response shifts after reward more often than placebo. Our results reveal independent deleterious effects of age group and exogenous dopamine on reward learning, suggesting a more complex scenario than predicted by the dopamine overdose hypothesis.

Quantitative evaluation of the tracer distribution in dopamine transporter SPECT for objective interpretation

PURPOSE

Quantification of the tracer distribution would add objectivity to the visual assessments of dopamine transporter (DAT) single photon emission computed tomography (SPECT) data. Our study aimed to evaluate the diagnostic utility of fractal dimension (FD) as a quantitative indicator of tracer distribution and compared with the conventional quantitative value: specific binding ratio (SBR). We also evaluated the utility of the combined index SBR/FD (SBR divided by FD).

MATERIALS AND METHODS

We conducted both clinical and phantom studies. In the clinical study, 150 patients including 110 patients with Parkinsonian syndrome (PS) and 40 without PS were enrolled. In the phantom study, we used a striatal phantom with the striatum chamber divided into two spaces, representing the caudate nucleus and putamen. The SBR, FD, and SBR/FD were calculated and compared between datasets for evaluating the diagnostic utility. Mann-Whitney test and receiver-operating characteristics (ROC) analysis were used for analysis.

RESULTS

ROC analysis revealed that the FD value had high diagnostic performance [the areas under the curve (AUC) = 0.943] and the combined use of SBR and FD (SBR/FD) delivered better results than the SBR alone (AUC, 0.964 vs 0.899; p < 0.001). The sensitivity, specificity, and accuracy, respectively, were 79.1, 85.0, and 80.7% with SBR, 84.5, 97.5, and 88.0% with FD, and 92.7, 87.5, and 91.3% with SBR/FD.

CONCLUSION

Our results confirmed that the FD value is a useful diagnostic index, which reflects the tracer distribution in DAT SPECT images. The combined use of SBR and FD was more useful than either used alone.

Training for Micrographia Alters Neural Connectivity in Parkinson's Disease

Despite recent advances in clarifying the neural networks underlying rehabilitation in Parkinson's disease (PD), the impact of prolonged motor learning interventions on brain connectivity in people with PD is currently unknown. Therefore, the objective of this study was to compare cortical network changes after 6 weeks of visually cued handwriting training (= experimental) with a placebo intervention to address micrographia, a common problem in PD. Twenty seven early Parkinson's patients on dopaminergic medication performed a pre-writing task in both the presence and absence of visual cues during behavioral tests and during fMRI. Subsequently, patients were randomized to the experimental (N = 13) or placebo intervention (N = 14) both lasting 6 weeks, after which they underwent the same testing procedure. We used dynamic causal modeling to compare the neural network dynamics in both groups before and after training. Most importantly, intensive writing training propagated connectivity via the left hemispheric visuomotor stream to an increased coupling with the supplementary motor area, not witnessed in the placebo group. Training enhanced communication in the left visuomotor integration system in line with the learned visually steered training. Notably, this pattern was apparent irrespective of the presence of cues, suggesting transfer from cued to uncued handwriting. We conclude that in early PD intensive motor skill learning, which led to clinical improvement, alters cortical network functioning. We showed for the first time in a placebo-controlled design that it remains possible to enhance the drive to the supplementary motor area through motor learning.

Normal aging and Parkinson's disease are associated with the functional decline of distinct frontal-striatal circuits

Impaired ability to shift attention between stimuli (i.e. shifting attentional ‘set’) is a well-established part of the dysexecutive syndrome in Parkinson's Disease (PD), nevertheless cognitive and neural bases of this deficit remain unclear. In this study, an fMRI-optimised variant of a classic paradigm for assessing attentional control (Hampshire and Owen 2006) was used to contrast activity in dissociable executive circuits in early-stage PD patients and controls. The results demonstrated that the neural basis of the executive performance impairments in PD is accompanied by hypoactivation within the striatum, anterior cingulate cortex (vACC), and inferior frontal sulcus (IFS) regions. By contrast, in aging it is associated with hypoactivation of the anterior insula/inferior frontal operculum (AI/FO) and the pre-supplementary motor area (preSMA). Between group behavioural differences were also observed; whereas normally aging individuals exhibited routine-problem solving deficits, PD patients demonstrated more global task learning deficits. These findings concur with recent research demonstrating model-based reinforcement learning deficits in PD and provide evidence that the AI/FO and IFS circuits are differentially impacted by PD and normal aging.

Levodopa medication improves incidental sequence learning in Parkinson's disease

Empirical evidence suggests that levodopa medication used to treat the motor symptoms of Parkinson's disease (PD) may either improve, impair or not affect specific cognitive processes. This evidence led to the 'dopamine overdose' hypothesis that levodopa medication impairs performance on cognitive tasks if they recruit fronto-striatal circuits which are not yet dopamine-depleted in early PD and as a result the medication leads to an excess of dopamine. This hypothesis has been supported for various learning tasks including conditional associative learning, reversal learning, classification learning and intentional deterministic sequence learning, on all of which PD patients demonstrated significantly worse performance when tested on relative to off dopamine medication. Incidental sequence learning is impaired in PD, but how such learning is affected by dopaminergic therapy remains undetermined. The aim of the current study was to investigate the effect of dopaminergic medication on incidental sequence learning in PD. We used a probabilistic serial reaction time task (SRTT), a sequence learning paradigm considered to make the sequence less apparent and more likely to be learned incidentally rather than intentionally. We compared learning by the same group of PD patients (n=15) on two separate occasions following oral administration of levodopa medication (on state) and after overnight withdrawal of medication (off state). Our results demonstrate for the first time that levodopa medication enhances incidental learning of a probabilistic sequence on the serial reaction time task in PD. However, neither group significantly differed from performance of a control group without a neurological disease, which indicates the importance of within group comparisons for identifying deficits. Levodopa medication enhanced incidental learning by patients with PD on a probabilistic sequence learning paradigm even though the patients were not aware of the existence of the sequence or their acquired knowledge. The results suggest a role in acquiring incidental motor sequence learning for dorsal striatal areas strongly affected by dopamine depletion in early PD.

A neurological perspective on the enhancement debate: Lessons learned from Parkinson's disease

Cognitive enhancement is signified by adaptive behavioural change following an intervention that targets the brain. Although much of the discussion and research into cognitive enhancement focuses on the effects of neural interventions in healthy individuals, it is useful to consider evidence from clinical populations. Diseases of the central nervous system represent the primary and richest source of evidence on the effects of brain manipulations, which are in the first instance therapeutic. Parkinson's disease (PD) is used as a model for understanding the effects of pharmacological agents that target systems with a central role in cognition. The mixed outcomes of deep brain stimulation on cognition will also be discussed. By illustrating the psychopharmacological principle of diverse and malleable neurochemical optima for different cognitive functions, and the role of individual differences, it will be argued that the entire spectrum of cognitive effects in any one individual following any given manipulation, such as the administration of a drug, often includes enhancement as well as impairment. Predicting these effects represents a complex multivariate problem, and the accuracy of this predictive effort, as well as the harm prevention it connotes, is determined by our evolving understanding of the brain and cognition. A manipulation can be said to confer cognitive enhancement; however, it is argued that using the global term cognitive enhancer to refer to such a manipulation without qualification is of limited utility.

Mood induction effects on motor sequence learning and stop signal reaction time

The neurobiological theory of positive affect proposes that positive mood states may benefit cognitive performance due to an increase of dopamine throughout the brain. However, the results of many positive affect studies are inconsistent; this may be due to individual differences. The relationship between dopamine and performance is not linear, but instead follows an inverted "U" shape. Given this, we hypothesized that individuals with high working memory capacity, a proxy measure for dopaminergic transmission, would not benefit from positive mood induction and in fact performance in dopamine-mediated tasks would decline. In contrast, we predicted that individuals with low working memory capacities would receive the most benefit after positive mood induction. Here, we explored the effect of positive affect on two dopamine-mediated tasks, an explicit serial reaction time sequence learning task and the stop signal task, predicting that an individual's performance is modulated not only by working memory capacity, but also on the type of mood. Improvements in explicit sequence learning from pre- to post-positive mood induction were associated with working memory capacity; performance declined in individuals with higher working memory capacities following positive mood induction, but improved in individuals with lower working memory capacities. This was not the case for negative or neutral mood induction. Moreover, there was no relationship between the change in stop signal reaction time with any of the mood inductions and individual differences in working memory capacity. These results provide partial support for the neurobiological theory of positive affect and highlight the importance of taking into account individual differences in working memory when examining the effects of positive mood induction.

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.

Postural motor learning in people with Parkinson's disease

Protective postural responses to external perturbations are hypokinetic in people with Parkinson's disease (PD), and improving these responses may reduce falls. However, the ability of people with PD to improve postural responses with practice is poorly understood. Our objective was to determine whether people with PD can improve protective postural responses similarly to healthy adults through repeated perturbations, and whether improvements are retained or generalize to untrained perturbations. Twelve healthy adults and 15 people with PD underwent 25 forward and 25 backward translations of the support surface, eliciting backward, and forward protective steps, respectively. We assessed whether: (1) performance improved over one day of practice, (2) changes were retained 24 h later, and (3) improvements generalized to untrained (lateral) postural responses. People with PD and healthy adults improved postural response characteristics, including center of mass displacement after perturbations (p < 0.001), margin of stability at first footfall (p = 0.001), step latency (p = 0.044), and number of steps (p = 0.001). However, unlike controls, improvements in people with PD occurred primarily in the first block of trials. Improvements were more pronounced during backward protective stepping than forward, and with the exception of step latency, were retained 24 h later. Improvements in forward-backward stepping did not generalize to lateral protective stepping. People with PD can improve protective stepping over the course of 1 day of perturbation practice. Improvements were generally similar to healthy adults, and were retained in both groups. Perturbation practice may represent a promising approach to improving protective postural responses in people with PD; however, additional research is needed to understand how to enhance generalization.

The Effect of Levodopa on Improvements in Protective Stepping in People With Parkinson's Disease

Background The effect of levodopa on postural motor learning in people with Parkinson's disease is poorly understood. In particular, it is unknown whether levodopa affects improvement in protective postural responses after external perturbations such as a slip or trip, a critical aspect of fall prevention. Objective Determine the effect of levodopa on postural motor learning in people with Parkinson's disease. Methods We assessed improvement in protective postural responses in people with Parkinson's disease over short-term (1 day) perturbation training on and off levodopa. We also assessed retention and generalization of improvement. Participants were 22 individuals with Parkinson's disease. The primary outcome was total center of mass (COM) displacement after perturbation. Secondary outcomes assessed first step performance and included margin of stability at first foot contact. Results People with Parkinson's disease improved COM displacement (P = .011) and margin of stability (P = .016) over training. Improvements in these outcomes were more pronounced after training while on levodopa than off levodopa. Levodopa State × Training interactions were not observed for other step performance variables (eg, step latency, length, total number of steps). Improvements were retained for 24 hours, and for margin of stability, retention was more pronounced while on levodopa than off (P = .018). Conclusions Individuals with Parkinson's disease are able to improve protective postural responses through short-term perturbation training, and improvements were more pronounced when on levodopa for some variables. Perturbation training may be more effective if completed while optimally medicated with levodopa.

Cognitive Contributions to Freezing of Gait in Parkinson Disease: Implications for Physical Rehabilitation

People with Parkinson disease (PD) who show freezing of gait also have dysfunction in cognitive domains that interact with mobility. Specifically, freezing of gait is associated with executive dysfunction involving response inhibition, divided attention or switching attention, and visuospatial function. The neural control impairments leading to freezing of gait have recently been attributed to higher-level, executive and attentional cortical processes involved in coordinating posture and gait rather than to lower-level, sensorimotor impairments. To date, rehabilitation for freezing of gait primarily has focused on compensatory mobility training to overcome freezing events, such as sensory cueing and voluntary step planning. Recently, a few interventions have focused on restitutive, rather than compensatory, therapy. Given the documented impairments in executive function specific to patients with PD who freeze and increasing evidence of overlap between cognitive and motor function, incorporating cognitive challenges with mobility training may have important benefits for patients with freezing of gait. Thus, a novel theoretical framework is proposed for exercise interventions that jointly address both the specific cognitive and mobility challenges of people with PD who freeze.

Distinct effects of dopamine vs STN stimulation therapies in associative learning and retention in Parkinson disease

Evidence has been provided in Parkinson's disease patients of cognitive impairments including visual memory and learning which can be partially compensated by dopamine medication or subthalamic nucleus stimulation. The effects of these two therapies can differ according to the learning processes involving the dorsal vs ventral part of the striatum. Here we aimed to investigate and compare the outcomes of dopamine vs stimulation treatment in Parkinson patient's ability to acquire and maintain over successive days their performance in visual working memory. Parkinson patients performed conditional associative learning embedded in visual (spatial and non spatial) working memory tasks over two consecutive days either ON or OFF dopaminergic drugs or STN stimulation depending on the group of patients studied. While Parkinson patients were more accurate and faster in memory tasks ON vs OFF stimulation independent of the day of testing, performance in medicated patients differed depending on the medication status during the initial task acquisition. Patients who learnt the task ON medication the first day were able to maintain or even improve their memory performance both OFF and ON medication on the second day after consolidation. These effects were observed only in patients with dopamine replacement with or without motor fluctuations. This enhancement in memory performance after having learnt under dopamine medication and not under STN stimulation was mostly significant in visuo-spatial working memory tasks suggesting that dopamine replacement in the depleted dorsal striatum is essential for retention and consolidation of learnt skill.

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.

Individual predictors of sensorimotor adaptability

There are large individual variations in strategies and rates of sensorimotor adaptation to spaceflight. This is seen in both the magnitude of performance disruptions when crewmembers are first exposed to microgravity, and in the rate of re-adaptation when they return to Earth's gravitational environment. Understanding the sources of this variation can lead to a better understanding of the processes underlying adaptation, as well as provide insight into potential routes for facilitating performance of "slow adapters". Here we review the literature on brain, behavioral, and genetic predictors of motor learning, recovery of motor function following neural insult, and sensorimotor adaptation. For example, recent studies have identified specific genetic polymorphisms that are associated with faster adaptation on manual joystick tasks and faster recovery of function following a stroke. Moreover, the extent of recruitment of specific brain regions during learning and adaptation has been shown to be predictive of the magnitude of subsequent learning. We close with suggestions for forward work aimed at identifying predictors of spaceflight adaptation success. Identification of "slow adapters" prior to spaceflight exposure would allow for more targeted preflight training and/or provision of booster training and adaptation adjuncts during spaceflight.

Cognitive impairment in Parkinson's disease

Cognitive impairment is a significant non-motor symptom of Parkinson's disease (PD). Longitudinal cohort studies have demonstrated that approximately 50% of those with PD develop dementia after 10 years, increasing to over 80% after 20 years. Deficits in cognition can be identified at the time of PD diagnosis in some patients and this mild cognitive impairment (PD-MCI) has been studied extensively over the last decade. Although PD-MCI is a risk factor for developing Parkinson's disease dementia there is evidence to suggest that PD-MCI might consist of distinct subtypes with different pathophysiologies and prognoses. The major pathological correlate of Parkinson's disease dementia is Lewy body deposition in the limbic system and neocortex although Alzheimer's related pathology is also an important contributor. Pathological damage causes alteration to neurotransmitter systems within the brain, producing behavioural change. Management of cognitive impairment in PD requires a multidisciplinary approach and accurate communication with patients and relatives is essential.

The effects of methylphenidate on resting-state striatal, thalamic and global functional connectivity in healthy adults

By blocking dopamine and norepinephrine transporters, methylphenidate affects cognitive performance and regional brain activation in healthy individuals as well as those with neuropsychiatric disorders. Resting-state connectivity evaluates the functional integrity of a network of brain regions. Here, we examined how methylphenidate effects resting-state functional connectivity of the dorsal striatum and thalamus, areas each with dense dopaminergic and noradrenergic innervations, as well as global cerebral connectivity. We administered a single, oral dose (45 mg) to 24 healthy adults and compared resting-state connectivity to 24 demographically matched adults who did not receive any medication. The results showed that methylphenidate alters seed-based and global connectivity between the thalamus/dorsal striatum with primary motor cortex, amygdala/hippocampus and frontal executive areas (p < 0.05, corrected). Specifically, while methylphenidate at this dosage enhances connectivity to the motor cortex and memory circuits, it dampens prefrontal cortical connectivity perhaps by increasing catecholaminergic signalling past the 'optimal' level. These findings advance our understanding of a critical aspect of the multifaceted effects of methylphenidate on brain functions. The results may also facilitate future studies of the aetiology and treatment of neurological and psychiatric disorders that implicate catecholaminergic dysfunction.

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.

Visuomotor adaptation in Parkinson's disease: effects of perturbation type and medication state

To perform simple everyday tasks, we use visual feedback from our external environment to generate and guide movements. However, tasks like reaching for a cup may become extremely difficult in movement disorders such as Parkinson's disease (PD), and it is unknown whether PD patients use visual information to compensate for motor deficiencies. We tested adaptation to changes in visual feedback of the hand in three subject groups, PD patients on daily levodopa (l-dopa) therapy (PD ON), PD patients off l-dopa (PD OFF), and age-matched control subjects, to determine the effects of PD on the visual control of movement. Subjects were tested on two classes of visual perturbations, one that altered visual direction of movement and one that altered visual extent of movement, allowing us to test adaptive sensitivity to changes in both movement direction (visual rotations) and extent (visual gain). The PD OFF group displayed more complete adaptation to visuomotor rotations compared with control subjects but initial, transient difficulty with adaptation to visual gain perturbations. The PD ON group displayed feedback control more sensitive to visual error compared with control subjects but compared with the PD OFF group had mild impairments during adaptation to changes in visual extent. We conclude that PD subjects can adapt to changes in visual information but that l-dopa may impair visual-based motor adaptation.

Differences in neural connectivity between the substantia nigra and ventral tegmental area in the human brain

Objectives: Many animal and a few human studies have reported on the neural connectivity of the substantia nigra (SN) and the ventral tegmental area (VTA). However, it has not been clearly elucidated so far. We attempted to investigate any differences in neural connectivity of the SN/VTA in the human brain, using diffusion tensor imaging (DTI).

Methods: Sixty-three healthy subjects were recruited for this study. DTIs were acquired using a sensitivity-encoding head coil at 1. 5T. Connectivity was defined as the incidence of connection between the SN/VTA and each brain regions in the brain.

Results: The connectivity of SN was higher than that of the VTA. This included in the primary motor cortex, primary somatosensory cortex, premotor cortex, prefrontal cortex, caudate nucleus, globus pallidus, putamen, nucleus accumbens, temporal lobe, amygdala, pontine basis, occipital lobe, anterior and posterior lobe of cerebellum, corpus callosum, and external capsule (p < 0.05). However, no significant differences were observed in the red nucleus, thalamus, pontine tegmentum, and medial temporal lobe between the SN and VTA (p > 0.05).

Conclusions: We found the differences in neural connectivity of the SN/VTA in the human brain. The method and results of this study can provide useful information for clinicians and researchers in neuroscience, especially who work for Parkinson’s disease and patients with brain injury.

Association of COMT val158met and DRD2 G>T genetic polymorphisms with individual differences in motor learning and performance in female young adults

Individuals learn new skills at different rates. Given the involvement of corticostriatal pathways in some types of learning, variations in dopaminergic transmission may contribute to these individual differences. Genetic polymorphisms of the catechol-O-methyltransferase (COMT) enzyme and dopamine receptor D2 (DRD2) genes partially determine cortical and striatal dopamine availability, respectively. Individuals who are homozygous for the COMT methionine (met) allele show reduced cortical COMT enzymatic activity, resulting in increased dopamine levels in the prefrontal cortex as opposed to individuals who are carriers of the valine (val) allele. DRD2 G-allele homozygotes benefit from a higher striatal dopamine level compared with T-allele carriers. We hypothesized that individuals who are homozygous for COMT met and DRD2 G alleles would show higher rates of motor learning. Seventy-two young healthy females (20 ± 1.9 yr) performed a sensorimotor adaptation task and a motor sequence learning task. A nonparametric mixed model ANOVA revealed that the COMT val-val group demonstrated poorer performance in the sequence learning task compared with the met-met group and showed a learning deficit in the visuomotor adaptation task compared with both met-met and val-met groups. The DRD2 TT group showed poorer performance in the sequence learning task compared with the GT group, but there was no difference between DRD2 genotype groups in adaptation rate. Although these results did not entirely come out as one might predict based on the known contribution of corticostriatal pathways to motor sequence learning, they support the role of genetic polymorphisms of COMT val158met (rs4680) and DRD2 G>T (rs 1076560) in explaining individual differences in motor performance and motor learning, dependent on task type.

Performance of a motor task learned on levodopa deteriorates when subsequently practiced off

Studies in animals and in people with Parkinson's disease (PD) demonstrate complex effects of dopamine on learning motor tasks; its effect on retention of motor learning has received little attention. Recent animal studies demonstrate that practicing a task in the off state, when initially learned in the on state, leads to progressive deterioration in performance. We measured the acquisition and retention of 3 different motor tasks in the presence and absence of levodopa. Twenty individuals with Hoehn and Yahr Stage 1.5 to 3 PD practiced the tasks daily for two 4-day weeks, one half practicing on L-dopa the first week and off the second week. The other half practiced off l-dopa both weeks. The tasks were (1) alternate tapping of 2 keys, (2) moving the body toward 2 targets on a posturography device, and (3) mirror drawing of a star. For the tapping and body movement tests, those who practiced on the first week had a progressive decline in performance with practice during week 2, while subjects off during week 1 maintained or improved. In contrast, for the mirror task, subjects on L-dopa initially had much more difficulty completing the task compared to subjects who practiced off. Both groups improved with practice the first week and had flat performance the second week. These data suggest that performance of speed-accuracy tasks learned in the on state may progressively worsen if subsequently practiced in the off state. In addition, performance, but not learning, of some tasks may be impeded by L-dopa.

Dopamine overdose hypothesis: evidence and clinical implications

About a half a century has passed since dopamine was identified as a neurotransmitter, and it has been several decades since it was established that people with Parkinson's disease receive motor symptom relief from oral levodopa. Despite the evidence that levodopa can reduce motor symptoms, there has been a developing body of literature that dopaminergic therapy can improve cognitive functions in some patients but make them worse in others. Over the past two decades, several laboratories have shown that dopaminergic medications can impair the action of intact neural structures and impair the behaviors associated with these structures. In this review, we consider the evidence that has accumulated in the areas of reversal learning, motor sequence learning, and other cognitive tasks. The purported inverted-U shaped relationship between dopamine levels and performance is complex and includes many contributory factors. The regional striatal topography of nigrostriatal denervation is a critical factor, as supported by multimodal neuroimaging studies. A patient's individual genotype will determine the relative baseline position on this inverted-U curve. Dopaminergic pharmacotherapy and individual gene polymorphisms can affect the mesolimbic and prefrontal cortical dopaminergic functions in a comparable, inverted-U dose-response relationship. Depending on these factors, a patient can respond positively or negatively to levodopa when performing reversal learning and motor sequence learning tasks. These tasks may continue to be relevant as our society moves to increased technological demands of a digital world that requires newly learned motor sequences and adaptive behaviors to manage daily life activities.

Striatal denervation pattern predicts levodopa effects on sequence learning in Parkinson's disease

Mild to moderate Parkinson's disease shows more denervation in the posterodorsal striatum and sparing of the anteroventral striatum. Dopaminergic medications can interfere with anteroventral striatum function by overdosing this relatively intact structure. The authors determined how regional striatal denervation affects medication-associated sequence learning impairment in Parkinson's disease. Eighteen Parkinson's patients performed motor sequence learning on and off levodopa. Patients underwent (11)C-dihydrotetrabenazine positron emission tomography scans to measure nigrostriatal denervation. Patients with more preserved putamen were more likely to exhibit levodopa-associated sequence learning impairments. Furthermore, the ratio of denervation in the anterior to posterior dorsal putamen predicted the level of learning differences on and off levodopa. These results demonstrate that the spatial pattern of nigrostriatal dopaminergic denervation predicts medication responsiveness for motor sequence learning.

Functional neuroimaging in Parkinson's disease

The use of functional imaging in neurodegenerative diseases has increased in recent years, with applications in research into the underlying pathophysiology, aiding in diagnosis, or evaluating new treatments. In Parkinson's disease (PD), these imaging methods have expanded our understanding of the disease beyond dopaminergic deficits. Moreover, functional imaging methods have described alterations in functional networks relating not only to the motor symptoms, but also to many nonmotor features of PD, such as cognitive dysfunction. From a clinical viewpoint, functional imaging methods can assist in monitoring disease progression, such as in the context of clinical trials, and holds the potential to aid in early diagnosis of PD and differentiation from other parkinsonian disorders.