Cognitive deficits in the early stages of Parkinson's disease

Striatal dopamine influences the default mode network to affect shifting between object features

Cognitive flexibility or the ability to change behavior in response to external cues is conceptualized as two processes: one for shifting between perceptual features of objects and another for shifting between the abstract rules governing the selection of these objects. Object and rule shifts are believed to engage distinct anatomical structures and functional processes. Dopamine activity has been associated with cognitive flexibility, but patients with dopaminergic deficits are not impaired on all tasks assessing cognitive flexibility, suggesting that dopamine may have different roles in the shifting of objects and rules. The goals of this study were to identify brain regions supporting object and rule shifts and to examine the role of dopamine in modulating these two forms of cognitive flexibility. Sixteen young, healthy volunteers underwent fMRI while performing a set-shift task designed to differentiate shifting between object features from shifting between abstract task rules. Participants also underwent PET with 6-[¹⁸F]-fluoro-l-m-tyrosine (FMT), a radiotracer measuring dopamine synthesis capacity. Shifts of abstract rules were not associated with activation in any brain region, and FMT uptake did not correlate with rule shift performance. Shifting between object features deactivated the medial PFC and the posterior cingulate and activated the lateral PFC, posterior parietal areas, and the striatum. FMT signal in the striatum correlated negatively with object shift performance and deactivation in the medial PFC, a component of the default mode network, suggesting that dopamine influences object shifts via modulation of activity in the default mode network.

Executive function in Parkinson's disease: contributions of the dorsal frontostriatal pathways to action and motivation

Disruption of the dorsal frontostriatal pathways in Parkinson's disease (PD) is associated with impairments in motivation, as well as in executive function. The goal of this study was to investigate whether these impairments are related and, if so, whether the disruption of frontostriatal pathways compromises the ability to process the motivational aspects of feedback in such tasks. In Experiment 1, informative feedback improved the performance of young, healthy participants in a task-switching paradigm. This task-switching paradigm was then used in Experiment 2 to test whether feedback would improve the performance of 17 PD patients and age-matched controls. The PD group benefitted from feedback to the same degree as control participants; however, depression scores on the Beck Depression Inventory were significantly related to feedback usage, especially when response selection demands were high. Regardless of feedback, PD patients were more impaired when response demands were high than in an equally difficult condition with low action demands. These results suggest that response selection is a core impairment of insufficient dopamine to the dorsal frontal striatal pathways.

Cognitive impairment and dementia in Parkinson's disease

Relatively subtle cognitive disturbances may be present from the initial stages of Parkinson's disease (PD) that progress in many patients to a more severe cognitive impairment and dementia. Several of the initial deficits are ascribed to failure in the frontal-striatal basal ganglia circuits and involve executive defects in planning, initiation, monitoring of goal-directed behaviors and working-memory. Other non-demented PD patients also exhibit visuospatial and memory deficits more representative of posterior cortical functioning and fail performing naming or copying tasks. Major differences in the overall rate of cognitive decline among PD patients support the co-existence of at least two patterns of involution, differentiating a relatively slow decline of fronto-striatal deficits from a more rapid decline of posterior-cortical deficits, with different pathophysiological substrates, genetics, prognosis and response to drugs used to treat the motor symptoms of PD.

Efficacy of tailored computer-based neurorehabilitation for improvement of movement initiation in Parkinson's disease

While Parkinson's disease (PD) is considered a motor disorder, motor signs of PD can be exacerbated by cognitive dysfunction. We evaluated the efficacy of a computer-based cognitive rehabilitation training program designed to improve motor-related executive function. Thirty people with PD and 21 controls participated in the 10-day training. Training consisted of a two-phase button press task. First, subjects produced an externally cued (EC) digit sequence, typing numbers displayed on the computer screen. Second, subjects were prompted to generate the same sequence in the absence of the number display (internally represented sequence, IR). Sequence length was automatically adjusted to maintain 87% correct performance. Participants were evaluated before and after training using a fixed version of the training task, and generalization of training was assessed using measures involving IR motor sequencing, switching and activities of daily living. PD participants were divided into two groups, those who showed impairment in IR motor sequence production prior to training (N=14) and those whose performance was similar to controls (N=16). Following training the impaired PD group showed significantly greater reduction in sequence initiation and completion time and in error rate for IR conditions compared to the unimpaired PD and control groups. All groups improved on Trails B-A, and pre-training Trails B was identified as a predictor of training-based improvement in IR sequence completion time and error rate. Our findings highlight the importance of neurorehabilitation tailored to the specific cognitive deficits of the PD patient.

White matter alteration of the cingulum in Parkinson disease with and without dementia: evaluation by diffusion tensor tract-specific analysis

BACKGROUND AND PURPOSE

In PD, the neurodegenerative process begins in the brain stem and extends to the limbic system and finally into the cerebral cortex. We used diffusion tensor tractography to investigate the FA of the cingulate fiber tracts in patients with PD with and without dementia.

MATERIALS AND METHODS

Fifteen patients with PD, 15 patients with PDD, and 15 age-matched healthy controls underwent diffusion tensor imaging with a 3T MR imager. Diffusion tensor tractography images of the anterior and posterior cingulate fiber tracts were generated. Mean diffusivity and FA were measured along the tractography of the anterior and posterior cingulate fiber tracts. One-way ANOVA with the Scheffé post hoc test was used to compare results among the groups.

RESULTS

FA was significantly lower in patients with PDD than in healthy controls in both the anterior and the posterior cingulate fiber tracts (P = .003, P = .015) and significantly lower in patients with PD than in healthy controls (P = .003) in the anterior cingulate fiber tract. There were no significant mean diffusivity differences among the groups. MMSE and FA values of the anterior cingulate fiber tracts in patients with PDD were significantly correlated (r = 0.633, P < .05).

CONCLUSIONS

The reduced FA in patients with PD and PDD might reflect neuropathologic changes such as Lewy body pathology in the cingulate fibers. This abnormality might contribute to the dementing process in PD.

Dopamine, affordance and active inference

The role of dopamine in behaviour and decision-making is often cast in terms of reinforcement learning and optimal decision theory. Here, we present an alternative view that frames the physiology of dopamine in terms of Bayes-optimal behaviour. In this account, dopamine controls the precision or salience of (external or internal) cues that engender action. In other words, dopamine balances bottom-up sensory information and top-down prior beliefs when making hierarchical inferences (predictions) about cues that have affordance. In this paper, we focus on the consequences of changing tonic levels of dopamine firing using simulations of cued sequential movements. Crucially, the predictions driving movements are based upon a hierarchical generative model that infers the context in which movements are made. This means that we can confuse agents by changing the context (order) in which cues are presented. These simulations provide a (Bayes-optimal) model of contextual uncertainty and set switching that can be quantified in terms of behavioural and electrophysiological responses. Furthermore, one can simulate dopaminergic lesions (by changing the precision of prediction errors) to produce pathological behaviours that are reminiscent of those seen in neurological disorders such as Parkinson's disease. We use these simulations to demonstrate how a single functional role for dopamine at the synaptic level can manifest in different ways at the behavioural level.

Nigral grafts in animal models of Parkinson's disease. Is recovery beyond motor function possible?

Parkinson's disease (PD) has long been considered predominantly to be a "movement disorder," and it is only relatively recently that nonmotor symptoms of PD have been recognized to be a major concern to patients. Consequently, there has been surprisingly little investigation into the feasibility of utilizing cell replacement therapies to ameliorate any of the nonmotor dysfunctions of PD. In this chapter, we identify nonmotor impairments associated predominately with dopaminergic dysmodulation, evaluate the few emerging studies that have identified a role for dopamine and nigral transplantation in nonmotor performance, and consider a number of outstanding questions and considerations dominating the field of nigral transplantation today. Preliminary results obtained from rodent models of PD, despite being limited in number, give clear indications of graft effects on striatal processing beyond the simple activation of motor output and promise a major, exciting, and fruitful new avenue of research for the next decade. We can now consider the prospect of rewriting the opportunities for treating patients, with new stem cell sources to be complemented by new targets for therapeutic benefit.

Progressive resistance exercise and Parkinson's disease: a review of potential mechanisms

This paper reviews the therapeutically beneficial effects of progressive resistance exercise (PRE) on Parkinson's disease (PD). First, this paper discusses the rationale for PRE in PD. Within the first section, the review discusses the central mechanisms that underlie bradykinesia and muscle weakness, highlights findings related to the central changes that accompany PRE in healthy individuals, and extends these findings to individuals with PD. It then illustrates the hypothesized positive effects of PRE on nigro-striatal-thalamo-cortical activation and connectivity. Second, it reviews recent findings of the use of PRE in individuals with PD. Finally, knowledge gaps of using PRE on individuals with PD are discussed along with suggestions for future research.

The Movement Disorder Society Evidence-Based Medicine Review Update: Treatments for the non-motor symptoms of Parkinson's disease

The Movement Disorder Society (MDS) Task Force on Evidence-Based Medicine (EBM) Review of Treatments for Parkinson's Disease (PD) was first published in 2002 and was updated in 2005 to cover clinical trial data up to January 2004 with the focus on motor symptoms of PD. In this revised version the MDS task force decided it was necessary to extend the review to non-motor symptoms. The objective of this work was to update previous EBM reviews on treatments for PD with a focus on non-motor symptoms. Level-I (randomized controlled trial, RCT) reports of pharmacological and nonpharmacological interventions for the non-motor symptoms of PD, published as full articles in English between January 2002 and December 2010 were reviewed. Criteria for inclusion and ranking followed the original program outline and adhered to EBM methodology. For efficacy conclusions, treatments were designated: efficacious, likely efficacious, unlikely efficacious, non-efficacious, or insufficient evidence. Safety data were catalogued and reviewed. Based on the combined efficacy and safety assessment, Implications for clinical practice were determined using the following designations: clinically useful, possibly useful, investigational, unlikely useful, and not useful. Fifty-four new studies qualified for efficacy review while several other studies covered safety issues. Updated and new efficacy conclusions were made for all indications. The treatments that are efficacious for the management of the different non-motor symptoms are as follows: pramipexole for the treatment of depressive symptoms, clozapine for the treatment of psychosis, rivastigmine for the treatment of dementia, and botulinum toxin A (BTX-A) and BTX-B as well as glycopyrrolate for the treatment of sialorrhea. The practical implications for these treatments, except for glycopyrrolate, are that they are clinically useful. Since there is insufficient evidence of glycopyrrolate for the treatment of sialorrhea exceeding 1 week, the practice implication is that it is possibly useful. The treatments that are likely efficacious for the management of the different non-motor symptoms are as follows: the tricyclic antidepressants nortriptyline and desipramine for the treatment of depression or depressive symptoms and macrogol for the treatment of constipation. The practice implications for these treatments are possibly useful. For most of the other interventions there is insufficient evidence to make adequate conclusions on their efficacy. This includes the tricyclic antidepressant amitriptyline, all selective serotonin reuptake inhibitors (SSRIs) reviewed (paroxetine, citalopram, sertraline, and fluoxetine), the newer antidepressants atomoxetine and nefazodone, pergolide, Ω-3 fatty acids as well as repetitive transcranial magnetic stimulation (rTMS) for the treatment of depression or depressive symptoms; methylphenidate and modafinil for the treatment of fatigue; amantadine for the treatment of pathological gambling; donepezil, galantamine, and memantine for the treatment of dementia; quetiapine for the treatment of psychosis; fludrocortisone and domperidone for the treatment of orthostatic hypotension; sildenafil for the treatment of erectile dysfunction, ipratropium bromide spray for the treatment of sialorrhea; levodopa/carbidopa controlled release (CR), pergolide, eszopiclone, melatonin 3 to 5 mg and melatonin 50 mg for the treatment of insomnia and modafinil for the treatment of excessive daytime sleepiness. Due to safety issues the practice implication is that pergolide and nefazodone are not useful for the above-mentioned indications. Due to safety issues, olanzapine remains not useful for the treatment of psychosis. As none of the studies exceeded a duration of 6 months, the recommendations given are for the short-term management of the different non-motor symptoms. There were no RCTs that met inclusion criteria for the treatment of anxiety disorders, apathy, medication-related impulse control disorders and related behaviors other than pathological gambling, rapid eye movement (REM) sleep behavior disorder (RBD), sweating, or urinary dysfunction. Therefore, there is insufficient evidence for the treatment of these indications. This EBM review of interventions for the non-motor symptoms of PD updates the field, but, because several RCTs are ongoing, a continual updating process is needed. Several interventions and indications still lack good quality evidence, and these gaps offer an opportunity for ongoing research. © 2011 Movement Disorder Society.

The case for testing memory with both stories and word lists prior to dbs surgery for Parkinson's Disease

Patients seeking deep brain stimulation (DBS) surgery for Parkinson's disease (PD) typically undergo neuropsychological assessment to determine candidacy for surgery, with poor memory performance interpreted as a contraindication. Patients with PD may exhibit worse memory for word lists than for stories due to the lack of inherent organization in a list of unrelated words. Unfortunately, word list and story tasks are typically developed from different normative datasets, and the existence of a memory performance discrepancy in PD has been challenged. We compared recall of stories and word lists in 35 non-demented PD candidates for DBS. We administered commonly used neuropsychological measures of word list and story memory (Hopkins Verbal Learning Test, Logical Memory), along with a second word list task that was co-normed with the story task. Age-corrected scores were higher for the story task than for both word list tasks. Compared to story recall, word list recall correlated more consistently with motor severity and composite measures of processing speed, working memory, and executive functioning. These results support the classic view of fronto-subcortical contributions to memory in PD and suggest that executive deficits may influence word list recall more than story recall. We recommend a multi-componential memory battery in the neuropsychological assessment of DBS candidates to characterize both mesial temporal and frontal-executive memory processes. One should not rely solely on a word list task because patients exhibiting poor memory for word lists may perform better with stories and therefore deserve an interdisciplinary discussion for DBS surgery.

Instrumental activities of daily living in Parkinson's disease dementia as compared with Alzheimer's disease: relationship to motor disability and cognitive deficits: a pilot study

OBJECTIVE

Our objective was to assess which clinical factors contribute most to the impairment of instrumental activities of daily living (IADL) in patients with dementia in Parkinson's disease (PDD) as compared to age- and dementia duration-matched patients with Alzheimer's disease (AD).

METHODS

Eighteen consecutive subjects (16 men, 2 women) with PDD and 30 age- and dementia duration-matched subjects with AD (13 men, 17 women) were recruited. The groups were not matched for their Mini-Mental Status Examination (MMSE) score. Neuropsychiatric, cognitive and motor data were collected in a routine clinical setting using the MMSE, four brief tests of the seven minute screen (i.e. the Benton Temporal Orientation, Grober and Buschke's enhanced cued recall, verbal fluency, and the clock drawing test), the Neuropsychiatric Inventory, a modified version of the Hoehn and Yahr scale and a Czech modified version of Lawton's IADL questionnaire. The IADL scores were correlated with clinical variables in each group.

RESULTS

There were no differences in IADL abilities between both groups. A significant association was found between IADL and cognitive impairment as measured by screening tests in the AD group. In the PDD group, IADL scores were particularly correlated with PD duration (r=-0.73, p<0.01) and the Hoehn and Yahr score (r=-0.59, p<0.01).

CONCLUSION

Our pilot study results show that motor deficits remain the major contributor to IADL impairment in PDD.

High-level language production in Parkinson's disease: a review

This paper discusses impairments of high-level, complex language production in Parkinson's disease (PD), defined as sentence and discourse production, and situates these impairments within the framework of current psycholinguistic theories of language production. The paper comprises three major sections, an overview of the effects of PD on the brain and cognition, a review of the literature on language production in PD, and a discussion of the stages of the language production process that are impaired in PD. Overall, the literature converges on a few common characteristics of language production in PD: reduced information content, impaired grammaticality, disrupted fluency, and reduced syntactic complexity. Many studies also document the strong impact of differences in cognitive ability on language production. Based on the data, PD affects all stages of language production including conceptualization and functional and positional processing. Furthermore, impairments at all stages appear to be exacerbated by impairments in cognitive abilities.

Is all cognitive impairment in Parkinson's disease "mild cognitive impairment"?

Cognitive impairment can be demonstrated in Parkinson's disease (PD) from the very beginning of the disease. Clinical manifestations range from slight deficits, only demonstrable by means of neuropsychological testing, up to dementia. If a linear involution is supposed for the cognitive worsening in PD, then the relatively subtle cognitive defects should be taken as the earliest signs of dementia implying that PD-MCI concept would be thoroughly equivalent to that used for the early prediction of other dementias among healthy population. Cognitive defects in PD, however, may not follow a normal distribution. While fronto-striatal deficits, such as working memory, set-shifting and free-recall verbal memory appear altered in most patients during long periods of time, certain functions depending on more posterior-cortical regions, such as copying or naming, usually characterize patients with dementia. Fronto-striatal and posterior-cortical cognitive defects may have a different pathophysiological substrates, evolution and prognosis. While fronto-striatal defects appear more related to dopaminergic defects, posterior-cortical defects may obey multiple neurotransmitter failure. Designing criteria to accurately diagnose PD-MCI is highly relevant for clinical treatment, research, care-giving and decision-making. Besides quantitative defects, an operative definition of MCI in PD should clearly distinguish a "risky cognitive profile" among the broad cognitive defects intrinsic to PD. Thus, along with other possible biological markers, from a neuropsychological point of view, posterior-cortical defects probably represent the very syndrome of MCI in PD.

Inverted-U-shaped dopamine actions on human working memory and cognitive control

Brain dopamine (DA) has long been implicated in cognitive control processes, including working memory. However, the precise role of DA in cognition is not well-understood, partly because there is large variability in the response to dopaminergic drugs both across different behaviors and across different individuals. We review evidence from a series of studies with experimental animals, healthy humans, and patients with Parkinson's disease, which highlight two important factors that contribute to this large variability. First, the existence of an optimum DA level for cognitive function implicates the need to take into account baseline levels of DA when isolating the effects of DA. Second, cognitive control is a multifactorial phenomenon, requiring a dynamic balance between cognitive stability and cognitive flexibility. These distinct components might implicate the prefrontal cortex and the striatum, respectively. Manipulating DA will thus have paradoxical consequences for distinct cognitive control processes, depending on distinct basal or optimal levels of DA in different brain regions.

Levodopa and the feedback process on set-shifting in Parkinson's disease

OBJECTIVE

To study the interaction between levodopa and the feedback process on set-shifting in Parkinson's disease (PD).

METHODS

Functional magnetic resonance imaging (fMRI) studies were performed on 13 PD subjects and 17 age-matched healthy controls while they performed a modified card-sorting task. Experimental time periods were defined based on the types of feedback provided. PD subjects underwent the fMRI experiment twice, once during "off" medication (PDoff) and again after levodopa replacement (PDon).

RESULTS

Compared with normal subjects, the cognitive processing times were prolonged in PDoff but not in PDon subjects during learning through positive outcomes. The ability to set-shift through negative outcomes was not affected in PD subjects, even when "off" medication. Intergroup comparisons showed the lateral prefrontal cortex was deactivated in PDoff subjects during positive feedback learning, especially following internal feedback cues. The cortical activations were increased in the posterior brain regions in PDoff subjects following external feedback learning, especially when negative feedback cues were provided. Levodopa replacement did not completely restore the activation patterns in PD subjects to normal although activations in the corticostriatal loops were restored.

CONCLUSION

PD subjects showed differential ability to set-shift, depending on the dopamine status as well as the types of feedback cues provided. PD subjects had difficulty performing set-shift tasks through positive outcomes when "off" medication, and showed improvement after levodopa replacement. The ability to set-shift through negative feedback was not affected in PD subjects even when "off" medication, possibly due to compensatory changes outside the nigrostriatal dopaminergic pathway.

Nonmotor symptoms in Parkinson’s disease: the dark side of the moon

Nonmotor symptoms may appear during the course of Parkinson’s disease, complicating the advanced phase in particular, but are also common in the premotor phase of Parkinson’s disease. The appearance of nonmotor manifestations represents a milestone, determining a worse prognosis and lower quality of life; however, they are often misdiagnosed and untreated. The spectrum of nonmotor symptoms encompasses mood disorders, psychosis, dementia, sleep disorders, impulse-control disorders and autonomic dysfunctions. This article describes these nonmotor symptoms and their management.

Cerebral cortical and subcortical cholinergic deficits in parkinsonian syndromes

OBJECTIVES

Cholinergic projections to cerebral cortical and subcortical regions are decreased in Parkinson disease (PD), but not evaluated in the parkinsonian syndromes of multiple system atrophy (MSA-P) and progressive supranuclear palsy (PSP). We studied cholinergic innervation in these disorders as compared to age-appropriate normal control subjects.

METHODS

We used PET with [(11)C]PMP to measure acetylcholinesterase (AChE) activity in multiple cerebral cortical and subcortical regions. We studied 22 normal controls, 12 patients with PD, 13 patients with MSA-P, and 4 patients with PSP.

RESULTS

We found significantly decreased AChE activity in most cerebral cortical regions in PD and MSA-P, and a similar but nonsignificant decrease in PSP. No differences were found between PD and MSA-P. Significantly decreased AChE activity was found in PD in striatum, cerebellum, and thalamus, with a marginally significant decrease in mesencephalon and no change in pons. Significantly greater declines in AChE activity in all subcortical regions were seen in MSA-P and PSP vs in PD. Decreased AChE activity in brainstem and cerebellum of all 3 disorders correlated with disturbances of balance and gait.

CONCLUSIONS

Cerebral cortical cholinergic activity is decreased to a similar level in Parkinson disease (PD), parkinsonian syndromes of multiple system atrophy (MSA-P), and progressive supranuclear palsy (PSP) as compared to normal controls. Subcortical cholinergic activity is significantly more decreased in MSA-P and PSP than in PD. The more substantial decrease reflects greater impairment in the pontine cholinergic group, which is important in motor activity, particularly gait. These differences may account for the greater gait disturbances in the early stages of MSA-P and PSP than in PD.

Top–Down Attentional Control in Parkinson's Disease: Salient Considerations

Cognitive dysfunction in Parkinson's disease (PD) has been hypothesized to reflect a failure of cortical control. In keeping with this hypothesis, some of the cognitive deficits in PD resemble those seen in patients with lesions in the lateral pFC, which has been associated with top–down attentional control. However, there is no direct evidence for a failure of top–down control mechanisms in PD. Here we fill this gap by demonstrating disproportionate control by bottom–up attention to dimensional salience during attentional set shifting. Patients needed significantly more trials to criterion than did controls when shifting to a low-salient dimension while, remarkably, needing significantly fewer trials to criterion than did controls when shifting to a high-salient dimension. Thus, attention was captured by bottom–up attention to salient information to a greater extent in patients than in controls. The results provide a striking reinterpretation of prior set-shifting data and provide the first direct evidence for a failure of top–down attentional control, resembling that seen after catecholamine depletion in the pFC.

The contribution of neuroimaging for the study of cognitive deficits in Parkinson's disease

The last few years have seen an increase in the number of studies using functional Magnetic Resonance Imaging (fMRI) along with receptor imaging and regional cerebral blood flow Positron Emission Tomography (PET) to understand the neurobiological underpinnings of cognitive deficits in Parkinson's disease (PD). These studies have shown evidence that the nigrostriatal degeneration solely cannot account for these deficits and that involvement of other neural systems such as the mesocortical dopamine may also play an important role. In this article, we provide a review of neuroimaging results regarding the role of possible compensatory activity, L-Dopa medication, and difference in genotypes on the cognitive deficits observed in PD. Finally, some future avenues for research are proposed.

Cognitive performance in nondemented nonpsychotic Parkinson disease patients with or without a history of depression prior to the onset of motor symptoms

In Parkinson disease (PD), cognitive impairment is common, occurs mainly in the form of milder deficits (as opposed to dementia), and commonly coincides with depression. In this cross-sectional study, we evaluated whether depression that existed before the onset of typical motor symptoms (pre-PD depression) reflected on the actual cognitive performance. Nondemented nonpsychotic PD patients with (test, n = 27) and without (control, n = 112) a history of pre-PD depression, caliper-matched for age, education, and disease duration were assessed for motor and nonmotor disease characteristics and in a battery of cognitive tests. Test patients had higher actual depression/anxiety levels. Gradual multivariate and mediation analysis indicated unfavorable effects of pre-PD depression on cognition: a direct effect on mental set shifting/response inhibition (independent of actual depression/ anxiety or other factors); and indirect effects on other cognitive domains mediated through the increased depression/anxiety. Data suggest that pre-PD depression favors poorer cognitive abilities in nondemented patients at a given time after PD has been diagnosed.

Dopaminergic drugs modulate learning rates and perseveration in Parkinson's patients in a dynamic foraging task

Making appropriate choices often requires the ability to learn the value of available options from experience. Parkinson's disease is characterized by a loss of dopamine neurons in the substantia nigra, neurons hypothesized to play a role in reinforcement learning. Although previous studies have shown that Parkinson's patients are impaired in tasks involving learning from feedback, they have not directly tested the widely held hypothesis that dopamine neuron activity specifically encodes the reward prediction error signal used in reinforcement learning models. To test a key prediction of this hypothesis, we fit choice behavior from a dynamic foraging task with reinforcement learning models and show that treatment with dopaminergic drugs alters choice behavior in a manner consistent with the theory. More specifically, we found that dopaminergic drugs selectively modulate learning from positive outcomes. We observed no effect of dopaminergic drugs on learning from negative outcomes. We also found a novel dopamine-dependent effect on decision making that is not accounted for by reinforcement learning models: perseveration in choice, independent of reward history, increases with Parkinson's disease and decreases with dopamine therapy.

Non-motor symptoms in Parkinson's disease

Parkinson's disease is a hypokinetic movement disorder with cardinal motor features of bradykinesia, resting tremor and rigidity. However, non-motor symptoms, such as cognitive, neuropsychiatric, sleep, autonomic and sensory disturbances are gaining increasing attention. These non-motor symptoms may be intrinsic to the disease pathology or may be results of treatment with dopaminergic agents. Given that most, if not all, patients with Parkinson's disease will experience non-motor symptoms, it is important to be sensitive to these phenomena, especially since some non-motor signs may precede motor impairment. Treatment may include interventions independent of traditional, dopaminergic anti-Parkinson therapy or may be tailored to increase or reduce dopamine responsiveness of the symptom.

Initial clinical manifestations of Parkinson's disease: features and pathophysiological mechanisms

A dopaminergic deficiency in patients with Parkinson's disease (PD) causes abnormalities of movement, behaviour, learning, and emotions. The main motor features (ie, tremor, rigidity, and akinesia) are associated with a deficiency of dopamine in the posterior putamen and the motor circuit. Hypokinesia and bradykinesia might have a dual anatomo-functional basis: hypokinesia mediated by brainstem mechanisms and bradykinesia by cortical mechanisms. The classic pathophysiological model for PD (ie, hyperactivity in the globus pallidus pars interna and substantia nigra pars reticulata) does not explain rigidity and tremor, which might be caused by changes in primary motor cortex activity. Executive functions (ie, planning and problem solving) are also impaired in early PD, but are usually not clinically noticed. These impairments are associated with dopamine deficiency in the caudate nucleus and with dysfunction of the associative and other non-motor circuits. Apathy, anxiety, and depression are the main psychiatric manifestations in untreated PD, which might be caused by ventral striatum dopaminergic deficit and depletion of serotonin and norepinephrine. In this Review we discuss the motor, cognitive, and psychiatric manifestations associated with the dopaminergic deficiency in the early phase of the parkinsonian state and the different circuits implicated, and we propose distinct mechanisms to explain the wide clinical range of PD symptoms at the time of diagnosis.

Enhanced frontal function in Parkinson's disease

We investigated the role of dopamine in working memory by examining effects of withdrawing dopaminergic medication in patients with Parkinson's disease. Resistance to distraction during a delayed response task was abnormally enhanced in Parkinson's disease patients OFF medication relative to controls. Conversely, performance on a backward digit span test was impaired in these same Parkinson's disease patients OFF medication. Dopaminergic medication reinstated susceptibility to distraction and backward digit span performance, so that performance of Parkinson's disease patients ON medication did not differ from that of controls. We hypothesize that the enhanced distractor resistance and impaired backward digit span in Parkinson's disease reflects low dopamine levels in the striatum, and perhaps upregulated frontal dopamine levels. Dopaminergic medication may reinstate distractibility by normalizing the balance between striatal and prefrontal dopamine transmission.

Neuropsychological aspects of Tourette syndrome: a review

Tourette syndrome (TS) is assumed to result from frontostriatal dysfunction, which would be expected to result in impairments in neuropsychological functions. This possibility has been explored in a number of studies that have assessed the performance of patients with TS within major cognitive domains and on tests involving executive functioning. We aim to summarize the main findings of these studies while evaluating the influence of task limitations and potentially critical confounding factors such as the presence of comorbidity. Although there is clearly a need for improved study design, we tentatively suggest that there is considerable evidence for cognitive impairment in a subgroup of patients, and that some difficulties seem to be intrinsic to TS. These impairments may reflect dysfunction of the anterior cingulate network within the frontostriatal pathway.

A deformation-based morphometry study of patients with early-stage Parkinson's disease

BACKGROUND AND PURPOSE

Previous volumetric magnetic resonance imaging (MRI) studies of Parkinson's disease (PD) utilized primarily voxel-based morphometry (VBM), and investigated mostly patients with moderate- to late-stage disease. We now use deformation-based morphometry (DBM), a method purported to be more sensitive than VBM, to test for atrophy in patients with early-stage PD.

METHODS

T1-weighted MRI images from 24 early-stage PD patients and 26 age-matched normal control subjects were compared using DBM. Two separate studies were conducted, where two minimally-biased nonlinear intensity-average were created; one for all subjects and another for just the PD patients. The DBM technique creates an average population-based MRI-average in an iterative hierarchical fashion. The nonlinear transformations estimated to match each subject to the MRI-average were then analysed.

RESULTS

The DBM comparison between patients and controls revealed significant contraction in the left cerebellum, and non-significant trends towards frontal, temporal and cingulate sulcal expansions with frontal and temporal white matter contractions. Within the patient group, the unified PD rating scores were highly correlated with local expansions in or near sulci bordering on frontal and temporal cortex.

CONCLUSION

Our results suggest that DBM could be a sensitive method for detecting morphological changes in early-stage PD.

Rivastigmine in Parkinson's disease dementia

Parkinson's disease has now evolved beyond what was considered to be a traditional motor disorder. It is being increasingly recognized that non-motor symptoms such as cognitive impairment, frank dementia, psychosis, depression, autonomic dysfunction and sleep disturbances are just as integral to the disease spectrum. The cholinergic system has been proposed to play a pivotal role in cognitive dysfunction. Based on interpretation of clinical studies in patients with Alzheimer's, cholinesterase inhibitors have also been studied for dementia associated with Parkinson's disease. Most of these include large and small placebo-controlled studies and several pilot studies have indicated that cholinesterase inhibitors have a favorable effect on cognition, psychiatric symptoms and global function in Parkinson's disease dementia. A large randomized placebo-controlled clinical trial showed that rivastigmine had moderate improvement in dementia associated with Parkinson's disease. The magnitude of effects in terms of scores for the cognitive subscale of the Alzheimer's disease assessment scale and Alzheimer's disease cooperative study-clinicians global impression of change were similar to those observed among patients with Alzheimer's disease who were treated with cholinesterase inhibitors. A transdermal patch which gradually releases rivastigmine over the application period is now available for use in mild to moderate dementia associated with Parkinson's disease and Alzheimer's disease.

Linguistic correlates of asymmetric motor symptom severity in Parkinson's Disease

Asymmetric motor severity is common in Parkinson's Disease (PD) and provides a method for examining the neurobiologic mechanisms underlying cognitive and linguistic deficits associated with the disorder. In the present research, PD participants (N=31) were assessed in terms of the asymmetry of their motor symptoms. Interviews with the participants were analyzed with the Linguistic Inquiry and Word Count (LIWC) program. Three measures of linguistic complexity - the proportion of verbs, proportion of function words, and sentence length - were found to be affected by symptom asymmetry. Greater left-side motor severity (and hence greater right-hemisphere dysfunction) was associated with the production of significantly fewer verbs, function words, and shorter sentences. Hence, the production of linguistic complexity in a natural language context was associated with relatively greater right hemisphere involvement. The potential neurobiological mechanisms underlying this effect are discussed.

[Assessment of hyper- and hypodopaminergic behaviors in Parkinson's disease]

The common perception that Parkinson's disease patients tend to be depressed, anxious, apathetic and harm-avoiding has currently been challenged by the recognition that they can also exhibit a hedonistic, novelty-seeking personality. Thus, Parkinson's disease patients may indulge in their passions in an irresponsible and disinhibited manner, and engage in repetitive, compulsive behaviors that may be harmful and destructive to their social or professional lives. The dopamine dysregulation syndrome includes hypersexuality, pathological gambling, and compulsive shopping; it is associated with addiction to dopaminergic medication. However, not all behavioral changes are necessarily accompanied by a dopaminergic addiction. After antiparkinson treatment is initiated, patients enter a 'honeymoon period' during which changes in mood and behavior reflect a return to the patients' premorbid personality. The increased motivation and higher level of activity in professional as well as leisure activities are considered positive changes by both the patients and their relatives. With prolonged and increased dopaminergic treatment, these positive behavioral changes can become excessive and evolve into nocturnal hyperactivity and stereotyped, repetitive and time consuming behaviors which ultimately disorganize the patient's everyday routine and herald behavioral addictions. These drug-induced behavioral changes are under-appreciated by neurologists and under-reported by the patients who neither complain about the behaviors nor understand the relationship between motivated behavior and dopaminergic medication. For these reasons, we propose a new scale for the assessment of behavior and mood to quantify and track changes related to Parkinson's disease, to dopaminergic medication, and to non-motor fluctuations. This scale is based on the concept of hypo- and hyperdopaminergic mood and behavior. The scale consists of 18 items addressing non-motor symptoms, grouped in four parts: general psychological evaluation, apathy, non-motor fluctuations and hyperdopaminergic behaviors. The rating in five points (0-4 from absent to severe) is carried out during a semi-structured interview. Open-ended questions introduce each item, allowing patients to express themselves as freely as possible. Close-ended questions permit the rating of severity and intensity. This new instrument can be used by psychologists, psychiatrists or neurologists familiar with Parkinson's disease. Designed to detect changes in mood and behavior of Parkinson's disease patients resulting either from the disease or its treatment, this tool can be used in conjunction with the neurocognitive evaluation, to help tailor the treatment of motor and non-motor symptoms to each individual's needs.

Dementia in Parkinson's disease

Parkinson's disease is the second most common neurodegenerative illness diagnosed in the United States. Dementia is recognized as a common component of advanced Parkinson's disease (PD). In patients with early PD, cognitive changes occur and primarily reflect impairment in executive function. It is unknown if the early cognitive changes detected on neuropsychological testing in Parkinson's disease are predictive of the subsequent development of Parkinson's disease with dementia (PDD). Many patients with PD develop dementia characterized by a wide range of cognitive deficits distinct from those seen in Alzheimer's disease (AD). Neuropsychiatric problems frequently accompany PDD. This chapter reviews the epidemiology, clinical characteristics of early and late cognitive changes, pathology, neuroimaging, diagnosis, and treatment of PDD.

Neuroprotective effect of PACAP on translational control alteration and cognitive decline in MPTP parkinsonian mice

Parkinson's disease (PD) is characterized by a triade of motor symptoms due to the degeneration of nigrostriatal pathway. In addition to these motor impairments, cognitive disturbances have been reported to occur in PD patients in the early stage of the disease. The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a neurotoxin widely used to produce experimental models of PD. In a previous work, we showed that MPTP altered the expression of proteins involved in mTOR antiapoptotic and PKR apoptotic pathways of translational control (TC) in neuroblastoma cells. In the present study, the results indicated that a subchronic MPTP intoxication in mice decreased the dopaminergic neuron number, produced an activation of PKR way and an inhibition of mTOR way of TC especially in striatum and frontal cortex associated with a great activation of PKR in hippocampus. Moreover, in parallel to biochemical analysis, the mnesic disturbances induced by MPTP were characterized in C57Bl/6 mice, by testing their performance in three versions of the Morris Water Maze task. Behavioral results showed that the MPTP lesion altered mice learning of a spatial working memory, of a cued version and of a spatial reference memory task in the water maze. Furthermore, we previously demonstrated that the neuropeptide pituitary adenylate cyclase activating polypeptide (PACAP) could counteract the MPTP toxicity on TC factors in neuroblastoma cells. Thus, the second objective of our study was to assess the PACAP effect on MPTP-induced TC impairment and cognitive deficit in mice. The pretreatment with PACAP27 by intravenous injections partially protected TH-positive neuron loss induced by MPTP, prevented the MPTP-induced protein synthesis control dysregulation and mnesic impairment of mice. Therefore, our results could indicate that PACAP may be a promising therapeutic agent in Parkinson's disease.

L-dopa medication in Parkinson's disease restores activity in the motor cortico-striatal loop but does not modify the cognitive network

Background

The goal of this study was to evaluate the effects of L-Dopa medication in Parkinson's disease (PD) on brain activation during the performance of a set-shifting task. Using fMRI, we have previously studied the patterns of activity observed in patients with PD after overnight removal of dopaminergic medication compared with control participants during the performance of different stages of the Wisconsin Card Sorting Task (WCST). The results revealed decreased cortical activity in the PD group compared to controls in the conditions that significantly required striatum, while increased cortical activity was observed when striatum was not involved. However, the effect of dopaminergic medication in PD patients on those patterns of activity has not yet been studied.

Methodology/Principal Findings

Here, eleven PD patients at early stage of the disease taking L-Dopa medication were recruited and underwent two fMRI sessions while performing the WCST: one session while taking their normal dose of medication and the other following overnight dopaminergic medication withdrawal. We found that L-dopa medication helped restoring a normal pattern of activity when matching and not planning was required, by increasing cortical activity in the premotor cortex. This effect was even stronger in the motor loop, i.e. when the putamen was required for controls, when matching following negative feedback. However, the medication did not change the pattern of activity in conditions relying primarily on a cognitive loop, i.e. when the caudate nucleus was required.

Conclusions/Significance

These studies provide explanation at the neural level regarding the relatively poor effects of L-Dopa on the cognitive deficits observed in PD.

White matter involvement in idiopathic Parkinson disease: a diffusion tensor imaging study

BACKGROUND AND PURPOSE

Diffusion tensor imaging (DTI) offers a unique window on the connectivity changes, extending beyond the basal ganglia, which accompany the cognitive symptoms of Parkinson disease (PD). The primary purpose of this study was to assess the microstructural damage to cerebral white matter occurring in idiopathic PD.

MATERIALS AND METHODS

Our sample included patients with PD without dementia (n = 10; Hoehn and Yahr stages I and II; Unified Parkinson Disease Rating Scale, 20.5 +/- 8.3; and Mini-Mental State Examination, 28.3 +/- 1.5) and age-matched healthy control subjects (n = 10). DTI was performed on a 1.5T scanner, and mean diffusivity (MD) and fractional anisotropy (FA) maps were obtained. Regions of interest (ROIs) were drawn on the major fiber bundles as well as on gray matter nuclei.

RESULTS

In patients, the MD was increased at borderline significance in the substantia nigra but was unaltered in the thalamus, globus pallidus, putamen, and in the head of the caudate nucleus. The FA and MD were unaltered in the corticospinal tract in the midbrain and at the level of the internal capsule, and in the splenium of the corpus callosum. By contrast, the MD was increased and the FA was decreased in the genu of the corpus callosum and in the superior longitudinal fasciculus; in the cingulum, only the MD was altered. The observed changes were not significantly lateralized.

CONCLUSIONS

Widespread microstructural damage to frontal and parietal white matter occurs already in the early stages of PD.

Rule-based category learning in patients with Parkinson's disease

Measures of explicit rule-based category learning are commonly used in neuropsychological evaluation of individuals with Parkinson's disease (PD) and the pattern of PD performance on these measures tends to be highly varied. We review the neuropsychological literature to clarify the manner in which PD affects the component processes of rule-based category learning and work to identify and resolve discrepancies within this literature. In particular, we address the manner in which PD and its common treatments affect the processes of rule generation, maintenance, shifting and selection. We then integrate the neuropsychological research with relevant neuroimaging and computational modeling evidence to clarify the neurobiological impact of PD on each process. Current evidence indicates that neurochemical changes associated with PD primarily disrupt rule shifting, and may disturb feedback-mediated learning processes that guide rule selection. Although surgical and pharmacological therapies remediate this deficit, it appears that the same treatments may contribute to impaired rule generation, maintenance and selection processes. These data emphasize the importance of distinguishing between the impact of PD and its common treatments when considering the neuropsychological profile of the disease.

Substantia nigra hyperechogenicity in depressive subjects relates to motor asymmetry and impaired word fluency

BACKGROUND

Substantia nigra hyperechogenicity (SNH) is a characteristic transcranial sonography (TCS) finding in Parkinson's disease (PD). SNH, found also in about 10% of healthy adults, was related to a subclinical malfunction of the nigrostriatal dopaminergic system on positron emission tomography studies. Both, liability for developing PD and frequency of SNH were found to be increased in depressed subjects. Here, we investigated whether SNH in depression is related to motor or cognitive abnormalities resembling early PD.

METHODS

Fourty-one patients with major depressive disorder and 15 with adjustment disorder with depressed mood were studied clinically and with TCS.

RESULTS

Frequency of SNH was similar in both groups (39, 33%; Chi-square test, P = 0.70). Larger SN echogenic size correlated with larger right-to-left asymmetry of finger tapping (Spearman test, r = 0.37, P = 0.009) and lower verbal fluency (r = -0.35, P = 0.038). These correlations were stronger in patients at ages >/= 50 years (r = 0.52, P = 0.007; r = -0.50, P = 0.020), and, independently from age, in patients with reduced echogenicity of brainstem raphe suggested to reflect alteration of the serotonergic system (r = 0.40, P = 0.045; r = -0.51, P = 0.044). Whereas bilateral sum score of finger tapping was negatively correlated with severity of depression on the beck depression inventory (r = -0.50, P = 0.001) and the Hamilton depression rating scale (r = -0.34, P = 0.019), no correlation was found between depression severity and tapping asymmetry, or between depression severity and verbal fluency.

CONCLUSION

Data suggest that TCS detects a subgroup of patients with depression characterized by symptoms of early parkinsonism who are possibly at an elevated risk of later developing definite PD.