Review Article - (2018) Volume 8, Issue 2

Broad Impairment of Executive Functions in Patients with Parkinson’s disease: A Meta-Analysis

Corresponding Author:

Jin H Yan
Laboratory of Neuromotor Control and Learning, Shenzhen University, 3688 Nan Hai Avenue, Shenzhen, 518060, China
Tel: 86-0755-8617-2031
Fax: 86-0755-2671-6888

Abstract

Abstract

Objective: Executive functions (EF) impairments have been observed in patients with Parkinson’s disease (PD). However, the pattern of EF deficits in this population remains unclear. This article aimed to examine the influence of PD on different EF domains through meta-analysis of published data.

Methods: This article aimed to compare the EFs of PD patients with those of healthy controls (CON) in different EF domains. We searched electronic databases for articles reporting comparisons of EF performance between non-demented/non-depressed PD patients and CON. Accordingly, we identified 140 studies investigating 6 EF domains (attention, inhibition, planning, reasoning, shifting and working memory) in 4683 PD patients and 4247 CON.

 Results Results showed that PD patients exhibited impaired attention (Hedges’ g= -0.48), inhibition (Hedges’ g=-0.48), planning (Hedges’ g = -0.49), reasoning (Hedges’ g = -0.31), shifting (Hedges’ g = -0.55) and working memory (Hedges’ g = -0.53). They exhibited a moderately impaired overall EF (Hedges’ g = -0.49). EF deficits were not moderated by age, years of education, disease severity, motor deficits, disease duration, medication dose or global cognition.

Conclusions: The findings suggest that among PD patients, EFs in which reasoning is least affected are broadly impaired

Keywords

Executive functions, Meta-analysis, Movement disorder, Parkinson’s disease

Introduction

Parkinson’s disease (PD) is a progressive neurodegenerative disorder characterized by dopamine depletion subsequent to the loss of dopaminergic neurons in the substantia nigra pars compacta [1]. The most salient symptoms exhibited by PD patients are motor impairments, including bradykinesia, rigidity, resting tremor, gait problems and postural instability [2-4]. In industrialised countries, PD affects 1% of older adults (age > 60 years) [5], and estimates suggest that the population of PD patients will reach 8.7–9.3 million worldwide by 2030 [6]. This increase is expected to place significant burdens on caregivers and healthcare systems in the future.

Recent research has demonstrated that in addition to motor impairments, PD patients exhibit decreases in cognitive capabilities, particularly executive functions (EF) [7]. EFs comprise a set of inter-related, effortful cognitive processes directed toward goal-directed behaviors [8]. According to the unity/diversity model, 3 core EFs exist: inhibition, working memory and shifting [9]. Inhibition refers to the suppression and control of attention, thoughts and responses required to reach a goal. Working memory describes the ability to retain information temporarily for processing and manipulation [10]. Shifting is defined as changing between task sets or response rules. The core EFs serve as foundation for higher-order EFs, such as planning and reasoning [11]. Planning involves the identification and organization of a sequence of steps needed to achieve a goal [12], whereas reasoning describes the application of knowledge to draw logical inferences [13]. As EFs enable us to address a variety of everyday tasks in a flexible manner, EF impairments can reduce the quality of life and functional outcomes of PD patients.

In addition to declines in behavioral performances, EF dysfunctions exhibited by PD patients may be related to abnormal activity in the basal ganglia and dorsolateral prefrontal cortex [14-17]. As a result, compensatory brain activity is often observed in regions related to EFs. For instance, in the Tower of London task, PD patients can normally activate the prefrontal cortex, despite harbouring subcortical lesions, and can additionally activate the hippocampus as a compensatory mechanism [14]. Regarding set shifting, the behavioral performances of PD patients and healthy controls were comparable, although PD patients exhibited increased activation in the inferior parietal cortex and superior frontal gyrus that likely indicated certain neuro-compensatory mechanisms [18]. During the n-back task, PD patients exhibited reduced connectivity between the dorsolateral prefrontal cortex and other task-related regions, indicating deficits in the executive network. These findings suggested that hyperactivity in the dorsolateral prefrontal cortex, caudate nucleus and inferior parietal cortex plays a crucial role in counteracting decreases in the EF performance in PD patients [19]. Hence, EF dysfunctions in PD patients can be associated with dysfunctional frontostriatal loops caused by dopamine pathway abnormalities.

According to a previous meta-analysis, PD patients exhibited dysfunctional EFs with effect sizes (Hedges’ g) ranging from -0.43 to -0.94 [7]. However, that meta-analysis failed to adequately address EF deficits in PD patients. First, the numbers of included studies related to different EF tasks were rather small (k = 2–14). Second, the authors did not report results for separate EFs, and thus it was difficult to compare the extents of deficits among different EFs. To overcome these limitations in our understanding, we conducted a meta-analysis to quantitatively summarize the existing results and compare different EFs between PD patients and healthy controls. Moreover, as a handful of studies have suggested the moderating effects of age, education, overall cognitive ability and PD-associated motor deficits on EF [20-24], we also examined the potential moderators of cognitive declines in PD patients. The results might provide insights for practitioners and clinicians that would allow them to focus on the most impaired cognitive abilities and devise suitable strategies to improve the functional abilities and quality of life of PD patients.

Methods

▪ Inclusion and exclusion criteria

In this review, we included studies that measured EFs in PD patients and healthy controls. Inclusion was limited to English-language research articles published in peer-reviewed journals that provided sufficient data for effect size computation. Additionally, patients in the included studies should not have been clinically depressed or demented or exhibited other neurological diseases. Commentaries, review articles and studies including PD patients experiencing deep brain stimulation were excluded.

▪ Literature search

The studies were identified by searching electronic databases and scanning the bibliographies of articles published from the first available date to May 10, 2016. The electronic databases Web of Knowledge, PubMed, Medline and PsycINFO were searched for literature using the term ‘Parkinson’s disease’ in combination with the terms ‘executive functions’, ‘working memory’, ‘cognitive control’, ‘inhibition’, ‘set shifting’, ‘flexibility’, ‘planning’, ‘reasoning’ or ‘task switching’.

▪ Data extraction

We developed an electronic spreadsheet for data extraction. The screening and eligibility assessment was performed by 3 reviewers in a non-blinded, standardized manner. During training, the reviewers assessed 300 articles and achieved a rate of agreement above 90%. Disagreements between the reviewers were resolved by consensus. Subsequently, the remaining articles were divided into 3 groups and each was assigned to a reviewer who performed data extraction and coding. The spreadsheet completed by each reviewer was double-checked by another reviewer.

The following information was extracted from each included article: (1) participants’ characteristics, including the mean age, years of education, global cognition (Mini-Mental State Examination score, MMSE) [25], disease severity (Hoehn and Yahr stage, HY) [26], motor deficits (motor score of the Unified Parkinson’s Disease Rating Scale motor, UPDRS) [27], disease duration and dose of medication (levodopa equivalent daily dose, LEDD); (2) EF domain and tasks used and (3) behavioral outcomes (EF performance).

Statistical Analyses

EF was the primary outcome measure. Mean data were converted to Hedges’ g for the metaanalysis. Multiple tests that assessed the same EF in one study were combined. The data analysis was conducted in Comprehensive Meta-Analysis 2.0. A random-effects model was used to account for variability among the samples and assessment tools across the included studies. A positive Hedges’ g indicated a better EF performance among PD patients relative to healthy controls.

▪ Heterogeneity

Cochran’s Q test was used to assess heterogeneity. An I² statistic was included to quantify the proportion of heterogeneity across studies that could not be explained by chance (I² values of 25%, 50%, and 75% corresponded to low, moderate, and high heterogeneity, respectively) [28].

▪ Publication bias

Publication bias was assessed through a visual inspection of funnel plots and Egger’s asymmetry test [29]. In a funnel plot, asymmetry can result from the non-publication of null or negative results. In Egger’s asymmetry test, the standardized effect estimate (effect size/standard error) is regressed on precision (1/standard error). A significant deviation of the y-intercept from zero might indicate the presence of publication bias. In addition, a fail-safe N test was also used to determine the number of hypothetical missing studies required to nullify the overall effects. A fail-safe N that exceeds the threshold (N ≥ 5k+10) has been well accepted as an indicator of a meta-analytic result robust to publication bias [30]. The trim-and-fill method was used to calculate an adjusted effect size corrected for the effects of missing studies in asymmetrical funnel plots [31].

▪ Meta-regression

The influences of age, years of education, disease severity, motor deficits, disease duration and global cognition and dose of medication on the study outcome were assessed through a mixedeffects meta-regression analysis based on an unrestricted maximum likelihood model.

Results

One hundred and forty articles involving 4639 PD patients and 4219 healthy controls were retrieved and included in the meta-analysis. Figure 1 shows the number of studies retained at different stages of the literature search and screening process. Details of the individual studies are presented in Table 1. The mean ages of the PD patients and controls were 64.34 and 63.81 years, respectively. A total of 275 effect sizes were included in the meta-analysis: tapping on attention (k = 11, N_PD = 556, N_CON = 521), inhibition (k = 56, N_PD = 1837, N_CON = 1572), planning (k = 22, N_PD = 771, N_CON = 698), reasoning (k = 13, N_PD = 429, N_CON = 419), shifting (k = 82, N_PD = 2827, N_CON = 2648) and working memory (k = 91, N_PD = 2901, N_CON = 2650).

Table 1: Summary of the included studies.

Figure 1: Flowchart of literature search.

▪ Pooled effect size

The pooled effect sizes, heterogeneity and publication bias results are summarized in Table 2. A summary forest plot of all the relevant EFs is presented in Figure 2. The pooled effect size suggested a fairly moderate deficit in overall EF among PD patients. Small to moderate effect sizes were observed for attention (Hedges’ g = -0.48, 95% CI: -0.62 to -0.35), inhibition (Hedges’ g = -0.48, 95% CI: -0.59 to -0.36), planning (Hedges’ g = -0.49, 95% CI: -0.62 to -0.36), reasoning (Hedges’ g = -0.31, 95% CI: -0.45 to -0.18), shifting (Hedges’ g = -0.55, 95% CI: -0.68 to -0.42) and working memory (Hedges’ g = -0.49, 95% CI: -0.53 to -0.44).

Table 2: Summary of the meta-analytic results.

Figure 2: Meta-analysis results for different executive functions. Positive effect size indicates better behavioral performance of Parkinson’s patients (PD).

▪ Heterogeneity

Heterogeneity was significant among the studies of overall EF (Cochran’s Q = 820.94, p< 0.001). After grouping by domain, however, significant heterogeneity was not observed among studies of attention (Cochran’s Q = 12.56, p= 0.32), planning (Cochran’s Q = 27.97, p = 0.14) and reasoning (Cochran’s Q = 6.44, p= 0.89), but remained significant among studies of inhibition (Cochran’s Q = 135.85, p< 0.001), shifting (Cochran’s Q = 414.86, p< 0.001) and working memory (Cochran’s Q = 215.28, p< 0.001). Heterogeneity was moderate for inhibition (I² = 59.51%), working memory (I² = 58.19%) and overall EF (I² = 66.5%), but high for shifting (I² = 80.48%).

▪ Publication bias

The fail-safe N exceeded the thresholds for all EF domains except reasoning, indicating that the meta-analytic results were generally robust to publication bias. Funnel plot asymmetry appeared to be present for all EF domains (Figure 3). However, Egger’s asymmetry test yielded significant results only for inhibition (t = 2.80, p < 0.01), reasoning (t = 2.99, p < 0.01), shifting (t = 2.02, p < 0.05), working memory (t = 2.64, p < 0.01) and overall EF (t = 3.93, p < 0.001). After a trim-and-fill procedure, the adjusted effect sizes generally decreased, and no significant measure became non-significant. On average, the mean effect size (Hedges’ g) decreased (i.e., became less negative) in the trimmed analyses by a value of 0.13 (Table 2).

Figure 3: Funnel plots of executive functions.

▪ Meta-regression

The results of meta-regression analyses showed that age (β = -0.0046, p = 0.44), years of education (β = -0.0036, p = 0.83), Hoehn and Yahr stage (β = -0.11, p = 0.32), disease duration (β = -0.0025, p = 0.15), UPDRS motor score (β = -0.0046, p = 0.44), MMSE (β = 0.065, p = 0.21) and LEDD (β = 0.000009, p = 0.54) did not significantly predict the study outcomes.

Discussion

▪ Summary of evidence

This meta-analysis quantitatively compared the EFs of PD patients and healthy controls. A fairly moderate impairment of overall EF was observed among PD patients, consistent with the results of a previous meta-analysis [7]. The smaller effect size observed in this meta-analysis might have resulted from the inclusion of a greater number of studies. Small to moderate effect sizes were observed in different EF domains (Hedges’ g = -0.31 to -0.55), among which reasoning skill was the least affected. This finding was compatible with the results of an earlier study in which abstract thinking was demonstrated to be less affected than attentional control in PD patients [32]. Other EF domains were similarly affected to a similar extent. Furthermore, the EF deficits experienced by PD patients were not found to be influenced by age, years of education, disease severity, motor deficits, disease duration, medication dose or global cognition.

▪ Heterogeneity

Moderate heterogeneity was observed for overall EF. After grouping by EF domain, however, high heterogeneity was observed for shifting, whereas moderate heterogeneity was observed for inhibition and working memory. Heterogeneity may have resulted from variability among the assessment tools and performance indexes used by the included studies. In addition, the constructs of inhibition, shifting and working memory could possibly be divided into heterogeneous sub-constructs. For instance, PD patients might exhibit impairment in only one type of working memory. Thus, heterogeneity may arise if the different types of inhibition, shifting and working memory are treated as single constructs.

▪ Publication bias

According to Egger’s asymmetry test, the overall EF was subject to publication bias. Even after grouping by EF domain, obvious publication bias remained for inhibition, reasoning, shifting and working memory, suggesting that null results or results indicating better EF among PD patients may not have been published. The failsafe N indicated that the meta-analytic results for attention, inhibition, planning, shifting and working memory were robust to publication bias. However, the fail-safe N for reasoning was smaller than the threshold, indicating that the meta-analytic results for this domain were susceptible to publication bias. The effect sizes generally decreased after the trim-out-fill procedure was applied, but remained significantly different from zero. Hence, broad EF impairments were observed in PD patients, and reasoning remained the least affected EF domain.

▪ Implications

The current meta-analysis provided evidence indicating that PD patients exhibit substantial dysfunctions across different EF domains. In this population, the impairments in the attention, inhibition, planning, and reasoning, shifting and working memory domains were not moderated by age, years of education, disease severity, motor deficits, disease duration, medication dose or global cognition. These results better enable us to understand the EF profiles of PD patients, and this information will assist clinicians and caregivers with devising suitable strategies to improve the functional outcomes and quality of life of PD patients.

EFs are essential for performing daily tasks; accordingly, impairments in these domains can greatly degrade a PD patient’s quality of life and functional capability [33]. The existing literature suggests that EF can be altered through deliberate training and intervention. For instance, physical exercise has been shown to improve inhibition, planning and working memory in PD patients [34-36]. Computerized cognitive training and video games were also found to improve EF in PD patients [37,38]. The selection of an appropriate remedial approach could halt a decline in or even improve the EF of PD patients, thus reducing the challenges faced during daily life and the burdens placed on caregivers.

▪ Limitations and future research direction

This meta-analysis was limited to studies published in peer-reviewed journals. Although this criterion ensured the quality of the included studies, we risked missing relevant studies that had been published elsewhere. Additionally, some identified studies were excluded from the meta-analysis because they contained insufficient data for an effect size computation. This reduced the number of included studies and potentially introduced a non-random bias.

In addition, the selection of EF domains and tasks may have been arbitrary. According to an existing consensus, inhibition, working memory and shifting are the core EFs [9]. Although attention, planning and reasoning are crucial for goal-directed behaviors, their inclusion in EF is debatable. For instance, reasoning may depend on one’s prior knowledge and experience, rather than the ability to follow rules of logic. Moreover, more than one performance index (such as reaction time, accuracy and error rate) might apply to an EF task, and the index selection may be subjective. To ensure that the included performance indices were representative of the assessed EFs, we attempted to select the most appropriate indicators based on suggestions in the literature.

Furthermore, as most included studies failed to report the medication statuses of PD patients during testing, it was difficult to determine whether performance was affected by medication. Therefore, future reports should provide more details about the medication statuses of PD patients, which would allow a decoupling of genuine EF deficits from medication-influenced performance.

Conclusions

PD patients exhibit impairments in the EF domains of attention, inhibition, planning, reasoning, shifting and working memory; of these, reasoning is the least affected. Furthermore, these deficits are not influenced by age, years of education, disease severity, motor deficits, disease duration, medication dose or global cognition. However, as the results for reasoning were more susceptible to publication bias, compared to other EF domains, additional studies of this domain should be conducted and included in future meta-analyses. Finally, although numerous studies have demonstrated the plasticity of EF in PD patients, the efficacies of remediate EF training and intervention strategies require further verifications.

Funding

This paper was supported by the Natural Science Foundation of SZU to JHY (JCYJ20170302143406192).

Conflict of Interest

No conflict of interest is to be disclosed.

References

1. Petzinger GM, Fisher BE, McEwen S, et al. Exercise-enhanced neuroplasticity targeting motor and cognitive circuitry in Parkinson's disease. Lancet. Neurol 12(7), 716-726 (2013).
2. Parkinson J. An essay on the shaking palsy. 1817. J. Neuropsychiatry. Clin .Neurosci 14(2), 223-236; discussion 222 (2002).
3. Strafella AP, Lozano AM, Lang AE, et al. Subdural motor cortex stimulation in Parkinson's disease does not modify movement-related rCBF pattern. Mov. Disord 22(14), 2113-2116 (2007).
4. Strafella AP, Lozano AM, Ballanger B, et al. rCBF changes associated with PPN stimulation in a patient with Parkinson's disease: A PET study. Mov. Disord 23(7), 1051-1054 (2008).
5. De Lau LM, Breteler MM. Epidemiology of Parkinson's disease. Lancet. Neurol 5(6), 525-535 (2006).
6. Dorsey E, Constantinescu R, Thompson J, et al. Projected number of people with Parkinson disease in the most populous nations, 2005 through 2030. Neurology 68(5), 384-386 (2007).
7. Kudlicka A, Clare L, Hindle JV. Executive functions in Parkinson's disease: Systematic review and meta-analysis. Mov. Disord 26(13), 2305-2315 (2011).
8. Diamond A. Executive functions. Ann. Rev. Psychol 64135-168 (2013).
9. Miyake A, Friedman NP, Emerson MJ, et al. The unity and diversity of executive functions and their contributions to complex "Frontal Lobe" tasks: A latent variable analysis. Cogn. Psychol 41(1), 49-100 (2000).
10. Baddeley AD, Hitch GJ. Developments in the concept of working memory. Neuropsychology 8(4), 485-493 (1994).
11. Collins A, Koechlin E. Reasoning, learning, and creativity: Frontal lobe function and human decision-making. PLoS. Biol 10(3), e1001293 (2012).
12. Lezak MD. Neuropsychologica.l assessment. Place: Oxford University Press, USA.
13. Wason PC, Johnson-Laird PN. Psychology of reasoning: Structur. and content. Place: Harvard University Press.
14. Dagher A, Owen AM, Boecker H, et al. The role of the striatum and hippocampus in planning: a PET activation study in Parkinson's disease. Brain 124(Pt 5), 1020-1032 (2001).
15. Dirnberger G, Frith CD, Jahanshahi M. Executive dysfunction in Parkinson's disease is associated with altered pallidal–frontal processing. Neuroimage 25(2), 588-599 (2005).
16. Marklund P, Larsson A, Elgh E, et al. Temporal dynamics of basal ganglia under-recruitment in Parkinson's disease: transient caudate abnormalities during updating of working memory. Brain 132(Pt 2), 336-346 (2009).
17. Owen AM, Doyon J, Dagher A, et al. Abnormal basal ganglia outflow in Parkinson's disease identified with PET. Implications for higher cortical functions. Brain 121 ( Pt 5)949-965 (1998).
18. Gerrits NJ, van der Werf YD, Verhoef KM, et al. Compensatory fronto-parietal hyperactivation during set-shifting in unmedicated patients with Parkinson's disease. Neuropsychologia 68107-116 (2015).
19. Trujillo JP, Gerrits NJ, Veltman DJ, et al. Reduced neural connectivity but increased task-related activity during working memory in de novo Parkinson patients. Hum. Brain. Mapp 36(4), 1554-1566 (2015).
20. Bott NT, Johnson ET, Schuff N, et al. Sensitive measures of executive dysfunction in non-demented Parkinson's disease. Parkinsonism. Relat. Disord 20(12), 1430-1433 (2014).
21. Dujardin K, Defebvre L, Krystkowiak P, et al. Executive function differences in multiple system atrophy and Parkinson's disease. Parkinsonism. Relat. Disord 9(4), 205-211 (2003).
22. Huizinga M, Dolan CV, van der Molen MW. Age-related change in executive function: Developmental trends and a latent variable analysis. Neuropsychologia 44(11), 2017-2036 (2006).
23. McKinlay A, Grace RC, Dalrymple-Alford JC, et al. Characteristics of executive function impairment in Parkinson's disease patients without dementia. J. Int. Neuropsychol. Soc 16(2), 268-277 (2010).
24. Wu Q, Chen L, Zheng Y, et al. Cognitive impairment is common in Parkinson’s disease without dementia in the early and middle stages in a Han Chinese cohort. Parkinsonism. Relat. Disord 18(2), 161-165 (2012).
25. Folstein MF, Folstein SE, McHugh PR. "Mini-mental state". A practical method for grading the cognitive state of patients for the clinician. J. Psychiatr. Res 12(3), 189-198 (1975).
26. Hoehn MM, Yahr MD. Parkinsonism: Onset, progression and mortality. Neurology 17(5), 427-442 (1967).
27. Ramaker C, Marinus J, Stiggelbout AM, et al. Systematic evaluation of rating scales for impairment and disability in Parkinson's disease. Mov. Disord 17(5), 867-876 (2002).
28. Higgins JP, Thompson SG, Deeks JJ, et al. Measuring inconsistency in meta-analyses. Br. Med. J 327(7414), 557-560 (2003).
29. Egger M, Smith DG, Schneider M, et al. Bias in meta-analysis detected by a simple, graphical test. Br. Med. J 315(7109), 629-634 (1997).
30. Rosenthal R. The file drawer problem and tolerance for null results. Psychol. Bull 86(3), 638 (1979).
31. Duval S, Tweedie R. Trim and fill: A simple funnel-plot-based method of testing and adjusting for publication bias in meta-analysis. Biometrics 56(2), 455-463 (2000).
32. Kudlicka A, Clare L, Hindle JV. Pattern of executive impairment in mild to moderate Parkinson's disease. Dement. Geriatr. Cogn. Disord 36(1-2), 50-66 (2013).
33. Yogev-Seligmann G, Hausdorff JM, Giladi N. The role of executive function and attention in gait. Mov.Disord 23(3), 329-342; quiz 472 (2008).
34. Cruise KE, Bucks RS, Loftus AM, et al. Exercise and Parkinson's: Benefits for cognition and quality of life. Acta. Neurol. Scand 123(1), 13-19 (2011).
35. Ridgel AL, Kim CH, Fickes EJ, et al. Changes in executive function after acute bouts of passive cycling in Parkinson's disease. J. Aging. Phys. Act 19(2), 87-98 (2011).
36. Tanaka K, Quadros AC, Jr., Santos RF, et al. Benefits of physical exercise on executive functions in older people with Parkinson's disease. Brain. Cogn 69(2), 435-441 (2009).
37. dos Santos Mendes FA, Pompeu JE, Modenesi Lobo A, et al. Motor learning, retention and transfer after virtual-reality-based training in Parkinson's disease--effect of motor and cognitive demands of games: A longitudinal, controlled clinical study. Physiotherapy 98(3), 217-223 (2012).
38. Paris AP, Saleta HG, de la Cruz Crespo Maraver M, et al. Blind randomized controlled study of the efficacy of cognitive training in Parkinson's disease. Mov. Disord 26(7), 1251-1258 (2011).
39. Agosta F, Galantucci S, Svetel M, et al. Clinical, cognitive, and behavioural correlates of white matter damage in progressive supranuclear palsy. J.Neurol 261(5), 913-924 (2014).
40. Colman KS, Koerts J, van Beilen M, et al. The impact of executive functions on verb production in patients with Parkinson's disease. Cortex 45(8), 930-942 (2009).
41. Crescentini C, Marin D, Del Missier F, et al. Interference from retrieval cues in Parkinson's disease. Neuropsychology 25(6), 720-733 (2011).
42. Crescentini C, Mondolo F, Biasutti E, et al. Preserved and impaired task-switching abilities in non-demented patients with Parkinson's disease. J. Neuropsychol 6(1), 94-118 (2012).
43. Duncan GW, Firbank MJ, Yarnall AJ, et al. Gray and white matter imaging: A biomarker for cognitive impairment in early Parkinson's disease? Mov. Disord 31(1), 103-110 (2016).
44. Elwan OH, Baradah OH, Madkour O, et al. Parkinson's disease, cognition and aging. Clinical, neuropsychological, electrophysiological and cranial computerized tomographic assessment. J. Neurol. Sci 143(1-2), 64-71 (1996).
45. Ibarretxe-Bilbao N, Zarei M, Junque C, et al. Dysfunctions of cerebral networks precede recognition memory deficits in early Parkinson's disease. Neuroimage 57(2), 589-597 (2011).
46. Lord S, Galna B, Coleman S, et al. Cognition and gait show a selective pattern of association dominated by phenotype in incident Parkinson’s disease. Front. Aging. Neurosci 6 (2014).
47. Murray LL, Rutledge S. Reading comprehension in Parkinson's disease. Am. J. Speech. Lang. Pathol 23(2), S246-258 (2014).
48. Poletti M, Frosini D, Pagni C, et al. Mild cognitive impairment and cognitive-motor relationships in newly diagnosed drug-naive patients with Parkinson's disease. J. Neurol. Neurosurg. Psychiatry 83(6), 601-606 (2012).
49. Rodríguez‐Ferreiro J, Cuetos F, Herrera E, et al. Cognitive impairment in Parkinson's disease without dementia. Mov. Disord 25(13), 2136-2141 (2010).
50. Abe N, Fujii T, Hirayama K, et al. Do parkinsonian patients have trouble telling lies? The neurobiological basis of deceptive behaviour. Brain 132(Pt 5), 1386-1395 (2009).
51. Anderson RJ, Simpson AC, Channon S, et al. Social problem solving, social cognition, and mild cognitive impairment in Parkinson's disease. Behav. Neurosci 127(2), 184-192 (2013).
52. Baggio HC, Segura B, Ibarretxe-Bilbao N, et al. Structural correlates of facial emotion recognition deficits in Parkinson's disease patients. Neuropsychologia 50(8), 2121-2128 (2012).
53. Baggio HC, Segura B, Sala-Llonch R, et al. Cognitive impairment and resting-state network connectivity in Parkinson's disease. Hum. Brain. Mapp 36(1), 199-212 (2015).
54. Barnes J, Boubert L. Executive functions are impaired in patients with Parkinson's disease with visual hallucinations. J. Neurol. Neurosurg. Psychiatry 79(2), 190-192 (2008).
55. Beste C, Willemssen R, Saft C, et al. Error processing in normal aging and in basal ganglia disorders. Neuroscience 159(1), 143-149 (2009).
56. Beyer MK, Bronnick KS, Hwang KS, et al. Verbal memory is associated with structural hippocampal changes in newly diagnosed Parkinson's disease. J. Neurol. Neurosurg. Psychiatry 84(1), 23-28 (2013).
57. Bezdicek O, Michalec J, Nikolai T, et al. Clinical validity of the Mattis Dementia Rating Scale in differentiating mild cognitive impairment in Parkinson's disease and normative data. Dement. Geriatr. Cogn. Disord 39(5-6), 303-311 (2015).
58. Bohlhalter S, Abela E, Weniger D, et al. Impaired verbal memory in Parkinson disease: Relationship to prefrontal dysfunction and somatosensory discrimination. Behav. Brain. Funct 549 (2009).
59. Bohnen NI, Kaufer DI, Hendrickson R, et al. Cognitive correlates of cortical cholinergic denervation in Parkinson's disease and parkinsonian dementia. J. Neurol 253(2), 242-247 (2006).
60. Broeders M, Velseboer DC, de Bie R, et al. Cognitive change in newly-diagnosed patients with Parkinson's disease: A 5-year follow-up study. J. Int. Neuropsychol. Soc 19(6), 695-708 (2013).
61. Cohen RG, Klein KA, Nomura M, et al. Inhibition, executive function, and freezing of gait. J.Parkinsons. Dis 4(1), 111-122 (2014).
62. Dujardin K, Blairy S, Defebvre L, et al. Deficits in decoding emotional facial expressions in Parkinson's disease. Neuropsychologia 42(2), 239-250 (2004).
63. Edelstyn NM, Mayes AR, Condon L, et al. Recognition, recollection, familiarity and executive function in medicated patients with moderate Parkinson's disease. J. Neuropsychol 1(Pt 2), 131-147 (2007).
64. Dujardin SW. Targeted training of the decision rule benefits rule-guided behavior in Parkinson's disease. Cogn. Affect. Behav. Neurosci 13(4), 830-846 (2013).
65. Fales CL, Vanek ZF, Knowlton BJ. Backward inhibition in Parkinson's disease. Neuropsychologia 44(7), 1041-1049 (2006).
66. Fling BW, Cohen RG, Mancini M, et al. Asymmetric pedunculopontine network connectivity in parkinsonian patients with freezing of gait. Brain 136(Pt 8), 2405-2418 (2013).
67. Galtier I, Nieto A, Lorenzo JN, et al. Cognitive impairment in Parkinson's disease: More than a frontostriatal dysfunction. Span. J.Psychol 17E68 (2014).
68. Gawrys L, Falkiewicz M, Pilacinski A, et al. The neural correlates of specific executive dysfunctions in Parkinson's disease. Acta. Neurobiol. Exp 74(4), 465-478 (2014).
69. Green J, Woodard JL, Sirockman BE, et al. Event-related potential P3 change in mild Parkinson's disease. Mov. Disord 11(1), 32-42 (1996).
70. Hausdorff JM, Doniger GM, Springer S, et al. A common cognitive profile in elderly fallers and in patients with Parkinson's disease: The prominence of impaired executive function and attention. Exp. Aging. Res 32(4), 411-429 (2006).
71. Hsieh YH, Chen KJ, Wang CC, et al. Cognitive and motor components of response speed in the stroop test in Parkinson's disease patients. Kaohsiung. J .Med .Sci 24(4), 197-203 (2008).
72. Koerts J, Van Beilen M, Leenders KL, et al. Complaints about impairments in executive functions in Parkinson's disease: The association with neuropsychological assessment. Parkinsonism. Relat. Disord 18(2), 194-197 (2012).
73. Koerts J, Meijer HA, Colman KS, et al. What is measured with verbal fluency tests in Parkinson’s disease patients at different stages of the disease? J. Neural. Transm 1-9 (2013).
74. Koerts J, Tucha L, Leenders KL, et al. Neuropsychological and emotional correlates of personality traits in Parkinson's disease. Behav. Neurol 27(4), 567-574 (2013).
75. Lewis SJ, Shine JM, Duffy S, et al. Anterior cingulate integrity: Executive and neuropsychiatric features in Parkinson's disease. Mov. Disord 27(10), 1262-1267 (2012).
76. Marzinzik F, Herrmann A, Gogarten J, et al. Dysfunctional action control as a specific feature of Parkinson's disease. J. Neural. Transm 122(8), (2015).
77. McNamara P, Durso R, Brown A. Relation of “sense of self” to executive function performance in Parkinson's disease. Cogn. Behav. Neurol 16(3), 139-148 (2003).
78. McNamara P, Durso R, Harris E. Life goals of patients with Parkinson's disease: A pilot study on correlations with mood and cognitive functions. Clin. Rehabil 20(9), 818-826 (2006).
79. McNamara P, Stavitsky K, Durso R, et al. The impact of clinical and cognitive variables on social functioning in Parkinson's disease: Patient versus examiner estimates. Parkinsons. Dis 2010 (2010).
80. Miller IN, Neargarder S, Risi MM, et al. Frontal and posterior subtypes of neuropsychological deficit in Parkinson's disease. Behav. Neurosci 127(2), 175-183 (2013).
81. Mitchell RL, Boucas SB. Decoding emotional prosody in Parkinson's disease and its potential neuropsychological basis. J. Clin. Exp. Neuropsychol 31(5), 553-564 (2009).
82. Obeso I, Wilkinson L, Jahanshahi M. Levodopa medication does not influence motor inhibition or conflict resolution in a conditional stop-signal task in Parkinson's disease. Exp. Brain. Res 213(4), 435-445 (2011).
83. O'Callaghan C, Naismith SL, Shine JM, et al. A novel bedside task to tap inhibitory dysfunction and fronto-striatal atrophy in Parkinson's disease. Parkinsonism. Relat. Disord 19(9), 827-830 (2013).
84. Pellicano C, Assogna F, Piras F, et al. Regional cortical thickness and cognitive functions in non-demented Parkinson's disease patients: A pilot study. Eur. J. Neurol 19(1), 172-175 (2012).
85. Pereira JB, Ibarretxe-Bilbao N, Marti MJ, et al. Assessment of cortical degeneration in patients with Parkinson's disease by voxel-based morphometry, cortical folding, and cortical thickness. Hum. Brain. Mapp 33(11), 2521-2534 (2012).
86. Pettit L, McCarthy M, Davenport R, et al. Heterogeneity of letter fluency impairment and executive dysfunction in Parkinson's disease. J. Int. Neuropsychol. Soc 19(9), 986-994 (2013).
87. Pillon B, Ertle S, Deweer B, et al. Memory for spatial location is affected in Parkinson's disease. Neuropsychologia 34(1), 77-85 (1996).
88. Pillon B, Ertle S, Deweer B, et al. Memory for spatial location in ‘de novo’ parkinsonian patients. Neuropsychologia 35(3), 221-228 (1997).
89. Ranchet M, Paire-Ficout L, Uc EY, et al. Impact of specific executive functions on driving performance in people with Parkinson's disease. Mov. Disord 28(14), 1941-1948 (2013).
90. Raskin SA, Woods SP, Poquette AJ, et al. A differential deficit in time- versus event-based prospective memory in Parkinson's disease. Neuropsychology 25(2), 201-209 (2011).
91. Segura B, Baggio HC, Marti MJ, et al. Cortical thinning associated with mild cognitive impairment in Parkinson's disease. Mov. Disord 29(12), 1495-1503 (2014).
92. Stavitsky K, Neargarder S, Bogdanova Y, et al. The impact of sleep quality on cognitive functioning in Parkinson's disease. J. Int. Neuropsychol. Soc 18(1), 108-117 (2012).
93. Theilmann RJ, Reed JD, Song DD, et al. White-matter changes correlate with cognitive functioning in Parkinson's disease. Front. Neurol 437 (2013).
94. van Spaendonck KP, Berger HJ, Horstink MW, et al. Card sorting performan ce in Parkinson's disease: A comparison between acquisition and shifting performance. J. Clin. Exp .Neuropsychol 17(6), 918-925 (1995).
95. Wild LB, de Lima DB, Balardin JB, et al. Characterization of cognitive and motor performance during dual-tasking in healthy older adults and patients with Parkinson's disease. J. Neurol 260(2), 580-589 (2013).
96. Wylie SA, Stout JC, Bashore TR. Activation of conflicting responses in Parkinson's disease: Evidence for degrading and facilitating effects on response time. Neuropsychologia 43(7), 1033-1043 (2005).
97. Wylie SA, van den Wildenberg WP, Ridderinkhof KR, et al. The effect of speed-accuracy strategy on response interference control in Parkinson's disease. Neuropsychologia 47(8-9), 1844-1853 (2009).
98. Zgaljardic DJ, Borod JC, Foldi NS, et al. An examination of executive dysfunction associated with frontostriatal circuitry in Parkinson's disease. J. Clin. Exp. Neuropsychol 28(7), 1127-1144 (2006).
99. JR, Chen J, Yang ZJ, et al. Rapid eye movement sleep behavior disorder symptoms correlate with domains of cognitive impairment in Parkinson's disease. Chin. Med.J 129(4), 379-385 (2016).
100. Altgassen M, Phillips L, Kopp U, et al. Role of working memory components in planning performance of individuals with Parkinson's disease. Neuropsychologia 45(10), 2393-2397 (2007).
101. Cipresso P, Albani G, Serino S, et al. Virtual multiple errands test (VMET): A virtual reality-based tool to detect early executive functions deficit in Parkinson's disease. Front. Behav. Neurosci 8405 (2014).
102. Dujardin K, Defebvre L, Grunberg C, et al. Memory and executive function in sporadic and familial Parkinson's disease. Brain 124(2), 389-398 (2001).
103. Engels G, Weeda WD, Vlaar AM, et al. Clinical Pain and Neuropsychological Functioning in Parkinson's Disease: Are They Related? Parkinsons. Dis 20168675930 (2016).
104. Foster ER, McDaniel MA, Repovs G, et al. Prospective memory in Parkinson disease across laboratory and self-reported everyday performance. Neuropsychology 23(3), 347-358 (2009).
105. McKinlay A, Kaller CP, Grace RC, et al. Planning in Parkinson's disease: A matter of problem structure? Neuropsychologia 46(1), 384-389 (2008).
106. Miah IP, Olde Dubbelink KT, Stoffers D, et al. Early-stage cognitive impairment in Parkinson's disease and the influence of dopamine replacement therapy. Eur. J. Neurol 19(3), 510-516 (2012).
107. Monetta L, Grindrod CM, Pell MD. Irony comprehension and theory of mind deficits in patients with Parkinson's disease. Cortex 45(8), 972-981 (2009).
108. Morris RG, Downes JJ, Sahakian BJ, et al. Planning and spatial working memory in Parkinson's disease. J. Neurol. Neurosurg. Psychiatry 51(6), 757-766 (1988).
109. Muslimovic D, Post B, Speelman JD, et al. Motor procedural learning in Parkinson's disease. Brain 130(Pt 11), 2887-2897 (2007).
110. Parrao T, Chana P, Venegas P, et al. Olfactory deficits and cognitive dysfunction in Parkinson’s disease. Neurodegener. Dis 10(1-4), 179-182 (2012).
111. Pell MD, Monetta L, Rothermich K, et al. Social perception in adults with Parkinson's disease. Neuropsychology 28(6), 905-916 (2014).
112. Perfetti B, Varanese S, Mercuri P, et al. Behavioural assessment of dysexecutive syndrome in Parkinson's disease without dementia: A comparison with other clinical executive tasks. Parkinsonism. Relat. Disord 16(1), 46-50 (2010).
113. Rosen JB, Rott E, Ebersbach G, et al. Altered moral decision-making in patients with idiopathic Parkinson's disease. Parkinsonism. Relat. Disord 21(10), 1191-1199 (2015).
114. Schomaker J, Berendse HW, Foncke EM, et al. Novelty processing and memory formation in Parkinson's disease. Neuropsychologia 62124-136 (2014).
115. Basic J, Katic S, Vranicc A, et al. Cognition in Parkinson's disease. Croat. Med. J 45(4), 451-456 (2004).
116. Benke T, Bosch S, Andree B. A study of emotional processing in Parkinson's disease. Brain. Cogn 38(1), 36-52 (1998).
117. Bodden ME, Mollenhauer B, Trenkwalder C, et al. Affective and Cognitive Theory of Mind in patients with parkinson's disease. Parkinsonism. Relat. Disord 16(7), 466-470 (2010).
118. Brand M, Labudda K, Kalbe E, et al. Decision-making impairments in patients with Parkinson's disease. Behav. Neurol 15(3-4), 77-85 (2004).
119. Costa A, Peppe A, Carlesimo GA, et al. Neuropsychological correlates of alexithymia in Parkinson's disease. J. Int. Neuropsychol. Soc 13(6), 980-992 (2007).
120. Costa A, Zabberoni S, Peppe A, et al. Time-based prospective memory functioning in mild cognitive impairment associated with Parkinson's disease: Relationship with autonomous management of daily living commitments. Front. Hum. Neurosci 9333 (2015).
121. Euteneuer F, Schaefer F, Stuermer R, et al. Dissociation of decision-making under ambiguity and decision-making under risk in patients with Parkinson's disease: A neuropsychological and psychophysiological study. Neuropsychologia 47(13), 2882-2890 (2009).
122. Mioni G, Meligrana L, Rendell PG, et al. Event-based prospective memory in patients with Parkinson's disease: The effect of emotional valence. Front. Hum. Neurosci 9427 (2015).
123. Natsopoulos D, Katsarou Z, Alevriadou A, et al. Deductive and inductive reasoning in Parkinson's disease patients and normal controls: Review and experimental evidence. Cortex 33(3), 463-481 (1997).
124. Akamatsu T, Fukuyama H, Kawamata T. The effects of visual, auditory, and mixed cues on choice reaction in Parkinson's disease. J. Neurol. Sci 269(1-2), 118-125 (2008).
125. Aksan N, Anderson SW, Dawson J, et al. Cognitive functioning differentially predicts different dimensions of older drivers' on-road safety. Accid. Anal. Prev 75236-244 (2015).
126. Alonso-Recio L, Martin-Plasencia P, Loeches-Alonso A, et al. Working memory and facial expression recognition in patients with Parkinson's disease. J. Int. Neuropsychol. Soc 20(5), 496-505 (2014).
127. Bogdanova Y, Cronin-Golomb A. Neurocognitive correlates of apathy and anxiety in Parkinson's disease. Parkinsons. Dis 2012793076 (2012).
128. Bokura H, Yamaguchi S, Kobayashi S. Event-related potentials for response inhibition in Parkinson's disease. Neuropsychologia 43(6), 967-975 (2005).
129. Broussolle E, Dentresangle C, Landais P, et al. The relation of putamen and caudate nucleus 18F-Dopa uptake to motor and cognitive performances in Parkinson's disease. J. Neurol. Sci 166(2), 141-151 (1999).
130. Brown RG, Marsden CD. An investigation of the phenomenon of "set" in Parkinson's disease. Mov. Disord 3(2), 152-161 (1988).
131. Camicioli RM, Wieler M, de Frias CM, et al. Early, untreated Parkinson's disease patients show reaction time variability. Neurosci. Lett 441(1), 77-80 (2008).
132. Cooper JA, Sagar HJ, Jordan N, et al. Cognitive impairment in early, untreated Parkinson's disease and its relationship to motor disability. Brain 114(5), 2095-2122 (1991).
133. Dalrymple-Alford JC, Kalders AS, Jones RD, et al. A central executive deficit in patients with Parkinson's disease. J.Neurol. Neurosurg. Psychiatry 57(3), 360-367 (1994).
134. Delazer M, Sinz H, Zamarian L, et al. Decision making under risk and under ambiguity in Parkinson's disease. Neuropsychologia 47(8-9), 1901-1908 (2009).
135. Drag LL, Bieliauskas LA, Kaszniak AW, et al. Source memory and frontal functioning in Parkinson's disease. J. Int. Neuropsychol. Soc 15(3), 399-406 (2009).
136. Fama R, Sullivan EV, Shear PK, et al. Extent, pattern, and correlates of remote memory impairment in Alzheimer's disease and Parkinson's disease. Neuropsychology 14(2), 265-276 (2000).
137. Goebel S, Atanassov L, Kohnken G, et al. Understanding quantitative and qualitative figural fluency in patients with Parkinson's disease. Neurol. Sci 34(8), 1383-1390 (2013).
138. Hartikainen P, Helkala EL, Soininen H, et al. Cognitive and memory deficits in untreated Parkinson's disease and amyotrophic lateral sclerosis patients: A comparative study. J. Neural. Transm 6(2), 127-137 (1993).
139. Katzen HL, Levin BE, Llabre ML. Age of disease onset influences cognition in Parkinson's disease. J. Int. Neuropsychol. Soc 4(3), 285-290 (1998).
140. Kobayakawa M, Koyama S, Mimura M, et al. Decision making in Parkinson's disease: Analysis of behavioral and physiological patterns in the Iowa gambling task. Mov. Disord 23(4), 547-552 (2008).
141. Lin CH, Ou YK, Wu RM, et al. Predictors of road crossing safety in pedestrians with Parkinson's disease. Accid. Anal. Prev 51202-207 (2013).
142. Miura K, Matsui M, Takashima S, et al. Neuropsychological characteristics and their association with higher-level functional capacity in Parkinson's disease. Dement. Geriatr. Cogn.Dis. Extra 5(2), 271-284 (2015).
143. Muller U, Wachter T, Barthel H, et al. Striatal [123I]beta-CIT SPECT and prefrontal cognitive functions in Parkinson's disease. J. Neural. Transm 107(3), 303-319 (2000).
144. Partiot A, Verin M, Pillon B, et al. Delayed response tasks in basal ganglia lesions in man: Further evidence for a striato-frontal cooperation in behavioural adaptation. Neuropsychologia 34(7), 709-721 (1996).
145. Sagar HJ, Sullivan EV, Cooper JA, et al. Normal release from proactive interference in untreated patients with Parkinson's disease. Neuropsychologia 29(11), 1033-1044 (1991).
146. Stolwyk RJ, Charlton JL, Triggs TJ, et al. Neuropsychological function and driving ability in people with Parkinson's disease. J. Clin. Exp. Neuropsychol 28(6), 898-913 (2006).
147. Tamura I, Kikuchi S, Otsuki M, et al. Deficits of working memory during mental calculation in patients with Parkinson's disease. J. Neurol. Sci 209(1-2), 19-23 (2003).
148. Uc EY, Rizzo M, Anderson SW, et al. Impaired visual search in drivers with Parkinson's disease. Ann. Neurol 60(4), 407-413 (2006).
149. Uc EY, Rizzo M, Anderson SW, et al. Impaired navigation in drivers with Parkinson's disease. Brain 130(Pt 9), 2433-2440 (2007).
150. Vandenbossche J, Deroost N, Soetens E, et al. Impaired implicit sequence learning in Parkinson's disease patients with freezing of gait. Neuropsychology 27(1), 28-36 (2013).
151. Werheid K, Koch I, Reichert K, et al. Impaired self-initiated task preparation during task switching in Parkinson's disease. Neuropsychologia 45(2), 273-281 (2007).
152. Yogev-Seligmann G, Rotem-Galili Y, Dickstein R, et al. Effects of explicit prioritization on dual task walking in patients with Parkinson's disease. Gait. Posture 35(4), 641-646 (2012).
153. Yu RL, Wu RM, Tai CH, et al. Feeling-of-knowing in episodic memory in patients with Parkinson's disease with various motor symptoms. Mov. Disord 25(8), 1034-1039 (2010).
154. Yu R-L, Wu R-M, Chiu M-J, et al. Advanced Theory of Mind in patients at early stage of Parkinson’s disease. Parkinsonism. Relat. Disord 18(1), 21-24 (2012).
155. Zamarian L, Visani P, Delazer M, et al. Parkinson's disease and arithmetics: The role of executive functions. J. Neurol. Sci 248(1-2), 124-130 (2006).
156. Beato R, Levy R, Pillon B, et al. Working memory in Parkinson's disease patients: Clinical features and response to levodopa. Arq. Neuropsiquiatr 66(2A), 147-151 (2008).
157. Benito-Leon J, Louis ED, Posada IJ, et al. Population-based case-control study of cognitive function in early Parkinson's disease (NEDICES). J. Neurol. Sci 310(1-2), 176-182 (2011).
158. Breitenstein C, Van Lancker D, Daum I, et al. Impaired perception of vocal emotions in Parkinson's disease: Influence of speech time processing and executive functioning. Brain Cogn 45(2), 277-314 (2001).
159. Bublak P, Müller U, Grön G, et al. Manipulation of working memory information is impaired in Parkinson's disease and related to working memory capacity. Neuropsychology 16(4), 577 (2002).
160. Crawford TJ, Higham S, Mayes J, et al. The role of working memory and attentional disengagement on inhibitory control: Effects of aging and Alzheimer's disease. Age 35(5), 1637-1650 (2013).
161. Fournet N, Moreaud O, Roulin JL, et al. Working memory in medicated patients with Parkinson's disease: The central executive seems to work. J. Neurol. Neurosurg. Psychiatry 60(3), 313-317 (1996).
162. Gilbert B, Belleville S, Bherer L, et al. Study of verbal working memory in patients with Parkinson's disease. Neuropsychology 19(1), 106-114 (2005).
163. Goebel S, Mehdorn HM, Leplow B. Strategy instruction in Parkinson's disease: Influence on cognitive performance. Neuropsychologia 48(2), 574-580 (2010).
164. Koivisto M, Portin R, Rinne JO. Perceptual priming in Alzheimer's and Parkinson's diseases. Neuropsychologia 34(5), 449-457 (1996).
165. Lee TM, Chan CC, Ho SL, et al. Prose memory in patients with idiopathic Parkinson's disease. Parkinsonism. Relat. Disord 11(7), 453-458 (2005).
166. Lee E-Y, Cowan N, Vogel EK, et al. Visual working memory deficits in patients with Parkinson's disease are due to both reduced storage capacity and impaired ability to filter out irrelevant information. Brain 133(9), 2677-2689 (2010).
167. Martin RC, Okonkwo OC, Hill J, et al. Medical decision-making capacity in cognitively impaired Parkinson's disease patients without dementia. Mov. Disord 23(13), 1867-1874 (2008).
168. Owen AM, Iddon JL, Hodges JR, et al. Spatial and non-spatial working memory at different stages of Parkinson's disease. Neuropsychologia 35(4), 519-532 (1997).
169. Peavy GM, Salmon D, Bear PI, et al. Detection of mild cognitive deficits in Parkinson's disease patients with the WAIS-R NI. J. Int. Neuropsychol .Soc 7(5), 535-543 (2001).
170. Peigneux P, Meulemans T, Van der Linden M, et al. Exploration of implicit artificial grammar learning in Parkinson's disease. Acta. Neurol. Belg 99(2), 107-117 (1999).
171. Poliakoff E, Smith-Spark JH. Everyday cognitive failures and memory problems in Parkinson's patients without dementia. Brain. Cogn 67(3), 340-350 (2008).
172. Pollux PM. Advance preparation of set-switches in Parkinson's disease. Neuropsychologia 42(7), 912-919 (2004).
173. Siepel FJ, Bronnick KS, Booij J, et al. Cognitive executive impairment and dopaminergic deficits in de novo Parkinson's disease. Mov. Disord 29(14), 1802-1808 (2014).
174. Stebbins GT, Gabrieli JD, Masciari F, et al. Delayed recognition memory in Parkinson's disease: A role for working memory? Neuropsychologia 37(4), 503-510 (1999).
175. Xu D, Cole MH, Mengersen K, et al. Executive function and postural instability in people with Parkinson's disease. Parkinsons. Dis 2014684-758 (2014).