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Stroop test performance in impulsive and non impulsive patients with Parkinson’s disease

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Abstract

Impulsive personalities are considered to have a general impairment in cognitive flexibility and cortical inhibition. To examine this hypothesis we used a trial by trial Stroop task in impulsive and non impulsive patients with Parkinson’s disease (PD) and recorded errors and reaction times (RT). We tested 28 impulsive PD (PD + ICB) and 24 non impulsive PD (PD-ICB) patients prior to and after dopaminergic medication. These results were compared with 24 age matched normal controls.

We found an increased error rate in all PD patients prior to their usual medication which resolved after medication. Furthermore patients on medication showed enhanced cognitive flexibility and shorter RT. There was no difference between non impulsive and impulsive PD patients. This suggests that the impulsive behaviours may not affect response inhibition tasks and the response inhibition required in the Stroop test does not engage the same processes that differentiate impulsive and non-impulsive PD patients, which likely involve mesolimbic dopamine.

Introduction

The Stroop Colour Word test is a simple but reliable and well researched test for examining cognitive flexibility. The task requires participants to respond to the ink colour and suppress the more familiar word identity. Whilst responses in congruent settings are relatively automatic, incongruency between the letters and ink colour requires keen attention and leads to slower responses.

Impairment in the Stroop test has been described in patients with frontal lobe damage, drug abusers [1], patients with schizophrenia [2] and patients with Parkinson’s disease (PD) [3]. However, an item by item Stroop test has never been used in PD patients who have developed impulsive compulsive behaviours (ICB) such as pathological gambling, compulsive shopping, hypersexuality, and binge eating. These patients have poorer working memory [4] but it is unclear whether fast cognitive updating as required in the Stroop test will be also impaired. As dopaminergic medications are strongly implicated in the development of ICBs [5], we tested all patients before and after dopaminergic medication to assess its effect on cognitive flexibility. We anticipated that PD patients with ICBs (PD + ICB) would perform worse than non impulsive PD (PD-ICB) patients and normal controls on a task that requires inhibition of competing responses. We also thought that all patients would show improvement in cognitive flexibility, reflecting an improved ability to respond to changing task demands, after dopaminergic medication.

Section snippets

Patients and methods

Twenty-four PD-ICB, 28 PD + ICB patients and 24 normal controls were tested on an item by item Stroop test. The presence of ICBs was defined using proposed criteria [5], [6]. Most PD + ICB patients had more than 1 addictive behaviour, which is in line with the hypothesis that all ICBs share common risk factors regardless of their type of impulsive compulsive behaviour [7]. The ICBs included compulsive sexual behaviour (13 patients), pathological gambling (11 patients), compulsive buying

Statistical analysis

A mixed model ANOVA was performed. The dependent variable was either the error rate or the reaction time, averaged by condition. Condition (off vs. on and 1st and 2nd session in normal controls) were modelled as within subject factors and group (PD-ICB, PD + ICB and normal controls) was modelled as a between subject factor. Subject was included as a random factor. Since there was a significant age-difference between the groups we have added age as a cofactor in all analyses.

Results

We found a significant effect of age between the 3 groups (F(2,74) = 7.0, p = 0.002). Post hoc analysis revealed that the PD-ICB group was older than the PD + ICB (p = 0.001) and a trend to be older than the control group (p = 0.058). We found a significant effect of age of onset between the patient groups (t49 = 3.1, p = 0.03). There was no difference in the LEU dose, disease duration and UPDRS motor score (part 3), across the groups (Table 1).

Analysis of Stroop test

We compared the PD-ICB and PD + ICB groups off and on medication to normal controls, pair-wise (Bonferroni corrected for 4 comparisons). For errors (Fig. 1A) we found a main effect of group for PD-ICB patients off medication vs. normal controls (F(1, 34) = 7.18, p = 0.037) and ICB off medication vs. normal controls (F(1, 36) = 8.25, p = 0.022). Thus, off medication all patients made more errors than normal controls, but on medication there was no difference between patients and normal controls (p > 0.05).

Discussion

We found a significant Stroop interference effect in all participants. Furthermore we demonstrated that all PD patients made more errors than normal controls when off medication. We found that in their “on state” patients had a shorter RT on switching behaviour between congruent and incongruent trials, in keeping with previous studies [8], [9]. Our findings are also consistent with previous studies showing improvement of Stroop performance in PD patients with and without deep brain stimulation

Acknowledgement

The authors wish to thank the patients and families who participated in the study. This work was supported in part by the Intramural Research Program of the NIH, NIMH.

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    Citation Excerpt :

    However, the authors highlighted that this study lacks a direct assessment of all the neuropsychological features previously found to be selectively impaired in PD with ICDs, including the Trail Making Test, decision-making, and impulsivity. More recently, a deficit in decision-making has been largely described in PD gamblers patients (Angioletti, Siri, Meucci, Pezzoli, & Balconi, 2018; Balconi, Angioletti, Siri, Meucci, & Pezzoli, 2018; Balconi, Siri, Meucci, Pezzoli, & Angioletti, 2018; Djamshidian, O’Sullivan, Lees, & Averbeck, 2011; Rao et al., 2010; Rossi et al., 2010). In healthy people, it is well-known that metacognitive abilities—the high order function of self-monitoring, updating and adjusting maladaptive behaviors and “to have an insight about the quality of one’s decision” (Brevers et al., 2013)—have an important role in the regulation of decision-making.

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The review of this paper was entirely handled by an Associate Editor, Robert Rodnitzky.

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