The influence of contextual constraint on verbal selection mechanisms and its neural correlates in Parkinson's disease.

A small number of studies have described verbal selection deficits in Parkinson's disease (PD) when selection must occur among competing alternatives. However, these studies have largely focused on single-word processing, or have utilised sentence stems that carry high contextual constraint, thus reducing selection demands. The present study aimed to determine the influence of variable contextual constraint on the selection of a verbal response in PD. This was achieved using an adaption of the Hayling Sentence Completion Task whereby PD participants and matched controls were required to provide a single word to complete a cloze probability sentence stem that carried a low, medium, or high degree of contextual constraint. Results revealed no main effect of group in terms of response time or accuracy, though a group-by-condition interaction in accuracy was noted. This was characterised by a significant difference in accuracy between low and medium levels of constraint for control participants, but no significant difference for the PD group. Functional MRI data revealed marked between-group differences in underlying neural activity. The control group showed increased recruitment of the dorsal striatum and the vlPFC under conditions that placed greater demands upon selection (i.e. low and medium constraint), and greater activity overall in the left dlPFC and right vlPFC. However, in the PD group, behavioural performance appeared to be maintained despite underlying decreases in frontostriatal activity, suggesting other compensatory mechanisms that may include changes in functional connectivity or an over-medication effect in frontal networks in response to loss of signalling in cortico-subcortical pathways.

Functional correlates of strategy formation and verbal suppression in Parkinson's disease.

Individuals with Parkinson's disease (PD) have shown impaired performance on the verbal suppression component of the Haylings Sentence Completion Test (HSCT). The present study aimed to determine whether this performance related to (i) the inability to suppress a pre-potent response or (ii) difficulty in the generation of a strategy to facilitate task execution. The study adopted a novel variation of the HSCT that isolated each process and employed fMRI to examine the associated neural correlates in a comparison of individuals with PD and matched healthy controls. No significant behavioral differences were detected between these two groups. However, fMRI results revealed atypical underlying neural activity in the PD group. Controls exhibited increased activation in the left dorsolateral prefrontal cortex and striatum when generating a response independently, relative to generation when a supporting strategy was provided. The PD group demonstrated the opposite pattern of activation, in addition to greater recruitment of right hemisphere regions. This pattern of activation was postulated to be evidence of compensatory mechanisms, acting to bolster the output of frontostriatal circuits compromised by disease pathology.

The Suppression of Irrelevant Semantic Representations in Parkinson's Disease.

The impairment of lexical-semantic inhibition mechanisms in Parkinson's disease (PD) remains a source of contention. In order to observe whether people with PD are able to suppress irrelevant semantic information during picture naming, the present study employed an object-based negative priming paradigm with 16 participants with PD and 13 healthy controls. The task required participants to name a red target image while ignoring a superimposed, green distractor image. The semantic relationship between the distractor image and the target image of the subsequent trial was manipulated, such that the distractor image was identical, semantically related, or semantically unrelated to said target image. The PD group and the control group were slower in naming a target image that had previously served as a distractor image, relative to naming a target image that was unrelated to the previous distractor image. Thus, a negative priming effect was present in both groups. Furthermore, no significant difference in the magnitude of this effect was observed between the control and PD groups. When considered in the context of existing literature surrounding negative priming in PD, these results suggest that inhibition is subserved by multiple, domain-specific mechanisms and that the inhibitory processing of visual-semantic stimuli is intact in PD.

Assessing cognition after stroke. Who misses out? A systematic review.

BACKGROUND: Cognitive impairments post-stroke are common. Assessment of cognition typically involves pen-and-paper tasks, which are often reliant on linguistic and motor function, creating barriers for many stroke survivors. The characteristics of stroke survivors excluded from cognitive assessments have never been investigated. AIMS: (1) To determine if the stroke samples included in studies evaluating clinimetric properties of cognitive assessments represent the stroke population, (2) to identify the different modes of cognitive assessments, and (3) to ascertain whether the different modes of cognitive assessments influence the stroke samples used in the studies. SUMMARY OF REVIEW: We systematically reviewed studies that evaluated at least one clinimetric property of a cognitive assessment in adult stroke survivors from January 2000 to October 2013. Eligibility criteria, reasons for drop-outs and missing data were extracted. A theming process was employed to synthesize the data. From the initial yield of 3731 articles, 109 were included. Six broad categories describing reasons for exclusion were identified. Cognitive impairments were the most common (68%), then communication issues (62%), endurance problems (42%), sensory loss (39%), psychiatric illness (38%) and motor limitations (27%). The most prevalent assessment mode was pen-and-paper (73%), then virtual reality (11%), computer (6%), observational functional performance (5%), informant (3%) and telephone (3%). Regardless of mode, issues with cognition and communication were the most frequently used exclusion criteria. CONCLUSIONS: Our findings indicate that cognitive assessments are not tested in representative stroke samples. Research is needed to identify valid and reliable cognitive assessments that are feasible in a wider range of stroke survivors.