In addition to typical motor symptoms, Parkinson’s disease is characterized by numerous cognitive symptoms. Identifying early cognitive changes and the course of their progression remains an important topic in current research. Among the most frequently studied changes are those associated with disturbances in the fronto-striatal network. Although language is generally considered relatively preserved in the broader discourse on cognitive symptoms, numerous reviews of language function point to specific, subtle changes across various linguistic domains, while the causes of these changes remain a frequent topic of debate. In the area of syntax, difficulties in understanding complex sentence structures have received considerable attention, as they can appear as early as the disease's initial stages. The primary aim of this study is to shed light on the nature of comprehension difficulties in non-demented patients by using relative clauses to examine the relationship between syntactic-grammatical and other cognitive factors in comprehension performance. A review of the literature shows that the prevailing interpretation suggests a cognitive basis for these difficulties. This has been supported by numerous methodologically diverse studies using neuroimaging and behavioral data from non-demented patients. These studies confirmed relatively preserved syntactic-grammatical knowledge, while deviations were identified in other supporting cognitive resources, including limitations in attentional control, working memory, and task switching, as well as slowed information processing speed. These resources are likely involved in overcoming the processing complexity of relative clauses and can be interpreted as reflecting general disturbances in the fronto-striatal network. However, a critical review also highlights certain limitations of the cognitive hypothesis, particularly its restriction to a single type of complex sentence structure, as well as the lack of data from typologically diverse language groups, which is needed for more reliable cross-linguistic comparisons aimed at providing additional support for the proposed universality of the hypothesis.
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