From Words to Diagnosis: Exploring the Predictive Potential of Language Competence and Implicit Learning in Parkinson's Disease

Parkinson’s disease (PD) is traditionally considered a motor disorder, but current medical research increasingly adopts a systemic perspective on this condition, acknowledging that non-motor symptoms are also present. Albeit more subtle, they precede motor symptoms in the time course of disease progression. For this reason, focusing on non-motor symptoms is critical to accelerate disease detection and might represent a paradigm shift in PD diagnosis, which is currently clinical- and motor-based. One specific type of non-motor symptom concerns language competence. Starting from the ‘60s, in the last decades much emphasis has been put on Parkinsonian dysarthria, a motor speech disorder that affects articulation and results in changes in voice quality and imprecise speech. This type of motor-related linguistic change is acknowledged by the Movement Disorder Society (MDS) in its Unified Parkinson’s Disease Rating Scale (UPDRS), the gold standard for assessing PD severity and progression. However, linguistic symptoms are not limited to those of the motor type. On the contrary, they also encompass non-motor, cognitive aspects of language, as PD has been shown to affect multiple linguistic domains (see Panozzo Chiomento, Vender, & Delfitto, 2026, for a systematic review). Another non-motor domain of interest is implicit learning, a domain-general skill flexibly recruited during multiple processes, including language. The extent to which implicit learning is impaired in PD remains controversial. Evidence suggests that implicit learning is not globally impaired but depends on task characteristics and on participant-specific factors, such as cognitive status and treatment state. In this interdisciplinary work, bridging linguistics and medicine, we devised an experimental paradigm to evaluate language competence and implicit learning in a cohort of individuals with PD and in neurotypical controls of two age groups. One group was matched to the clinical population on the key demographic variables (age, biological sex, education), while the second consisted of younger adults. This design enabled us to investigate how the two cognitive capacities evolve over time in healthy ageing and PD pathology. The ultimate goal of this work is to evaluate language competence and implicit learning as potential predictors of PD, thereby addressing the challenge of diagnostic delay. While the present dissertation focuses on individuals already diagnosed on clinical and motor grounds, future research could extend these findings by assessing the robustness of the identified predictors, examining their accuracy, sensitivity, specificity, positive predictive value, negative predictive value. Additional studies could investigate their role in differential diagnosis between idiopathic PD and other neighbouring pathologies, as well as their presence in individuals bearing genetic mutations associated with familial forms of PD. Such work would allow us to see if language competence and implicit learning are also predictive in at-risk populations. Taken together, these measures, particularly when combined with other critical non-motor symptoms (e.g., sleep disturbances, olfactory impairment, neuropsychiatric manifestations such as depression, apathy, or anxiety, and autonomic or gastrointestinal dysfunction) in a symptom-clustering approach, may accelerate diagnosis, prevention, monitoring, and access to treatment. A validated collection of symptoms with high predictive value could ultimately serve as a scalable screening tool for the general population. In the broader context, the world population has never been older and has never aged at such a rapid pace. In these unprecedented times, research on age-related conditions is more crucial than ever. Beyond its immediate clinical implications, this work also addresses broader theoretical questions, such as: What is the role of subcortical structures (particularly the basal ganglia) in the language system? What is the role of dopamine in modulating linguistic and cognitive functions? How do language processing and domain-general mechanisms interact in both healthy ageing and pathological conditions? How does the trajectory of healthy ageing compare to PD pathology in shaping language and implicit learning mechanisms, and what compensatory processes might be at play? What insights does evidence from the PD population provide on theories on grounded cognition, which explore the relationship between language and physical experience? This dissertation is organised into three parts: Part I provides the theoretical background; Part II presents three experimental studies; Part III offers a general discussion, integrating theoretical perspectives with our novel experimental findings to draw conclusions and outline directions for future research.