Exploring the Use of Cohesive Devices in Dementia within an Elderly Italian Semi-spontaneous Speech Corpus
Authors: Giorgia Albertin, Elena Martinelli (Università di Bologna, 2024)
Topic: Multimodal

Abstract
The study of language disruption in dementia, aimed at individuating which features correlate with cognitive impairment, is a growing area in computational linguistic research. Still, it needs a further development in analyzing some discourse phenomena that also undergo deterioration, and can help expand our understanding of dementia-related speech and refine automatic tools. This paper explores the discourse property of cohesion by investigating three types of cohesive devices: reference, lexical iteration, and connectives. Ten features related to these categories have been defined and automatically extracted from an Italian corpus of semi-spontaneous speech collected from dementia patients and healthy controls. Some of the designed features have proven significant for the binary classification of the two groups and further quantitative analysis highlight interesting differences in the use of cohesive devices, that seem to be associated with cognitive decline.

Introduction
Linguistics deficits commonly characterized neurodegenerative diseases from their onset. In Dementia, or Major Neurocognitive Disorder (DSM-5 AUTHOR), a syndrome of acquired and progressive impairment in cognitive function that interfere with independence in everyday life, language deterioration manifests itself within a broader framework of cognitive impairment, which could affects memory, visuo-spatial skills, executive functions and reasoning. Deficits both in verbal production and comprehension have been observed, despite the specificity of different Dementia's etiological subtypes, among which the most common is Alzheimer's Disease (AD), characterized with a primary impairment in episodic memory. In AD, for example, among the well-established linguistic deficits there are word-finding problems, which include anomia, the production of semantic paraphasias AUTHOR and the "on the-tip-of-the tongue" experience AUTHOR, low speech rate, poor word comprehension AUTHOR and, as the disease worsen, a generalized simplification of syntax AUTHOR. Also discourse and pragmatic level is affected by cognitive decline. Errors in referential cohesion has been registered, in particular regarding ambiguous use of pronouns AUTHOR. Coherence is compromised, especially in spontaneous speech: the discourse appears with an abundance of irrelevant details and the overt difficulty to mention the key concept or to refer to the topic, resulting in a lack of informativeness in communication AUTHOR. In recent years, speech analysis in cognitive decline has gained increasing importance in the development of low-cost and portable tools for dementia screening, also supported by the remarkable advancements in Natural Language Processing (NLP) and Machine Learning (ML) technologies AUTHOR. The refinement of classification systems goes hand in hand with the operationalization of linguistic features computed from oral productions, that need to be adapted to different languages. Regarding Italian, the OPLON (OPportunities for active and healthy LONgevity) [2014-2016] project was devoted to the automatic extraction of an extensive group of linguistic features from acoustic, rhythmic, readability, lexical, morpho-syntactic and syntactic levels, from a speech corpus of cognitively impaired patients and healthy peers AUTHOR. Analysis of the significance of the features highlighted that the acoustics ones largely correlated with the cognitive state of the subjects AUTHOR. Expanding the list of language levels covered to include speech properties would enrich the features used for classification and, in addition, could broaden our understanding of how cognitive decline manifests itself in verbal competence. Nevertheless, defining specific features of higher-level and complex phenomena is not trivial. Drawing inspiration from works that propose a "stratified" approach to discourse analysis, which individually considers macro-phenomena that intersect with one another AUTHOR, this paper will examine cohesion, the property of the superficial form of the text to reflect its internal unity AUTHOR. Cohesion assures continuity in discourse through a network of cohesive devices, which are mainly words or morphemes, that contribute to maintain semantic relations occurring in the text AUTHOR. Therefore, we proposed a method to design and formalize a set of cohesion features, with the aim of observing whether they contribute to discriminate the speech of individuals with dementia from healthy peers. Specifically, three types of elements, which Halliday \& Hasan AUTHOR indicate among the major contributors to cohesion, were taken into consideration: reference, lexical iteration and connectives. The implementation of measures based on cohesive devices is the first step towards the attempt to include discourse properties in the automatic analysis of language in cognitive decline. The study of their interaction with features of other linguistic levels is crucial to observe whether they have a positive impact on discrimination between dementia subjects and healthy subjects. The work presented in this paper, therefore, has to be intended as a preliminary analysis that will serve to pursue more sophisticated ML classification in the future.

Conclusion
In this work, we present a methodology for delineating linguistic features of cohesion to track and study changes in discourse properties in the speech of individuals with cognitive impairment compared to healthy peers. The research focused on three types of cohesive devices, i.e., reference, lexical iteration, and connectives, that were automatically extracted from a Italian corpus of semi-spontaneous speech from dementia subjects and controls, collected in Basilicata. Statistical significance for binary discrimination was computed applying the Kolmogorov-Smirnov test, and then adjusting the results with Bonferroni's method. The test shows that a feature of the repetitions of lemmas and the one related to the set of cohesive devices jointly considered contribute to distinguish the two groups. Moreover, the quantitative distribution of the cohesive devices reveals differences in the use of elements within the considered categories between PG and CG, which seem to highlight a general deterioration in discursive competencies associated with dementia. The results obtained provide a preliminary basis for further study of discourse properties in cognitive decline, with the aim of expanding the set of linguistic features that can be automatically extracted to other levels of language. This expansion is intended to refine digital systems that could be employed as support for the early diagnosis and monitoring of neurodegenerative diseases, potentially improving timely interventions for patients and their caregivers.