Several aspects of language comprehension and production at different linguistic levels (syntactic, semantic and discourse) were investigated in this study. The observed effects of performance accuracy and speed, along with qualitative characteristics of linguistic processing became the basis for comparing healthy and clinical populations of Russian speakers. The current research was focused on the following fundamental scientific problem: to what extent are linguistic effects observed in patients with language pathology specific to this clinical population and independent from universal mechanisms of language processing, characteristic of healthy language users.
The Goal of the Research was to provide support for the continual nature of normative language processing and the existence of universal linguistic mechanisms in healthy and clinical populations. The research was grounded in the functional approach to language that regards impaired language performance as expression of the same mechanisms and strategies that characterize normative language processing as well. The specific aims of the research concerned the empirical determination of characteristics and extent of variation in language behavior in healthy controls and patients with acquired neurogenic language impairments – with the overall goal of modeling general mechanisms of language processing in these two populations.
The Empirical Base of the Research involved a series of psycho- and neurolinguistic experiments combined in five tracks: behavioral, eye-tracking, electrophysiological, hemodynamic, and one focused on language test development. This enabled the investigation of the scientific problem from a multi-modal perspective. The behavioral track included two studies aimed at uncovering the characteristics of language processing in an ecologically valid situation of spontaneous speech and in a constrained psycholinguistic experiment. The eye-track direction involved advanced analysis of previously collected eye-tracking data on lexical-semantic and syntactic processing in healthy controls and patients with aphasia, as well as an investigation into the influence of other cognitive functions, such as working memory, on these aspects of language comprehension. The electrophysiological track, aimed at determining electrophysiological markers (event-related potentials) of speech processing, encompassed three control behavioral experiments and development of an electrophysiological paradigm for parallel – Russian and Spanish – experiments. The hemodynamic track, focused on investigating the plasticity of the neural substrate of language, included one control and three neuroimaging experiments. Finally, the track on tests development centered on the creation of standardized language assessment tools for different populations of Russian language users.
The Results of the Research provided stronge empirical support for the lack of qualitative specificity of language processing in individuals with language impairments compared to healthy language users. Behavioral experiments showed similar processing strategies at the discourse and grammatical levels utilized by patients with different types of aphasia and healthy controls. Eye-tracking experiments in healthy and clinical populations again demonstrated that similar mechanisms are at play in on-line language processing, with differences stemming from the variation in efficacy and temporal dynamics. Furthermore, it was shown that working memory impacts different levels of language processing in patients with nonfluent aphasia. In control experiments of the electrophysiological track it was demonstrated that comprehension of linguistic categories in patients with different types of aphasia and in healthy controls is of similar nature and differs only quantitatively, and that its attrition is observed in both Russian monolingual individuals with aphasia and in Russian speaking bilinguals. These findings served as the foundation of a longer-term research project exploring language attrition in bilingual individuals and patients with aphasia, for which electrophysiological paradigms have been created. In the hemodynamic track the effect of semantic complexity was observed in both individuals with aphasia and in healthy controls; in patients it was registered at the level of response accuracy, while in healthy controls – in reaction times and patterns of neural activation. Finally, to provide a sound methodological basis for current and future psycho- and neurolinguistic experiments, several standardized language assessment tests have been developed.
Recommendations on the implementation of the obtained results stem from their vast significance for further refinement of theoretical models of language processing in healthy and lesioned brains, and consequently, for a potential improvement of methods of differential assessment and targeted therapy for patients with acquired neurogenic language disorders.
The Field of the Application of the results is fundamental and applied research on language processing, as well as the rehabilitation of language disorders. The results of conducted work can become the basis for further empirical investigations of language, aimed at determining novel manifestations of the detected norm-pathology continuum in linguistic processing, and serve as an initial step for the development of assessment and therapeutic language tools.