Neural correlates of multi-level language processes (lexical, semantic, grammatical, discourse) as well as cognitive functions (memory, attention) that affect speech in healthy people and patients with brain pathologies were studies. The observed effects regarding the relationship between behavioral outcome and neuronal representations of language processing served as the basis for comparing healthy and clinical populations of language speakers. The present research aimed to solve the critical problem concerning the neural mechanisms of language: are these mechanisms fundamentally different in patients with speech pathology and healthy speakers?
Goal of Research: The goal of the present research was to demonstrate that language norm is a continuum as far as neural representations are concerned and that healthy people and patients share common brain mechanisms of speech. Specifically, the study used empirical methods to identify multimodal neural representations of language and related cognitive functions. A functional involvement of various brain areas during language processing in normative and non-normative language speakers was examined; temporal characteristics of language-related neural activity were established; a brain substrate critically involved in the execution of certain linguistic aspects was detected.
Empirical Base of Research: The empirical basis for the present research was a series of neurolinguistic experiments divided into three research lines: hemodynamic, electrophysiological and structural-anatomical. Such approach allowed examining the problem at different levels, using the multimodal neural representations of language. Each of the three types of experiments involved a comprehensive literature review (both Russian and non-Russian sources); a hands-on experience with research methods (functional magnetic resonance imaging, the event-related brain potentials, voxel-based lesion-symptom mapping) necessary for the successful execution of the project, and the group’s previous research results on this topic. The hemodynamic line of research included three Russian experiments which set out to test and compare the nature of speech manifestations and related cognitive functions in normative and non-normative language users. Regarding the electrophysiological experiments, the analysis of evoked brain potentials for language processing in Dutch and Portuguese has been carried out; analogous paradigms for electrophysiological experiments has been designed in Russian and Spanish. Structural-anatomical direction has focused on the identification of the brain regions involved in word generation.
Results of Research: The results of the research project have shown that neural mechanisms of language processing do not differ qualitatively between healthy people and people with speech impairment. Despite the fact that patients’ speech is characterized by a number of peculiarities, their mechanisms for speech comprehension and production share much in common with those of healthy people. Our hemodynamic experiments demonstrated that intact brain regions normally involved in the realization of speech are usually activated in patients with brain damage and speech dysfunction. Some observed changes in activation are related to the compensatory reorganization of language in the brain and can be explained by an individual patient’s behavioral profile. The electrophysiological line of research yielded data showing that language operations that are difficult for patients are also more cognitively taxing for healthy people, which is reflected in their more pronounced electrophysiological reaction. Finally, a clinical experiment with the voxel-based lesion-symptom mapping showed a critical involvement of the left-hemisphere basal structures (normally responsible for movements) in verb generation, suggesting a mediation of linguistic functions by nonlinguistic factors.
Recommendations on implementation of Results: Recommendations on implementation of the results of the present research project concern the use of the new knowledge about the neural representations of different language aspects in normative and non-normative groups of language users and the development of the methodological apparatus for clinical practices. The observed effects are crucial for specifying speech mechanisms in healthy and damaged brains and should be put to use for optimizing the differential diagnostics and treatment of patients with speech dysfunction.
Field of application: The observed results can be applied in the fields of basic and applied language research as well as rehabilitation of the speech function in patients with brain damage. The results of our work can serve as a basis for future experimental studies examining new manifestations of the observed continuum between the norm and the pathology in other non-normative populations. In addition, the results can potentially be useful for the design of new practical methods of diagnostics and treatment of speech impairment.