PhD Project 28
The Role of Subcortical Structures in Language
(last update 2019-07-01)
Understanding the internal brain organisation for language not only requires studying cortical structure and function, as is typically done, but also investigating subcortical structures such as the basal ganglia, cerebellum, and thalamus. This PhD project investigates the role of the basal ganglia in language. More specifically, the aim is to study the basal-ganglia-thalamocortical and cerebellum-thalamocortical loops in spoken language tasks. fMRI will be used to examine the contributions of both subcortical loops to language in healthy participants. Moreover, focusing on the basal ganglia, language performance of patients with Parkinson’s disease on and off deep-brain stimulation will be examined (in collaboration with the RUMC).
Inflection is the modification of a word to express a different grammatical category. Considering that the literature on the contribution of the basal ganglia to inflection is still not clear, a series of experiments was designed to test whether inhibition plays a role in inflectional encoding.
The Role of Basal Ganglia Mediated Inhibition in Producing Inflections
Team members: Ferreira, Roelofs, and Piai
This project investigates the role of basal ganglia in language. In particular, inflectional encoding is studied as a proxy for inhibition mediated by the basal ganglia. Dual-system theory proposes that regular forms (walk – walked) are constructed by rule, while irregulars (swim – swam) are stored and retrieved from memory. During retrieval of irregular forms, rule application is assumed to be inhibited.
When speakers switch between tasks, an asymmetrical switch-cost is obtained which has been attributed to overcoming previous inhibition of the predominant response. Capitalizing on this, participants in the experiment alternated between producing regular and irregular past tense forms. Results displayed no difference in reaction time between regulars and irregulars, offering no support for the proposal that the production of irregulars involves (basal ganglia mediated) inhibition of the regular rule. A control experiment showed clear switch-costs when switching between reading and inflecting, suggesting that the paradigm had sufficient power to detect switch-costs. These findings challenge the assumption that inhibition is the mechanism by which rule application is blocked in producing the past tense of irregular verbs.
The task-switching paradigm being developed has potential to be used as a comparative measure of inhibition during language function in controls and patients. Specifically, it could be used in combination with fMRI to probe basal ganglia circuits responsible for hypothesized inhibition.
This project combines expertise in cognitive modelling, language production, task switching and inhibition, neuroimaging, and patient studies. This unique combination of expertise/knowledge is essential for developing novel paradigms to assess understudied sub-cortical structures in the context of language processing.
A first behavioural experiment was conducted. Using this rationale, a behavioural experiment was designed where participants alternated between producing regular and irregular past tenses in Dutch. The results displayed no difference in reaction time between producing regulars and irregulars, so no support was found for the proposal of Pinker and Ullman (2002) that the production of irregulars involves the inhibition of the regular rule.
A second follow-up behavioural experiment was also carried out during which participants had to switch between inflectional encoding of regulars and irregulars in one part of the experiment and to switch between reading and inflectional encoding in the other part of the experiment. The results showed clear switching costs when switching between reading and inflecting, but no regularity effects were observed in the two parts of the experiment. These findings challenge the assumption that inhibition is the mechanism by which rule application is blocked in producing the past tense of irregular verbs.
This project brings together researchers with expertise in cognitive modelling, language production, the role of subcortical structures in task switching and inhibition, neuroimaging, and patient studies. This unique combination of expertise and knowledge holds great promise in tackling the question of what the role of subcortical structures might be in language processing.
The use of a task-switching paradigm to shed light on the validity of a core assumption of the dual-system theory of how the language system deals with irregular forms (i.e., inhibition of grammatical rule application) is novel. The paradigm developed can be used to investigate the role of basal ganglia circuits in this inhibition using fMRI, and has the potential to be developed for use as a comparative measure of language function in controls and patients with various forms of basal ganglia dysfunction.