The role of the basal ganglia in inflectional encoding: An fMRI study of producing the past tense

Abstract

According to a prominent account of inflectional encoding (Pinker & Ullman, 2002), regular forms are encoded by a rule-governed combination of stems and affixes, whereas irregular forms are retrieved from memory while inhibiting rule application. Previous research has suggested that the basal ganglia play a role in rule application as well as in domain-general inhibition. Ullman et al. (1997) observed that patients with Parkinson’s disease had difficulties with regulars, suggesting a rule application problem. However, Macoir et al. (2005) found no specific difficulties with regulars in these patients. Moreover, whereas Oh et al. (2011) observed in an fMRI study the left caudate to be more active in regular than irregular production, Desai et al. (2006) found the opposite. In a previous series of behavioral experiments, we tested whether domain-general inhibition, involved in task and language switching, is also involved in irregular production. We found a cost in reaction times for switching from inflecting to reading but not from irregulars to regulars. However, we employed an indirect measure of inhibition, since we measured its effect on subsequent trials. In our current ongoing fMRI experiment we replicated the design of our behavioral experiments. In the inflection-only part, participants alternated between producing regular and irregular past tenses in Dutch. The infinitive form of a verb was presented on a screen and participants had to produce the first-person singular past-tense form of the verb as quickly and accurately as possible. The verb type changed every second trial. In the task-switching part, participants alternated between inflecting and reading aloud. The procedure was similar to the inflection-only part, with the addition of a colored frame around the word cuing participants which task to perform. The task changed every second trial. Regulars and irregulars were presented in small mini-blocks of 24 trials of the same regularity type, hence switch costs in the task-switching part concern task and not verb regularity. Our preliminary (n = 8) analysis of the inflection-only part showed that irregulars recruit the right middle frontal gyrus, and regulars the left superior temporal gyrus. Moreover, the regularity of the previous verb seems to modulate the cortical response, with repeat trials recruiting the pre-SMA and switch trials showing SMA activation, especially for irregulars. At a behavioral level we had observed that the inflection of a verb is not affected by the regularity of the previous verb (Ferreira et al., 2020). However, at a neuronal level, our preliminary results suggest that regular and irregular production rely on different cortical structures. Our preliminary results did not show significant basal ganglia involvement in any of the contrasts, but data collection is still ongoing. Furthermore, results of the inflection-only part will be compared with those of the task-switching part, which will shed light onto whether the same cortical and subcortical regions are involved when switching between tasks and verbs.

Publication
Poster presentation at SNL 2020, Sandbox Series
João Ferreira
João Ferreira
PhD candidate

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