Dissociating cerebellar regions involved in formulating and articulating words and sentences

Abstract

This fMRI study of healthy volunteers investigated which parts of the cerebellum are involved in formulating and articulating sentences using three sentence-based tasks: (i) a sentence production task that involved describing simple events in pictures (e.g. “The goat is eating the hat”); (ii) An auditory sentence repetition task involving the same sentence articulation but not sentence formulation, and (iii) An auditory sentence-topicture matching task that involved the same pictorial events and no overt articulation. Activation for each of these tasks was compared to the equivalent word processing tasks: noun production (object naming), verb production (naming the verb in pictorial events), auditory noun repetition, and auditory noun-to-picture matching. Auditory and visual semantic association tasks were also included, in the same within-subjects design, to control for visual and auditory working memory and semantic processing. Three distinct cerebellar regions were activated by sentence production compared to noun and verb production. First, we associate activation in bilateral cerebellum lobule VIIb with sequencing words into sentences, as well as phonemes into words because it increased for sentence production compared to all other conditions, including sentence repetition and sentence-to-picture matching; and was also activated by word production compared to word matching. Second, we associate a paravermal part of right cerebellar lobule VIIIb with overt motor execution of speech, because activation was higher during (i) production and repetition of sentences compared to the corresponding noun conditions, and (ii) noun and verb production compared to all matching tasks; with no activation relative to fixation during any silent (non-speaking) matching task. Third, we associate activation within right cerebellar Crus II with covert articulatory activity because it activated for (i) all speech production more than matching tasks, and (ii) sentences compared to nouns during silent (non-speaking) matching as well as sentence production and sentence repetition. As all three regions were activated during word production tasks, our study serendipitously segregated, for the first time, three distinct functional roles for the cerebellum in generic speech production; and demonstrates how sentence production enhanced the demands on these three cerebellar speech production regions.