Cerebellar Contributions to Speech Motor Control and Auditory Verbal Imagery: Acoustic / Kinematic Analyses of Ataxic Dysarthria and Functional Magnetic Resonance Imaging in Healthy Subjects

Acoustic analyses of sentence utterances noted slowed speech tempo, concomitant with a tendency towards syllable isochrony, in patients with impaired vocal tract control due to ataxic disorders. Furthermore, kinematic measurements revealed decreased maximum velocity of articulatory orofacial gestures with respect to a given movement amplitude. Syllable lengthening seems to develop into a plateau effect at a speaking rate of about 3 Hz both during sentence productions and oral diadochokinesis tasks. Most noteworthy, functional magnetic resonance imaging (fMRI) of cerebellar activation during syllable repetitions of varying frequencies in healthy subjects revealed a threshold effect of the calculated rate / response functions between 3 and 4 Hz. Recent psycholinguistic models of speech production assume a repository of overlearned syllable-sized articulatory routines housed within the premotor cortex (mental syllabary). Conceivably, the cerebellum participates in the retrieval of these syllable templates at a subject’s habitual speaking rate (4 to 6 Hz) and contributes to their temporal organization in terms of a rhythmically structured utterance. Unexpectedly, a threshold effect of cerebellar activation even occurred during covert (silent) syllable productions (auditory verbal imagery). Presumably, thus, the cerebellum even contributes to the temporal organization of “internal speech”, i.e., a prearticulatory, but fully parsed representation of the sound structure of sentences. As a consequence, cerebellar disorders may compromise any cognitive operations insofar as speech code-based information processing such as working memory rehearsal mechanisms or verbal thought associated with executive functions are involved.