Left frontal white matter links to rhythmic processing relevant to speech production

Recent evidence suggests that a shared neurocomputational mechanism tracks the temporal structure of auditory input and speech output. We investigate the link between speech production and rhythmic abilities in 24 healthy controls and 23 patients with primary progressive aphasia, including 12 non-fluent variant patients (NFV) with apraxia of speech. Seventy-five percent of NFV cases displayed poorer rhythmic abilities. Rhythmic abilities and speech rhythm were significantly correlated in both controls and patients. Deformation-based morphometry and diffusion tensor imaging were used to identify white matter atrophy related to impaired rhythmic abilities in NFV. Left frontal white matter volume adjacent to the supplementary motor area (SMA) correlated with rhythmic abilities. Metrics of the left Aslant tract, which is typically damaged in NFV cases, also correlated with rhythmic abilities. Our results support the existence of a common temporal scaffolding mechanism structuring perceptual input and speech output. This mechanism partly explains abnormal speech in NFV.