When rhythm guides speech production: Insights from Parkinson's disease and spinocerebellar ataxia.

Rhythmic auditory exposure modulates subsequent speech processing, yet its effect on speech production remains poorly understood. People with Parkinson's disease (PD) or spinocerebellar ataxia (SCA) suffer from dysarthria, for which there is no treatment. Using a rhythmic priming approach, we investigated whether brief rhythmic primes modulate speech production and facilitate it in people with PD and SCA considered here as clinical models of basal ganglia dysfunction and cerebellar dysfunction, respectively. Fifty-eight native French speakers (20 controls, 24 people with PD, 14 people with SCA) read sentences following a regular prime, an irregular prime, or silence. We measured accuracy and the temporal features of speech production including speech initiation time, speech rate, and rhythmic metrics. Bayesian mixed-effects models were used for the statistical analyses of these outputs. Accuracy showed no effect of rhythmic priming. Regular primes consistently shortened speech initiation time in all populations, but the benefit of regular primes was smaller in people with PD than in controls. In People with SCA, speech rate decreased after regular primes. The impact of rhythmic priming on rhythmic metrics was limited. Together, these results indicate that perception-to-production rhythmic priming mainly affects global, rather than fine-grained, timing control of speech during speech production. They also shed light on the implications of the basal ganglia and cerebellum in the modulation of speech by rhythm. They also provide insights for speech production models, which currently lack an explicit mechanism for external rhythmic inputs. Clinically, they help inform rhythm-based speech and language therapy methods.