It has been proposed that the cerebellum serves to generate predictions about the sensory consequences of future movements. Complete or over reliance on sensory feedback is thought to result in unstable movements. Patients with cerebellar ataxia are known for their deficits in visually guided movement. Tellingly, their movements are known to improve in the absence of visual feedback. Thus it is suggested that patients with damage to the cerebellum are less able to make accurate predictions about the sensory consequences of movements and have to rely on reafferent information which ultimately leads to unstable movements. Whilst the majority of this work has been done in relation to visually guided movements of the upper limbs, speech and voicing are both strongly affected in cerebellar ataxia yet relatively little is known about the nature of vocal behaviour in this patient population. The present study aimed to investigate the nature of auditory feedback processing in patients with cerebellar degeneration by measuring various aspects of vocal behaviour. 16 cerebellar patients were tested on a battery of vocal assessments designed to probe different aspects of vocalisation: we investigated ability to produce spontaneous voicing, pitch tracking of a moving pitch target and pitch perturbation.

We confirmed previous findings that patients with cerebellar damage display increased variability in spontaneous pitch resembling vocal tremor. We then investigated the hypothesis that reducing auditory feedback during vocalisation would improve vocal stability. We report that under auditory masking conditions, variability in vocal pitch is significantly reduced.  In order to investigate this idea further, a third experiment was carried out where we investigated how patients responded to perturbations in pitch production whereby auditory feedback is pitch shifted during vocalisation. As predicted, patients with cerebellar damage displayed significantly altered responses to the pitch shift compared to healthy age matched controls indicating an alteration in the way reafferent information is utilised.

Together, these three experiments provide compelling evidence in favour of the idea of the cerebellum as a prediction system, the dysfunction of which leads to over reliance on sensory feedback and hence unstable auditorily guided vocal movements. These data will be discussed in relation to the function of the cerebellum in the neural control of vocal behaviour and current models of speech production.