Speech volume regulation in Parkinson's disease: effects of implicit cues and explicit instructions

This study examined the regulation of speech volume in hypophonic subjects with Parkinson's disease (PD) and age- and gender-matched controls. The first two experiments investigated the ability of subjects with PD to automatically regulate speech volume in response to two types of implicit cue: (i) background noise (BGN) and (ii) instantaneous auditory feedback (IAF). Control subjects demonstrated the Lombard effect by automatically speaking louder when competing against increasing levels of background noise. They also showed the reverse effect, decreasing speech volume when increasing levels of facilitative instantaneous auditory feedback were provided. Subjects with PD demonstrated decreased overall speech volume; they were less able than controls to appropriately increase volume as background noise increased, and to decrease volume as IAF increased. Thus, subjects with PD demonstrated over-constancy of speech volume and failed to respond to the implicit cues integral to volumetric scaling. A further experiment (3) was carried out to contrast the regulation of volume in response to implicit cue with an explicit attention-driven cue (i.e. instructions regarding volume level). As in Experiments 1 and 2, subjects with PD exhibited reduced speech volume. Under explicit volume instructions, the ability of subjects with PD to regulate volume was normalised. These findings suggest that subjects with PD have the capacity to speak with normal volume provided they consciously attend to speaking loudly. In subjects with PD, overall speech volume was always lower than for control subjects, suggesting a reduction of cortical motor set in the articulatory system similar to that demonstrated by the reduced amplitude of limb movements (hypokinesia) in the motor system.