Elsevier

Neuropsychologia

Volume 37, Issue 13, December 1999, Pages 1453-1460
Neuropsychologia

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

https://doi.org/10.1016/S0028-3932(99)00067-6Get rights and content

Abstract

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.

Introduction

Idiopathic Parkinson’s disease is a late-onset neurodegenerative movement disorder which presents clinically as tremor, bradykinesia (slowness of movement), hypokinesia (reduced movement), rigidity and postural instability; and possibly some subtle cognitive changes suggestive of frontal lobe dysfunction [15]. Though less striking than skeletal motor impairment, reduced speech volume (hypophonia) is a common aspect of speech dysfunction in people with idiopathic PD [12]. Parkinsonian speech or hypokinetic dysarthria is characterised by monotony of pitch and speech intensity, reduced stress, variable rate, short phrases, short rushes of speech and imprecise consonants [5]. Perceptual studies [1], [5] have typically described Parkinsonian speech as being reduced in volume. This finding has been supported by acoustic measures [2], [10] documenting the decreased intensity of Parkinsonian speech. To date, however, studies in Parkinsonian speech have been largely descriptive, and have not yet investigated experimentally the phenomenon of reduced volume. Although there are other disorders where speech volume may be compromised, e.g. amyotrophic lateral sclerosis, myasthenia gravis and pseudobulbar dysphonia, comparisons with the present study are inappropriate given the different neurogenic origin of these disorders. We were particularly interested in the organisation of speech motor control and therefore focussed the study on PD as it is the best known model of basal ganglia dysfunction.

This paper investigates the regulation of Parkinsonian speech volume in response to two types of implicit auditory cue. The first was provision of background noise of various intensities, resulting in the ‘Lombard effect’ [13]. The second was provision of instantaneous auditory feedback of various levels, resulting in what we term ‘the reverse-Lombard effect’. The Lombard effect [13] refers to the inevitable increase in speech intensity associated with increased levels of background noise, such as might occur at rock concerts. This is due to the necessity to make speech audible to the listener [11]. To our knowledge, this method has not been used to examine speech volume regulation in any speech disordered population. Healthy individuals consistently increase speech volume progressively as competing background noise (BGN) increases. The opposite effect could occur with a facilitative stimulus instead, such as the amplification of one’s own voice heard simultaneously when using a microphone. Speech volume should then be gradually reduced as the level of instantaneous auditory feedback (IAF) increases, thus resulting in the reverse-Lombard effect.

This study deliberately used the Lombard paradigm to manipulate speech intensity because the effect has been shown to be largely involuntary as the appropriate volume level is selected automatically without conscious deliberation [20]. The role of attention has been shown to greatly influence the performance of a motor task in subjects with PD. Parkinsonian gait [18] and handwriting [19] are more markedly hypometric when attentional resources are diverted to a concurrent task performed simultaneously with the primary motor task. Hence it was of interest to examine the effect of implicit auditory cues (BGN, IAF) on speech volume in a Parkinsonian population.

This paper investigates the regulation of speech volume (for reading and conversation) in hypophonic (reduced speech volume) subjects with PD and age-matched controls via the reverse-Lombard effect by using increasing levels of IAF (Experiment 1), and via the Lombard effect by using increasing levels of pink noise i.e. white noise with high frequencies filtered out (Experiment 2). It was expected that controls would show the expected trend in regulating volume, but that by analogy with problems in upper-limb scaling of extent [16], speech-volume scaling in subjects with PD might be disordered. In particular we predicted that subjects with PD would be unresponsive to automatic cues to vary speech volume. Experiment 3 was then performed in order to determine whether the abnormal Parkinsonian speech pattern could be normalised using explicit instructions.

Section snippets

Participants

Twelve idiopathic PD subjects (mean=74.6 years, sd=5.5 years) with hypophonia and 12 healthy age and sex-matched controls (mean=76.3 years, sd=3.0 years) participated in this study. Subjects with PD were stabilised on anti-Parkinson medication and remained on their usual medication regime when participating in the experiment, and were tested between one and three hours of receiving medication. Participants were screened for hearing-impairment, and those requiring the use of hearing aids were

Participants

The same participants as in Experiment 1 participated in Experiment 2 after a rest period.

Apparatus

The testing conditions were the same as in Experiment 1. Participants wore a head-set (David Clark) and their speech was recorded via the head-set microphone (positioned 8 cm from the mouth) connected to a Marantz (PMD222) tape recorder. A noise generator was used to deliver pink noise (i.e. white noise with high frequencies filtered out) at various dB levels. A lapel microphone relayed the experimenter’s

Experiment 3

The ability of Parkinsonian subjects to regulate volume according to an explicit cue (i.e. verbal instructions regarding speech intensity) was examined.

General discussion

This study examined volume regulation in response to two types of implicit cue. It was found that Parkinsonian subjects’ speech samples were generally reduced in volume; subjects with PD were also largely unresponsive to these implicit cues, failing to vary speech volume appropriately, and thus demonstrated an abnormally over-constant pattern of volume regulation. Under explicit instructions regarding speech volume, subjects with PD were now able to achieve a normal pattern of volume

Acknowledgements

We would like to acknowledge the assistance of the Linguistics Department of Monash University for the use of the CSL system, Kingston Centre for the use of the sound-proof room, and the participants who volunteered for this study.

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    For example, when individuals with PD are instructed to speak loudly and clearly or to imitate the normal speech of others, many aspects of their speech improve significantly, including louder speech volume, higher speech intelligibility, more variable prosodic pitch, and more precise articulatory movements (Clark et al., 2014; Dromey and Ramig, 1998; Goberman and Elmer, 2005; Tjaden et al., 2013; Weir-Mayta et al., 2017). This approach has been shown to be particularly effective for improving the regulation of speech loudness such that individuals with PD can control their loudness and healthy controls when given explicit loudness instructions (Ho et al., 1999, 2000) or when they are trained to speak loudly with external cueing while receiving the Lee Silverman Voice Treatment (LSVT) LOUD (Ramig et al., 1995, 2001). These findings parallel the use of external cues to improve limb motor control (e.g., handwriting, stride length, and gait velocity) in individuals with PD (Cerasa et al., 2006; Lewis et al., 2000; Morris et al., 1996; Oliveira et al., 1997; Smith et al., 2014; Thaut et al., 1996).

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