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