INTRODUCTION

Musician’s dystonia (MD), a task-specific movement disorder, results in the deterioration of fine motor control at the instrument and threatens professional careers. Two main forms of MD are distinguished: MD of the upper extremity and embouchure dystonia (ED) [1]. In hand MD, the affected fingers are usually identifiable during instrument-specific examination, allowing an audiovisual diagnosis based on the assessment of sound quality and abnormal movements. In ED, however, the identification of dystonic movements is more challenging. In addition to the involvement of perioral muscles [2], abnormal tongue movements and, in some cases, pharyngeal and laryngeal movements play a role [3,4]. Therefore, the diagnosis of ED relies mainly on assessing the acoustic quality of playing, although previous studies have shown that the quantification of sound features provides a more objective tool for investigating ED [5,6]. Most studies on MD have investigated abnormalities in rhythm and pitch. However, alongside the abdominal support provided by the diaphragm and the thoraco-abdominal musculature, the tongue is crucial in regulating and fine-tuning the speed of airflow through the oral cavity and thus in the evenness of the power in a sustained tone, which is perceived by the listener as loudness fluctuation [7,8]. To date, whether ED impairs the ability to control the loudness of a note remains unknown. Extending our previous findings [9], we hypothesized that the ability to play a note with even loudness is impaired in ED. Our aims were to 1) assess loudness variability in patients with ED; 2) replicate the findings of abnormal F0 fluctuations in our first study 9 with a larger sample size; and 3) assess the relationship between the fluctuations in loudness and F0.

The fundamental frequency (F0) determines the pitch of a note and reflects the rate of vibration of the sound source; in wind instruments, air pressure oscillations are generated by the interaction of the air column with the embouchure [7]. F0 fluctuations indicate small pitch variations and therefore serve as a sensitive marker of instability in orofacial motor control [6].

MATERIALS & METHODS

Participants

We investigated 35 professional musicians: 17 with ED (patients) and 18 healthy controls (controls). Demographic data and music-related data were acquired (Supplementary Table 1). This study was approved by the Ehtics Committee of the Technical University of Munich (approval number: 5173/11S), and all participants provided written informed consent in accordance with the Declaration of Helsinki.

Sound-recording and analysis

Each participant played one note twice in three pitch registers at medium loudness for 5 seconds and was asked to maintain the loudness and pitch as precisely as possible, without vibration. We discarded the first 500 ms of each recording to avoid onset-related loudness fluctuations. Owing to technical difficulties and low signal-to-noise ratios, only the high and middle pitch registers could be analyzed.

F0 fluctuations were analyzed using Harvest [10]. A sustained F0 signal of 1 s was extracted. The standard deviation was computed from the F0 signal and defined as the variable representing the fluctuation of the time-varying F0 signal.

The fluctuation in time-varying loudness was then calculated using the same interval as that used for the F0 fluctuation. As an approximation of perceived sound intensity, loudness was calculated from the summation of the spectral envelope obtained by the WORLD vocoder [11], and its standard deviation (in dB) was defined as the fluctuation of the time-varying loudness signal.

Statistics

Between-group differences in fluctuations in F0 and loudness were analyzed by a two-way mixed-design analysis of variance (ANOVA), with group (patient, control) and pitch register (medium, high) serving as independent variables. Post hoc tests were performed using a Wilcoxon test with correction for multiple comparisons (false discovery rate). Hypothesizing a positive correlation, we performed a one-sided Spearman’s rank correlation to assess the correlation between F0 and loudness fluctuations in patients. The significance level was α=0.05.

RESULTS

Among the 17 patients (mean age 45.2±12.7 years), four played the trumpet, four the horn and nine the trombone. Of the 18 controls (mean age 44.2±11.6 years), four played the trumpet, ten the horn, and four the trombone. The mean age of dystonia onset was 38.8±12.3 years, and the average disease duration was 5.4± 4.6 years at the time of the study. All pitch registers were affected in all patients, albeit to different degrees. Further participant characteristics are provided in Supplementary Table 1.

Loudness fluctuation

ANOVA revealed a significant main effect for group but not for pitch register (F_(1,28)=15.3, p=0.0005). Wilcoxon post hoc tests revealed significant group differences in both pitch registers (middle: p=0.004; high: p=0.0027), indicating greater loudness fluctuations in both pitch registers in patients (Figure 1A).

F0 fluctuation

ANOVA revealed a significant main effect for group but not for pitch register (F_(1,28)=23.5, p<0.0001). A Wilcoxon post hoc test revealed significant group differences in both pitch registers (middle: p=0.00015; high: p=0.00015), indicating a greater fluctuation in F0 in both pitch registers in patients (Figure 1B).

Correlation

Fluctuations in F0 and loudness in patients were significantly correlated for both pitch registers combined (S=26,724, p=0.00001, ρ=0.49) (Figure 2). The correlation for each separate pitch register was likewise significant (high pitch: S=4,042, p=0.013, ρ=0.38; medium pitch: S=2,482, p=0.00006, ρ=0.62).

DISCUSSION

With a relatively large sample size, we could confirm for the first time that the fluctuation in the loudness of a sustained note is greater among musicians with ED. All the musicians reported that a certain pitch register was affected; however, no register could be played to their satisfaction.

By demonstrating greater variability in tongue movement, Hellwig et al. [3] reported abnormal tongue involvement in patients with ED, indicating a loss of fine motor control. This was discussed as an attempt to maintain a stable air guidance through the oral cavity, which controls stable pitch and loudness. Whether the loss of tongue control—and, consequently, the control of stable air guidance—is a primary feature of dystonia or a compensatory response to impaired fine-motor control of the embouchure and adjacent muscles remains unsolved. In either case, a greater variability of loudness and F0 would result.

One limitation of the present study is that we cannot comment on the low register. However, Hellwig et al. [3] reported abnormal tongue movements across all pitch registers. With respect to F0, our previous work revealed that F0 fluctuation was significantly higher in patients in the low register [5]. Finally, we found a significant correlation between F0 and loudness fluctuations. These findings suggest that the low register should also be affected by loudness instability.

Previous studies have established objective acoustic metrics for ED. Beyond F0 variability during sustained tones [5], Morris et al. [6] proposed a composite acoustic severity score combining pitch, tremor, sound breaks, and timing irregularities. While such quantitative acoustic measures can be obtained using (freely available) software in the research context, no automated, clinically validated tool is currently available that would allow the full integration of such analyses into the clinical workflow. The development of user-friendly, standardized software is desirable to enable broader clinical application and complement standardized clinical rating scales [12] and subjective performance evaluations. This approach provides movement disorder specialists and neurologists with quantifiable markers of disease severity and treatment response.

No patient had any other neurological disorders. Three patients had previously received trihexyphenidyl for their dystonia but stopped 4–12 weeks before the study (Supplementary Table 1). No participant had ever received botulinum neurotoxin (BoNT). Although BoNT injections have been reported to alleviate symptoms of lingual dystonia, including ED [13], another study investigating the efficacy of BoNT in MD revealed no effect on ED [14]. Future work could investigate whether F0 and loudness variability change after BoNT treatment depending on whether tongue involvement is primary or compensatory.

Three questions arise from our findings and reports of altered tongue kinematics in ED: First, is the overactivation of cortical sensorimotor areas of the lip region in ED [15], which has been discussed as a correlate of reduced inhibition [16], also detectable in the tongue? Second, are differences in the somatotopic representation between patients and healthy controls for the lip region [17,18] also detectable for the tongue? Third, can loudness instability be predicted by altered tongue-related cortical activity, as shown for F0 fluctuations? [14]

Furthermore, in response to the repeated call for replication studies [19,20], we also confirmed higher F0 variability in ED in a larger sample, thus establishing it as a robust and reliable objective marker of ED.

In conclusion, we present loudness fluctuation as a novel supplementary quantitative measure for ED that is easily applicable in both research and clinical settings. The incorporation of such acoustic tools into future diagnostic and therapeutic frameworks could improve the objectivity and reproducibility of ED assessment. Further studies should address the questions raised by our findings.

Supplementary Materials

Notes

Conflicts of Interest

The authors have no financial conflicts of interest.

Funding Statement

This work was supported by the University of Music, Drama and Media Hannover, Germany; the Department of Neurology, Klinikum rechts der Isar, Technical University of Munich, Germany; and the Japan Science and Technology Agency (JST) Moonshot R&D Program (Grant No. JPMJMS2012).

Acknowledgments

We thank all musicians for taking part in this study.

Author Contributions

Conceptualization: André Lee, Tobias Mantel. Data curation: André Lee, Tobias Mantel. Formal analysis: André Lee, Masanori Morise. Investigation: Tobias Mantel. Methodology: André Lee, Masanori Morise, Shinichi Furuya. Project administration: Bernhard Haslinger. Resources: Eckart Altenmüller, Bernhard Haslinger. Software: Masanori Morise. Supervision: Bernhard Haslinger. Validation: Tobias Mantel. Visualization: André Lee. Writing—original draft: André Lee. Writing—review & editing: all authors.

Figure 1.

Mean loudness fluctuation (A) and mean pitch fluctuation (B) for the middle and high pitch registers for patients (blue) and healthy brass players (red). Levels of significance: **p<0.01. F0, fundamental frequency; SD, standard deviation.

Figure 2.

Correlation between F0 fluctuation and loudness fluctuation in patients. SD, standard deviation, F0, fundamental frequency.

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