Finger-specific loss of independent control of movements in musicians with focal dystonia
Introduction
The independent control of finger movements is a key feature of the dexterous use of the hand. A superior independence of finger movements in humans over non-human primates indicates the development of this motor function in parallel with the evolution of an ability to manipulate various tools (van Duinen and Gandevia, 2011). However, damages in cortical and subcortical regions caused by stroke (Lang and Schieber, 2004), cerebellar dysfunction (Brandauer et al., 2012), or Parkinson’s disease (Park et al., 2012) yield a loss of individuated finger movements, which severely impairs the quality of life. To unravel its neural mechanisms is therefore essential.
As a further highly interesting example of central nervous sensory-motor dysfunction, focal hand dystonia in musicians (MD) is a task-specific movement disorder that frequently manifests as a deterioration of dexterous finger movements. This is due to the loss of voluntary control of extensively trained movements that musicians performed during playing their instrument (Altenmüller and Jabusch, 2010, Altenmüller et al., 2012). For those who are affected, the disorder is highly disabling and often terminates their professional musical careers (Jabusch et al., 2004). Patients with focal hand dystonia displayed atypical activity in motor-related cortical and subcortical regions during individuated finger movements (Oga et al., 2002, Garraux et al., 2004, Lerner et al., 2004, Moore et al., 2012) and reduced central nervous surround inhibition in the finger muscles when investigated with motor cortex stimulation (Rosenkranz et al., 2005, Beck and Hallett, 2010). These findings suggest an impairment of individuated finger movements that play a crucial role in expert musical performance (Slobounov et al., 2002, Furuya et al., 2011, Furuya and Soechting, 2012, Furuya and Altenmüller, 2013). However, it has not been characterised how MD influences the individuated finger movements.
The present study aims to identify the pathophysiological impact of MD on the individuated movements of each of the fingers. Individuated finger movements require moving one particular finger while keeping the remaining fingers voluntarily immobilised. A lack of independence across fingers hinders the dynamic finger movement due to a spill-over of the stabilising force of production, and vice versa. MD can therefore disrupt the spatio-temporal features of the movements. However, potential problems to evaluate it reliably were limited sample size of the patient group and large number of candidate movement features affected by MD. The latter renders it difficult to determine with conventional statistical techniques whether a particular combination of multiple movement features characterises MD. In addition it implies the problem of multiple comparisons. Here, a novel approach using principal component analysis (PCA) and cluster analysis is proposed as a solution for these problems. It allows us not only to determine whether MD degrades the individuated finger movements by accounting for abundant spatio-temporal features of movements, but also to identify a particular set of movement features that are specifically affected by MD. Therefore, we were particularly interested first to assess whether individuated finger movements are affected by MD using a cluster analysis, and second whether the effect of MD on movements is similar or different across fingers using PCA. The results of these analyses may well contribute to correctly diagnosing and monitoring the effect of treatment of MD. This is of practical relevance, because frequently the symptoms are difficult to recognise for clinicians. Especially the correct identification of the affected finger can be problematic, because non-affected fingers also move atypically to compensate for the symptoms (Elbert et al., 1998, Candia et al., 2005). However, misdiagnosing the dystonic movement can have severe consequences, since for example successful treatment with local injections of botulinum-toxin A into the cramping finger flexors or extensors relies on correct identification of the dystonic movements (Schuele et al., 2005).
Section snippets
Patients and controls
Pianists who had one or two of their fingers affected by MD were recruited from the outpatient clinic of the Institute of Music Physiology and Musicians’ Medicine at the Hannover University of Music, Drama, and Media. Each patient underwent a thorough neurological examination and was diagnosed by one of the authors (E.A.) who is specialised in movement disorders of musicians. Patients with other neurological, orthopaedic or psychiatric disorders were excluded from the study. In accordance with
Comparison across affected and non-affected fingers
To describe the movement characteristics of the affected and unaffected fingers, the mean and SD of each of the six movement variables within a trial during tapping with each of the four fingers were computed (Fig. 2 and Table 1). As a representative result, Fig. 2 displays the group means of mean and SD of the IKI across the healthy pianists (white), the MD pianists without symptoms at the tapping finger (grey), and the MD pianists with symptoms at the tapping finger (black). The results
Discussion
The present study demonstrated a successful classification of individual fingers between with and without symptoms of MD according to the movement features that characterised constrained and speeded finger tapping. This confirms the impact of MD on individuated finger movements. In addition, the results of PCA and an iteration of the cluster analysis using different variables determined a distinct set of movement features that contributed to the identification of MD, which differed across
Ackowledgement
We appreciate Prof. Kentaro Katahira at Tokyo Univ. for his invaluable comments on the manuscript and suggestions for cluster analyses. The authors declare no conflict of interest.
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