The need for coping with Parkinson’s disease

Developing new theoretical frameworks and testing innovative therapeutic approaches is a priority according to the International Parkinson and Movement Disorder Society [20]. Due to the lack of curative approaches, strategies for coping and neuroprotection and personalized management of PD are called for [21,22]. In addition to the chronic degenerative course, the extremely high complexity of PD is a major challenge for medicine. PD was recently conceptualized as a complex neuropsychiatric disorder [23], including emotion (i.e., depression and anxiety), perception and thinking (i.e., psychosis), and motivation (i.e., impulse control disorders and apathy). Multiple non-motor symptoms often precede the leading motor symptoms, sometimes by several years. Patients rate these non-motor symptoms as an even greater burden than the motor symptoms [24]. By reviewing the clinical characteristics and neural substrates of various symptoms (negative effects on gait, handwriting, grip, and speech) and highlighting how PD and its medical and surgical treatments effect motor symptoms, Moustafa and colleagues [25], offered a “unified framework” explaining the range of motor symptoms in PD. They argued that various motor symptoms in PD “reflect dysfunction of neural structures responsible for action selection, motor sequencing, and coordination and execution of movement” (p.727) [25]. However, an integrative understanding of motor and non-motor symptoms is currently not possible.

Prenger and colleagues [26] report that PD leads to a variety of emotional and communicative changes disrupting social functioning. These would include problems producing emotional facial expressions (i.e., facial masking) and emotional speech (i.e., dysarthria) as well as recognizing verbal and nonverbal emotional cues from others. A previous systematic mixed methods review [27] described how PD symptoms (e.g., tremor, facial masking, and neuropsychiatric symptoms) compromise relationships (e.g., partnership, friends, and family) and lead to early retirement. Moreover, they pointed out that conventional PD therapy rarely has a positive impact on the functioning of social roles. According to Prenger and colleagues, social symptoms of PD can result in “severe negative consequences, including stigma, dehumanization, and loneliness, which might affect the quality of life to an even greater extent than more well-recognized motor or cognitive symptoms” (p. 1) [26]. So far, dealing with PD has continued to present new challenges for clinicians and practitioners. The disorder appears to gain in complexity, making the search for a linear, causal approach to overcoming it increasingly unlikely.

Singing with patients of Parkinson’s disease

Di Benedetto and colleagues started to investigate a voice and choral singing treatment to improve speech and voice disorders in PD with “an amusing, agreeable, and collective approach” (p. 13) [28]. Semi-structured interviews revealed that singing was helpful to PD and stroke patients. Singing supported patients’ coping with disease burden, including social isolation, low mood, and communication difficulties [29].

Stegemöller and colleagues [30–33] explored the benefits of singing on a range of symptoms experienced by people with PD. They began by recording voice, respiratory, and quality-of-life measures before and after eight weeks of singing [30] and examined the acute effects of one hour of group therapeutic singing on physiological measures of stress [32] and clinical motor symptoms [32,33] in patients with PD. They found significant improvements in maximum inspiratory and expiratory pressures and phonation time when singing. Notably, both voice-related and overall quality of life improved in patients [30]. In addition, their study showed a significant decrease in mean scores for gait, posture, and tremor, but not for speech and facial expression, or bradykinesia [33]. Contrary to expectations, they found no significant changes in cortisol levels or motor improvement, nor any significant correlation between both variables in PD patients in a single session [32]. However, the singing group showed a tendency to feel less sad compared to the control group. In addition, the heart rate increased in the singing group and decreased in the control group after the session. From this single session, Stegemöller and colleagues [32] conclude that singing without sheet music, accompanied by vocal exercises, is a promising yet exploratory intervention for people with PD. Tamplin’s team went further and developed an interdisciplinary ParkinSong model, comprising high-effort vocal and respiratory tasks, speech exercises, group singing, and social communication opportunities. Results from a controlled trial with active control condition at two dosage levels (weekly vs. monthly) over three months [34], followed by a four-armed controlled clinical trial over 12 months [35], demonstrated promising potential to counteract deterioration in voice, speech, respiratory strength, voice-related quality-of-life outcomes and well-being from mild to moderate PD. In a randomized crossover study, Butala and colleagues [36] found that weekly group singing may improve conversational voice volume and aspects of quality of life, including emotional well-being and body discomfort, in people with PD, although baseline group differences limited attribution of effects solely to singing.

Moreover, Yeo and colleagues [37] investigated the effects of individual singing-induced swallowing exercises in patients with advanced PD in comparison to a matched usual care control group. Therapeutic singing exercises did not improve but preserved swallowing function in advanced PD patients compared to the controls and improved swallowing-related quality of life in those with advanced-stage diseases. There is a first hint that an online format of singing is practicable and appreciated by patients of PD but more thought needs to be given to the delivery format of group singing programs in the context of telemedicine [38,39].

In their review of the current state of studies, Barnish and colleagues [40] cite significant methodological limitations, such as a lack of control groups. However, in their assessment, the studies suggest a “broader benefit” of singing with effects on functional communication, cognitive status, motor function, and quality of life that should be evaluated through further studies. Besides singing, dancing is becoming of greater interest to serve as an integrative therapeutic intervention “via music, metaphorical language, and a socially reinforced reality of art-partaking” (p.1) [41]. The interest in performing arts as a therapeutic medium in PD is increasing [42].

Polyvagal theory as a new leading concept for coping

We refer to Polyvagal Theory by Porges [9–19] as an integrative framework for this study. Porges postulated a neurophysiological model describing neural circuits involved in the regulation of autonomic states and how these, in turn, shape adaptive behavior and social engagement (e.g., the face-heart connection [10,17–19], gut-brain interactions [43]). Within this framework, the autonomic nervous system is conceptualized as a phylogenetically ordered hierarchy comprising the ventral vagal complex (State I, see Fig 2) the sympathetic system (State II, see Fig 2), and the dorsal vagal complex (State III, see Fig 2). These subsystems support, respectively, social engagement and restoration (I), mobilization in fight/flight responses (II), and immobilization with freezing and shutdown in the face of extreme threat (III). Ontogeny is proposed to mirror phylogeny, such that postnatally these systems mature in the order of the hierarchy, from dorsal to ventral. In line with Jacksonian dissolution [10,17–19], acute threat, illness, or injury is hypothesized to trigger hierarchical deactivation from the newest to the oldest systems.

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Fig 2. Activation pattern of the autonomic nervous system before and after a singing intervention with patients of Parkinson’s disease (PD), according to the Polyvagal Theory from SW Porges [10–13].

Note: I Ventral Vagal Complex: a state in which one feels safe, takes part in social interaction, and feels connected with oneself and with others (including social engagement system); II Sympathetic System: a state of feeling danger in which mobilization occurs. People show fight or flight responses, that is, they enact active defensive (fight-flight responses); III Dorsal Vagal Complex: a state in which people experience life threat, immobilization in form of a shutdown as a passive defense (freezing, fainting).

https://doi.org/10.1371/journal.pone.0337210.g002

It is important to note that the autonomic state functions as an intervening variable that mediates physiological regulation, behavioral expression, emotional reactivity, and health outcomes. Central to this model is the notion that the face–heart connection allows social interactions to regulate visceral states [10,17–19] and that gut–brain interactions provide an additional pathway linking autonomic regulation to subjective experience [43]. Signals of safety, danger, and life threat from within the body and from the external environment may trigger different autonomic states through a process termed “neuroception” [10,17–19]. Consequently, each autonomic state alters an individual’s capacity to feel safe and to flexibly navigate between defensive modes and states of connection (see Fig 2, [10,17–19]). Consistent with this view, previous studies have shown that disruptions in the regulation of autonomic states are associated with hyperarousal, fatigue, and a range of psychosomatic and psychiatric symptoms [44–46].

As hierarchically organized, the evolutionary later developed systems constraint the preceding ones, so that the ventral vagal complex (State I, see Fig 2) has the most important effect on health. When it overrides the older, more basal systems, individuals may operate within a broader window of tolerance [47,48], characterized by well-being, coping capacity, and resilience [10–13,17–19].

Why and how might a singing format strengthen states of the ventral vagal complex? The Singing Hospitals format is designed to foster perceived safety, shared attention, and coordinated action through music and interpersonal encounter among participants [3–6]. Thus, singing may stimulate the ventral vagal complex both directly—via neurophysiological effectors [13,14,17–19]—and indirectly, by creating a socially reinforced context that enables bidirectional signaling and co-regulation in line with the hierarchical model of autonomic regulation [15–19,49,50]. Ventral vagal complex activity is associated with rapid, flexible cardiac regulation, which can be facilitated by prolonged, slow exhalation phases characteristic of singing. Beyond its roles in ingestion and orienting, the complex is hypothesized to regulate state-dependent social behaviors, including nursing, bonding, vocal communication, and facial prosody. Special visceral efferent components of cranial nerves (V, VII, IX, X, and XI) are thought to coordinate facial expression, vocal prosody, head–neck orientation, posture, and middle-ear muscle tone, thereby functionally linking social communication with autonomic regulation, named social engagement system [10,17–19,50]. Accordingly, there are multiple ways in which singing could modulate participants’ states (see subscales of the self-developed rating scale):

• From physical rigidity to flexibility (subscale 1): through deeper diaphragmatic breathing with prolonged exhalation, tension-releasing vocal resonance, alongside music-induced axial rotation and postural realignment;
• From mental dullness to clarity (subscale 2): by shifting hyper-/hypoarousal toward socially engaged attention, and by using structured rhythm and phrasing for re-orientation, present-moment focus, and sustained attention;
• From being emotionally uninvolved to being touched (subscale 3): through self-congruent lyrics and vocal prosody that deepen self-expression and support affect regulation and embodied processing (e.g., via tempo, rhythm, and dynamics);
• From social isolation to connection (subscale 4): through experiences of togetherness fostered by synchronization, shared attention, and co-creation among participants.

Taken together, the Singing Hospitals format may function as a multi-domain regulatory resource—simultaneously supporting autonomic regulation, emotional expression, attentional focus, and social connectedness—and thereby help individuals with PD to cope more effectively with everyday challenges.

Within Polyvagal Theory, the state-dependent responsiveness of the autonomic nervous system to cues of safety, danger, and life threat is called “autonomic reactivity” by Cabrera and colleagues [51]. They developed a self-report questionnaire to look into these states by seeking key sensations of autonomic reactivity. In contrast to interoceptive accuracy, this questionnaire captures people’s interoceptive sensibility as “an individual’s beliefs about their interoceptive ability and the degree to which they feel engaged by interoceptive signals” (p. 66) [51]. Interoception comprises the process by which individuals’ sense, interpret, and integrate signals from within the body, thus detecting their current state across conscious and unconscious levels [52]. Enhancing individuals’ awareness of and capacity to regulate their autonomic state in response to signals of safety, fight-flight, and freezing is getting a key role in therapy. Thus, reinforcing interoception abilities is a relevant focus for clinical interventions [53]. Furthermore, vagal nerve stimulation is proposed as a method to promote calming, enhance social engagement, and support physiological balance through activation of the ventral vagal pathway [54]. According to Polyvagal Theory, all resource-oriented, preventive and supportive formats—such as self-help or coping—should also aim to foster patient’s awareness of these states [54].

In this pilot study, we refer to Polyvagal Theory to talk about how singing in the format of Singing Hospitals interacts with autonomic states. We expected that a weekly singing group activity (one hour per week for eight weeks) would positively affect PD patients’ well-being during the singing sessions (H1). Furthermore, we explored whether this would be a reproducible effect within another context, expecting that a singing group activity in the format of an eight-hour workshop would have similar positive effects on PD patients (H2). Finally, we assumed that singing could have more long-lasting effects on stress regulation through repeated stimulation of the social engagement system by the singing activity. Thus, we expected the weekly singing intervention to reduce patients’ body awareness of autonomous reactivity compared to a no singing control group (H3).

Participants and procedure

The study was approved by the Ethics Committee of the University of Trier (Germany) in May 28^th 2019 (EK 04/2019). We did not determine our sample size in advance but collected data from as many patients as possible from June 18^th 2019 until November 30^th 2019. Study data are available online: https://pasa.psycharchives.org/reviewonly/45567bb6ba636cc5943e6ccd48d5222c757322bb8942c57f6f07267b9634cbc8. We confirm that all ongoing and related trials for this intervention are registered at the German Clinical Trials Register (DRKS-IDDRKS00035851). Since we missed the prospective registration of clinical trials as a prerequisite for publication in leading medical journals, we registered our pilot study during the submission process. All participants included in the study provided written informed consent. There were no activities to increase compliance. Participants continued their current PD medication and concomitant therapies and were recruited for two different single-arm feasibility studies (Fig 1).

Materials

Descriptive statistics

According to the norms of the 12-item Body Awareness Very Short Form [51,56], at the beginning of the pilot study, our samples of PD patients reported scores within a normal range. In the weekly singing group (N = 13), the body awareness of M = 27.92 (SD = 9.12) corresponds to a T-value of 51.5 [51], or 50.6 [56]. In the non-singing control group (N = 22), the body awareness of M = 26.64 (SD = 7.77) corresponds to a T-value of 50.7 [51] or 49.7 [56].

Statistical analyses

To test our hypotheses, we planned to conduct three types of analyses of variance (ANOVA) using SPSS.14/IBM. First, in Sample 1, we planned to use 2 (time) x 3 (sessions) ANOVAs to test the effects of hourly singing across time (within: pre vs. post) and sessions (within: 3–4 vs. 5–6 vs. 7–8) on physical flexibility, clarity of mind, emotional involvement, and social connectedness, respectively. Second, in Samples 1 and 2, we planned to use a 2 (time) x 2 (group) ANOVA to test the effect of singing across time (within: pre vs. post) and group (between: weekly singing vs. workshop singing) on well-being (i.e., the mean across the four ratings mentioned above). Third, in Samples 1 and 3, we planned to use a 2 (time) x 2 (group) ANOVA on autonomic reactivity to test the effect of singing across time (within: pre vs. post) and group (between: weekly singing vs. no singing).

Effects of weekly singing

To examine psychological changes during the weekly singing group activity, we conducted four separate 2 (time: pre vs. post treatment) x 3 (sessions: 3–4, 5–6, and 7–8) ANOVAs with repeated measures on both factors. Fig 3 illustrates the results. On all four dimensions of our self-developed rating scale, there were highly significant main effects of Time (Physical: F(1, 12) = 69.06, p < .001, =.852; Mental: F(1, 12) = 18.83, p < .001, =.611; Emotional: F(1, 12) = 25.41, p < .001, =.679; Social: F(1, 12) = 15.15, p < .002, =.558). Findings indicate that, across sessions, there were significant improvements from pre to post singing in patients’ self-rated physical flexibility (M_pre = 0.03 vs. M_post = 2.10, p < .001), clarity of mind (M_pre = 0.85 vs. M_post = 2.26, p < .001), emotional involvement (M_pre = 0.59 vs. M_post = 2.21, p < .001), and social connectedness (M_pre = 1.06 vs. M_post = 2.31, p < .002). On all four dimensions, there was no significant main effect of Session (all Fs(2, 24) < 0.74, p > .490, <.058) indicating no continuous improvements from early to later sessions. Furthermore, on all four dimensions, there was no significant Time x Session interaction (all Fs(2, 24) = 1.18, p < .323, <.090) indicating that the improvement through singing did not change over the course of the weekly sessions but was visible right from the start (i.e., session 3–4) and did not fade out towards the end (i.e., session 7–8). Taken together, the hourly singing activity improved not only mental clarity, social connectedness, and emotional involvement, but also patients’ physical flexibility.

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Fig 3. Means (and standard errors) of self-reported states pre- and post-one-hour long weekly singing activities among patients with Parkinson’s disease (N = 13).

https://doi.org/10.1371/journal.pone.0337210.g003

Weekly singing versus workshop singing

To explore if PD patients showed the same positive effects when singing was practiced in a different format, we conducted a 2 (time: pre, post) x 2 (group: weekly singing, workshop singing) ANOVA with repeated measures on the first factor. In Sample 1, we aggregated ratings across sessions 3–8 and across dimensions. In Sample 2, we aggregated ratings across dimensions. Again, there was a highly significant main effect of Time, F(1, 25) = 31.63, p < .001, =.596 (i.e., d = 2.429; see https://www.escal.site/) indicating significant improvements from pre to post singing in patients’ physical flexibility, mental clarity, emotional involvement, and social connectedness (M_pre = 1.12, SD_pre = 1.25 vs. M_post = 2.65, SD_pre = 1.13, p < .001). There was a significant main effect of Group, F(1, 25) = 6.59, p = .017, =.208, but no significant Time x Group interaction, F(1, 25) = 0.72, p = .791, =.003. Thus, patients of PD benefited from the two different singing activity settings in the same way (see Fig 4). A posthoc sample size calculation (ANOVA, repeated measures, within factors, effect size f = 1.215, α = .025, N = 27, 2 groups, 2 measurements, r = .50) indicated that we achieved a power (1 – β) = 1.00 for detecting a main effect of time.

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Fig 4. Means (and standard errors) of well-being (physical flexibility, mental clarity, emotional involvement, social connectedness) pre- and post-singing among patients with Parkinson’s disease in a weekly singing group (Sample 1, N = 13) and a workshop singing group (Sample 2, N = 14).

https://doi.org/10.1371/journal.pone.0337210.g004

Weekly singing versus no singing

To test whether the weekly singing group activity reduced autonomic reactivity compared to a control group without singing, we conducted 2 (time: T1, T2) x 2 (group: weekly singing group, no singing group) ANOVA with repeated measures on the first factor. There were no significant main effects of Time, F(1, 33) = 0.80, p = .378, <.024, and Group, F(1, 33) = 1.61, p = .214, <.046. More importantly, the Time x Group interaction was significant, F(1, 33) = 7.22, p = .011, =.180 (i.e., d = 0.937; see https://www.escal.site/). Fig 5 illustrates the interaction. Independent t-Tests revealed that, at the beginning of the intervention, the singing group (M_pre = 27.92, SD_pre = 9.19) did not differ in autonomic reactivity from the control group (M_pre = 26.64, SD_pre = 7.77), t(33) = 0.44, p = .661, Diff = 1.29, 95% CI [−4.63, 7.20]; after eight weeks of singing, the singing group (M_post = 22.38, SD_post = 4.25) showed lower body awareness compared to the control group (M_post = 29.41, SD_post = 8.61), t(33) = −2.74, p = .010; Diff = −7.02, 95% CI [−12.24, −1.81]. Dependent t-Tests revealed that the singing group’s decrease in the markers of body awareness was (marginally) significant, t(12) = 2.18, p = .050; Diff = 5.54, 95% CI [0.01, 11.09], whereas the control group’s increase was not significant, t(21) = −1.51, p = .147; Diff = −1.51, 95% CI [−6.60, 1.06]. A posthoc sample size calculation (ANOVA, repeated measures, within-between interaction, effect size f = 0.469, α = .025, N = 35, 2 groups, 2 measurements, r = .30) indicated that we achieved a power (1 – β) =.98 for detecting a Time x Group interaction.

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Fig 5. Means (and standard errors) in body perception and autonomic reactivity at baseline (Time 1) and after eight weeks (Time 2) among patients with Parkinson’s disease in a weekly singing group (Sample 1, N = 13) and a no singing control group (Sample 3, N = 22).

https://doi.org/10.1371/journal.pone.0337210.g005

Effects in weekly singing and workshop singing groups

In line with what other studies have found [28–37,40–42], singing once a week improved physical functioning, including mood and social participation, however, these gains were typically transient, with participants returning to a lower level between sessions. Specifically, we introduced four dimensions (physical, mental, emotional, and social) in our ratings before and after the group singing activity (see Fig 3) to distinguish specific effects of singing from an overall effect. Our statistical analysis revealed a significant uniform improvement emerging across all four dimensions through singing. When comparing weekly singing with a one-day singing workshop, we found a similar singing effect across all four dimensions (Fig 4). This consistent finding across two different settings indicates a general effect of singing. The main effect of singing had a large effect size (d = 2.43). In summary, the present results reinforce the findings of the meta-analysis by Barnish and colleagues [40] who suggest that singing could make a valuable contribution to disease management in PD.

Effects on interoceptive sensibility and autonomous reactivity through singing

Consistent with our hypothesis, the weekly group singing activity resulted in significantly lower scores in the 12-item Body Awareness Very Short Form compared to the control group (Fig 5). This means that patients with PD were less aware of body discomfort after the singing activity. Referring to the principles of Polyvagal Theory [14,18,50], we assume that our form of group singing—characterized among others by eye contact, deepened breathing, and an expanded range of vocalization—may foster an autonomic state associated with the social engagement system (Fig 2). According to Polyvagal Theory, this system provides the psychophysiological foundation for self-regulation, co-regulation through social interaction, and mutually cooperative behavior. In previous studies, the activity of the social engagement system promoted physiological processes and emotional states associated with coping and health [57,58]. Thereby, the stimulation of the social engagement system has led to positive effects, especially in the treatment of emotional trauma [59].

Toward a model of singing effects according to Porges’ polyvagal theory

Our pilot study encourages further investigation into the extent to which group singing may facilitate a shift in autonomic state regulation toward ventral vagal activation and a physiological state of safety and social engagement, conducive to symptom reduction and enhanced well-being. At the end of the singing group, patients felt more physically flexible, mentally clearer, emotionally involved, and socially connected (Figs 3 and 4). How can we explain such a general shift through singing? We might gain insights from looking at research on dancing among patients with PD [60–62]. Koch and colleagues [62], for example, found that dancing positively influenced embodied identity. Singing might have a similar effect. In a review of medical and social science articles including group singing and rhythm in music as well as personal experience and reasoning, Buetow and colleagues [63] assume that Parkinson’s disease disrupts both social and biological rhythms. They suggest that participating in group singing may foster psychological states such as connectedness and flow, which could support rhythmic synchronization. This, in turn, may help alleviate motor timing difficulties and emotional processing challenges, ultimately enhancing quality of life.

It is important to note that in these previous explanations, dancing is treated as well as singing in an either/or manner: either as a primarily physical activity, thus focusing on body-related parameters or motor symptoms of PD, or as a form of psychosocial support, thus focusing on non-motor aspects such as quality of life. Only recently have there been initial attempts to take an integrative view. For example, Fontanesi and DeSouza [41] compared dance to an exercise intervention of matched intensity and found different outcomes indicating that dance entails intrinsic artistic elements—hypothesized to influence affective responses, experiences of beauty, self-efficacy, and gait performance—that are absent in exercise.

We propose that the concept of autonomic reactivity might contribute to describing the highly dysfunctional dynamics in symptom progression in PD. Consistent with Polyvagal Theory, signals of inner or outer threat might create those crucial boundary conditions leading to the cascade of motor, non-motor, and socioemotional symptoms as seen in PD. The advantage of our explanatory approach proposed here is that it could overcome this split between physical and social aspects in creative interventions such as dancing and singing. In general, embodied experience—including expressive gesture and movement, perceived social safety and connection, and emotional involvement within our singing format—may co-vary with psychophysiological balance and be paralleled by activation of the ventral vagal complex and the social engagement system; these processes are mutually reinforcing. Consistent with Polyvagal Theory, Porges emphasizes bottom-up influences whereby the autonomic state constrains or enables higher-order perception, attention, and volition. Rather than treating symptoms one by one, a polyvagal-informed approach targets the prevailing autonomic state, matching interventions to the individual’s momentary capacity for regulation, connection, and learning. Leveraging the body’s adaptive tendencies is intended to yield more enduring, intrinsic adaptive effects [18,19]. This aligns with the view that restoring dynamic homeostasis is a central goal of effective physiological regulation, as emphasized by Billman [64]. Accordingly, we highlight a close interconnection between physical and social processes that promote stress regulation and foster learning-oriented, cooperative, and restorative behavior.

Similar to trauma-exposed people, patients with Parkinson’s disease (PD) may show persistent alterations of autonomic balance and sensorimotor control that culminate in chronic autonomic dysregulation, expressed as hypervigilance, dissociative tendencies, anxiety, depression, and diverse somatic symptoms [18,19]. Within a polyvagal perspective, such patterns could reflect a long-standing shift toward defensive states—i.e., withdrawal of ventral vagal influence, recruitment of sympathetic mobilization, and dorsal vagal shutdown—even in the absence of immediate threat as a chronic dominance of the basal states (II + III, see Fig 2).

Several PD-specific mechanisms might maintain this anchoring. First, weakening of the social engagement system due to hypomimia and reduced vocal prosody may diminish cues of safety and interpersonal co-regulation. Second, attenuated orienting responses—for example restricted axial rotation and narrowed visual scanning (“tunnel vision”)—may limit environmental appraisal and the detection of safety signals. Third, impaired motor expression of arousal and startle—manifesting as festination and freezing—may hinder the resolution of defensive activation, thereby perpetuating state dysregulation.

To our knowledge, no PD studies have systematically operationalized the body therapy notion of “muscular armor” [65] or examined whether singing-based interventions can release such bracing patterns and thereby shift autonomic state toward ventral-vagal engagement. These hypotheses warrant targeted, state-aware investigation.

The earliest link between PD and the vagus nerve traces back to the original descriptions of Lewy-type inclusions in brainstem nuclei, including the dorsal motor nucleus of the vagus in 1912/1913 [66]. In later accounts, the vagus moved to the foreground with the Braak staging hypothesis from 2003 [67]. Recently, researchers studied the effects of stimulating the vagus nerve [68]. However, polyvagal assumptions regarding the hierarchical organization of the autonomic nervous system and the role of co-regulation have not yet been considered. Only in a brief essay in 2012, Diederich and Parent referenced Polyvagal Theory and suggested that symptoms such as impaired automatic gait or olfactory loss may reflect dysfunction in evolutionary ancient neural networks in PD [69], however, this hypothesis has not yet been systematically pursued in PD research.

We propose using Polyvagal Theory [9–19] to develop an integrative view on motor and non-motor symptoms in PD. Beside preventing degenerative nerve damage or substitute endogenous substances (e.g., dopamine), modulating psychophysiological states as described above could be an intervening variable whether symptoms escalate or decrease. The way autonomic states are organized in a hierarchy (see Fig 2) means that activating the social engagement system would constraint autonomic reactivity. Group singing could strengthen a highly adaptive self- and co-regulatory response to stressors from the inner and outer world. Conversely, chronic inactivation of the social engagement system could lead to a psychophysiological regression to basal systems with corresponding stress flooding and rigidification reactions, as observed in the course of PD. Singing could help to overcome this regression by strengthening the social engagement system. In general, it can be assumed that the reduction of autonomic reactivity in favor of the activity of the ventral vagal complex will lead to an alleviation of symptoms. Interoceptive sensitivity could help patients in gaining body information about which global affect state they are in and how this is changed by singing.

In other patient groups, researchers found a vicious cycle between autonomic dysregulation and chronic disease [70], autonomic reactivity and the buffering effects of adolescent friendships [71], social co-regulation of the autonomic nervous system between infants and their caregivers [72], atypical autonomic regulation and poor psychological well-being in college females with maltreatment histories [73], autonomic dysregulation and diabetes-relates distress [74], and cardiac autonomic dysregulation and joint hypermobility in adolescent with functional abdominal pain disorders [75]. To study coping with PD, we propose to refer to these insights, for example, the mutual influence of traumatic stress and the autonomic brain-gut connection. In this field, Polyvagal Theory is already used as an integrative framework for psychosocial and gastrointestinal pathology [43,76,77]. Here, singing was theorized to be suitable to cause a favorable “switch” in the system of the autonomic nervous system. In the following, we outline the theoretical and practical implications of a polyvagal perspective in coping with PD.