Language and movement are intertwined. This is true when children learn words and rules about their language, and it remains true for the moment-to-moment formulation of sentences in verbal communication (Wang et al., 2019). Voice and speech are the components of the motor system that are the vehicles on which oral language is expressed. It stands to reason, then, that the neural systems underlying movement and those underlying language production may be intertwined. However, motor and language systems are frequently considered separately, particularly when approaching remediation of either.

An example of this is in individuals with Parkinson’s disease (PD) who have characteristic impairments in vocal motor control secondary to dopaminergic deficiencies (Hallett and Khoshbin, 1980; Berardelli et al., 2001; Fox et al., 2002). PD results in hypokinetic dysarthria in up to 89% of patients (Logemann et al., 1978; Hartelius and Svensson, 1994; Ho et al., 1998; Miller et al., 2007; Schalling et al., 2017), affecting vocal quality [i.e., reduced loudness, hoarse and/or breathy quality, monotone prosody (Fox and Ramig, 1997; Ho et al., 1998; Baumgartner et al., 2001; Midi et al., 2008; Plowman-Prine et al., 2009)] as well as speech clarity (i.e., articulatory precision, vowel centralization) and speech rate (Logemann and Fisher, 1981; Forrest et al., 1989; Ackermann and Ziegler, 1991; Tjaden and Wilding, 2004; Sapir et al., 2007; Skodda and Schlegel, 2008; Sapir et al., 2010; Tjaden et al., 2013).

Language is also altered in individuals with PD. Action word or verb processing is diminished (Péran et al., 2009; Rodríguez-Ferreiro et al., 2014; Salmazo-Silva et al., 2017; Johari et al., 2019), particularly for those that are associated with an individual’s location of greatest motor impairment[i.e., upper vs. lower limbs; (Roberts et al., 2017)]. Recognition, comprehension, or production of highly actionable nouns (e.g., screwdriver, toothbrush) is represented in the brain across semantic and motor domains (Noppeney et al., 2006), but is minimally impaired in PD, if at all (Bocanegra et al., 2017). Differences in action word processing tend to be verb-specific, are seen early in the PD pathogenesis, and affect access to lexical representations for the comprehension and production of action words in isolation and in contextual interactions [e.g., (Cardona et al., 2013)]. However, verb processing in individuals with PD is not necessarily associated with motor impairment severity (D’Ascanio et al., 2023), unlike in other motor disorders [e.g., amyotrophic lateral sclerosis (ALS) in (Cousins et al., 2018); cervical dystonia in (Bayram and Akbostanci, 2018)]. Nonetheless, there may be a unique interaction between the motor and language systems in PD, likely centered in cortico-striatal loops (Silveri et al., 2012; Cardona et al., 2013). Investigations of several motor disorders have parsimoniously localized aspects of action concepts to cortico-striatal loops [as impaired in individuals with PD (Cardona et al., 2014)], rather than to primary motor or premotor cortex [as is impaired in individuals with ALS, (Cousins et al., 2018)]. The latter finding highlights a potential for effector-specific representations of the semantics for action concepts, a hypothesis that is best couched in the theories of embodied cognition (Pulvermüller, 2005; Barsalou, 2010; Binder and Desai, 2011; Barsalou, 2020).

What remains unclear is whether action word or verb processing in PD is attributable to a motor system impairment influencing language processing specifically, or if it is related to the presence of cognitive impairment (Murray, 2000; Bocanegra et al., 2017; Salehi et al., 2024). Subtle cognitive differences are observed in PD, particularly in executive functioning and processing speed (Schapira et al., 2017; Smith and Caplan, 2018), which could explain impaired language in contexts of higher processing load like discourse. Thus, observations of action word differences for PD may be attributable more generally to the cognitive deficits influencing sentence processing and the executive planning and working memory demands for syntactically complex stimuli.

One approach to investigating the motor system and its relationship to language processing in PD is through observation of performance changes when individuals are on or off dopaminergic medications. Individuals with PD on dopaminergic medications produce more action verbs and action content in verbal fluency tasks compared to those not taking their medications (Herrera et al., 2012). Herrera and colleagues concluded that “the dopamine network from basal ganglia to brain motor areas might play a role in retrieving action verbs with specific semantic representations” (p. 72). This would suggest that the deficit influences the early stages of message formulation, at the level of lexical semantics (Ferreira, 2000), but is also likely influenced by the task employed (e.g., verbal fluency for which lexical retrieval is the target behavior).

A similar approach to investigating dopaminergic influence on language has been used to investigate individuals with PD who have undergone surgical implantation of deep brain stimulators. A small number of studies of language performance assessed pre- and post-surgical stimulator placement in the subthalamic nucleus (Zanini et al., 2003) or the pedunculopontine nucleus (Zanini et al., 2009) have reported improvements in morphosyntax with stimulator on versus off. That is, word structure (morphology) and grammar (syntax), both of which are thought to be guided by verb selection (Ferreira, 2000), improved with stimulation. Despite the small sample sizes, no changes (good or bad) were observed pre- to post-surgery or on/off stimulation for neuropsychological measures or lexical semantics (words and meanings), suggesting discourse production effects were specific to later stages of utterance planning – i.e., surface structure for grammatical encoding, function assignment, or constituent assembly (Bock and Levelt, 1994; Ferreira, 2010).

These findings suggest that language production may vary with modulation of the motor system in individuals with dopaminergic degeneration. A potential extension of this theory is that improving motor system function could improve language function in PD. The current study aims to determine whether a treatment targeting improved motor (voice) function in PD is associated with concomitant improvements in verbal language production, specifically in morphosyntax. This study is a secondary analysis of narrative language data from a randomized controlled trial of the Lee Silverman Voice Treatment (LSVT LOUD®). This intervention targets change at the level of motor function (respiratory-laryngeal-oromotor support) and has been shown to effectively improve vocal loudness in individuals with PD (Ramig et al., 2018; Perry et al., 2024) as well as other areas of speech motor function and communication not targeted, including speech articulation (Sapir et al., 2007), intonation (Ramig et al., 2001), speech intelligibility (Levy et al., 2020; Schulz et al., 2021), facial expression (Dumer et al., 2014), swallowing (El Sharkawi et al., 2002; Miles et al., 2017), and neural functioning related to speech (Narayana et al., 2010; Baumann et al., 2018). Given these documented system-wide improvements, this intervention might also effect changes in language production. Thus, the aims of this study are to (a) replicate previously reported differences in discourse-level language production and information content in individuals with PD relative to neurotypical, age-matched healthy controls, and (b) determine whether participation in LSVT LOUD associates with improvements in discourse-level language production or information content. We hypothesize that differences in discourse-level language production may be observed between individuals with PD and controls at Baseline as found in previous studies. Further, we hypothesize that the intervention may yield improvements in discourse-level language. If such improvements are specific to verb production – either in terms of retrieval or use of verbs in context (morphosyntax), this result may lend support to the idea that motor and language domains are intertwined.

Participants are those reported in the LSVT LOUD protocol (Ramig et al., 2018). Participants with PD were diagnosed by a neurologist, were between stages I to IV on the Hoehn and Yahr scale for symptom severity (Hoehn and Yahr, 1967) and were stable on antiparkinsonian medications. The PD participants’ levodopa equivalence is reported in (Ramig et al., 2018). All were 45–85 years of age with normal hearing for age, mental status [scored ≥25 on the Mini-Mental Status Examination (MMSE, Folstein et al., 1975)], and reported no greater than moderate symptoms of depression [based on the Beck Depression Inventory II ≤ 24, (Beck et al., 1996)]. Individuals were excluded if they had PD with other neurological conditions or atypical PD at the time of screening, presented with speech or voice disorders unrelated to PD, had undergone neurosurgical treatment, had laryngeal pathology or surgical history, or had swallowing impairment requiring treatment. Additionally, individuals with PD who had undergone intensive speech treatment within the 2 years prior to the study or who had participated in LSVT LOUD previously were excluded.

Study procedures were approved by the institutional review board at the University of Colorado at Boulder and University of Colorado Health Science Center and are shared at ClinicalTrials.gov identifier: NCT00123084. All participants provided informed written consent for participation. Please see Ramig et al. (2018) for details regarding screening and randomization for the LSVT LOUD treatment trial.

Twenty participants with idiopathic PD underwent LSVT LOUD (TXPD) for the full 4-week intensive dose. Participants received 16, one-hour sessions of voice treatment, delivered 4 days a week over 4 weeks (see details in Supplementary materials). From the 22 TXPD and 22 UNTXPD participants reported in Ramig et al. (2018), three were later found to have MSA or palilalia and were excluded from this study. Additionally, one TXPD participant was an outlier for several language variables (i.e., words per minute, MLU words, verbs per utterance, density, and content units), and thus his data were removed from the study, leaving a total of 19 TXPD participants. A second group of 20 participants with idiopathic PD served as an untreated comparison PD group (UNTXPD), and 20 age-matched, neurotypical participants served as healthy controls (Controls).

The present study tested whether linguistic information produced during the BDAE Cookie Theft picture description task differed in individuals with PD compared to age-matched Controls. The study also assessed whether completion of an intensive intervention targeting increased vocal loudness might improve the motor system engagement for speech and, in turn, strengthen access to action concepts, improving retrieval or production of verbs and syntactic complexity in these narratives. Elicited from a convenience sample of treated and untreated participants with PD along with age-matched controls from a randomized controlled trial of LSVT LOUD, these narratives were used to test whether (1) lexical semantic or morphosyntactic language variables differed in the PD groups compared to Controls at Baseline, and (2) whether treatment-related change in vocal loudness (i.e., motor system) corresponded with changes in lexical semantic or morphosyntactic language production.

The original contributions presented in the study included in the article/Supplementary material, further inquiries can be directed to the corresponding author.

Study procedures were approved by the institutional review board at the University of Colorado at Boulder and are shared at ClinicalTrials.gov identifier: NCT00123084. All participants provided informed written consent for participation. Please see Ramig et al. (2018) for details regarding screening and randomization for the LSVT LOUD treatment trial. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study. Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

AR: Conceptualization, Data curation, Formal analysis, Methodology, Project administration, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing. KG: Formal Analysis, Methodology, Resources, Writing – original draft, Writing – review & editing. KC: Methodology, Writing – original draft, Writing – review & editing. JL: Methodology, Writing – original draft, Writing – review & editing. CMF: Conceptualization, Writing – original draft, Writing – review & editing. AH: Data curation, Writing – original draft, Writing – review & editing. LR: Conceptualization, Data curation, Funding acquisition, Writing – original draft, Writing – review & editing.