The inclusion criteria for enrolment into the study were as follows: (1) clinically established PD (criteria by Postuma et al. 2016), (2) right-handedness, (3) presence of HD symptoms based on the assessment of a speech therapist and the results of a 3F Test Dysarthric profile total score (Kostalova 2013), (4) Czech as their first language. Exclusion criteria were (1) alcohol or drug abuse, (2) hallucinations, (3) any diagnosed psychiatric disorder (4) dementia, based on the Montreal Cognitive Assessment (MoCA) test for dementia, MoCA > 20 (Biundo et al. 2014), and on a clinician’s interview with a caregiver (5) cardio pacemaker or any MRI-incompatible metal in the body, (6) epilepsy.

The disease severity was assessed using the Unified Parkinson’s Disease Rating Scale (UPDRS), part III (Motor Examination) scale. All participants were on a stable dopaminergic medication at least 4 weeks prior to baseline assessment and during the whole study. The patients were tested in the ON medication state without dyskinesias since we wanted to modulate HD symptoms in a real-life scenario in patients on dopaminergic medication. All patients signed an informed consent form that was approved by the local ethics committee.

Participants underwent tDCS over the right posterior STG. At the baseline visit, each participant underwent a speech assessment using the 3F Test Dysarthric profile (Kostalova 2013) (for details see Table S1 in Supplementary materials). Structural MRI scans (T1 MPRAGE) were performed for frameless stereotactic navigation of the electrode placement. After the baseline visit, each participant underwent three stimulation sessions (anodal, cathodal, and sham stimulation), separated by one day without stimulation. A crossover double-blind design was used, and stimulation protocols were randomized across subjects and sessions.

A protocol consisting of speech tasks lasted up to 10 min and was performed prior to and immediately after each stimulation session.

The speech protocol contained a special reading task (reading a phonetically balanced paragraph containing 150 words; patients were allowed read the text in advance). HD symptoms were assessed using speech parameters of interest based on our previous research.(Brabenec et al. 2019) More specifically, we quantified tongue and jaw rigidity (relF1SD and relF2SD), monopitch (relF0SD), duration of silences (DurMED), and irregular speech rhythms (SPIR) (see Table 1 for detailed description of the parameters).

Stimulation was applied through a battery-driven device (DC-Stimulator Plus, neuroConn GmbH, Germany). Both electrodes were positioned over the right and left posterior superior temporal gyrus (STG) (MNI coordinates: X = 40, Y = − 38, Z = 14; X = − 40, Y = − 38, Z = 14; based on our previous research) (Brabenec et al. 2021). We used the T1 MRI scan-based frameless stereotactic neuronavigation to specify the exact location of the electrode center in each individual. For anodal stimulation, the anode was placed over the right STG and the cathode over the left STG. For cathodal stimulation, the cathode was placed over the right STG and the anode over the left STG.

A current of 2 mA was delivered using two rubber electrodes (5 × 5 cm) for 20 min. The electrode was held in place by a conductive gel. The sham stimulation was applied with the same settings, but the stimulator was turned off after 30 s.

We used linear mixed models (LMM) to evaluate the effects of each stimulation condition on the relative changes in acoustic parameters. The stimulation condition was a fixed factor in LMM. Post-hoc pairwise comparisons of estimated marginal means were made with the Bonferroni correction. Age, gender, and levodopa equivalent dose (LED) were used as covariates in all LMMs. Wilcoxon signed-rank tests were used to compare the values of these parameters prior to and after each stimulation condition. A Spearman correlation analysis was used to assess associations between the tDCS-induced changes and the simulation of the electric field in right STG. These statistical procedures were performed with IBM SPSS Version 25.0 (IBM Corp., Armonk, NY, USA).

SimNIBS software (version 4.0.1) was used to calculate the electric field induced by tDCS, based on the finite element method and individualized tetrahedral head meshes generated from the structural T1 images of the participant. Electric field simulations were computed for both cathodal and anodal montages. Both electrodes were positioned based on the MNI coordinates mentioned previously. The mean electric field in the right STG (sphere radius = 10 mm) was calculated using a MATLAB script.