Effect of Transcranial Direct Current Stimulation on the Voice
Phase: NA
Conditions: Voice and Resonance Disorders, Transcranial Direct Current Stimulation, Voice Disturbance
Sponsor: Universidade Federal de Pernambuco

Effect of Transcranial Direct Current
Stimulation on the Voice
Document date: 07/15/2024

Study protocol
The present study was approved by the Research Ethics Committee of the institution where
the research was conducted (n. 5.758.856). All included volunteers signed the written
informed consent prior to the experiment.
Vocally healthy individuals were recruited from the university community. We included
volunteers of both sexes, aged 18 to 45 years, who scored up to 16 points on the Vocal
Symptoms Scale (VSS), Brazilian version. The Brazilian Dysphonia Screening Tool (Br-DST)
was also applied, and those who answered "yes" to both questions would be excluded.
Through a crossover study design, the volunteers underwent four experimental sessions
separated by 7 days-minimum washout period to prevent cumulative effects of stimulations.
In each session, a single target area was stimulated: (i) left laryngeal M1, (ii) left S1 of the
larynx, (iii) left DLPFC, and (iv) cerebellum. The order of sessions was randomized in order
counterbalanced by an external researcher to ensure blinding of both volunteer and therapist
during vocal assessment. The allocation of sessions was securely concealed within opaque,
sealed envelopes that were accessible only to the researcher responsible for tDCS.
The electrode setup during the tDCS sessions was as follows: (i) For left laryngeal M1 , the
anode was positioned over C5 (international 10-20 EEG system) and the cathode over the
contralateral supraorbital region; (ii) For the left S1 of the larynx, the anode was positioned 3
cm posterior to C5 and the cathode over the contralateral supraorbital region; (iii) For left
DLPFC, the anode was positioned over F3 and the cathode over the contralateral supraorbital
region; (iv) For the cerebellum, the anode was placed over inion and the cathode over the
right deltoid muscle. During the tDCS session, volunteers were instructed to sit in a chair and
find a comfortable position. tDCS was delivered by an electrical stimulator (Neuroconn,
Germany) through a pair of saline-soaked sponge electrodes (35 cm²) with a current intensity
of 2mA for 20 minutes.
Before and after each session, volunteers were systematically evaluated for vocal acoustic
parameters and laryngeal diadochokinesis. The vocal assessment analysis was performed
immediately before and after tDCS using the Voxmetria and Vocalgrama software from CTS
Informatics. Voice recordings were conducted using an HP Notebook Acer Aspire 5 PC with a
Karsect HT-2º headset microphone and Andrea PureAudio™USB-AS sound signal amplifier
equipment. The microphone was positioned four centimeters away from the mouth at a

45-degree angle, with the participant seated and hands resting on their legs, forming a
90-degree angle.
The following tasks were requested: (i) Evaluation of vocal quality through sustained vowel
/Ɛ/ for 5 seconds at weak intensity (below 70dB), habitual intensity (between 70 and 80dB),
and strong intensity (above 80dB), controlled with a decibel meter. (ii) Cepstral analysis with
sustained vowel /Ɛ/ for 5 seconds and the phrase "olha lá o avião azul" at weak, habitual, and
strong intensities (as defined above), controlled with a decibel meter. (iii) Diadochokinesis
assessment with the task of emitting vowels /a/ and /i/ separately, with rapid and interrupted
repetitions of each vowel as fast as possible for eight seconds. (iv) Pitch range assessment
through ascending and descending glissandos up to the maximum vocal extension in weak
and strong emissions. (v) Assessment of reported vocal effort with sustained vocal effort
emission. For phase 1, participants were asked to reproduce a note using a virtual keyboard (3
tones below their maximum glissando frequency) and then maintain that tone while counting
from 21 to 30, Immediately afterwards, the participants answered the Adapted Borg CR10 for
Vocal Effort Ratings (BR)17. Like the original protocol, the Adapted Borg CR10 for Vocal
Effort Ratings (BR) maintained a scale from 0 to 10, where 0 means “no vocal effort at all”,
and 10 “maximum vocal effort”.
For Vocal range profile, the participants were instructed to produce ascending (from low to
high) and descending (from high to low) glissandos of the vowel /ɛ/ as weak as possible and
as strong as possible intensities. As the participant produced the vowel /ɛ/ in ascending and
descending glissando, the graph was marked with dots that corresponded to the frequency
(abscissa) and intensity (ordinate) of the vocal productions. The participants were asked to
emit the loudest possible tone during the vocal productions of glissando. The assessment of
diadochokinesis was performed by counting peaks per second in the spectrogram, excluding
the initial and final two seconds for analysis. For the analysis of acoustic parameters, the
recording of the vowel /ɛ/ was edited by discarding the first and last minutes of emission (as
these segments showed greater instability), considering approximately three seconds of
emission for analysis. The assessment of diadochokinesis was performed by counting peaks
per second in the spectrogram, excluding the initial and final two seconds for analysis.

Statistical Analysis
The sample size was determined using the Sample Size Calculators tool designed by David
Schoenfeld with the support of the Massachusetts General Hospital - Mallinckrodt General
Clinical
Research
Center
(http://hedwig.mgh.harvard.edu/sample_size/js/js_crossover_quant.html).
The calculation was carried out with 12 volunteers, considering the minimum detectable
difference (MDD: 1.96StandardError√2) and the standard deviation of the primary outcome
measures. A statistical power (β) of 80% and a significance level (α) of 5% were considered.
For data analysis, normality was tested using the Shapiro-Wilk test. To compare pre and
post stimulation results, the Student's t-test was applied for parametric data and the Wilcoxon
signed-rank test for non-parametric data. To compare the effects of tDCS across different time
points, a Friedman ANOVA for repeated measures was conducted, followed by pairwise post
hoc comparisons with a significance level of 5% (p < 0.05). All analyses were performed in
SPSS software (version 26.0, SPSS Inc, Chicago, USA).