Duffy (2013) defines speech intelligibility as the “degree of accuracy with which a listener recovers the acoustic signal or message produced by a speaker.” Kent (1992) considers intelligibility as the paramount issue in speech pathology, “the behavioral standard of communication.”
Speech intelligibility can be severely reduced in Parkinson’s disease (PD); it can be among the main concerns of people with PD (Miller et al., 2007).
The validity and reliability in assessing speech intelligibility in PD may be affected by several factors:
- Reduced loudness SPL impacts audibility
- External Cueing/prompting enhances speech performance
- Multi-tasking reduces speech performance/real world
- Performance diminishes across utterance
- Patients with PD have an auditory perception problem; they deny speech problems
It is well established the vocal loudness significantly improves in PD following LSVT LOUD® (e.g., Perry et al.,2024; Ramig et al.,2018). Increases in vocal loudness are associated with an increase in audibility. Thus, it could logically follow that with increases in loudness and audibility, speech intelligibility would improve as well.
We designed a study to evaluate whether LSVT LOUD improves speech intelligibility in PD. Our study controlled for loudness changes and external cueing and diminishing performance and we used spontaneous speech production.
Unlike typical intelligibility studies where loudness is boosted so the signal is audible and then intelligibility is rated, we added 10-talker babble noise to yield a 0 signal to noise ratio.
Study Design
This study was reported following CONSORT guidelines for non-pharmacological treatment (Boutron et al, 2017).
64 Subjects with PD (4 Groups)
- Two groups received intensive treatment
- (LSVT LOUD or LSVT ARTIC)
- Two groups remained untreated
- (PD and non-PD)
- Sample size 20 per group, statistical power .80
- 3 PD groups reported here
Speech data collection
Narrative speech was collected to approximate daily self-generated speech. Sentences from a 90 sec personal narrative were selected for analysis. Data from 57 subjects were analyzed.
Select Dependent Variables
Single, primary outcome variable for RCT:
- dB Sound Pressure Level (SPL) (@ 30 cm) for reading passage and monologue
Secondary outcome variable:
- *Speech intelligibility* of spontaneous monologue
Stimulus preparation
To evaluate impact of SPL on intelligibility:
- Pre- and post-treatment sentences mixed with 10 talker babble.
- Noise level was chosen to yield 0 dB SNR for pre-treatment phrases
- Same noise level was used for pre- and post-treatment stimuli, so SNR differed.
This is NOT the Lombard Effect
- Noise was mixed with the speakers’ voices for presentation to the listeners
- Speakers with PD did not hear noise
Listeners
There were 117 listeners with normal hearing, ages 18-35 years. Listeners were blinded to treatment conditions.
Data collection procedure
Randomized stimuli were presented via insert earphones at average of 80 dB SPL (range = 66-86 SPL). The listening task (with order counterbalanced) was performed using customized software.
- Orthographic transcription
- Dependent variable: % of words transcribed accurately (transcription accuracy)
Scoring and analysis
Scoring and analysis were blinded.
Transcription accuracy
Listeners’ transcriptions of phrases in noise were compared to American English listeners’ transcription of utterances without noise. Spelling errors, homonyms scored as correct.
Intra-Rater Reliability: .688-.998, except one listener whose reliability was .078; Inter-Rater Reliability: 0.9936.
Results
Transcription accuracy (See Figure below)
For treatment targeting voice, within group increases in TA from baseline to post-treatment were significant (p<0.0001). For treatment targeting articulation, increases in TA from baseline to post-treatment were not significant (p=0.18). For the no treatment group, decreases in TA were significant (p=0.0115).
Between-group comparisons of changes in TA from baseline to post-treatment indicated that increases in the voice group were significantly greater than those for both the articulation group (p=0.04) and the no treatment group (p=0.0002). Differences in TA changes between the articulation group and the non-treatment group were not significant (p=0.147).
Summary
Descriptively, there were treatment effects for both treatments, but only statistically significant increases in LSVT LOUD group. No improvement for the untreated group. When speech was evaluated in background noise, speakers with PD who have received LSVT LOUD increased in intelligibility. We hypothesize that their SPL increase is a significant variable in improvements.
Treatment outcomes were consistent with stimulability (“cueing/prompting”) studies. This was conversational speech and blind transcription. With all its variability in length, complexity, and predictability. There were large standard deviations, yet statistically significant and reliably measured.
Why would increasing SPL increase intelligibility in conversational speech?
- Speech becomes more audible!
- Greater mouth opening and articulatory displacement
- Slower
- More time for speaker to achieve articulatory targets
- More time for listener to process the speech
- Spreading of effects
- Vowel space, facial expression, swallow function
Significance of these findings
These findings provide the first RCT evidence that intensive speech treatment targeting voice improves speech intelligibility in PD. Taken together with the previous gains reported following treatment targeting voice (vocal loudness, voice quality, articulation, intonation, patient reported communication effectiveness, consistent with neural findings (PET and Functional MRI)), there is substantial evidence that this treatment results in meaningful improvements in communication.
These improvements may positively impact health-related quality of life for patients with PD globally. Data from implementation studies in “real world” clinical settings, are consistent with these findings.
Select References
Duffy JR. Motor speech disorders: substrates, differential diagnosis, and management. St. Louis: Mosby, 2019.
Kent RD, ed. Intelligibility in speech disorders: theory, measurement, and management. Amsterdam: John Benjamin Publishing 1992: 119-155.
Miller, N et al. How do I sound to me? Perceived changes in communication in Parkinson’s disease. Clin Rehabil 2008; 22(1);14-22.
Perry, S et al. Behavioral Management of Respiratory/Phonatory Dysfunction for Dysarthria Associated with Neurodegenerative Disease: A systematic Review. Amer J Speech-Language Pathology 2024;33 (2):1069-1097.
Ramig, LO, et al. Speech treatment in Parkinson’s disease: randomized controlled trial (RCT). Mov Disorders 2018; 33:1777-91.
Boutron, I et al. CONSORT statement for randomized trials of nonpharmacologic treatments: a 2017 update and a CONSORT extension for nonpharmacologic trial abstracts. Ann Intern Med 2017; 167: 40-47.
This research was funded in part by the National Institutes of Health-National Institute of Deafness and Other Communication Disorders (NIH-NIDCD) and LSVT Global.
Clinical Trials.gov Identifier: NCT00123084.
All members of this research team have fully disclosed any conflict of interest. The conflict-of-interest management plan has been approved by the Office of Conflict of Interest and Commitment at the University of Colorado-Boulder.
