Kinematics of Loud, Soft, and Whispered Speech.

INTRODUCTION: We examined the effects of four types of voicing on vocal tract movements by comparing the articulatory kinematics of whispered speech to habitual, loud, and soft conditions. METHODS: Participants included 10 males and 10 females with no history of communication disorders. They read six stimulus sentences in habitual, loud, soft, and whispered conditions. An electromagnetic articulograph tracked the tongue, jaw, and lip movements. Analysis focused on the words we do from a longer sentence. Vertical tongue/jaw and horizontal lip movements were measured during the production of the retracted and rounded front and back vowels in we do. RESULTS: Soft speech led to smaller and slower lip movements than in the habitual condition. Displacement increased for the tongue and jaw in loud and whispered speech compared to the habitual condition. Tongue and jaw velocity increased for loud but not for whispered speech compared to the habitual condition. Utterance duration increased for loud and whispered conditions. DISCUSSION: The increasing tongue and jaw displacement and velocity from soft to habitual to loud speech reported here is consistent with previous accounts. Whispering was less intense than soft speech, yet it involved larger tongue and jaw movements than habitual speech, possibly reflecting a speaker's focus on greater articulatory clarity when the acoustic signal is the weakest.

Telling Stories in Noise: The Impact of Background Noises on Spoken Language for People With Aphasia.

PURPOSE: The purpose of this study was to determine how different background noise conditions affect the spoken language of participants with aphasia during a story retell task. METHOD: Participants included 11 adults with mild to moderate aphasia and 11 age- and gender-matched controls. Participants retold stories in a silent baseline and five background noise conditions (conversation, monologue, phone call, cocktail, and pink noise). Dependent measures of speech acoustics (fundamental frequency and mean intensity), speech fluency (speech rate and disfluent words), and language production (correct information units [CIUs], lexical errors, lexical diversity, and cohesive utterances) were compared between groups and across conditions. RESULTS: Background noise resulted in higher fundamental frequency (fo) and increased mean intensity for control participants across all noise conditions but only across some conditions for participants with aphasia. In relation to language production, background noise interfered significantly more with communication efficiency (i.e., percent CIUs) for participants with aphasia than the control group. For participants with aphasia, the phone call condition led to decreased lexical diversity. Across groups, condition effects generally suggested more interference on speech acoustics in conditions where continuous noise was present and more interference on language in conditions that presented continuous informational noise. CONCLUSIONS: Although additional research is needed, preliminary findings suggest that background noise interferes with narrative discourse more for people with aphasia (PWA) than neurologically healthy adults. PWA may benefit from therapy that directly addresses communicating in noise. SUPPLEMENTAL MATERIAL: https://doi.org/10.23641/asha.23681703.

Corticosteroid-LABA inhalers increase phonation threshold pressure (PTP) and flow (PTF) in rabbits.

OBJECTIVES: This study examined the effects of a combination corticosteroid plus long-acting beta2 -adrenergic agonist inhaler (IC) on rabbit phonation. METHODS: White New Zealand male rabbits were assigned randomly to experimental and control groups (n = 11 per group). The experimental group received twice-daily doses of Advair HFA™ (fluticasone propionate 45 mcg and salmeterol 21 mcg) via a veterinary facemask with 1-way valve and spacer; the control group received aerosolized saline. After 8 weeks, animals were euthanized, larynges excised, frozen, and subsequently thawed and mounted on a standard bench apparatus. Phonation was elicited during 15 successive trials, and phonation threshold pressure (PTP; cmH2 O) and flow (PTF; L/min) were quantified. RESULTS: Repeated measures analysis of variance indicated significant differences between the experimental and control groups (p < 0.05). Mean PTP and PTF values were higher (worse) for rabbits that received Advair HFA™. CONCLUSION: Following 8-week exposure to ICs, rabbit larynges required greater air pressure and flow to initiate phonation. Because even modest phonation onset differences can have a meaningful clinical impact on voice function, these findings suggest that LABA ICs may put patients at risk for voice disorders. Furthermore, these voice disorders may occur within a relatively short timeframe. The results from this study have important clinical implications for voice care in those who use ICs. LEVEL OF EVIDENCE: NA Laryngoscope, 133:2680-2686, 2023.

Kinematic and Acoustic Changes to Vowels and Diphthongs in Bite Block Speech.

Purpose We examined the effect of bite block insertion (BBI) on lingual movements and formant frequencies in corner vowel and diphthong production in a sentence context. Method Twenty young adults produced the corner vowels (/u/, /ɑ/, /æ/, /i/) and the diphthong /ɑɪ/ in sentence contexts before and after BBI. An electromagnetic articulograph measured the movements of the tongue back, middle, and front. Results There were significant decreases in the acoustic vowel articulation index and vowel space area following BBI. The kinematic vowel articulation index decreased significantly for the back and middle of the tongue but not for the front. There were no significant acoustic changes post-BBI for the diphthong, other than a longer transition duration. Diphthong kinematic changes after BBI included smaller movements for the back and middle of the tongue, but not the front. Conclusions BBI led to a smaller acoustic working space for the corner vowels. The adjustments made by the front of the tongue were sufficient to compensate for the BBI perturbation in the diphthong, resulting in unchanged formant trajectories. The back and middle of the tongue were likely biomechanically restricted in their displacement by the fixation of the jaw, whereas the tongue front showed greater movement flexibility.

Effects of Background Noise on Speech and Language in Young Adults.

Purpose The aim of this study was to investigate how different types of background noise that differ in their level of linguistic content affect speech acoustics, speech fluency, and language production for young adult speakers when performing a monologue discourse task. Method Forty young adults monologued by responding to open-ended questions in a silent baseline and five background noise conditions (debate, movie dialogue, contemporary music, classical music, and pink noise). Measures related to speech acoustics (intensity and frequency), speech fluency (speech rate, pausing, and disfluencies), and language production (lexical, morphosyntactic, and macrolinguistic structure) were analyzed and compared across conditions. Participants also reported on which conditions they perceived as more distracting. Results All noise conditions resulted in some change to spoken language compared with the silent baseline. Effects on speech acoustics were consistent with expected changes due to the Lombard effect (e.g., increased intensity and fundamental frequency). Effects on speech fluency showed decreased pausing and increased disfluencies. Several background noise conditions also seemed to interfere with language production. Conclusions Findings suggest that young adults present with both compensatory and interference effects when speaking in noise. Several adjustments may facilitate intelligibility when noise is present and help both speaker and listener maintain attention on the production. Other adjustments provide evidence that background noise eliciting linguistic interference has the potential to degrade spoken language even for healthy young adults, because of increased cognitive demands.

Age-Related Changes in Speech and Voice: Spectral and Cepstral Measures.

Purpose This study examined differences in selected acoustic measures of speech and voice according to age and sex and across families. Method Participants included 169 individuals, 79 men and 90 women, from 18 families, ranging in age from 17 to 87 years. Participants reported no history of articulation disorders, stroke or active neurologic disease, or severe-to-profound hearing loss. They read aloud two passages to facilitate examination of the following speech and voice acoustic parameters: fricative spectral moments (center of gravity, standard deviation, skewness, and kurtosis), the proportion of time spent speaking, mean speaking fundamental frequency, semitone standard deviation (STSD), and cepstral peak prominence smoothed. Results The results indicated a significant age effect for fricative spectral center of gravity, spectral skewness, and speaking STSD. There was a significant sex effect for spectral center of gravity, spectral kurtosis, and mean fundamental frequency. Familial relationship was significant for spectral skewness, STSD, and cepstral peak prominence smoothed. Conclusions These findings revealed that certain speech and voice features change with age and some change differently for men and women. Additionally, speakers from the same family units may demonstrate similar patterns for prosody, voicing, and articulatory behavior. The results also demonstrated normal differences in speech and voice variation across age, sex, and family unit. Understanding patterns and differences across these demographic variables in healthy speakers is important to distinguishing more confidently between normal and disordered speech and voice patterns clinically.

Bidirectional Interference Between Simulated Driving and Speaking.

Purpose This study relied on acoustic measures of connected speech and several indices of driving performance to quantify interference between speaking and simulated driving. Method Three groups of 20 younger (ages 20-30 years), middle-age (ages 40-50 years), and older (ages 60-71 years) adults produced monologues and completed a simulated driving task, which involved maintaining a constant speed and lane position on a freeway. Both tasks were completed separately and concurrently. Results There were significant divided attention effects, with a reduced speaking time ratio, and increases in vocal intensity, speed variability, and steering wheel adjustments. There was a significant between-subjects age effect for intensity and fundamental frequency as the younger group had less variation with these variables compared to the other age groups across conditions. There was a significant between-subjects age effect for lane position, steering wheel position, and speed as the younger group had less variation in lane position compared to the other 2 groups, and the older group had more variation in speed and steering wheel position compared to the other 2 groups across the experimental conditions. Conclusion These findings reveal that divided attention conditions can impact both speech and simulated driving performance. The results also shed some light on the effects of age on speech and driving tasks, although the degree of interference from divided attention did not differ by age.

Examining Acoustic and Kinematic Measures of Articulatory Working Space: Effects of Speech Intensity.

Purpose: The purpose of this study was to examine the effect of speech intensity on acoustic and kinematic vowel space measures and conduct a preliminary examination of the relationship between kinematic and acoustic vowel space metrics calculated from continuously sampled lingual marker and formant traces. Method: Young adult speakers produced 3 repetitions of 2 different sentences at 3 different loudness levels. Lingual kinematic and acoustic signals were collected and analyzed. Acoustic and kinematic variants of several vowel space metrics were calculated from the formant frequencies and the position of 2 lingual markers. Traditional metrics included triangular vowel space area and the vowel articulation index. Acoustic and kinematic variants of sentence-level metrics based on the articulatory-acoustic vowel space and the vowel space hull area were also calculated. Results: Both acoustic and kinematic variants of the sentence-level metrics significantly increased with an increase in loudness, whereas no statistically significant differences in traditional vowel-point metrics were observed for either the kinematic or acoustic variants across the 3 loudness conditions. In addition, moderate-to-strong relationships between the acoustic and kinematic variants of the sentence-level vowel space metrics were observed for the majority of participants. Conclusions: These data suggest that both kinematic and acoustic vowel space metrics that reflect the dynamic contributions of both consonant and vowel segments are sensitive to within-speaker changes in articulation associated with manipulations of speech intensity.

Speech Adaptation to Kinematic Recording Sensors: Perceptual and Acoustic Findings.

Purpose: This study used perceptual and acoustic measures to examine the time course of speech adaptation after the attachment of electromagnetic sensor coils to the tongue, lips, and jaw. Method: Twenty native English speakers read aloud stimulus sentences before the attachment of the sensors, immediately after attachment, and again 5, 10, 15, and 20 min later. They read aloud continuously between recordings to encourage adaptation. Sentence recordings were perceptually evaluated by 20 native English listeners, who rated 150 stimuli (which included 31 samples that were repeated to assess rater reliability) using a visual analog scale with the end points labeled as "precise" and "imprecise." Acoustic analysis began by segmenting and measuring the duration of the fricatives /s/ and /ʃ/ as well as the whole sentence. The spectral center of gravity and spectral standard deviation of the 2 fricatives were measured using Praat. These phonetic targets were selected because the standard placement of sensor coils on the lingual surface was anticipated to interfere with normal fricative production, causing them to become distorted. Results: Perceptual ratings revealed a decrease in speech precision after sensor attachment and evidence of adaptation over time; there was little perceptual change beyond the 10-min recording. The spectral center of gravity for /s/ decreased, and the spectral standard deviation for /ʃ/ increased after sensor attachment, but the acoustic measures showed no evidence of adaptation over time. Conclusion: The findings suggest that 10 min may be sufficient time to allow speakers to adapt before experimental data collection with Northern Digital Instruments Wave electromagnetic sensors.

Effects of voice-sparing cricotracheal resection on phonation in women.

INTRODUCTION: Individuals with idiopathic subglottic stenosis (SGS) are at risk for voice disorders prior to and following surgical management. This study examined the nature and severity of voice disorders in patients with SGS before and after a revised cricotracheal resection (CTR) procedure designed to minimize adverse effects on voice function. METHOD: Eleven women with idiopathic SGS provided presurgical and postsurgical audio recordings. Voice Handicap Index (VHI) scores were also collected. Cepstral, signal-to-noise, periodicity, and fundamental frequency (F0 ) analyses were undertaken for connected speech and sustained vowel samples. Listeners made auditory-perceptual ratings of overall quality and monotonicity. RESULTS: Paired samples statistical analyses revealed that mean F0 decreased from 215 Hz (standard deviation [SD] = 40 Hz) to 201 Hz (SD = 65 Hz) following surgery. In general, VHI scores decreased after surgery. Voice disorder severity based on the Cepstral Spectral Index of Dysphonia (KayPentax, Montvale, NJ) for sustained vowels decreased (improved) from 41 (SD = 41) to 25 (SD = 21) points; no change was observed for connected speech. Semitone SD (2.2 semitones) did not change from pre- to posttreatment. Auditory-perceptual ratings demonstrated similar results. CONCLUSION: These preliminary results indicate that this revised CTR procedure is promising in minimizing adverse voice effects while offering a longer-term surgical outcome for SGS. Further research is needed to determine causal factors for pretreatment voice disorders, as well as to optimize treatments in this population. LEVEL OF EVIDENCE: 4. Laryngoscope, 127:2085-2092, 2017.

Exploring the Clinical Utility of Relative Fundamental Frequency as an Objective Measure of Vocal Hyperfunction.

Purpose: Vocal hyperfunction, related to abnormal laryngeal muscle activity, is considered the proximal cause of primary muscle tension dysphonia (pMTD). Relative fundamental frequency (RFF) has been proposed as an objective acoustic marker of vocal hyperfunction. This study examined (a) the ability of RFF to track changes in vocal hyperfunction after treatment for pMTD and (b) the influence of dysphonia severity, among other factors, on the feasibility of RFF computation. Method: RFF calculations and dysphonia severity ratings were derived from pre- and posttreatment recordings from 111 women with pMTD and 20 healthy controls. Three vowel-voiceless consonant-vowel stimuli were analyzed. Results: RFF onset slope consistently varied as a function of group (pMTD vs. controls) and time (pretherapy vs. posttherapy). Significant correlations between RFF onset cycle 1 and dysphonia severity were observed. However, in many samples, RFF could not be computed, and adjusted odds ratios revealed that these unanalyzable data were linked to dysphonia severity, phonetic (vowel-voiceless consonant-vowel) context, and group (pMTD vs. control). Conclusions: RFF onset appears to be sensitive to the presence and degree of suspected vocal hyperfunction before and after therapy. The large number of unanalyzable samples (related especially to dysphonia severity in the pMTD group) represents an important limitation.

Laryngeal Desiccation Challenge and Nebulized Isotonic Saline in Healthy Male Singers and Nonsingers: Effects on Acoustic, Aerodynamic, and Self-Perceived Effort and Dryness Measures.

OBJECTIVES: This study examined the effects of a laryngeal desiccation challenge and nebulized isotonic saline on voice production in young, healthy male singers and nonsingers. STUDY DESIGN: This is a prospective, double-blind, within-subjects experimental design. METHODS: Participants included 10 male university-trained singers and 10 age-matched nonsingers (mean age, 21.8 years; range, 18-26 years) who underwent a 30-minute oral breathing laryngeal desiccation challenge using medical grade dry air (<1% relative humidity) on two occasions in consecutive weeks. After the challenge, participants received either 3 mL or 9 mL of nebulized isotonic saline (0.9% Na+Cl-); order of administration was counterbalanced. Phonation threshold pressure (PTP), the cepstral spectral index of dysphonia (CSID) for sustained vowels and connected speech, and self-perceived vocal effort, mouth dryness, and throat dryness were measured at each recording (baseline, after challenge, and at 5, 35, and 65 minutes after treatment). RESULTS: Self-perceived effort and dryness measures increased (worsened) after desiccation challenge and decreased (improved) after nebulized treatment (P < 0.05). No consistent changes were observed for PTP or CSID over time. Overall, singers demonstrated significantly lower vocal effort and CSID as compared with nonsingers. CONCLUSIONS: Young, vocally healthy men may not experience physiologic changes in voice production associated with laryngeal desiccation and nebulized saline treatments; however, self-reported increases in vocal effort which are associated with dryness symptoms might improve with nebulized treatments. Future hydration research should consider age and sex variables.

Bidirectional Interference Between Speech and Nonspeech Tasks in Younger, Middle-Aged, and Older Adults.

PURPOSE: The purpose of this study was to examine divided attention over a large age range by looking at the effects of 3 nonspeech tasks on concurrent speech motor performance. The nonspeech tasks were designed to facilitate measurement of bidirectional interference, allowing examination of their sensitivity to speech activity. A cross-sectional design was selected to explore possible changes in divided-attention effects associated with age. METHOD: Sixty healthy participants were separated into 3 groups of 20: younger (20s), middle-aged (40s), and older (60s) adults. Each participant completed a speech task (sentence repetitions) once in isolation and once concurrently with each of 3 nonspeech tasks: a semantic-decision linguistic task, a quantitative-comparison cognitive task, and a manual motor task. The nonspeech tasks were also performed in isolation. RESULTS: Data from speech kinematics and nonspeech task performance indicated significant task-specific divided attention interference, with divided attention affecting speech and nonspeech measures in the linguistic and cognitive conditions and affecting speech measures in the manual motor condition. There was also a significant age effect for utterance duration. CONCLUSIONS: The results increase what is known about bidirectional interference between speech and other concurrent tasks as well as age effects on speech motor control.

The effects of emotional expression on vibrato.

OBJECTIVES/HYPOTHESIS: The purpose of this study was to investigate the effect of emotional expression on several acoustic measures of vibrato, including its rate, extent, and steadiness. We hypothesized that singing a passage with emotional content would influence these variables. STUDY DESIGN: This study used a within-subjects, repeated-measures design. Singer performance under different conditions was analyzed. METHODS: Ten graduate student singers (eight women, two men) completed a series of tasks including sustained sung vowels at several pitch and loudness levels, an assigned song that was judged to have relatively neutral emotion, and a personal selection that included passages of intense emotion. Vowel tokens were extracted from the recordings and averaged for each task. Dependent measures included the mean fundamental frequency (F0), mean intensity, frequency modulation (FM) rate, FM extent, and measures of FM rate and extent variability. RESULTS: The FM rate and extent were higher and the modulation variability was lower for the more emotional song than for the sustained vowels. Mean F0 and intensity were higher for the emotional song than for the neutral song. CONCLUSIONS: Singing an emotional passage influences acoustic features of vibrato when compared with isolated, sustained vowels. The wider dynamic and pitch ranges for emotional passages only partly explain vibrato differences between emotional and neutral singing.

Effects of age and syntactic complexity on speech motor performance.

PURPOSE: This study investigated the effect of age on articulatory movement and stability in young, middle-age, and older adults. It also examined the potential influence of linguistic complexity on speech motor control across utterances that differed in their length and grammatical complexity. METHOD: There were 60 participants in 3 age groups: 20-30 years, 40-50 years, and 60-70 years, with equal numbers of men and women in each group. The speakers produced 10 repetitions of 5 different stimuli-each of which included the same bilabial-loaded phrase in different grammatical contexts-while their lip movements were recorded. RESULTS: Participants from the 60-year-old group had significantly longer utterance durations, whereas those from the 20-year-old group had the highest jaw spatiotemporal index (STI) values. There were significant differences in the upper lip STI, displacement, and velocity as well as in vocal intensity for the longer, complex conditions compared with the shorter, phrase-only task. Overall, the differences in performance were minimal across grammatical complexity levels that were equal in length. CONCLUSION: These findings suggest that speech motor control matures beyond young adulthood and that linguistic complexity in a repetitive task does not appear to have a consistent effect on measures of speech movement.

The effects of practice on the concurrent performance of a speech and postural task in persons with Parkinson disease and healthy controls.

Purpose. Persons with Parkinson disease (PD) demonstrate deficits in motor learning as well as bidirectional interference (the performance of one task concurrently interferes with the performance of another task) during dual-task performance. Few studies have examined the practice dosages necessary for behavioral change in rehabilitation relevant tasks. Therefore, to compare the effects of age and PD on motor learning during dual-task performance, this pilot study examined persons with PD as well as neurologically healthy participants during concurrent performance of postural and speaking tasks. Methods. Seven persons with PD and 7 healthy age-matched and 10 healthy young control subjects were tested in a motion capture facility. Task performances were performed concurrently and recorded during 3 time periods (acquisition (beginning and ending), 48-hour retention, and 1-week retention). Postural control and speech articulatory acoustic variables were measured. Results. Healthy young participants consistently performed better than other groups on all measured postural and speech variables. Healthy young participants showed decreased variability at retention, while persons with PD and healthy age-matched controls were unable to consistently improve their performance as a result of practice. No changes were noted in the speech variables. Conclusion. The lack of consistent changes in motor performance in any of the tasks, except in the healthy young group, suggests a decreased efficiency of motor learning in the age-matched and PD groups and argues for increased practice dosages during balance training.

Frequency response of synthetic vocal fold models with linear and nonlinear material properties.

PURPOSE: The purpose of this study was to create synthetic vocal fold models with nonlinear stress-strain properties and to investigate the effect of linear versus nonlinear material properties on fundamental frequency (F0) during anterior-posterior stretching. METHOD: Three materially linear and 3 materially nonlinear models were created and stretched up to 10 mm in 1-mm increments. Phonation onset pressure (Pon) and F0 at Pon were recorded for each length. Measurements were repeated as the models were relaxed in 1-mm increments back to their resting lengths, and tensile tests were conducted to determine the stress-strain responses of linear versus nonlinear models. RESULTS: Nonlinear models demonstrated a more substantial frequency response than did linear models and a more predictable pattern of F0 increase with respect to increasing length (although range was inconsistent across models). Pon generally increased with increasing vocal fold length for nonlinear models, whereas for linear models, Pon decreased with increasing length. CONCLUSION: Nonlinear synthetic models appear to more accurately represent the human vocal folds than do linear models, especially with respect to F0 response.

A preliminary report on disordered speech with deep brain stimulation in individuals with Parkinson's disease.

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) has proven effective in treating the major motor symptoms of advanced Parkinson's disease (PD). The aim of this study was to learn which laryngeal and articulatory acoustic features changed in patients who were reported to have worse speech with stimulation. Six volunteers with PD who had bilateral STN electrodes were recorded with DBS turned on or off. Perceptual ratings reflected poorer speech performance with DBS on. Acoustic measures of articulation (corner vowel formants, diphthong slopes, and a spirantization index) and phonation (perturbation, long-term average spectrum) as well as verbal fluency scores showed mixed results with DBS. Some speakers improved while others became worse on individual measures. The magnitude of DBS effects was not predictable based on the patients' demographic characteristics. Future research involving adjustments to stimulator settings or electrode placement may be beneficial in limiting the negative effects of DBS on speech.

The acoustic effects of vowel equalization training in singers.

Vowel equalization is a technique that can be used by singers to achieve a more balanced vocal resonance, or chiaroscuro, by balancing corresponding front and back vowels, which share approximate tongue heights, and also high and low vowels by means of a more neutral or centralized lingual posture. The goal of this single group study was to quantify acoustic changes in vowels after a brief training session in vowel equalization. Fifteen young adults with amateur singing experience sang a passage and sustained isolated vowels both before and after a 15-minute training session in vowel equalization. The first two formants of the target vowels /e, i, ɑ, o, u/ were measured from microphone recordings. An analysis of variance was used to test for changes in formant values after the training session. These formant values mostly changed in a manner reflective of a more central tongue posture. For the sustained vowels, all formant changes suggested a more neutral tongue position after the training session. The vowels in the singing passage mostly changed in the expected direction, with exceptions possibly attributable to coarticulation. The changes in the vowel formants indicated that even a brief training session can result in significant changes in vowel acoustics. Further work to explore the perceptual consequences of vowel equalization is warranted.

Bidirectional interference between speech and postural stability in individuals with Parkinson's disease.

Because people frequently talk while engaged in other activities, and because Parkinson's disease (PD) is known to diminish multi-tasking performance, this study examined dual task interference between speaking and postural stability in nine individuals with PD, seven age-matched, and 10 healthy young controls. Participants repeated a target utterance and performed a rise to toes task in both single and dual task conditions. Diphthong transitions were measured from audio recordings and postural variables reflecting planning, coordination, and stability were derived from a multi-camera motion capture system and force plate recordings. Thus, sensitive measures of both speech and postural control were obtained. The group with PD performed more poorly than both control groups for the isolated postural task, but their single task speech measures did not differ from the controls, in spite of listener ratings which indicated mild-to-moderate dysarthria severity. The group with PD showed evidence of bidirectional dual task interference in that there were reduced diphthong extents and slopes along with smaller, slower, and less stable postural movements. These results indicate that concurrent performance of speech and a challenging postural control task impairs speech and postural stability in persons with PD and may result in greater risk during daily activities.