Perception of Vocal Tremor During Sustained Phonation Compared With Sentence Context *Amy Lederle, *,†Julie Barkmeier-Kraemer, and ‡Eileen Finnegan, *Tucson, Arizona, ySacramento, California, and zIowa City, Iowa Summary: Objectives/Hypothesis. Vocal tremor is an acoustical phenomenon characterized by relatively peri- odic modulations in fundamental frequency and intensity. Although vocal tremor is considered easier to perceive during sustained phonation than during connected speech, systematic comparison between these speech contexts has not been investigated. This investigation compared vocal tremor perception during sustained phonation and connected speech contexts. Study Design. This is a prospective, controlled study with randomized conditions. Methods. Audio recordings from five speakers diagnosed with essential vocal tremor were used for this study. Twenty-four naive adult listeners rated the overall severity (ie, aberrance) of the voice and the degree of shakiness (ie, tremor) during sustained phonation of /i/. A different group of 21 naive adult listeners rated sentence stimuli con- sisting of two different sentences, one loaded with voiced and the other with voiceless speech sounds. Results. All speakers were rated by listeners to have similar levels of shakiness and overall severity during sustained phonation. However, significantly higher levels of shakiness and overall severity were perceived during sustained pho- nation than during sentence context. A nonsignificant trend was shown for higher average ratings for shakiness and over- all severity on voice-loaded compared with voiceless-loaded sentences. Conclusions. This study demonstrated that vocal tremor is perceived as significantly more severe during sustained phonation than during connected speech. More importantly, individual speakers differed in their ability to reduce vocal tremor perception during connected speech. Thus, sustained phonation does not necessarily offer a valid estimation of the impact of a vocal tremor on an individual’s connected speech. Key Words: Vocal tremor–Essential vocal tremor–Perceptual–Speech context. INTRODUCTION Vocal tremor is an acoustical event that can be defined by the presence of nearly periodic modulations of fundamental fre- quency (F0) and intensity in the voice. Perhaps, the most fasci- nating aspect of vocal tremor is that a predictable acoustic output occurs even when different structures within the speech mechanism exhibit tremor out of phase from each other. 1 Both perceptual and acoustic features identified during sustained phonation are used to characterize clinical signs of vocal tremor. This clinical evaluation method stems from earlier re- ports that vocal tremor is most readily detected during sustained phonation compared with speaking contexts with shorter voic- ing durations such as occurs during connected speech. 2 The bulk of research investigating vocal tremor has focused on us- ing sustained phonation to study its acoustic patterns. However, it remains unknown how well information regarding an individ- ual’s vocal tremor during sustained phonation applies to its impact on their connected speech. The most common etiology associated with vocal tremor is essential tremor. 3,4 Essential tremor is one of the most common movement disorders, affecting approximately 4–5.6% of peo- ple over the age of 40 years. 5 Essential tremor is characterized by a postural and action tremor with a frequency of 4–12 Hz that typically involves the upper extremities. 5–17 Tremor occurs during volitional movements or sustained posture and is often progressive, resulting in spread to additional sites such as the head and abdomen after its initial onset. 5–17 In addition, the am- plitude of the tremor may increase over time. Although both sexes are equally affected, head tremor is typically more severe in women. 16 Approximately, 30% of individuals with essential tremor ex- hibit vocal tremor with most being female. 3 Vocal tremor as a result of essential tremor is considered to be an action- induced tremor occurring during speech. Perception of vocal tremor is characterized by rhythmic modulations of pitch and loudness that correspond with acoustic modulations of F0 and intensity ranging from 3 to 12 Hz. 2,18 Acoustic patterns associated with vocal tremor are consid- ered the result of involuntary muscle contractions resulting in periodic modulations of structures within the speech mecha- nism. The affected structures may reside anywhere within the speech mechanism including respiratory, phonatory, and artic- ulatory subsystems. 19–23 Recent research has demonstrated that affected musculature within the speech mechanism does not necessarily modulate synchronously to produce vocal tremor. 1 Finnegan et al 1 demonstrated that laryngeal muscula- ture in individuals with vocal tremor did not synchronously modulate tremor contractions. Rather, timing differences in the onset and rate of tremor patterns associated with affected la- ryngeal musculature occurred. Thus, different laryngeal muscle activation patterns within the same organ are associated with the characteristic acoustic output perceived as vocal tremor. Accepted for publication November 2, 2011. From the *Department of Speech, Language, Hearing Sciences, University of Arizona, Tucson, Arizona; yDepartment of Otolaryngology, University of California – Davis, Sacramento, California; and the zDepartment of Speech Pathology and Audiology, The University of Iowa, Iowa City, Iowa. Address correspondence and reprint requests to Julie Barkmeier-Kraemer, Department of Otolaryngology, The University of California – Davis, 2521 Stockton Blvd, Ste 6200, Sacramento, CA 956817. E-mail: [email protected]Journal of Voice, Vol. 26, No. 5, pp. 668.e1-668.e9 0892-1997/$36.00 Ó 2012 The Voice Foundation doi:10.1016/j.jvoice.2011.11.001
Transcript
Perception of Vocal Tremor During Sustained
Phonation Compared With Sentence Context
*Amy Lederle, *,†Julie Barkmeier-Kraemer, and ‡Eileen Finnegan, *Tucson, Arizona, ySacramento, California,and zIowa City, Iowa
Summary: Objectives/Hypothesis. Vocal tremor is an acoustical phenomenon characterized by relatively peri-
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odic modulations in fundamental frequency and intensity. Although vocal tremor is considered easier to perceive duringsustained phonation than during connected speech, systematic comparison between these speech contexts has not beeninvestigated. This investigation compared vocal tremor perception during sustained phonation and connected speechcontexts.Study Design. This is a prospective, controlled study with randomized conditions.Methods. Audio recordings from five speakers diagnosed with essential vocal tremor were used for this study.Twenty-four naive adult listeners rated the overall severity (ie, aberrance) of the voice and the degree of shakiness(ie, tremor) during sustained phonation of /i/. A different group of 21 naive adult listeners rated sentence stimuli con-sisting of two different sentences, one loaded with voiced and the other with voiceless speech sounds.Results. All speakers were rated by listeners to have similar levels of shakiness and overall severity during sustainedphonation. However, significantly higher levels of shakiness and overall severity were perceived during sustained pho-nation than during sentence context. A nonsignificant trend was shown for higher average ratings for shakiness and over-all severity on voice-loaded compared with voiceless-loaded sentences.Conclusions. This study demonstrated that vocal tremor is perceived as significantly more severe during sustainedphonation than during connected speech. More importantly, individual speakers differed in their ability to reduce vocaltremor perception during connected speech. Thus, sustained phonation does not necessarily offer a valid estimation ofthe impact of a vocal tremor on an individual’s connected speech.Key Words: Vocal tremor–Essential vocal tremor–Perceptual–Speech context.
INTRODUCTION
Vocal tremor is an acoustical event that can be defined by thepresence of nearly periodic modulations of fundamental fre-quency (F0) and intensity in the voice. Perhaps, the most fasci-nating aspect of vocal tremor is that a predictable acousticoutput occurs even when different structures within the speechmechanism exhibit tremor out of phase from each other.1 Bothperceptual and acoustic features identified during sustainedphonation are used to characterize clinical signs of vocaltremor. This clinical evaluation method stems from earlier re-ports that vocal tremor is most readily detected during sustainedphonation compared with speaking contexts with shorter voic-ing durations such as occurs during connected speech.2 Thebulk of research investigating vocal tremor has focused on us-ing sustained phonation to study its acoustic patterns. However,it remains unknown how well information regarding an individ-ual’s vocal tremor during sustained phonation applies to itsimpact on their connected speech.
The most common etiology associated with vocal tremor isessential tremor.3,4 Essential tremor is one of the most commonmovement disorders, affecting approximately 4–5.6% of peo-
ted for publication November 2, 2011.he *Department of Speech, Language, Hearing Sciences, University of Arizona,Arizona; yDepartment of Otolaryngology, University of California – Davis,nto, California; and the zDepartment of Speech Pathology and Audiology, Thety of Iowa, Iowa City, Iowa.ss correspondence and reprint requests to Julie Barkmeier-Kraemer, Departmentyngology, The University of California – Davis, 2521 Stockton Blvd, Ste 6200,nto, CA 956817. E-mail: [email protected] of Voice, Vol. 26, No. 5, pp. 668.e1-668.e9997/$36.002 The Voice Foundation.1016/j.jvoice.2011.11.001
ple over the age of 40 years.5 Essential tremor is characterizedby a postural and action tremor with a frequency of 4–12 Hzthat typically involves the upper extremities.5–17 Tremor occursduring volitional movements or sustained posture and is oftenprogressive, resulting in spread to additional sites such as thehead and abdomen after its initial onset.5–17 In addition, the am-plitude of the tremor may increase over time. Although bothsexes are equally affected, head tremor is typically more severein women.16
Approximately, 30% of individuals with essential tremor ex-hibit vocal tremor with most being female.3 Vocal tremor asa result of essential tremor is considered to be an action-induced tremor occurring during speech. Perception of vocaltremor is characterized by rhythmic modulations of pitch andloudness that correspond with acoustic modulations of F0 andintensity ranging from 3 to 12 Hz.2,18
Acoustic patterns associated with vocal tremor are consid-ered the result of involuntary muscle contractions resulting inperiodic modulations of structures within the speech mecha-nism. The affected structures may reside anywhere within thespeech mechanism including respiratory, phonatory, and artic-ulatory subsystems.19–23 Recent research has demonstratedthat affected musculature within the speech mechanism doesnot necessarily modulate synchronously to produce vocaltremor.1 Finnegan et al1 demonstrated that laryngeal muscula-ture in individuals with vocal tremor did not synchronouslymodulate tremor contractions. Rather, timing differences inthe onset and rate of tremor patterns associated with affected la-ryngeal musculature occurred. Thus, different laryngeal muscleactivation patterns within the same organ are associated withthe characteristic acoustic output perceived as vocal tremor.
Amy Lederle, et al Vocal Tremor Perception and Speech Context 668.e2
The impact of vocal tremor on communication in affected in-dividuals is significant. Investigation of patient reports of vocaltremor impact show that this disorder is associated with in-creased communication difficulty that eventually leads to re-duced social participation. Individuals with vocal tremorcommonly report symptoms of increased effort when speaking3
and increased tremor severity with anxiety.24,25 Speaking tasksthat require greater effort and evoke anxiety (eg, speaking ona telephone or at a conference) are often difficult for peoplewith vocal tremor.25 In addition, individuals with vocal tremoroften exhibit increased intensity, strained voice quality, andslower rate of speaking than normal.26 Adjustments to theirspeaking patterns may result from individual attempts to com-pensate for the increased sense of effort and ongoing vocalperturbation experienced during speaking.
To date, few treatment methods have demonstrated beneficialoutcomes in managing vocal tremor. The pharmaceutical orsurgical management approaches currently used to treat tremoroffer mixed outcomes in their benefit to individuals with vocaltremor. For example, pharmaceutical management using sys-temic medications (eg, propranolol, primidone) have been re-ported to offer a success rate of 25–40% when treating vocaltremor due to essential tremor.27,28 In contrast, a more localizedapplication of pharmaceutical treatment via Botulinum toxin(ie, Botox Allergen, Inc., Irvine, CA) injection has been re-ported as beneficial in reducing symptoms in as many as 80%of individuals with vocal tremor involving the larynx.3,22,29–33
Although generally beneficial in reducing vocal tremor and ef-fort levels during speaking, Botox injections have also been as-sociated with excessive breathiness and impaired swallowingfor up to 6 weeks following treatment in some individ-uals.22,29,32,33 These individuals may consequently choose todiscontinue Botox treatment.
There are only two reports of surgical treatment using deepbrain stimulation (DBS) to manage limb tremor that was alsoreported to benefit vocal tremor.34,35 However, the paucity ofliterature associated with DBS and vocal tremor may reflectthe primary goal of this management approach; to regain mobil-ity or improved control of limbs and hands.
Unlike tremor affecting the limbs, little attention has been fo-cused on the use of behavioral management for vocal tremor.Two studies have reported benefit to individuals with vocaltremor using management of speaking patterns.36,37 The earli-est of these studies did not offer details regarding speech treat-ment methods used to benefit one individual with vocaltremor.37 A recent case study reported on the development ofa speech treatment program that was successful in reducingthe perceived severity of vocal tremor in a 55-year-old femalediagnosed with essential vocal tremor (EVT).36 The speechtreatment program was based on observed changes in worsen-ing or lessening of the individual’s vocal tremor at differentpitches and intensities, and maladaptive speaking patternssuch as prolongation of vowel sounds and slowed rate of speechduring conversation. Modifications in voicing duration, speak-ing rate, pitch inflections, and degree of breathiness duringconnected speech were targeted. Improvements were found inpre- to posttreatment perception of vocal tremor by naive
listeners blind to speech sample condition. In addition, the cli-ent rated her overall satisfaction with the treatment as favorable.Thus, manipulation of connected speech patterns can be helpfulin reducing the perception of vocal tremor, although investiga-tion of this treatment approach with a larger group of individ-uals with vocal tremor is needed to reach a definitiveconclusion.
To date, comparison of vocal tremor patterns duringsustained phonation and connected speech contexts has beenlimited to informal clinical observations or case-based descrip-tion.2,36,38 However, a systematic comparison of speech con-texts is imperative for contributing to evaluative methods usedto determine the presence/absence, nature, and severity of vocaltremor in individuals presenting with communication impair-ment.39 At present, clinical evaluation of vocal tremor is typi-cally characterized during sustained phonation withoutcomparable description during connected speech. Although vo-cal tremor has been demonstrated to be more prominent duringprolongation of voiced sounds,2,38 consideration as to the sever-ity of a vocal tremormay also require consideration of its impacton connected speech. For example, individuals with vocaltremor have been reported to exhibit slower speaking rate andprolongation of vowels as an adaptation to the cyclic perturba-tion of vocal tremor.26 Such changes in speaking patterns maygive greater opportunity for listeners to hear the presence ofvocal tremor during connected speaking. In contrast, vocaltremor has been described as less evident during productionof connected speech at normal rate with a typical distributionof voiced and voiceless speech sounds.2,38 Such a differencecould relate to the rapid onset and offset of the voice during con-nected speech at a faster rate than the slower modulation rate ofthe vocal tremor. That is, the vocal tremor cycle may be disrup-ted more frequently during connected speech making it lessevident than occurs during sustained phonation. What remainsunknown is whether evaluation of vocal tremor during sustainedphonation can be generalized to the impact of vocal tremor onconnected speaking.
The purpose of this study was to compare the perception ofvocal tremor between different speech contexts. It was hypoth-esized that the perception of vocal tremor would be associatedwith more severe ratings during sustained phonation than dur-ing connected speech. In addition, it was hypothesized thatvocal tremor would be associated with more severe ratings dur-ing a sentence loaded with all-voiced compared with voicelessspeech sounds.
METHODS
Participants
This study was approved by the Institutional Review Board atthe University of Arizona.
Naive listeners. The primary language was English for allnaive listeners. Listeners denied a history of speech, language,and/or hearing disorders, and recent illness that might affectoverall listening performance (eg, ear infections). In addition,all participants passed a pure-tone auditory threshold screeningbilaterally, screened via headphones at 25 decibels at 1000,
Journal of Voice, Vol. 26, No. 5, 2012668.e3
2000, and 4000 Hz. Most of the participants denied experiencelistening to individuals with vocal tremor. At most, listeners re-ported having exposure to short clips of vocal tremor as part ofa class lecture. All participants were undergraduate and gradu-ate students at the University of Arizona except for onewho hadno college education. Two groups of listeners participated inthis study. The first group included 24 naive, adult listeners be-tween the ages of 20 and 30 years, with an average of 22 yearsof age (2 males, 22 females). This group of listeners judged sus-tained phonation speech samples only (see Section SpeechStimuli). The second group of participants included 21 naive,adult listeners between the ages of 19 and 58 years, with anaverage of 27 years of age (4 males, 17 females). This groupperformed ratings on the sentence stimuli only (see SectionSpeech Stimuli).
Expert listeners. Three speech-language pathologists withan average of 26 years of experience ranging from 17 to 34years assessing and treating voice disorders served as expert lis-teners. These expert listeners were not affiliated with the studyand were naive to the hypotheses of the investigation.
Speakers. Speakers included five individuals with vocaltremor associated with a diagnosis of essential tremor. De-identified recordings of these five speakers were used for bothsustained phonation and connected speech ratings. Speakerswith vocal tremor were all diagnosed with essential tremor bya neurologist with expertise in neuromotor disorders at the Uni-versity of IowaHospitals andClinics. None of the speakers werediagnosed with a primary EVT. Vocal tremor was diagnosedbased on findings from a voice evaluation consisting of acoustic,aerodynamic, videoendoscopic, and electromyographic testingby speech-language pathology and otolaryngology.
De-identified recordings of these vocal tremor speakers wereobtained from the Department of Speech Pathology and Audi-ology at the University of Iowa. Perceptual characteristics ofthese speakers were judged by the expert listeners. The expertlisteners were asked to assign a severity rating (ie, mild, mod-erate, or severe) to each sustained phonation produced by thefive speakers with vocal tremor. All were in agreement thateach of the five speakers exhibited a moderate to severe vocaltremor.
Speech stimuli recording methods
Speaker recordings consisted of sustained phonation of /i/ andproduction of two sentences, characterized by having predom-inantly voiced versus voiceless speech sounds. The voice-loaded sentence was, ‘‘We were away a year ago,’’ and ‘‘Poptook his socks off’’ was the voiceless-loaded sentence.
All speakers were instructed to produce the vowel and senten-ces at comfortable pitch and loudness levels in their typicalspeaking voice. Each vowel and sentence was repeated threedifferent times, for a total of nine recordings per speaker. All vo-cal tremor speech stimuli were recorded as part of a standardclinical evaluation at the University of Iowa Hospitals andClinics, Department of Otolaryngology—Head and Neck Can-cer using a professional grade dynamic microphone placed12 cm directly in front of the mouth in a quiet room.
The speech signals were amplified and recorded using a dig-ital tape recorder (PC108-M; Sony Corporation of America,New York, NY). Signals were not recorded in a soundproofbooth, so some background noise was present (ie, due to com-puters, EMG instrumentation, and fluorescent lights) in the re-cordings. The sound pressure level of all speech recordings wasnormalized using a flat C scale to minimize differences betweensamples. All speech stimuli were presented to listeners between78 and 85 dB sound pressure level (SPL).
Speech stimuli characteristics
Acoustic analyses were completed using PRAAT software(version 5.2.35, 2011)40 to characterize each speaker’s vocaltremor during one representative sustained phonation andconnected speech patterns. The rate and magnitude of eachspeaker’s tremor was determined from a one-second segmentof the midportion of each sustained vowel that exhibited cy-clic modulation of the acoustic signal. The number, or rate,of modulation cycles was counted within each selected one-second segment. The latter was determined by counting thenumber of cycles of modulation in F0 or intensity using a ‘‘val-ley-to-valley’’ or ‘‘peak-to-peak’’ method (see Figure 1). Themaximum and minimum values of F0 and relative intensityduring each modulation cycle were also recorded and usedto determine magnitude, or extent, of modulation in the F0and intensity domains. The relative extent of each F0 modula-tion was calculated using the formula (F0max� F0min)/(F0max + F0min)3 100. Relative intensity extent was deter-mined after converting SPL values into a linear unit, Pascals.Thereafter, the same formula for determining the relative ex-tent of intensity was used: (Amplitudemax�Amplitude min)/(Amplitudemax + Amplitudemin) 3 100 (see Table 1).Measures of articulation rate and average voicing duration
during connected speech were calculated for each sentencetype (voice loaded and voiceless loaded) and speaker usingthe same methods described in a prior publication.36 Articula-tion rate was determined by calculating the number of syllablesspoken per second for each sentence type. The duration ofvoiced segments corresponding with the audible voiced soundswithin a word or contiguous production of a sequence of voicedsounds for each sentence type was determined using a combina-tion of visual and auditory methods. First, onset of each voicedsegment was determined by visually inspecting an acousticwaveform of each sentence. Next, each onset and offset seg-ment was played aloud to determine the point at which the pe-riodic waveform was associated with perceptible sound. Oncethe visual and audible onset and offset of each segment of voic-ing within each sentence type was determined, the duration ofeach segment was measured. The duration of these segmentswas then averaged for each sentence type. Acoustic measuresfor each sentence type and speaker are provided in Table 2.
Listener experimental procedures
Listening training and experimental sessions. Individ-ual participants were seated in a sound-treated booth to com-plete the listening task. Stimuli were presented to listenersusing a laptop computer via the Alvin software program
FIGURE 1. Sustained phonation segments used to characterize vocal tremor modulation of F0 and relative intensity for each speaker. The down-
ward facing arrows indicate the beginning or end of each intensity modulation cycle. The upward facing arrows indicate the beginning or end of each
F0 modulation cycle.
Amy Lederle, et al Vocal Tremor Perception and Speech Context 668.e4
(version 1.27, 2007) as an interface.41 Stimuli were presentedvia headphones (Sennheiser HD 265 linear) at a comfortablelistening level.
Each listener completed a training session prior to the exper-imental portion of the listening session. The training session en-tailed a detailed explanation of the study’s procedure includingoperational definitions of the two dependent variables being
TABLE 1.
Sustained Phonation Measures of Vocal Tremor Rate and Mag
Speaker
Rate of F0
Modulation (Hz)
F0 Max
(Hz)
F0 Min
(Hz)
Average
Extent
F0 (%)
Rat
(A
Mo
1 4 239.4 172.6 16
2 5 208.3 161.6 12
3 3 235.7 200.3 8
4 4.5 198.2 147.3 15
5 5 145 109.4 12
rated, shakiness and overall severity of voice aberrance, andpractice using the Alvin software interface to rate nonexperi-mental speaker audio recordings. The training session alsoserved to familiarize listeners with speakers ranging from nor-mal to having obvious vocal tremor using speech stimuli similarto the experimental listening session. Practice ratings were con-ducted using the same protocol as the experimental listening
nitude Across Speakers
e of Intensity
mplitude)
dulation (Hz)
Average
Amplitude
Max (Pascals)
Average
Amplitude
Min (Pascals)
Average
Intensity
Extent (%)
4 0.34 0.18 31
5 0.27 0.14 37
3 0.26 0.04 71
5 0.28 0.19 19
5 0.31 0.21 19
TABLE 2.
Average Measures of Articulation Rate and Voicing
Duration for Each Sentence Type Across Speakers
Speaker Sentence Type
Average
Voicing
Duration (s)
Articulation
Rate
(Syllables/s)
1 Voice loaded 0.15 3.3
Voiceless loaded 0.11 1.7
2 Voice loaded 0.34 3.3
Voiceless loaded 0.16 2.8
3 Voice loaded 0.86 4.4
Voiceless loaded 0.13 3.3
4 Voice loaded 0.87 2.9
Voiceless loaded 0.14 2.2
5 Voice loaded 0.4 2.8
Voiceless loaded 0.19 2.3
Journal of Voice, Vol. 26, No. 5, 2012668.e5
session (described later) using nonexperimental speech stimuli.Training session speech stimuli included audio recordings ofindividuals with vocal tremor and a normal control speaker pro-ducing a prolonged vowel and voiceless-loaded or all-voicedsentences. The P.I. (AL) conducted the training session and pro-vided feedback to listeners regarding their ratings of the firsttwo training session speech stimuli. Feedback was no longerprovided beyond the first two training session items as partici-pants accurately verbalized that they detected the presence ofvocal tremor in nonexperimental speakers with vocal tremor.
Next, experimental recordings were presented to listeners inrandomized order, including two repetitions of each stimulus sothat intrarater reliability could be determined. The randomizedsequence of stimuli differed for each listener and was generatedby the Alvin software program. A different group of listenerswas used to judge each speech context of recorded utterancefor speakers. The first group of naive listeners judged 30 sus-tained phonation samples (3 trials of /i/3 5 speakers3 2 pre-sentations). The second group of naive listeners rated 60sentence stimuli (2 sentence types3 3 trials3 5 speakers3 2presentations).
Perceptual rating procedures. Listeners were asked to rateeach speech stimulus on two visual analog scales with anchorpoints at zero and 10, displayed one above another on a com-puter screen. Listeners were able to listen to speech stimuli asmany times as needed or return to prior stimuli previously ratedduring the experiment. Listeners were asked to rate overall se-verity using the instructions, ‘‘How abnormal does the voicesound to you?’’ They were asked to rate shakiness using the in-structions, ‘‘How unsteady does the voice sound to you duringspeaking?’’ The first rating parameter, overall severity, is typi-cally used by several perceptual rating instruments such as theConsensus Auditory-Perceptual Evaluation of Voice (CAPE-V)42 and Grade, Roughness, Breathiness, Asthenia, Strain Scale(GRBAS).43 The second rating parameter, shakiness, was de-rived from an unpublished preliminary study on vocal tremor.In the latter study, 70 naive listeners were asked to use theirown words to describe the voice quality of speakers with vocaltremor. Every naive listener used the term, ‘‘shaky,’’ as one of
their self-generated terms describing vocal tremor. Thus, thisterm was used in the present study to assure that naive listenerswould attend to the vocal tremor component of the audio signalsby rating shakiness.
Experimental measures. The dependent measures in thisstudy consisted of listener’s visual analog perceptual ratingsof overall severity and shakiness. Overall severity was definedas the degree of abnormality or aberrance of the voice, in gen-eral, perceived by the listener. Shakiness was defined as the de-gree of perceived unsteadiness of the voice, or vocal tremor.Listeners were instructed to select a point along the visual ana-log scale for each of the two dependent measures (ie, shakinessand overall severity) using the Alvin software program inter-face. The location of the point selected on each visual analogscale was then converted by the Alvin software program intoa whole number between 0 and 10 and saved. Ratings were ob-tained from listeners twice for each speech context (ie, vowel vsvoice-loaded and voiceless-loaded sentence context) and foreach of the three trial productions per speech production.
Statistical analysis
The P.I. (AL) and supervising scientist (J.M.B.-K.) reachedconsensus on all sustained phonation and sentence acousticmeasures that characterized each speakers’ vocal tremor andconnected speech patterns. A multiple factor mixed analysisof variance was completed for the two dependent variables,overall severity and shakiness ratings. The independent vari-ables for this analysis included speaker (n¼ 5), speech context(ie, sustained phonation, voice-loaded sentence, and voiceless-loaded sentence stimuli) (n¼ 3), trial (n¼ 3), and repetition(n¼ 2). The variable, repetition, was used to determine listenerintrarater reliability on ratings of overall severity and shakiness.Interrater reliability was not determined as listeners completedthe experimental listening task on one occasion. An alpha levelof P < 0.05 was used as the criterion level for significance.
RESULTS
Overall severity ratings
The results for ratings of overall severity demonstrated a signif-icant within-subject main effect for the independent variable,speaker (F(3.3,207.8)¼ 141.4, P < 0.0001). Significant inter-actions were also demonstrated for trial3 speech context(F(3.7,117.3)¼ 6.2, P < 0.0001) and for speaker3 trial3speech context (F(2.9,402.9)¼ 12.8, P < 0.0001). The three-way and two-way interactions appeared linked to differentspeakers showing ranges of ratings across trials for each speechcontext. However, all but two speakers exhibited a small differ-ence in average ratings across three trials (ie, 0.2–0.8). Twospeakers showed a larger difference between average ratingsacross three trials (ie, 1 and 1.4). Speaker 5 exhibited an averagerating range across three trials from 7.4 to 8.4 on the sustainedphonation speech context. Speaker 4 exhibited an average rat-ing range across three trials from 2.8 to 4.2. Thus, the interac-tion of speaker3 trial3 speech context did not appear linkedto an overall pattern other than two individuals showed greatervariability on two different speech contexts than the other
FIGURE 3. Average overall severity ratings for each of the three dif-
ferent speech contexts.
Amy Lederle, et al Vocal Tremor Perception and Speech Context 668.e6
speakers. A similar conclusion was made for the two-way inter-action of trial3 speech context. Figure 2 shows the averageoverall severity ratings for each speaker and speech context.The general trend in overall severity ratings fit the hypothesizedpattern that sustained phonation would receive the most severeratings followed by voice-loaded and voiceless-loaded senten-ces, respectively. One speaker (Speaker 4) exhibited the oppo-site pattern from that predicted.
A significant between-subjects effect was found for speechcontext (F(2,63)¼ 20, P < 0.0001). Post hoc testing showedsignificant differences between sustained phonation context(average rating of 7.6 (±2.4)) and each of the different sentencecontexts (average rating of 5.4 (±3.1) and 4.5 (±3.3) on thevoice-loaded and voiceless-loaded sentences, respectively).However, there was no significant difference, on average, be-tween overall severity ratings between the two sentence types(see Figure 3).
Shakiness ratings
The results for shakiness ratings showed one within-subjectsignificant main effect for speaker (F(3.1,194.7)¼ 96,P < 0.0001). Several interactions were also found to be sig-nificant. Three two-way interactions were found significant:speaker3 speech context (F(6.2,194.7)¼ 30, P < 0.0001),trial3 speech context (F(4,124.8)¼ 7.3, P < 0.0001), andspeaker3 trial (F(5.8,368)¼ 8, P < 0.0001). In addition, twothree-way interactions were significant: speaker3 repetition3 speech context (F(7.1,222.3)¼ 3, P¼ 0.005) and speaker3trial3 speech context (F(11.7,368)¼ 4.6, P < 0.0001).
The interaction between speaker and other variables appearslinked to significantly different ratings of shakiness acrossspeakers, depending on the speech context. As shown in
FIGURE 2. Average overall severity ratings for each speaker and
speech context.
Figure 4, the sustained vowel context showed the least variabil-ity in shakiness ratings between speakers and trials. In contrast,the voice-loaded and voiceless-loaded sentence contexts dem-onstrate a range of shakiness ratings across speakers and trials.The significant interaction found for the variable, repetition (ie,intrarater reliability between stimulus ratings during the firstpresentation compared with the second presentation), mayhave related to Speakers 1 through 4 being rated worse, on av-erage, during the second presentation of the voiceless sentencecontext than on the first. However, the average rating on the firstand second speaker stimulus presentation, overall, was notfound to be significantly different. Thus, post hoc pair-wisecomparisons were completed to test the significant interactionfound between speech context and speaker using an alphacriterion of P < 0.05. Outcomes demonstrated that shakinessratings for the sustained phonation context were significantlyhigher than both sentence types. However, the two sentencetypes were not significantly different from each other (seeFigure 5). The average rating across all speakers during sus-tained phonation context was 8.3 (±1.7) compared with anaverage rating of 5.5 (±2.4) and 4.0 (±2.3) for voice-loadedand voiceless-loaded sentences, respectively.
DISCUSSION
The purpose of this study was to compare the perception of vo-cal tremor during sustained and connected speech contexts.Based on ratings obtained from naive listeners, the sustainedphonation context received significantly more severe ratingsthan connected speech contexts on overall severity and shaki-ness. This finding supports Brown and Simonson’s (1963) clin-ical observation that vocal tremor was more easily perceivedduring sustained phonation than during shorter utterances.Thus, our findings demonstrate that the best context for detect-ing and characterizing vocal tremor is during sustainedphonation.
The unique contribution of this study was the experimentalfinding that perceptual ratings of tremor were significantlyworse, on average, for sustained phonation than for connectedspeech. Such a systematic comparison of listener perceptionsof vocal tremor speaker differences during connected speechand sustained phonation has not previously been reported. All
FIGURE 5. Average shakiness ratings by speaker and speech
context.
FIGURE 4. Shakiness ratings for each speaker across three trials for
each sentence context.
Journal of Voice, Vol. 26, No. 5, 2012668.e7
five speakers with vocal tremor in this study were judged to ex-hibit a similar vocal tremor severity and pattern during sus-tained phonation by speech-language pathology voice experts.Naive listeners also judged the speakers to be similar on overallseverity and shakiness during sustained phonation. In contrast,each speaker received a range of perceptual ratings across allspeech contexts. That is, each speaker showed differing patternsin their ratings between connected speech and sustained phona-tion contexts. Speaker 1 exhibited no difference betweenratings received across all three speech contexts for overallseverity and shakiness. As seen in Table 2, Speaker 1 was theonly speaker to show similar voicing duration between thevoice-loaded and voiceless-loaded sentence contexts. Althougha shorter voicing duration would be expected to reduce percep-
tion of vocal tremor in the connected speech context, this par-ticular speaker’s pattern occurred due to voice stoppagesassociated with the vocal tremor. Thus, the shorter voicingduration did not benefit this speaker because of the abnormalstoppages in voicing during speaking. Speaker 2 received sim-ilar ratings of overall severity and shakiness for sustained pho-nation and voice-loaded sentence context but received lowerratings on the voiceless-loaded sentence context. The lattermay have been related to the reduced voicing duration mea-sured for Speaker 2 in the voiceless-loaded sentence contextcompared with the voice-loaded sentence (see Table 2). The re-duced voicing duration during production of the voiceless-loaded sentence context may have made it more difficult forlisteners to perceive vocal tremor in this context. In contrast,Speaker 4 also showed a reduced voicing duration during pro-duction of the voiceless-loaded sentence compared with thevoice-loaded sentence. However, this speaker was given higherratings on the voiceless-loaded sentence than the voice-loadedsentence on overall severity. Shakiness was rated similarly be-tween the two sentences for Speaker 4. One possible explana-tion for Speaker 4 exhibiting the opposite pattern from thepredicted outcome may have related to the slower articulationrate exhibited on the voiceless-loaded sentence comparedwith the voice-loaded sentence. This would elucidate why lis-teners judged this speaker to sound more abnormal duringvoiceless-loaded compared with voice-loaded sentence produc-tion, but comparable in the degree of shakiness on both sentencetypes. Speakers 3 and 5 exhibited ratings in the predicted direc-tion for both overall severity and shakiness. That is, thesespeakers were rated worst for the sustained phonation context,next worst on the voice-loaded sentence, and best on thevoiceless-loaded sentence context. Interestingly, these twospeakers exhibit similar shakiness ratings to Speaker 1 for sus-tained phonation. Yet, they showed the greatest difference inratings between all three speech contexts.
Amy Lederle, et al Vocal Tremor Perception and Speech Context 668.e8
The differences observed in perceptual ratings across all fivespeakers and speech contexts suggest that sustained phonationby itself does not offer a valid estimation of the impact of a vocaltremor on an individual’s connected speech. A comparison ofvocal tremor patterns across sustained phonation and connectedspeech contexts appears necessary to determine the effect of vo-cal tremor on an individual’s speech production. Based on theoutcomes of this study, it is hypothesized that individualsjudged as having high ratings of overall severity and shakinessacross all three speech contexts would be considered the mostseverely affected. Those who exhibit differences in overall se-verity and shakiness levels during sustained phonation and thetwo types of connected speech contexts are hypothesized tohave less severe vocal tremor. The latter group of individualswould further be predicted to be amenable to speech treatmentto help manage their vocal tremor. Unfortunately, the speakersused in the present study were not enrolled in a behavior man-agement program to compare speech context patterns to treat-ment outcomes. However, future such research may elucidatewhether sustained phonation and connected speech patternspredict behavior management outcomes.
In general, vocal tremor was predicted to be less evident dur-ing connected speech than during sustained phonation due tothe rapid onset and offset of the voice and contiguous articula-tory movements. The outcomes of this study were consistentwith this hypothesis. The general trend in average ratings ofshakiness during connected speech further offered supportthat voicing duration may be an important phonetic componentfor the perception of vocal tremor. This was indicated by the ob-served trend in ratings of overall severity and shakiness sug-gesting that vocal tremor perception decreased as the durationof voicing decreased. Of interest is the source of perceptual rat-ings that varied from this predicted pattern. Articulation ratepatterns did not appear different across all but one of thespeakers rated differently than predicted. In addition, patternsin F0 and intensity contributions to the vocal tremor did not ap-pear predictive. One possible consideration that was not inves-tigated here might have been speaker differences in formanttransition patterns during connected speech. Future researchmay need to consider the role of abnormal acoustic patternsassociated with articulation distortion or aberrant patterns inphonetic transitions between speech sounds.
The clinical application of the findings from this study relatesto methods we use to evaluate individuals with vocal tremor.Specifically, outcomes of this study support that sustained pho-nation context is the best speech task to use when determiningwhether an individual exhibits a perceptible vocal tremor. How-ever, the outcomes of this study also suggest that clinicians needto consider the impact of vocal tremor on connected speech pat-terns. Such additional consideration may prove useful in deter-mining whether speaking patterns in those with vocal tremorcan be modified to reduce perception of vocal tremor.
Individuals exhibiting reduced perception of vocal tremorduring connected speech may benefit from speech treatmentfocusing on shortening the duration of voiced sounds duringconnected speech. As reported in the introduction, individualswith vocal tremor tend to reduce their rate of speech and pro-
long voiced speech sounds. This compensatory strategy wouldpromote the perception of more severe vocal tremor rather thanreduce it. Thus, an effective treatment approach might be tofocus on reduction of prolonged voicing patterns during con-nected speech to reduce the perception of vocal tremor. Futureresearch addressing the link between perception of vocal tremoracross different speech contexts and treatment outcomes is im-portant for testing our hypothesized outcomes. In addition, suchresearch may determine the best methods for identifying candi-dates for behavioral management of vocal tremor.
Acknowledgments
We thank Wesley Tolliver for his assistance with data organiza-tion, Mark Borgstrom for his assistancewith statistical analysis,Sarah Sullivan for her assistance with research design, andmembers of the University of Arizona Speech PhysiologyLab for critical feedback regarding the study design andmethods.
