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Transcript of LARYNGEAL DYNAMICS IN STUTTERING.pdf / KUNNAMPALLIL GEJO
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LARYNGEAL DYNAMICS IN STUTTERING
LARYNGEALONSETANDREACTIONTIMEOFSTUTTERERS:
Historically, larynx has been considered to play a central role, if not exclusive role in
stuttering (Yates, 1800/1839), Hunt, 1861; Kenyon, 1943). With recent advances in
technology, new and more sophisticated measuring devices have been developed, the
purpose being, to investigate into the laryngeal behaviors of stutterers and and the role of
larynx in stuttering.
This area of research has, taken 3 distinct directions:
1. Studies of stutterers voice onset time (VOT), voice initiation time (VIT) andspeech initiation time (SIT).
2. Electromyographic investigation of stutterers laryngeal muscle activity.3. Fiberoptic studies.
1. Voice onset time (VOT):
VOT has been defined as the time that elapses from the release of the consonant burst
to the onset of periodic glottal vibration for the production of the vowel that follows
the production of the vowel that follows the consonant (Lisker and Abramson, 1964).
METHODOLOGIES AND INSTRUMENTATION
:VOT can thus be measured with any instrument that:
1. Reliably senses and records the end of consonantal implosion and the initiation ofglottal vibration for phonation.
2. And provides means of determining the time lapsed between these 2 events.
Three main methods have emerged, over the years, which are well suited for such
measurements:
1. Spectrography.2. Detecting the sizeable rise in intraoral air pressure that occurs during the
implosion phase of stop consonant production.
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3. X-ray motion picture and voice recorder. The former, we can see the start of theconsonantal release on the X-ray film and the latter tells us when phonation starts.
The difference between these 2 events, expressed in temporal units, is VOT.
Findings:
The measurements and comparisons of the VOTs of stutterers and normal speaking
control subjects mainly included investigation of fluent productions of simple, isolated
CV syllables, during the generation of longer syllable sequences, and during the
production of stop consonant plus vowel combinations in continuous oral reading. The
results of studies of stutterers and normal VOTs are given in the following table:
Authors Method Subjects Results
Angello and
Wingate (1972)
Pressure-sensor and
voice-recorder; CV
utterances.
Matched groups of
12 adult stutterers
and 12 normals.
Stutterers VOT
were longer.
Wendell (1973) Spectrographic
analysis of CVs.
Matched groups of
12 child stutterers
and 12 normals.
Stutterers VOTs
were longer.
Metz, Conture, and
Caruso (1979).
Spectrographic
analysis of 18
different soundclusters in words.
5-young adult
stutterers and 5-
normals.
Stutterers VOT
were longer on only
6 of the 18 clusters(p
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VOICE AND SPEECH INITIAION TIMES (VIT and SIT)
VIT is defined as the time lapse between the appearance of some experimenter-controlled
external stimulus (e.g., a pure tone of flash of light), and the subjects initiation of glottal
vibration for phonation. Thus, VIT represents the time lapse between the onset of non-
speech event and the starting of voicing.
In similar fashion, some investigators have required subjects to utter a response of one
word or longer, beginning with a voiced sound. These studies are viewed as measuring
speech initiation time (SIT).
METHODOLOGIES AND INSTRUMENTATION:
Though there have been some minor variations across experiments, most VIT/SIT
investigations have employed highly similar methods and designs. In a typical project, a
subject is presented with a warning signal, waits for the appearance of a cueing stimulus,
and then generates a desired response as soon as possible. The following table presents
the summary of both the VIT and SIT studies:
Authors Characteristics
of subjects
External
signal(s)used
Subjects
response
Findings
Adams and
Hayden (1976)
10 adult
stutterers and
10 age-andsex matched
normals.
1000Hz pure
tone.
Phonated
/a/.
Both groups shortened VIT
from the beginning to end of
the experiment. Stuttererswere slower on two of three
comparisons made.
Starkweather,
Hirschman and
Tannenbaum
(1976).
11 adult
stutterers and
11 age-and-sex
matched
normals.
Green light
presented on
the screen.
26 test
syllables
reflecting a
wide range
in place and
manner of
articulation.
Both groups shortened VIT
from the beginning to end of
the experiment. Stutterers
were slower acroos all test
trials and across all syllable
types investigated.
Reardon (1977) 5 adult
stutterers and 5
sex-matched
Auditory
stimulus.
Phonated /a/
and 6 other
test.
Stutterers had longer VITs
on every utterance. Stutterers
longestVITs occurred on the
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normals. syllables. six test syllables. Their
shortest VITs appeared on
the isolated vowel.
Cross, Shaden,
and Luper
(1979).
10 adult
stutterers and
10 age-and-sex
matched
normals.
4000 Hz
presented in
each ear in
separate
condition.
Phonated / /. No difference in stutterers
VIT when tested tone was
presented to left as compared
to the right ear. Overall,
stutterers were slower than
normals.
Cross and
Luper (1979).
9 stutterers
each, at ages 5
and 7 years+9
adults age-and
sex matched
with like
numbers of
normals.
1000 Hz pure
tone.
Phonated / /. In both groups, VIT
shortened as age inceased. At
all age levels studied,
stutterers were slower than
normals.
Lewis, Ingham,
and Gervens(1979)
10 adult
stutterers and alike number of
normals.
1000 Hz pure
tone and alight flash;
presented in
separate
condition.
Phonate an
isolatedvowel.
Stutterers were slower than
normals in both the auditoryand visual cueing.
Prosek,
Montgomery,
Walden
(1979).
10 adult
stutterers and
10 age-and-sex
matched
normals.
Light flash, a
1000 Hz pure,
and spoken
words;
presented in
separate
conditions.
16 VC
words (e.g,
ape).
Stutterers were slower than
normals in all cueing
conditios.
Cross, and 8 adult Visual Phonate /a/ Stutterers were slower than
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Cooke (1979) stutterers and 8
normal
speakers.
stimulus, and
an auditory
stimulus,
presented in
separate
conditions.
normals across all
experimental conditions. The
greatest difference between
groups appeared in the
auditory-vocal response
condition.
Adler and
Starweather
(1980)
A group of
stutterers and a
group of non-
stutterers.
A visual
stimulus.
A laryngeal
gesture.
The stutterers were slower
than the control subjects in
all experimental condition.
Cullinan and
Springer
(1980).
11 child
stutterers with
articulation and
language
problems; 9
pure
stutterers; and
20 age-and-sex
matched normalchildren.
1000 Hz pure
tone.
Phonate /a/. The two groups of stutterers
combined, had slower VITs
than did normals. However,
this difference was a function
of the extreme slowness of
the stutterers with the
associated articulation and
language problems. VIT did
not differ between the purestutterers and the normals.
Murphy and
Baumgartner
(1981)
6 child
stutterers and 7
normal
speaking
children.
1000 Hz pure
tone.
Phonated
/a/.
No differences were found
between the groups.
Reich, Till, and
Goldsmith
(1981)
13 adult
stutterers and
13 age-and-sex
matched
normals.
1000 Hz pure
tone.
Phonted /a/
and the
word
upper.
Stutterers were slower than
normals on the isolated
vowel production and on the
words production.
Watson, and 8 adult 1000 Hz pure Phonated No differences were found
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Alfonso (1982) stutterers and
13 age-and-sex
matched
normals.
tone, and a
light flash
presented in
separate
conditions.
/a/, and a
nonsense
syllable
phrase.
between groups in either
condition, either the vowel or
the nonsense syllable phrase
response.
Hayden,
Adams, and
Jordahl (1982)
10 adult
stutterers and
10 ex-matched
normal adults.
1000 Hz pure
tone.
Production
of 9
sentences,
all
beginning
with a
vowel (e.g.,
Almonds
are nuts)
Stutterers were slower than
the normals.
INTERPRETATION:
In four of the six VOT studies, stutterers had longer (slower) scores than normal speaking
control subjects. In the SIT/VIT investigations that were reviewed, significant slowness
among the stutterers was noted unequivocally in 11 of 17 projects. Mixed findings wereobtained in two studies. Non significant differences were observed between stutterers and
control subjects in just 4 of the 17 experiments. From these outcomes we may conclude
that stutterers as a group are likely to have slower VOTs and VIT/SITs than matched
normal subjects.s
Beyond the broad interpretation, these studies tell us even more.
1. Stutterers slowness in VOT cuts across productions of isolated CV syllables toprose material being read aloud (Hillman and Gilbert, 1977).
2. Stutterers slowness in producing isolated vowels (VIT) appears also to be presentin the production of single words (Reich, Till and Goldsmith, 1981), and sentence
length utterances that are initiated with vowels (SIT; Hayden, Adams, and
Jordahl, 1982).
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Shortly after the completion of the first several VOT and VIT experiments, there was
considerable conjecture that the slowness was caused by an individuals history of
stuttering. In other words, having spent years as a stutterer, a person would quite likely to
approach speech or speech-acts with an excess of muscular tension in the larynx. Such
muscular tension, a result of a history of stuttering, might then act to retard VOT and
VIT.
At two predictions can be drawn from this framework.
1. We could forecast that young stutterers, with relatively short histories ofstuttering, would be less likely to approach to speech and speech-like acts with
excess muscular tension.
2. it should also follow that young stutterers would have shorter VOT and VITvalues as compared to adult stutterers because the children had briefer histories of
stuttering, and hence had less time to develop higher levels of muscular tension in
the larynx.
The results of studies cited in the table, fail to bear out these predictions, both VOT and
VIT scores for younger stutterers were slower than those of control subjects (Wendell,
1973, and Cross and Luper, 1979). It was also shown that stutterers VIT improved with
age (Cross and Luper, 1979). Neither of these findings would be likely if stuttering were
the cause of the slowness. Rather, such slowness probably coincided with the onset of the
disorder. Indeed, it is even possible that difficulty in quickly initiating voicing is one of
the immediate causes of stutterers repetitions and prolongations of articulatory gestures
(Adams, 1974), viewed here and elsewhere as core characteristics of stuttering (Wingate,
1964).
The next explanation that was developed pertained only to VIT. I this account, stutterers
slowness is causally related to a specific defect in the auditory system that retards the
reception or processing of stimuli used to cue vocal responses. Needless to say, this
interpretation was abandoned when stutterers were found slower than normal VITs to
visual signals as well (Starkweather, Hirschman, and Tannenbaum, 1976).
Noting this slowness in both auditory and visual stimulation, thought was give to
attributing it to some central disturbance that would reduce the speed with which
stutterers organized and started transmitting neural signals to the periphery for voice
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production. Inherent to this formulation is the idea that stutterers neural organization and
transmission are both normal with the exception of the speech with which they take
place. Recently, some experimenters have measured stutterers reaction times for
nonspeech tasks, such as button pressing, by using lights and/or tones. Stutterers neural
reaction times have also been assessed (McFarlane and Prins, 1978). There are only a few
of these investigations and their findings are mixed. Therefore, it would be premature to
interpret them at this point.
Finally in review, Adams (1981) offered an elaboration on the position that stutterers may
be slow to organize and transmit normal neural commands to their musculature.
Specifically, it was suggested that in addition to integrating and sending commands more
slowly, stutterers may also send inappropriate commands to the periphery. This would
activate muscles in ways that could delay voicing. It is interesting to note that stutterers
VIT and SITs improve when voicing and speech are initiated in synchrony with a
rhythmic stimulus (Hayden, Adams, and Jordahl, 1982). This finding is procative because
we have known for years that rhythmic speech improves fluency. Perhaps rhythm
enhances fluency by helping a speaker with the timing of events that are integral to
speech production (Brayton and Conture, 1978; Hayden, Jordahl and Adams, 1982). Such
an event could be voice initiated.
Suchitra (1985) conducted a study to find the acoustic parameters VOT, SIT, STT, Foand rate of speech in stutterers in pre and post therapy condition and found:
1. Non stutterers values are smaller when compared to pre and post therapy VOTvalues of stutterers.
2. Speech initiation time less than that of Stutterer in pre and post therapy condition.3. Stutterer post therapy VOT value smaller than pre-therapy value.4. Reduced post therapy SIT value of Stutterer compared to pre-therapy.5. Both show no difference in Fo does not vary as compared to pre-therapy.
Sebastian (1997) studied the acoustic parameter in stutterers and non-stutterers
and came to the following conclusion.
1. The VOT values were less for non-stutterers compared to stutterers.2. No significant difference between the stutterers and non-stutterers in terms
of formant frequencies was found.
3. Significant differences were found between stutterers and non-stutterers inword duration and vowel duration.
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Then she came to the conclusion that the laryngeal mechanisms during speech
in different for stutterers to that of non-stutterers.
LARYNGEALMUSCLEACTIVITYOFSTUTTERERS
Electromyographic (EMG) studies of stuttering are important because they provideinformation about a different level of the speech production process. The
electromyography amplifies and records the minute electrical voltages generated each
time a motor unit fires in response to a neural impulse. As motor units fire more
rapidly or as many motor units fire in close succession, electrical activity in a muscle or
muscle group increases. EMG recordings reflect the level of contractile activity in
muscle tissue and the variations in this activity over time.
When EMG recordings are combined with other information, such as acoustic
analyses of the speech produced, and knowledge of the anatomy and physiology of the
muscles under study, some inferences may be made regarding movements and/or levels
of muscle tension.
Electromyography in Stuttering Research
Most of the early EMG studies conducted with stutterers were designed to
investigate basic neurophysiological difference between stutterers and nonstutterers
(Morley, 1937; Steer, 1937; Travis, 1934). More recent experiments have focused on
the moment of stuttering and compared EMG patterns during fluent utterances with
those generated during stuttering.
A number of studies of stuttering have attempted to use electromyography as an
index of psychological status, for example arousal, anxiety, vigilance, anticipation, or
expectancy.
One study, which did not directly measure intrinsic laryngeal muscle activity,
does offer valuable insight into general throat area muscle activity related to stuttering.
Shrum (1967) used silver disc surface electrodes to record from several sites including
two bilateral masseter (jaw) muscle sites, two bilateral platysma (neck) muscle sites, and
one leg muscle site. He measured the duration of muscle activity from moment A, when
muscle activity was elevated over the resting state, to moment B when initiation of
phonation was recorded. He found that the interval between moments A and B (duration
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of prephonatory muscle activity) was significantly longer for stutterers than for
nonstutterers. For stutterers, this interval was longest before words on which they
stuttered, shorter before words on which they expected to stutter (but did not), and
shortest before words spoken without anticipation or stuttering. Shrum interpreted these
findings as indicating that stutterers began to tense earlier than nonstutterers. An
alternate interpretation is that initiation of phonation was delayed in stutterers. This
second interpretation of Shrums findings is consistent with recent research
demonstrating longer VOTs and slower initiation of phonation.
OBSERVING LARYNGEAL MOVEMENTS OF STUTTERERS
Development of the flexible fiber optic endoscope (fiberscope) a flexible tube
containing bundles of glass or plastic fibers has had a great impact on otolaryngology,
speech science, and speech pathology. The fiberscope contains two bundles of optical
glass or plastic strands / fibers with one bundle carrying a cold, bright ligh t (e.g. xenon)
to illuminate the area under investigation and the other bundle returning a color image
back for visualization and / or recording (Boyd,1982).Because a fiberscope can be readily
passed through a bodily orifice, routine activities of heretofore inaccessible parts of the
body, such as the vocal folds, can be visualized. Its use in the study of laryngeal activity
associated with stuttering is the basis of this discussion (Conture, 1977, 1982a, 1983;
Conture, McCall & Brewer, 1977. 1979; Freeman, 1975.
Fiberscope Investigations of Stuttering
Ushijima et al. (1966) who filmed both inappropriate glottal openings as well as
tightly adducted true/false vocal folds during different instances of stuttering. Fujita
(1966), using posterior-anterior X-rays of the laryngeal area, also reported nonpredictable
openings and closings of the pharyngolaryngeal cavity associated with stuttering.
Shortly thereafter, Conture and associates in Syracuse and Freeman and associates
at Haskins Laboratories publicly presented their fiberscopic and electromyographic
observations of the larynx during stuttering. Conture and associates work focused on
fiberscope observations, while that of Freeman and colleagues involved
electromyographic studies of stuttering.
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Conture et als 1977 work indicated that the larynx is often (1) inappropriately,
nonpredictably open or (2) inappropriately closed during instances of stuttering. These
findings were consistent with those of Ushijima et al. (1966) and, coupled with Freeman
and Ushijimas (1978) EMG findings, clearly implicated laryngeal involvement in the
disrupted speech physiology that characterize stuttering.
Conture (1982a), shows that laryngeal behavior was more variable during sound /
syllable repetitions than sound prolongations. Moreover, sound/syllable repetitions also
contained the greatest number of nonviewable/nonmeasurable videoframes. Still, these
findings, which are consistent with previous reports, indicate that laryngeal behavior not
only differs between stuttering and fluent productions but also between different types of
stuttering as well.
In a time-course description of laryngeal behavior from beginning to end of an
adult stutterers sound / syllable repetition, it is apparent that during a sound/syllable
repetition, laryngeal behavior is highly variable; the vocal fold open and closes
throughout the repetition. The larynx is not static; it oscillates between abductory and
adductory postures. Preliminary data also suggest that the height of the larynx during
stuttering varies. In fact, videofluoroscopic observations of laryngeal height during
stuttering (Conture, Gould & Caruso, 1980) indicate that many repetitions are
characterized by a descending or lowering of the larynx compared to its height during
fluent productions of a vowel.
For some sound prolongations, the ventricular folds are also compressed medially,
above the adducted vocal folds, as the epiglottis is pulled posteriorly. Sound
prolongations with some stutterers show constriction of the pharyngeal area at the level
of the larynx. Stutterers, who point to their throat and say that the word got stuck here,
may not only be sensing excessive laryngeal adduction but aerodynamic back pressure.
Conversely, some sound prolongations, particularly those on /s/ and /f/, are
associated with widely opened vocal folds. Of course, the vocal folds should be abducted
during production of these sounds since they are voiceless; however, the degree of
abduction is excessive and lasts far too long. Furthermore, a Stutterer who senses these
extended laryngeal abductions may still describe them in much the same way as overly
adducted laryngeal behavior; that is, the Stutterermay say the word got stuck.
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Effect of binaural masking noise on stutteringa spectrographic
analysis (NANDU 1982)
Masking does reduce the frequency and severity of stuttering and also alters the
usual manner of initiating phonation. It makes the Stutterer to speak more slowly and
loudly (Cherry and Sayers, 1953; Hanley and Steer, 1949; Adams and Hutchinson 1974).
Auditory masking by noise brings about a reduction in secondary behavior,
mostly while using automatic feedback masking. Expressionless speech and reduction inspeech intelligibility observed. (Dewar et al 1976)
Masking helps the stutterers to learn to monitor their speech primarily by
proprioceptive, tactile and kinesthetic feed backs. It also distracts the subjects attentionfrom his speech and reduces anxiety and also changes the manner of speech (Van Riper,
1973).
The purpose of the study was to find out the effects of masking noise on rate ofspeech, fluency, fundamental frequency, voice onset time, vowel duration and vocal level
in stutterers and normals.
Subjects: Four stutters and four non stutters with age and sex matched
All speech recordings were done using unidirectional microphone
Subject read the passage in the presence and absence of masking noise. This reading was
recorded using tape recorder.
Results
Both stutterers and non stutterers showed an increase in vocal intensity levelunder binaural masking noise, how ever, stutterers showed greater increase in vocal
intensity than non stutterers.
No significant difference in voice onset time (VOT) was observed in stutterers
and non stutterers- both in the presence and absence of binaural masking noise.
An increase in vowel duration was found in both stutterers and non stutterers
under binaural masking.
In a study, Geetha (1979) has attempted to find out the linguistic characteristics ofstuttering, in Kannada language, of 15 stutterers by analyzing their spontaneous speech
and reading samples. She has concluded that, The content words are stuttered ore often than the function words. The consonants in general were stuttered more often than vowels. However,
stuttering was found on vowels also and in a minority of cases vowel stuttering
was more than the consonant stuttering.
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There is no difference between the adult and child stutterers in terms of theirrepetition characteristics of various linguistic units as syllable, word, part word,
phrases and sentences. With respect to syllable structure, maximum stuttering was observed on CV
syllables and the next in the order was vowel syllable.
Analysis of speech of stutters (dissertation)
This study was taken up to find out the relationship between frequency ofoccurrence of stuttering and various linguistic factors and to note the relationship
between voicing and stuttering in adult and children.
Subjects: 71 adult stutterers age ranging from 12- 35 yrs and 11 children who werediagnosed as stutterers age ranging from 6- 12 yrs were made to read 2 passages (one
passage consisted of both voiced and unvoiced sounds and another passage had only
voiced sounds. The readings were recorded using tape recorder
Most of the stutterers showed normal rate of speech. It is found that the initial syllable in the word is most frequently stuttered than
the syllable in any other position. Stuttering was found most frequently on vowels than on consonants, both in
the case of adults and children in the present study. The adult stutterers showed more stuttering while reading combined passage
than the voiced passage. However, children did not show any such difference.
Adams & Reis (1971, 1974) have found that there was greater frequency of
stuttering in passage which had both voiced and unvoiced sounds than in the
passage which had only voiced sounds. They also found that the adaptation rate
was faster in the latter. They have concluded that the frequency of stuttering wasrelated to the onset of phonation required. But Manning & Coufal (1976) found
that the difficulty in both stutterers and non stutterers was during voiced to voicedtransition than, during voiceless to voiced, voiced to voiceless and voiceless to
voiceless phonatory transitions.
Another investigation by manning and Coufal (1976) compared the speech of 11adult stutterers and a matched group pf 11 non stutterers. Four types of phoneme
to phoneme transitions were compared. The results indicated that both the groups
had a lower percentage of disfluencies during voiced to voiced transition than
during voiceless to voiced, voiced to voiceless and voiceless to voiceless
phonatory transitions.
Gayathri (1980) investigated some aspects of phonatory behavior in stutterers. Theaim of her study was 1) to test if different degrees of voicing during repeated reading
of a passage bring about difference in the amount of adaptation in stutterers. 2) to test
if there is any relationship between the frequency of stuttering and the onset ofphonation invaried contexts: syllables, word list and passages in stutterers. 3) to test if
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there is any relationship between the frequency of stuttering and the occurrence of
stressed syllables in stutterers.
It was concluded from the three experiments that:
1)
Varying degree of voicing during rehearsals bring about significant differences inthe amount of adaptation. Greatest amount of adaptation occurs when there isinvolvement of voicing during the rehearsals as in aloud rehearsals and whispered
rehearsals.
2) Stutterers have greater frequency of stuttering when transition from voiced tovoiceless or voiceless to voiced consonants is required. This is indicated by
greater frequency of stuttering in combined passage and combined word list when
compared with the frequency of stuttering in voiced passage and voiced word list.
The adaptation rate is also faster in a voiced passage. Therefore the stutterers haveless lesser difficulty when transition from voiced to voiceless consonants are not
required and vice versa.
3)
Stutterer stutters more often on the non-stressed syllables preceding the stressedsyllables. This may be because, the stutters have difficulty in moving on the
following stressed syllables and keep repeating or prolonging the preceding
syllable. Stuttering is rare during the transitions from the stressed syllables to the
following non stressed syllables. Stuttering is also less frequent during thetransition from non stressed syllables to non stressed syllables.
Stuttering as a learnt extricatory response to a laryngeal abductor reflex
(Schwartz):
This is core of stuttering block model by Schwartz (1974, 1975a, 1975b). It was
his discovery of that physical cause of the stuttering block that him enabled him to
develop a relatively simple treatment. He stated that the core of the stuttering block is the
tendency, under conditions of psychological stress, for the loss of supra medullar,
inhibition controls upon the PCA in the presence of sub glottal air pressure associated
with speech.
Schwartz (1976) lists several kinds of stress which contributes to
stuttering. Baseline stress consists of speakers amount of psychological and muscle
tension.
Physical stress (fatigue), external stress (bad news) and speed stress (need to talk
in hurry) may add to Stutterer psychological stress. Finally other factors such as
situations of communicative stress, sound and word fears and verbal uncertainty, trigger
anticipation of stuttering which adds to psychological stress. As the Stutterer acquires
large repertoire of struggle and coping behaviors, anticipation of stuttering alone becomes
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sufficient to evoke a laryngospasm or a set of distracting or avoidance behaviors to
prevent its occurrence.
Comments:
1. Schwartzs model of stuttering and his approach to therapy have beencontroversial. The question whether or not PCA is the strongest intrinsic
muscle of the larynx as raised by Freeman, Ushijima and Hirose (1975). To
support his statement, Schwartz conceded that it was at least one of the
strongest laryngeal muscles.
2. Freeman et al (1975) raised an important question as to whether the PCA isreflexively active in controlling glottal width during exhalation.
3. Zimmerman and Allen (1975) wondered how the model could account forstuttering on voiceless sounds. for this Schwartz explained that an increase in
subglottal air pressure associated with such sounds was responsible for
conditioned laryngospasms.
4. This model does not account for the linguistic findings of stuttering and it wasprobably not meant to do so
5. It does not predict any general motor coordination deficits in stutterers. Mostof the respiratory and articulatory errors are seen as learnt excitatory
behaviors.
M.F. Schwartz (1974) proposed that Agnello & Wingates (1972) finding
that stutterers had longer than normal voice onset times in stop consonant vowel syllables
was due to neural inhibition of the PCA.
In summary, whereas any kind of laryngeal irregularities during stuttering could
be explained by Schwartzs model, direct evidence of the reflexive contraction of the
PCA prior to speech is lacking. Since his model hinges on that presumption, unqualified
acceptance of the model must await further empirical verification.
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REFERENCES:
Analysis and Synthesis of Speech of stutterers. The Spectrographic analysis of Stutters Speech under delayed auditory feed back.
Analysis of Speech of stutterers. Voice Onset Time for Stutterers and non Stutterers.
Edward G.Conture, Stuttering Second Edition