Perceptual and Motor Skills, 2005, 100,421-431. O Perceptual and Motor Skills 2005
CHORAL READING WITH FILTERED SPEECH:
EFFECT O N STUTTERING '
MANISH K. RAM1
Stuttering Research Laboratoy
University of North Dakota, Grand Forks
JOSEPH KALINOWSKI, MICHAEL P. RASTATTER,
DONALD HOLBERT. AND MARK ALLEN
East Carolina University, Greenville
Summay.-This study investigated use of choral reading with filtered components of speech and whispered speech on the frequency of stuttering. Three passages
read by a normal adult male were lowpass filtered with kneepoint frequencies at 100
Hz (approximate glottal source), 500 Hz (source and first formant), and 1 kHz
(source and the first two formants). Along with a whispered passage, a normal passage, and a control condition, these stimuli were used in a repeated-measures design
with 12 adult stutterers as they read passages while listening to one of the stimuli.
Frequencies of stuttering in each condition were analyzed. The choral speech, the 500Hz, the 1-kHz, and the whispered speech conditions all decreased the frequency of
stuttering while the 100-Hz stimuli did not. It is suggested that articulatory events,
chiefly the encoded speech output from the vocal tract, create effective cues and may
induce fluent speech in people who stutter.
Experimental conditions under which stuttering diminishes have been
investigated and reviewed by many researchers. These conditions that decrease the frequency of stuttering include choral reading (Johnson & Rosen,
1937; Barber, 1939; Pattie & Knight, 1944; Cherry & Sayers, 1956; Ingham
& Packman, 1979; Adams & Ramig, 1980; Andrews, Howie, Dozsa, & Guitar, 1982)) shadowing (Cherry & Sayers, 1956; Kelham & McHale, 1966;
Wingate, 1981; Andrews, et al., 1982), delayed auditory feedback (Lee, 1950;
Adamcyzk, 1959; Burke, 1975; Hutchinson & Norris, 1977; Martin & Haroldson, 1979), and frequency altered feedback (Howell, El-Yaniv, & Powell,
1987; Kalinowski, Armson, Roland-Mieszkowski, Stuart, & Gracco, 1993;
Hargrave, Kalinowski, Stuart, Armson, & Jones, 1994; Kalinowski, Stuart,
Sark, & Armson, 1996).
In the conditions of choral reading and shadowing, one listens to and
'Part of these data were presented in the Science and Research Career Forum at the Annual
Convention of the American Speech-Lan uage-Hearing Association at Washington, DC in November ZOO0 by the first author. The auttors thank the reviewers for their comments and suggestions on an earlier version of this manuscript. Direct correspondence to Manish K. Rami,
Ph.D., Department of Communication Sciences and Disorders, P.O. Box 8040, University of
North Dakota, Grand Forks, ND 58202-8040 or e-mail (manish.rami@und.edu).
422
M. K. RAMI, ET AL.
reads or speaks along with another speaker. It should be noted that truly
synchronous choral speech might result only after a great amount of rehearsal. Asynchronous choral reading continues to produce a reduction in stuttering frequency in the speech of a person who stutters as synchronous reading
(Cherry & Sayers, 1956). Under the effects of delayed auditory feedback and
frequency-altered feedback, one listens to his own productions that are electronically changed. In the case of delayed auditory feedback, the speech is
delayed and in the frequency altered feedback, the pitch of the speech is
shifted either up or down. In all of these conditions, one listens to speech
that is either not produced by oneself or is modified externally (usually electronically) either in the time or the frequency domain. These external stimuli
have been termed as the "second speech signal" (Kalinowski & Dayalu,
2001, p. 61). While explaining the possible mechanisms that could be involved in producing fluency, these authors suggested that the second speech
signal lends an appropriate speech gesture or an articulatory form to the
speech of people who stutter.
This study tests the proposed hypothesis. We filter the second speech
signal before it is delivered to the people who stutter to find out how much
of the articulatory cue (as provided by the second speech signal) is required
to decrease the stuttering frequency. We suggest that, if the second speech
stimulus, in some manner, is what induces a decrease in stuttering, then it is
necessary to examine its components to identify their individual role. To explore the components of the speech stimuli, the source-filter model of
speech production was used (Borden, Harris, & Rapheal, 1994). The source
filter model proposes that the sound produced by the vibration of the vocal
folds is the glottal source. This sound passes through the vocal tract. The
cavities of the vocal tract selectively filter the sound by either resonating at or
damping certain harmonics. The harmonics are the formant frequencies,
which aid in perception of speech. Three filter settings were used, one each
selected to approximate the glottal source, the glottal source along with the
first formant component, and the glottal source with the first and second formant components. Each of these settings included the source characteristics
with a varying amount of filter characteristics. With each filter level, we assumed more of the articulatory cue was intact in the speech signal. In addition, to separate the filter characteristics from the source characteristics,
whispered speech n7as used as one of the conditions. In whispered speech,
the periodic glottal source is absent, but the filter characteristics with aperiodic sources are present. This condition allowed examination of the articulatory cue in terms of the filter alone.
The purpose of this experiment was to explore the effects of choral
reading with the three filtered speech signals, a whispered speech signal, and
in a control condition on the frequency of stuttering. Participants were ask-
STUTTERING: CHORAL READING, FILTERED SPEECH
423
ed to read passages under the following six conditions: a control condition
of nonaltered auditory feedback, choral speech, lowpass filtered speech at 100
Hz (Filtered-loo), at 500 Hz (Filtered-500), and at 1000 Hz (Filtered- 1000),
and whispered speech. Except for the control condition of nonaltered auditory feedback, in all the other five conditions the second speech signal was
externally produced and delivered.
We postulated that, if the entire speech signal was necessary to produce
fluency, then all other conditions except the choral speech should not increase fluency since all the other conditions would provide only a part of the
signal. Were the source adequate, then the whispered speech condition,
which has no source, should not decrease stuttering. If complete filter characteristics are necessary for the fluency, then all conditions except the choral
speech and whispered speech should not decrease stuttering. If fluency occurs with any part of the filter but not the source, then filtered-100 should
not increase the amount of fluency. The control condition of nonaltered
feedback is predicted to not produce fluency.
METHOD
Participants
Twelve volunteers who stuttered (10 men, 2 women; age range: 19.253.1 years; M age=37.7 yr.) were participants. Comparable sample sizes in
similar experiments are common and are acceptable (Howell, El-Yaniv, &
Powell, 1987; Hargrave, et al., 1994; Kalinowski, Stuart Rastatter, Snyder, &
Dayalu, 2000). Case history information was used to screen participants with
speech disorders other than stuttering. One participant gave a history of onset of stuttering after an accident. No attempt was made to separate people
with developmental stuttering from people with neurogenic stuttering. All
participants but one reported normal hearing. The ratings of self-reported
severity of stuttering ranged from mild to severe. The data from two participants with exceptional case histories, one with a bilateral moderate high frequency hearing loss and another with stuttering onset after an accident, were
included after examining the data and finding them to be in line with the
rest of the participants.
A normal speaking adult male (age 28 years) volunteer read four 300-syllable passages and whispered an additional 300-syllable passages. These passages were from junior high texts (Taylor, 1985; Sims, 1987). All the signals
were recorded in a sound-treated booth (Ray Proof Sound Shield Model
803) on a Digital Audio Tape (DAT) recorder (Sony model ZA5ES) via a
microphone (Shure Model SM48) placed approximately 30 cm from the lips
with an orientation of 0" azimuth and -30" altitude. These signals were sampled at 44100 Hz. One unprocessed speech sample was transferred on a per-
424
M. K. RAMI, ET AL.
sonal computer (Apple model Power Macintosh 9600/300) using a portable
DAT recorder (Sony Model TCD 8) for the choral speech condition. The
three other signals were routed from the portable DAT into a commercially
available speech and sound signal analysis program (Infosignal Inc. Model
Signalyze 3.12), running on a personal computer (Apple model Power Macintosh 9600/300) for recording via a passive dual-channel analog filter (KrohnHite model 3328) using Buttenvorths filter characteristics. To create the low
passed filtered stimuli these signals were lowpass filtered at frequencies of
100 Hz, 500 Hz, and 1000 Hz with a rolloff of 48 dB/octave per channel to
generate three filtered speech signals. Both the channels of this filter were
used in series to obtain an effective slope of 96 dB/octave to ensure sharper
cut offs. The analog output of the filter was sampled at 22050 Hz and quantized at 16 bits. These signals were resampled at 44100 Hz, 16 bits, normalized using a sound-editing software (Macromedia Model Sound Edit 16 Ver.
2) and saved in Audio Interchange File Format (AIFF). The vowels with the
lowest first, second, and third formants are known to be /i/, /3/, and /u/,
respectively (Peterson & Barney, 1954). Five occurrences of these vowels in
the passages were analyzed to obtain values as a guide to the selection of
kneepoint frequencies for lowpass filtering. This formant-frequency analysis
was conducted by using software for phonetic analysis (Boersma & Weenink
Model Praat Ver. 3.8.46). The first formant for the vowel /i/ was at 253 Hz,
so a frequency lower than an octave below the first formant was selected to
filter out the first and other formants. The 100-Hz lowpass filtered signal
was used to produce an approximation of the glottal source and consisted of
the stimuli for first lowpass filtered speech conditions (Filtered-100). Similarly, five occurrences of the vowel /3/ in the recorded passages were examined
to specify the lowest second formant; this was an average of 1061 Hz. To filter out the second and other higher formants but to retain the glottal source
and the first formant, a lowpass frequency setting an octave below, 500 Hz
was used. The 500-Hz filtered signal contained the glottal source and the
first formant information and consisted of the second lowpass filtered speech
condition (Filtered-500). Five occurrences of the vowel /u/ in the recorded
passages were analyzed to obtain the lowest third formant which was 2204
Hz. A 1,000-Hz filter setting was used to filter out formants above the second formant, and this signal consisted of the third lowpass filtered-speech
condition (Filtered-1000). A second recording by the same speaker for the
same passages was made in whisper. This was to remove the effect of the periodic glottal source on the speech signal and constituted the stimuli for the
condition of whispered speech. It should be noted that the aperiodic source
present in the whispered speech signal can contribute to formant peaks and
aid speech perception (Tartter, 1991). The signals were recorded on an audio compact disk using a commercial software (Adaptec Model Toast Ver.
STUTTERING: CHORAL READING, FILTERED SPEECH
3.5.6 PPC) and a CD writer (La Cie Model CRW 4416s) for use as stimuli
in this experiment.
Apparatus
Participants were seated in a comfortable chair in front of a video camera in either a sound-treated room (Ray Proof Sound Shield Model 803) or
in a quiet room. The stimuli were presented to participants via insert earphones (Eartone Model 3A) at a comfortable listening level but not less than
75 dB SPL. The sound level was measured using a sound-level meter (Briiel
& Kj~erModel T223l) and a 1-in. condenser microphone (Briiel & Kjaer
Model 4144) with a 2-cc coupler (Briiel & K j ~ rModel DB0138). Participants were instructed about the reading task, and audio speech samples from
the participants were recorded with a wireless lapel microphone (Shure
Model LX-14 AZ) placed approximately 20 cm from the lips. A camera
(Sony Model DSR-PD100) and VCR (Samsung Model VR 8075) were used
for video recording. The microphone output was fed into the video camera,
and both the audio and video outputs of the camera were fed into the VCR.
A backup audio-only recording was also made using a cassette tape recorder
(Radioshack Model CTR 100).
Procedure
The experimenter set up individual meetings with the volunteers. After
the participants signed their informed consent, they were instructed for the
experiment and read a passage under each of the six conditions: nonaltered
auditory feedback, choral speech, Filtered-100, Filtered-500, Filtered-1000,
and whispered speech. To continue providing the stimuli in this experiment,
the participants were allowed to speak at their own rate while continuing to
hear the stimuli asynchronously. The six conditions were administered in an
order randomized across subjects in accordance with a nonsymmetrical digram balanced Latin square of six six. All the conditions were administered and audiovisual recordings were made in a single session. To control for
any carryover effects from one condition to another, all participants were engaged in a conversation for 2 min. between conditions, and they read an approximately 100-word passage backwards. Frequencies of stuttering were tabulated for all the six conditions using the audiovisual recordings, and means
for each condition were used for statistical analysis. Stuttering was operationally defined as occurrences of repetitions, prolongations, and blocks.
Twenty-five percent of the data were re-analyzed by the experimenter and a
trained observer to obtain point-to-point intrajudge and interjudge reliability
as indexed by Cohen Kappa.
RESULTS
Frequencies of stuttering were analyzed using SPSS and SUPER-
426
M. K. RAMI, ET AL.
ANOVA. Means and standard deviations of frequencies of stuttering per
300 syllables for all the feedback conditions are presented in Table 1. Intraand interjudge reliability as determined by Cohen Kappa were .77 and .69,
respectively, suggesting fair to good agreement (Fleiss, 1981).
TABLE 1
MEANSAND STANDARD
DEVIATIONS
FORNUMBER
OF STUTTERED
UTTERANCES
PER300 SYLLABLES
FORALLFEEDBACK
CONDITIONS
Feedback Condition
M
SD
Nonaltered auditory
Choral speech
Approximate glottal source
Glottal source and first formant
Glottal source and first two formants
Whispered speech
24.5
3.6
22.2
6.6
2.5
4.1
35.5
2.9
44.8
10.4
3.4
3.6
The planned comparisons were carried out to identify statistically significant differences among conditions. Five single-df comparisons of means were
undertaken to investigate statistically significant differences between the conditions. Null hypotheses were rejected for all at p < .05. The measure of w2 as
an effect size statistic was used since it is not affected by small sample sizes
(Keppel, 1991, p. 66). w2 effect size of .Ol is considered small, .06 is considered medium, and .15 or greater are considered large. To replicate the choral reading effect, nonaltered auditory feedback and choral speech were compared. This comparison was significant with a large effect (F,,,,=6.80, p=
.01, w2=0.28). To examine whether the approximate source, that is, Filtered-100, decreases stuttering, it was compared to the nonaltered auditory
feedback condition. This comparison was not significant (F,,,,= 0.79, ns, w2=
0). To investigate the effect of filter alone, that is, whispered speech, whispered speech was compared to the nonaltered feedback condition. This comparison was significant with a large effect (F,,,,= 6.48, p = .01, w2= 0.27). To
check the effect of the approximate source plus first formant stimuli, Filtered-500, it was compared to the nonaltered feedback condition and was significant with a large effect (F,,>,
= 4.99, p = .03, o2= 0.24). The last comparison
was between the effect of approximate glottal source plus the first and second formant, that is, Filtered-1000 and the nonaltered feedback condition
which was also significant with a large effect (F,,,, = 7.53, p = .008, w2= 0.3 1).
DISCUSSION
The choral speech effect (Barber, 1939; Pattie & Knight, 1944; Bloodstein, 1950; Cherry & Sayers, 1956; Ingham & Packman, 1979; Andrews, et
a!., 1982) was replicated as indicated by the significant difference and a large
effect between the conditions of nonaltered auditory feedback and choral
STUTTERING: CHORAL READING. FILTERED SPEECH
427
speech. That is, the frequency of stuttering was significantly reduced under
the choral speech condition as compared to the nonaltered auditory feedback condition. Of the four other hypotheses tested using single-df comparisons, three of them, (the choral reading effect, the effect of filter, and
the comparison of the effect of source with the filter) were supported by
statistical significance and large effect sizes, but the effect of periodic source
alone was not supported by the data.
We predicted that, if the speech signal in its entirety is necessary and is
the only stimulus that reduces stuttering in people who stutter, then all the
conditions except the choral speech condition should not decrease stuttering. This hypothesis was not supported since the whispered speech condition, as well as the periodic source plus varying amounts of filter, i.e., Filtered-500, Filtered-1000, led to significantly lower frequency of stuttering compared with nonaltered auditory feedback. Thus, parts of the speech signal
lowered the frequency of stuttering.
Secondly, we predicted that, if the approximate periodic glottal source
alone is necessary and sufficient to decrease stuttering, then only the conditions of nonaltered auditory feedback and the whispered speech conditions
should not decrease stuttering. The whispered speech, which had no periodic
glottal source, was associated with significantly fewer stuttered utterances
than in the nonaltered auditory feedback condition. However, the approximate glottal source did not affect stuttering. Thus, the glottal source seems
insufficient or unnecessary to decrease stuttering in people who stutter.
Thirdly, we predicted that, if complete filter characteristics are necessary to decrease stuttering, then all conditions except the choral speech and
the whispered speech conditions should not decrease stuttering. This hypothesis again, was not supported since the source plus the varying amount
of the filter stimuli, i.e., Filtered-500, Filtered-1000, significantly decreased
the frequency of stuttering. Therefore, the complete filter characteristics do
not seem to be essential either because partly filtered speech signals significantly lowered the frequency of stuttering.
Lastly, we predicted that, if any part of the filter characteristics is sufficient in yielding fluency, then only the nonaltered auditory feedback condition and the approximate periodic glottal source, i.e., Filtered-100, should
not decrease stuttering. This hypothesis was supported since Filtered-100
condition did not decrease the frequency of stuttering compared to the condition without auditory feedback. Thus the periodic glottal source does not
seem to decrease stuttering, but parts of the filter characteristics are sufficient in decreasing stuttering in people who stutter.
Since the approximate glottal source stimuli had no effect, filter characteristics and parts of filter characteristics (along with the aperiodic source in
428
M. K.RAMI, ET AL.
the whispered speech condition) may be the only cues that help in decreasing stuttering in people who stutter under these choral feedback conditions.
This leads us to conclude that the approximate source does not individually
decrease stuttering, while the approximate filter characteristics individually
do decrease stuttering. The presence of any part of the filter characteristics
seems to be sufficient and equally efficient in decreasing the frequency of
stuttering in people who stutter.
To make conjectures regarding possible sources for the fluency-inducing effects of these choral feedback conditions, several explanations may be
mentioned. First, during the time of early delayed auditory feedback research, it was suggested that the correct feedback might allow monitoring of
speech (Lee, 1950); however, it has been shown that the fluency-enhancing
effects remain when other passages are read (Barber, 1939; Bloodstein, 1950;
Cherry & Sayers, 1956). The present experiment suggests the effects of decreasing stuttering frequency remain even when the speech signal fed back is
modified and incomplete. We conclude that correct feedback is not essential
to decreasing stuttering in people who stutter and that various types of auditory signals may produce similar effects. This needs to be tested. Second, an
explanation for the decreased stuttering that seems to persist is that of distraction; however, several people have criticized this explanation since the
hypothesis is not testable (Cherry & Sayers, 1956; Fransella & Beech, 1965;
Fransella, 1967; Wingate, 1969; Stuart, 1999). Although distraction is difficult to measure, if there is any, these responses of differential decreases from
similar stimuli would not be expected. Third, another hypothesis is that of
external pace-setting (Pattie & Knight, 1944); however, choral reading conditions do not have the regularity of prosody to provide any pace-setting
mechanism (Wingate, 1969). If the choral reading condition does not provide
regular prosodic reference and aid external pace-setting, then lowpass filtered speech signals could not provide such cues either. So external pace setting does not seem to play any role in producing the fluent speech. Lastly,
Hargrave and colleagues (1994) suggested that some alteration of the feedback signal seems to be helpful in inducing effortless and natural fluency.
Yet, the understanding of the mode of action in such effects on speech of
people who stutter remains elusive.
Noting that the filter characteristics or any part of filter characteristics
are effective in decreasing stuttering in people who stutter, perhaps the encoded articulatory events seem to be the part of the speech signal which
could provide some cue or a set of cues. When these cues are available to a
person who stutters as a second signal, they are successful in decreasing stuttering. These cues could also be in the form of formants produced by the
aperiodic source (Tartter, 1991).
STUTTERING: CHORAL READING, FILTERED SPEECH
429
It must be kept in mind that the articulatory cues may also include visual cues as well as vowel information as occurs in lip reading. If encoded
articulatory cues within these signals decrease stuttering, they individually
might also be able to decrease stuttering. This has been reported in two experiments (Kalinowski, Dayalu, Stuart, Rastatter, & Rami, 2000; Kalinowski,
Stuart, Rastatter, Snyder, & Dayalu, 2000).
In the first experiment, people who stutter were required to memorize
short segments of text and recite them while focusing their gaze on the face,
lip, and jaw of a research assistant. The research assistant either did, i.e., experimental condition, or did not, i.e., control condition, silently mouth the
text in unison. The frequency of stuttering in the participants was reduced
by almost 80% in the experimental condition as compared to the control
condition (p = .003) (Kalinowski, Stuart, Rastatter, Snyder, & Dayalu, 2000).
These authors concluded that visual linguistic cues somehow reduced stuttering. In the second experiment, Kalinowski, Dayalu, Stuart, Rastatter, and
Rami (2000) investigated the effects of feedback of a continuous single vowel, a train of vowels, a continuous single consonant, and a train of consonants on the frequency of stuttering with participants who stuttered under
the feedback conditions of reading passages. They reported the two conditions with vowels were most effective in decreasing stuttering, and the two
consonant conditions were slightly less effective, but all the four conditions
were significantly more effective than the control condition of no auditory
feedback. Here again, it seems plausible that the cues or the encoded portion
of the speech gesture is received in all the four conditions in which stuttering in people who stutter was reduced.
W e speculate that the articulatory events within the speech gesture, including visual cues and vowel information, which essentially consist of the
encoded portion of the speech signal conveying crucial information on the
state of the articulators during the speech act, seem to be the only necessary
condition for decreasing stuttering in people who stutter. If this is the case,
it is important to test this hypothesis. It is also important to establish that
there are no competing explanations. We suggest that the effects of artificially generated cues be explored by using nonspeech signals or tones mimicking a vowel or noise that is filtered with the characteristics of a vocal tract,
etc. as stimuli. Such experiments may tell us about whether an encoded articulatory event alone is essential to decrease stuttering. It would also be interesting to see what happens when these cues are unfamiliar or belong to a
nonnative language, for example, when listening to a foreign language.
We further speculate that the information regarding the state of the articulators within the encoded portion of speech signal may, in a yet unknown manner, be responsible for engaging the speech motor areas of the
43 0
M . K. RAMI, ET AL.
cortex and ultimately responsible for entraining the speech-production system in people who stutter. If this be true, then the auditory system merely
conveys the information to the appropriate cortical areas like the visual system, which also can aid in producing the same effect. How long lasting and
how generalizable are such effects also requires study.
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