J. FLUENCY DISORD. 24 (1999), 127–136© 1999 Elsevier Science Inc. All rights reserved. 0094-730X/99/$–see front matter655 Avenue of the Americas, New York, NY 10010 PII S0094-730X(98)00021-7

DIFFERENTIAL EFFECTS OF FREQUENCY-SHIFTED FEEDBACK BETWEEN CHILD ANDADULT STUTTERERS

PETER HOWELL and STEVIE SACKIN

University College London

ROBERTA WILLIAMS

City University

It has been reported previously that presentation of an altered form of the voice enhances thefluency of people who stutter. One of these forms of alteration is frequency-shifted feedback.The effects of frequency-shifted feedback were compared between two speaker groups thatdiffered in age. The fluency-enhancing effects of frequency-shifted feedback was greater foradult speakers (mean age 21.3) than for children (mean age 9.11). The results are discussed interms of their implications for theory and treatment. © 1999 Elsevier Science Inc.

Key Words:

Stuttering; Development; Frequency-shifted feed back; Treatment

INTRODUCTION

Several types of altered auditory feedback (AAF) are successful at reducingdysfluencies in adult speakers who stutter. The forms of alteration shown toimprove stutterers’ speech include delayed auditory feedback (DAF) of thespeaker’s voice (Ryan & van Kirk, 1974), white noise maskers (Cherry &Sayers, 1956), speech-synchronous buzz maskers (Dewar, Dewar, Austin &Brash, 1979), a nondelayed frequency-shifted version of the speaker’s voice(Howell, El-Yaniv & Powell, 1987; Kalinowski, Stuart, Sark, & Armson,1996), a metronome click (Fransella & Beech, 1965), and the sound of one ormore other voice

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s (Cherry & Sayers, 1956).Theoretical work on AAF has lagged behind empirical findings about the

situations under which the techniques are effective at reducing dysfluencies.This can be illustrated by considering frequency-shifted feedback (FSF). Themain effects are a) it improves fluency almost immediately it is switched on;

Address correspondence to Peter Howell, Department of Psychology, University College London,Gower St., London WC1E 6BT, England.

128 P. HOWELL ET AL.

b) its effects are limited to the interval in which the FSF is delivered; c) it in-volves no conscious effort by the speaker (Joe Kalinowski and his associatesat East Carolina University have produced a compelling video demonstrationof a, b, and c); d) it is independent of direction in which the frequency spec-trum of the voice is shifted (Stuart, Kalinowski, Armson, Stenstrom, & Jones,1996); e) fluency is enhanced even when speakers are required to speak at anincreased rate (Kalinowski et al., 1996); and f) speakers do not raise theirvoice level under FSF (Howell, 1990).

Though the studies are mainly empirical, they do rule out certain explana-tions of how stuttering arises. For instance, the fact that the effects are so im-mediate rules out explanations that speakers replan the utterance to compen-sate for the effects of AAF. The argument that alterations produce a maskingsound (Howell, El-Yaniv, & Powell, 1987) seems unlikely as well since theeffects operate whether the sound is shifted up or down in frequency, whereasmasking effects would be expected to be greater when the masker is low fre-quency (Mayer, 1894). The report that FSF does not produce a Lombard effect(Howell, 1990) is also consistent with the conclusion that the effect is not at-tributable to masking. Ingham, Moglia, Frank, Ingham, and Cordes’ (1997)suggestion that FSF works by increasing temporal lobe activity toward normallevels is in doubt due to methodological difficulties associated with the PETscan data they use to support their argument (Howell, Sackin, Au-Yeung, Wil-liams & Rustin, submitted). The fact that no adequate explanation has beensupplied to account for the effects of FSF to date, is one reason for seekingnew theoretical explanations about the situations under which FSF does anddoes not improve fluency. A second is the potential role this and other relatedAAF manipulations may have in the treatment of stuttering (see Discussion).

A theory has been developed recently which has its main focus on explain-ing how stuttering originates (Au-Yeung, Howell & Pilgrim, 1998). The sub-sidiary goals of this theory are identification of factors and circumstances thatlead to fluency breakdown, the CNS structures associated with this break-down and, of principal interest here, how FSF can alter the way these struc-tures operate and lead to fluent speech (Howell et al., submitted). The part ofthe theory that predicts when FSF will be effective and when it will not con-cerns how stuttering originates and this is summarized briefly. The theory wasbased on two facts available in the literature: 1) Fluent speakers show thesame dysfluencies as people who stutter. 2) The words that cause problems forthese speakers changes over their lifespan. The first of these facts suggestedthat all speakers start with equivalent speech problems, while the latter may beinterpreted as suggesting that speakers get locked into stuttering during latedevelopment. To amplify this last point further, the most common type of dys-fluency exhibited early in development by all speakers is repetition (R) offunction words (Fwds). Such an episode of dysfluency is often followed by acontent word (Cwd), so the speaker frequently produces constructions like “a

EFFECTS OF FREQUENCY-SHIFTED FEEDBACK 129

a a a stream”. Au-Yeung et al. (1998) argue that production of this type ofdysfluency is used deliberately to delay production of the later Cwd. Cwds arephonologically more complex than Fwds (for example, their onsets often haveconsonant strings as in this example) and they require, therefore, more timefor planning the execution form. This time would become available if Fwdsthat precede Cwds are repeated (Clark & Clark, 1977). This assumes that exe-cution of an Fwd can be autonomously reexecuted one or more times (seeBlackmer & Mitton, 1991 for evidence on fluent speech repairs that supportsthis). While the repetition is taking place, planning for subsequent words cancontinue unhindered and the plan for the Cwd is produced fluently when it isready.

A large proportion of a child’s early life is involved in interaction withadults who play a social and educational role. The caregivers are tolerantwhen children are young and allow them ample time to express themselves.However, as the child gets older he or she must develop effective means toavoid being interrupted. The theory proposes that children who persist in theirstutter adopt a particular strategy to hold the floor. They cease repeating Fwdsto delay execution (regarding this as a strategy that runs the risk of interrup-tion) and attempt the next word (usually a Cwd) in the intended utterance. Theattempt at producing the Cwd will then be made using an incomplete plan un-less the remainder of the plan becomes available during its execution. Whenthe remainder of the plan arrives during execution, the word will be producedfluently (a form of busking). If the plan is incomplete at the time of its execu-tion, speech problems will occur. In the example used previously, this mightresult in a prolongation as in “a ssstream” or a part-word R as in “as.s.s.s.stream.” The advance to Cwds is intended to increase (in an abstractsense) speech rate. According to the theory, then, the point around where dys-fluency is most likely to occur is in the vicinity of rapidly produced words.

The theory predicts that before some age (Au-Yeung et al., 1998 suggest anage around 11), stuttering is no different to the normal nonfluencies that all speak-ers show. After this age speakers adjust their speech rate. These adjustments inspeech rate lead to a type of stuttering that cannot be reversed without aid ofsome form. FSF is one way known to make speech fluent in adult speakers. Itis proposed, furthermore, that FSF brings fluency about by slowing speechrate of the fast stretches that leads to planning and execution processes beingput back into temporal alignment. The theory predicts that FSF would not beas effective at making child stutterers fluent because they do not put this tem-poral pressure on planning and execution processes by increasing speech rateand advancing to the Cwd. In the current experiment, frequency of stutteringis assessed on two groups of speakers who have attended speech therapy clin-ics for their stutter. The stuttering rate of the two groups of speakers is testedunder normal speaking conditions and FSF. It is predicted that frequency ofstuttering will reduce more markedly for adults than children under FSF.

130 P. HOWELL ET AL.

METHOD

Subjects and Materials

Two groups of speakers participated in the experiment. There were eight sub-jects (all male) in each group. The groups were differentiated by age. The agesof the speakers in the youngest group ranged from 9 to 11 years (mean age9.11). The ages of the speakers in the adult group ranged from 20 to 24 (meanage 21.3). All the speakers had been assessed by therapists and were enrolledfor a two-week intensive therapy course. None of the speakers in either grouphad received therapy before. The recordings were made when they first at-tended the course, prior to any treatment. Frequency of stuttering (assessed asdescribed below) was approximately equal under normal listening conditionsfor the two groups: For the child group the mean rate per hundred words was12.1% (range 7.4–15.2) and for the adult group 13.1% (range 7.0–16.1).

Apparatus

Recordings of the speakers were made in an Amplisilence sound-attenuatedbooth (Howell, Staveley, Sackin & Rustin, in press). The speech was trans-duced with a Sennheiser K6 microphone positioned 6 inches in front of thespeaker in direct line with the mouth.

All listening tests were performed in this same sound-attenuated booth.Test sounds were played binaurally direct from computer for assessment via aFostex 6301B amplifier. Sound level was set at a comfortable level for listen-ing over headphones, and the level was checked routinely. The headphonesused were an RS 250-924 headset.

Procedure

Both groups of speakers read the “Arthur the rat” passage (Sweet, 1895,adapted by Abercrombie, 1964) under two listening conditions. The two con-ditions were FSF with speech shifted up by 1

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2 octave and normal auditoryfeedback (NAF). Speech was frequency shifted using commercially availablehardware (Digitech model studio 400). The sounds were delivered at the sub-ject’s most comfortable level over headphones. The headphones were alsoworn in the normal listening condition to keep the conditions as constant aspossible. The readings were recorded on DAT tape and transferred digitally tocomputer and down-sampled to 20 kHz for analysis. Two experienced speechresearchers transcribed the recordings independently by repeatedly listeningto the recording segmented into tone units. The transcriptions were made inorthographic format with dysfluencies marked as described below. The agree-ment between transcribers is high (95%) and so one transcriber’s version was

EFFECTS OF FREQUENCY-SHIFTED FEEDBACK 131

chosen at random for use in the experiment (the same transcriber’s versionwas used for assessing all speakers in all conditions).

Order of conditions (NAF and FSF) was counterbalanced across subjects topartially control for adaptation effects. In all conditions, the subjects were in-structed to read the story at as normal a rate as possible while attempting tomaintain maximum fluency.

Assessment Procedure

The recordings of the participants’ speech were transcribed using a broad pho-netic transcription in fluent regions and a narrow system in the region of dysflu-encies. Transcribers estimated the durations of pauses and prolonged segmentsto the nearest 50 ms and these were entered in the transcriptions. The stutteringtypes include segment, part-word, word, or phrase repetitions, segmental or syl-labic prolongations, and “other” dysfluencies. Other dysfluencies were mainlyinitiators or extraneous sequences (very often glottalic sounds or syllables ac-companied by high and

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or low pitch and stricture in the glottis). Howell, Sackin,and Glenn (1997) have reported that interjudge agreement for short-durationprolongations is low. Therefore, prolongations estimated to be 50 ms or less byeither judge were excluded as prolongations. Agreement between these judgeswas 79.82% for all dysfluencies, 82.59% for repetitions, 82.00% for prolonga-tions, and 61.18% on other dysfluencies. Other dysfluencies are a residual cate-gory (Wingate, 1988). This category is, by definition, heterogeneous, and thelow agreement was expected (Howell et al., 1997; Wingate, 1988). Therefore,other dysfluencies were excluded from the following analysis.

RESULTS

Dysfluency rates in percentages are shown for each speaker in both groups inTable 1. Overall, there was a small reduction under FSF for the children (from12.1 to 8.9%) but a much larger one for the adults (from 13.1 to 4.6%). Ananalysis of variance (ANOVA) was conducted on these data with age group(child versus adult) as a between-groups factor and listening condition (NAFversus FSF) as a within-groups factor. The difference between age groups wasnot significant (F1,28

5

3 .06,

p

.

0.05) which is due to frequency of stutter-ing under NAF being approximately equal across speaker groups (as requiredby the design). The difference between listening conditions was significant(F1,28

5

41.38,

p

,

0.001). The listening condition effect may be dominatedby the adult speakers who showed this reduction. The interaction between agegroups and listening conditions provides evidence about differential effects ofaltered feedback over age groups. This interaction was significant (F1,28

5

8.70,

p

,

0.01) which supports the prediction that children are less affected bythis form of altered feedback than are adults.

132 P. HOWELL ET AL.

DISCUSSION

According to the theoretical position presented in the introduction, dysfluen-cies in stuttered speech originate when speech plans are not available for exe-cution. Young speakers deal with this situation by delaying execution of a dif-ficult word (usually a Cwd) by repeating prior Fwds. Older speakers attemptthe difficult word on the basis of an incomplete plan. This theory predicts thatprocedures that slow speech will improve fluency of older speakers by allow-ing planning to get back into synchrony with execution. Since younger speak-ers have an alternative way of dealing with asynchrony between planning andexecution, they would gain less benefit from procedures, such as FSF, thatslow speech. The findings support the prediction by showing that there is lessreduction in dysfluency for young speakers than old speakers. According tothe theory, other ways of slowing speech rate would also improve fluency(particularly in older speakers) providing that they slow speech rate.

Speech rate varies throughout an utterance. An important aspect of the ac-count is that slowing needs to occur in local regions of speech (i.e. at the

Table 1.

Percentage dysfluencies in the “Arthur the rat passage” under normal auditory feedback (NAF) and frequency-shifted feedback (FSF)

Child NAF FSF

S1 11.9 6.1S2 7.4 7.5S3 14.4 8.9S4 13.1 11.1S5 15.2 12.5S6 9.7 5.1S7 10.9 8.8S8 14.4 10.9

— —12.1 8.9

Adult NAF FSF

S1 16.1 3.4S2 14.4 5.2S3 12.9 4.6S4 7.0 0.9S5 11.6 5.2S6 14.2 3.9S7 14.5 8.4S8 14.1 5.3

— —13.1 4.6

Note

: Data are shown for individual speakers with the dysfluency rates of the children at the top and theadults beneath.

EFFECTS OF FREQUENCY-SHIFTED FEEDBACK 133

points where planning gets out of synchrony with execution). Global speechrate measures, that have invariably been used in studies showing an associa-tion between speech rate and fluency, are relatively crude. Depending on theway that speech rate is slowed, it may or may not enhance fluency. For in-stance, speech rate can be reduced by slowing down all te speech proportion-ately or just by decreasing the rate of the slowest stretches. If there is a propor-tionate decrease of all stretches, this would reduce rate on the problematic faststretches, and fluency should increase. If the rate of the slow stretches aloneare affected, fluency improvement would not occur. Until metrics that allowlocal changes in rate to be measured are developed, it is difficult to interpretstudies that have claimed that rate changes do not occur under FSF despite flu-ency enhancement (Kalinowski et al., 1996). It is planned to develop localmeasures of speech rate and when they are available they will allow furthertests of the account presented here.

Other studies have noted that although FSF has dramatic and immediate ef-fects on fluency enhancement, these effects are mainly restricted to the periodin which the alteration is heard. It is the immediacy and the marked effects ofFSF that make it a potentially powerful procedure to use in conjunction withother techniques, particularly in conjunction with operant procedures. It hasbeen claimed that operant techniques produce long-term fluency enhance-ment; however, they require a long time for their effects to be established andthe points where they are effective cannot be controlled precisely (Onslow,Packman, Stocker, van Doorn, & Siegel, 1997).

Ingham et al. (1997) have reported a study which examined whether FSFproduces long-term improvements in fluency. These authors tested four subjectsover a number of 3-min trials. Though the data do not appear to have been sub-jected to statistical analysis, in their closing remarks, the authors conclude thatthe effects of FSF are not consistent across speakers. This conclusion is surpris-ing as Ingham et al. (1997) note some form of improvement under FSF for alltheir speakers. To quote the authors themselves, subject E.S. found “he couldspeak more easily during the FAF conditions . .”, subject F. G. “did show someindication of a reduction of stuttering,” subject A.G. “showed a dramatic reduc-tion in stuttering during both FAF conditions,” and subject E.O. “provided analmost textbook example of experimental effects . . .” Of particular relevance tothe current discussion are the data of subject E.S. who was a severe stuttererwho reported that FSF helped him. Ingham et al.’s (1997) analysis of his datawas not able to detect any improvement; however, inspection of the data of thissubject show that there is a ceiling effect with almost all intervals under normalauditory feedback or FSF being judged stuttered. This is expected as judgingwhether a 5 sec interval contains a stutter as in Ingham et al.’s (1997) study wouldprohibit detection of changes in disfluency rate (Howell et al., in press).

These comments are not meant to deny that the immediate effects of FSFare equivalent to, for instance, Onslow et al.’s (1997) more gradual effects

134 P. HOWELL ET AL.

that they achieve. There is obviously a marked difference in the time scale ofthese effects. There are two crucial things that need to be achieved by an ade-quate fluency treatment with stutterers: First, the speaker has to get out of thedysfluent behavior, and, second, he or she has to be able to sustain this overtime. It is our belief that FSF is an effective way of achieving the former re-quirement (as are other forms of AAF, slowed speech, regulated breathing andso on). Operant conditioning methods, long-term therapy, and so on are suit-able for achieving the latter. An adequate procedure needs to achieve both. Todevelop new effective treatments, the goals at each of these steps need to beexplicit. It seems wrongheaded to expect procedures like FSF and operantconditioning methods to meet both goals.

The preceding discussion leads to interesting possibilities for the combina-tion of procedures that are effective at these different levels as, for example,FSF and operant procedures. Moreover, these possibilities do not simply offerFSF a role but also raise issues about the best way of administering operantprocedures for getting potentially more rapid and sustainable improvementsthat have not been discussed previously. An effective conditioning procedureshould establish the required behavior and prevent it from extinguishing. FSFis rapid and highly effective at inducing fluency. This allows the experimenterto determine which of two types of response is made during stuttered epi-sodes; fluent speech (achieved with the aid of FSF) and dysfluent speech(when speakers are left with their NAF). If it is desired to prolong the formertype of behavior and reduce the latter during stuttered episodes, the partial re-inforcement extinction effect (PREE) suggests the way of doing this is to rein-force the required behavior intermittently rather than continuously (see Am-sel, 1992; Mackintosh, 1974 for reviews of the PREE effect on animals andMorely, 1979, for a review of studies on humans). The only question that re-mains is how to target reinforcement (FSF) on regions most likely to be stut-tered? The answer, according to the theory outlined in the introduction, is totarget regions spoken at a fast rate.

This research was supported by a grant from the Wellcome Trust. The authors

thank the representatives at East Carolina for supplying the Digitech hardware.

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