اثر دستکاری زمانی در درک ناروانی به صورت عادی و یا لکنت زبان

The purpose of this investigation was to study the effect of temporal features within repetition of speech segments on the perception of stuttering. Past research has provided evidence that certain temporal aspects of repetitions produced by people who stutter tend to be shorter than those produced by normally fluent speakers. The effect of these temporal factors on the perception of the disfluency as “stuttering” or “normal” has not yet been studied. Conversational speech of five children who stutter was recorded. Two short utterances, one containing part-word repetition (PWR) and one containing whole-word repetition (WWR), were identified in the speech of each child and then manipulated by the CSL and CSpeech computer softwares. Two selected elements within repetitions, namely the vowel of the repeated unit and the interval between the repeated units (e.g., but-but), were lengthened to simulate normal disfluency. Results indicated that both factors (interval duration and vowel duration) moderately affected listeners' perception. In general, repetitions with short vowel and interval durations were judged as more representative of stuttering, whereas repetitions with longer vowel and interval duration were judged as more representative of normal speech. Learning outcomes: As a result of this activity, the reader will learn about (1) various factors that influence the perception of disfluent segments as stuttering, (2) the special effect of duration of specific elements within repetitions on the perception of disfluency as stuttering, and (3) the possible implications of the new information for therapeutic considerations.

Research concerning differences between disfluencies that are “normal” and those characterized as “stuttering” has taken on two major tracks: studies that focused on disfluency output and studies that focused on listener perception. The present investigation pertains to perception, an issue that was thrust into prominence after the early exposition of the diagnosogenic theory (Johnson, 1942). With its basic tenet that stuttering is caused by listeners' erroneous judgment of a child's normal disfluencies as being “stuttering,” the theory has provided an impetus for many studies of factors that influence such perception. Studies concerning overall conditions and circumstances that affect listener judgment of a speaker as “a stutterer,” or specific disfluent events as “stuttering” or “normal,” have shown the impact of experience, background, psychological set, and experimental instructions on listeners' shifting judgment Cordes et al., 1992, Curlee, 1981, Tuthill, 1940, Tuthill, 1946 and Williams & Kent, 1958. Other perceptual studies have shown that certain types of disfluency, such as syllable repetitions and sound prolongations, are more likely to be judged as “stuttering” than other types (e.g., Boehmler, 1958 and Young, 1961) and that the number of disfluencies in a given sample, regardless of type, is also likely to influence stuttering judgment Curran & Hood, 1977, Hegde & Hartman, 1979a and Hegde & Hartman, 1979b. The number of repetition units per disfluent event was also found to influence listeners' perception. Sander (1963) found that it took fewer instances of double-unit repetition (e.g., bu-bu-but) than single-unit repetition (bu-but) to be judged by listeners as stuttering. Curran and Hood (1977) reported similar perceptual findings. Although perceptual studies have identified basic trends about disfluency typically perceived as stuttering or as normal, inconsistent judgments, overlap, ambiguities, and contradictory findings still exist (Curlee, 1981). For example, although changing instructions caused listeners to judge the same disfluencies as both stuttering and normal (Williams & Kent, 1958), some disfluent events were judged as stuttering more consistently than other disfluent events. Recently, Cordes (2000) reported that some disfluent events classified as “stuttering-like disfluencies” (SLD) were judged as normal. Thus, what makes a specific disfluent event more likely to be perceived as “normal” or “stuttering,” and what contributes to more stable judgment of a given disfluent event, is not yet understood. One reason for the difficulty in achieving better appreciation of features that influence stuttering–normal judgment is the fact that traditional disfluency classification systems have been grounded in linguistic terminology such as “part-word repetition (PWR), word-phrase repetition, sound prolongation,” etc. (Johnson et al., 1959). Although these descriptors are certainly useful for many purposes, they are apparently rather general and do not readily lend themselves to analysis of fine features. For example, it would seem reasonable to hypothesize that acoustic features, such as intonation, stress, etc., may help listeners differentiate between disfluencies produced by persons who stutter and fluent speakers and thus contribute to listeners' identification of stuttering Schwartz, 1995 and Van Riper, 1982. Attention to acoustic correlates of disfluencies regarded as stuttering or normal has been limited. The parameter of duration is of particular interest to the present study. Zebrowski (1991) compared overall duration of disfluencies in the speech of preschoolers who stutter to a control group and found no statistically significant differences. More promising findings, however, were reported for duration of segments within disfluencies. Howell and Vause (1986) analyzed speech samples of eight children who stutter and found that the duration of vowels produced during stuttered PWR was significantly shorter than the duration of vowels produced during the fluent speech of each speaker. They concluded that this shortening of duration led the listeners to believe that the vowel was neutralized to schwa. Howell, Williams, and Vause (1987) also reported that vowel duration within stuttered PWR was shorter than vowels within the fluent speech of the same speakers. Additionally, stuttered vowels were lower in amplitude than fluent vowels. In a third study, Howell and Williams (1992) continued to study this question from a perceptual perspective. Although the results also revealed durational differences between vowels produced in stuttered and fluent repetitions produced by the same speakers, manipulation of vowel duration, by itself, was not sufficient for changing listeners' perception of an intended vowel to a schwa. Another group of studies pursued a different temporal parameter of duration of subsegments within the disfluencies. Yairi and Hall (1993) reported a trend for the intervals between segments of a whole-word repetition (WWR) (e.g., the interval between “but-but”) of young children who stutter to be shorter than those in the repetitions of children who did not stutter. In a follow-up investigation, Throneburg and Yairi (1994) confirmed this trend, reporting significant group differences in the interval duration between repetition units in PWR and WWR. For example, normally fluent children had, on the average, intervals between the consecutive segments in PWR that were approximately three times longer than intervals in PWR produced by stuttering children. That is, children who stutter repeated faster than children who do not stutter did. At present, the effect of the duration of the interval on listener judgment of disfluencies is not known. In summary, it has been well documented that various conditions, instructions, type of disfluency, the amount of disfluency in the speech sample, and the number of repetition units influence listeners' perception of stuttering. It has been suggested that acoustic properties of disfluencies, such as duration, may differentiate between disfluencies produced by persons who stutter and fluent speakers. More recently, several investigators reported that stuttered vowels are shorter than fluent vowels in the speech of the same stuttering speakers and that durational features within repetitions produced by children who stutter differ from repetitions produced by normally fluent speakers. To date, however, there are not sufficient data to indicate whether these durational characteristics, evident in the speech output, also contribute to the perception of stuttering in speech of young children. The present study is an initial attempt to investigate this question. Consonant duration within repetition was not reported previously to differentiate stuttered speech from fluent speech. Thus, it was not included as a variable in this study. In addition, the use of natural speech rather than synthetic speech was deem desirable for this study in light of past findings regarding the presence of additional acoustic features that might contribute to listener identification of a disfluency as stuttering. Utilizing synthetic speech could, on one hand, isolate the parameters tested but, on the other hand, might present biased results. Listeners might respond differently to synthetic and natural disfluency. Both from a theoretical and clinical points of view, additional information regarding the relation between the physical features of early disfluency and listener's reactions to them should help clarify long-standing questions concerning the onset of stuttering. Are there factors beside the type and frequency of disfluency that register in listeners' minds as abnormal or stuttering? Is it possible that some apparently “simple, easy” repetitions contain additional elements that are not conveyed by their linguistic descriptions, making them different from similar disfluent events in the same class? Are some repetitions more “stuttering” than others are? In other words, what else is there that finally convinces parents and others that a child's repetition is a cause for concern? Additionally, data concerning the relation between physical characteristics of disfluent events and their perception may contribute to understanding of stuttering severity, provide clues for what might be important data to obtain in clinical evaluation, and might be useful in new development in the area of automatic recognition of stuttering. Information of this nature should also enhance formulating therapeutic objectives and, finally, help determine when and why children return to being normally fluent speakers. The present investigation was an exploratory study designed to assess the possibility that certain temporal acoustic features of specific components of disfluent events affect the perception of disfluencies as being normal or stuttering. Inasmuch as information on this parameter is scarce, we were looking for trends that might provide clues for further research concerning the relation between acoustic features and the normal/stuttering distinction. If interval and vowel duration within repetitions affect the perception of stuttering, stuttered vowels should sound more like normal disfluency if they are lengthened. To test this assumption, vowel duration and interval duration taken from speech samples of stuttering children were systematically manipulated. The effect of these manipulations was then evaluated perceptually by listeners. To limit the scope of the study, it was decided to manipulate only two types of disfluency: one-unit PWR and one-unit monosyllabic WWR. Incorporating other types of disfluency into this study would have required examination of multiple temporal factors as well as other acoustic features, thus utilizing different manipulation procedures. PWR and WWR were selected as the target disfluencies in this study because they are more likely to be found in normally fluent speech as well as in the speech of stuttering speakers. In addition, these disfluencies can be judged by listeners as both stuttering and normal disfluency.