Stuttering as “False Alarm” Responding CAROL A. SHERRARD, M.A.
Psychology Department, University of Durham*
Summary It is hypothesised that a stutter is a false alarm, or superfluous correction response to speech feedback signals, such that the overt behavioural response is the repetition of a speech unit. The experiment attempts to elicit stuttering in normals by setting up two conditions, anxiety and divided attention, in which false alarms in any perceptual-motor task would be predicted. The divided attention condition does increase stutters. The anxiety condition does not increase stutters, but increases the probability of a speech error being corrected, indicating that anxiety directs attention to speech feedback. Auditory masking reduces stutters in all conditions, demonstrating the crucial role of auditory feedback in normal as well as in pathological speech disfuencies. The distribution of elicited stutters is analogous to the distribution of false alarms in orthodox signal detection experiments. Introduction Stutterers can speak fluently under many conditions, such as for example when whispering, singing, speaking in time with a metronome, with masking noise: with delayed auditory feedback, speaking with an assumed accent or manner, shadowing another speaker, speaking in chorus, and so on almost ad infinitum. This characteristic condition-dependency of stuttering is sometimes cited as evidence that the speech mechanism itself is intact in the disorder, and that stuttering must have its cause in some other generalised but speech-related function such as perception (Cherry, Sayers and Marland, 1955). It is important to be aware that this argument is fallacious, as has been pointed out by Hamre (1972); for mere intermittency or condition-dependency in a disorder does not preclude the possibility of a localised organic cause, as examples such as epilepsy and Parkinsonism demonstrate. Nevertheless, it is impressive that no evidence has yet come to light of any defect of the speech or hearing mechanism in stutterers, despite a multitude of “physical” theories which have been put forward in the past (see Fransella, 1971), and which continue to be put forward (e.g. Tomatis, 1956; Shearer, 1964; Wyke, 1971). The evidence available to date suggests then that the cause of stuttering, if there is a unitary cause, is not to be sought in organic defects. If not organic, the disorder must be explained in terms of psychological variables, and it should be possible, once the relevant variables have been identified, to induce stuttering experimentally in normal subjects. The experiment reported here is such an attempt. There are two psychological variables strongly associated with stuttering, one as a result of empirical research, the other by virtue of accumulated clinical observation. These are auditory feedback and anxiety. The importance of auditory feedback is *Now at Psychology Department, Edinburgh University. 83
84
BRITISH JOURNAL OF DISORDERS OF COMMUNICATION
known from experiments showing that stuttering is largely eliminated if the speaker is prevented from hearing his own voice (Maraist and Hutton, 1957; Burke, 1969; Murray, 1969) and less directly from demonstrations of the delayed auditory feedback phenomenon (delayed auditory feedback is brought about in magnetic recording by interspersing a delay between an utterance and its playback to the speaker during ongoing speech, and this frequently produced a speech pattern strikingly similar to that of stuttering, with multiple repetitions, prolongations and blocks). The second variable, anxiety, is clinically observed to be a frequent concomitant of stuttering (in the form of both trait and state anxiety). State anxiety has also been shown to precipitate speech disturbances, including stutter-like disfluencies, in normals (Kanfer, 1958 ; Kasl and Mahl, 1965; Cook, 1969). Consideration of possible mechanisms whereby anxiety may produce stuttered speech leads to the postulation of a third variable, narrowed attention. Anxiety is a state of high arousal, and research on the effects of high arousal on information-processing has shown repeatedly that it produces a narrowing of attention (Easterbrook, 1959; Callaway and Stone, 1960; Zaffy and Bruning, 1966; Wachtel, 1968; Hockey, 1970). Depending on the number and distribution of signals in a task, this narrowing of attention may be useful, or it may mean that signals or other stimuli which are not relevant to the task cannot be ignored, and therefore interfere with performance (Callaway and Band, 1958; Broadbent, 1970). The possible role of narrowed attention in stuttering is suggested by considering that the one common factor uniting the otherwise bewildering variety of conditions which will temporarily suppress stuttering is that of not attending to one’s own normal voice. All of these conditions provide a novel external stimulus. This in turn suggests that attention may be habitually narrowed to some disruptive stimulus during stuttering. Since the only stimulus critically absent during noise masking of the voice (which, as was pointed out earlier, largely eliminates stuttering) is auditory feedback, this variable would seem to be the candidate for the disruptive stimulus. Undue attention to feedback can have two related consequences. Firstly, in the case of a skilled, automatised behaviour (of which speech is a paradigm example) anxious attention to feedback can disrupt smooth performance. This is an effect familiar to athletes and musicians (Posner and Keele, 1968) which has been explained as a variety of cognitive inhibition (Kimble and Perlmuter, 1970). Secondly, and more specifically, an anxious, over-aroused performer of a skill is likely to over-correct his performance (Davis, cited by Broadbent, 1958). Davis and Tune (1970) cite evidence that overaroused performers are likely to make a high proportion of false alarm responses. False alarm responses are responses to erroneous perceptions of signals. In any situation, all responses to the presence or absence of signals are constrained by the possibilities set out in Table 1.
Table 1 Possible responses to presence or absence of signals
present
Response Yes No HIT MISS
Signal absent
FALSE ALARM
CORRECT DENIAL
STUTTERING AS “FALSE ALARM” RESPONDING
85
It is possible therefore to interpret the “over-correction” of anxious performers as a tendency to make false alarm responses to performance feedback signals, i.e. to attempt to “correct” performance in response to erroneously perceived “error” signals. In the case of speech, feedback monitoring is held to consist of a vigilance for error signals (Fairbanks, 1954). A false alarm in this context will therefore be a response to a supposed speech error, when no error had in fact occurred. Now, the “correction” of an item erroneously supposed to be incorrect can only result in a repetition of that item. If the item is a phoneme, the effect will be (for example) “y. .. yes”. If the response unit concerned is a higher one, the false alarm may issue in a repeated word or a repeated phrase. All such types of repetition have been included within the symptomatology of stuttering. The false alarm concept, as used here, is derived from signal detection theory, and it is appropriate to consider the extent to which it can properly be applied outside the context of signal detection theory experimentation. The concepts of the theory have already been applied in quite diverse fields: motor skill, (Davis and Tune, 1970), attention (Moray and O’Brien, 1967), and stuttering (Martin, 1970), though with quite a different approach to the present study in this latter case. When translating a model from one field to another, the propriety of doing so may be judged in terms of the number of assumptions which are required or implied in making the translation. In the present case, there are two major assumptions: (i) That speech feedback error signals are responded to in a manner analogous to the responses to externally produced signals such as are used in signal detection theory experiments. This assumption a t least has the virtue of parsimony; it assumes that self-generated, “maintenance” activities of the subject do not need a separate psychology for their description. (ii) That a response which is orientated toward potential “correction” shares the essential features of a “detection” response. In fact, it is not necessary to postulate an extra “correction” process here, over and above “detection”. The correction process is the response to signals, analogous to pressing a key in an orthodox signal detection theory experiment. However, there is a diflerence, which is that the response postulated in the case of stuttering is a response to self-generated, rather than external, signals. The ultimate justification for translating the signal detection model must be the degree of success achieved in describing and accounting for the new phenomenon; the proof of the pudding will be in the eating. We have so far postulated three variables in association with stuttering:- auditory feedback, anxiety, and narrowed attention. In strict terms, the latter two variables are postulated as mediators of another variable, which is the probability of false alarm responses. However, supposing that an experiment which applied these three variables to normal subjects did produce stuttered speech, then the identity inferred between stutters and false alarm responses would not have been subjected to test. In the experiment to be reported now, it was therefore decided to include a control condition which has been found to increase the probability of false alarms, but which has not previously been associated with stuttering. This is a divided attention condition, in which the subject performs two tasks simultaneously (Wiener, Poock and Steele, 1964; Mackworth, 1970). In order to be able to attribute any stutters elicited to the presence of auditory feedback, it was also necessary to include a control condition of masked auditory feedback. It was hypothesised that (i) anxiety would increase stutters (ii) divided attention would increase stutters (iii) stutters in all conditions would be reduced by auditory masking,
86
BRITISH JOURNAL OF DISORDERS OF COMMUNICATION
and (iv) the distribution of stutters would match that expected for false alarms in an analogous orthodox signal detection experiment. Method Subjects Twelve subjects were obtained by advertising in the university, and they were paid for taking part in the experiment, which was described as an investigation of the effects of anxiety on speech. Eight were male, and four female. The mean age was 22&,the age range was 19-30. Subjects were assigned randomly to groups. Materials Four texts were selected from a book on modern history, ranging in length from 532 to 569 words. For presentation to subjects, each text was typed on two foolscap sheets. Apparatus Shocks were used in the anxiety condition, and were generated from a 6-volt accumulator via an inductance coil. In the masked auditory feedback condition, noise was generated from a Linear Diatonic generator equipped with a special “speech” setting for masking use, selecting noise of equal energy per cycle up to 1,000 Hz, thereafter falling off at 12dB per octave. This noise was relayed to subjects through Koss Pro4A headphones, together with a pure sine wave tone from a Feedback function generator. Noise and tone were mixed by a Uher Stereo Mix Unit for binaural presentation. A stylus maze was used in the divided attention condition. The anxiety group received shocks when certain points in the maze were contacted, triggering micro-switches which activated the shock apparatus. In order to prevent learning of this maze, it was hidden from subjects’ view by a box placed over it, which had an open front through which the subject’s right arm passed. Speech was recorded from a free-standing Uher microphone onto a Tandberg 1221X tape recorder. Design and procedure All subjects performed the same four tasks. These were: 1. A “base” task, in which a text was simply read out, and which provided a sample of subjects’ ordinary oral reading speech. (All texts were presented in random order within subjects.) 2. A divided attention task, in which subjects read out a text while at the same time following the stylus maze with their right hand (all subjects were right-handed). 3. A masked “base” task, identical with task 1, except that auditory feedback was masked with noise and a pure tone. In masked conditions, the experimenter adjusted the intensity of the noise and the intensity and frequency of the tone until the subject was certain that he could not hear his own voice. 4. A masked divided attention task, which was the same as task 2, with the addition of masking. Half of each group (anxiety group and non-anxiety group) performed the tasks with masking first, and this order was reversed for the other half. The base and maze tasks were done alternately throughout by each subject. Since two maze tasks with the same maze were thus required of each subject, a further precaution to prevent learning
STUTTERING AS “FALSE ALARM” RESPONDING
87
of the maze or of the positions of the shock micro-switches was to change the direction of the maze by turning it through 90 degrees. Anxiety group subjects had electrodes attached to the back of the left hand from the start of the experiment, when an individual shock level was set for each subject. This was done by giving successively greater shocks until the subject gave distinct signs of discomfort. Voltage on the coil could be varied from 1.3 to 6-5volts. Subjects were not informed when to expect shocks (which were in fact only applied during the maze condition), and so the threat of shock was present throughout for this group. Subjects were instructed to speak at a steady rate throughout, and were tapped on the arm in accordance with a pre-arranged signal if their rate of speech increased.
Results Treatment of results A stutter was defined as a part-word repeat, word repeat, within-word hesitation, or phrase repeat (two or more words were considered a phrase). All measures were independent. In the anxiety condition, it was observed that sometimes the effect of a shock was to abruptly halt the ongoing utterance of a word or phrase, which would then be re-started once the subject began speaking again, resulting in some repetition. It seemed clear that these were instances of linguistic units which could not easily be continued after an interruption, for reasons of breath-group, prosody, or syllable organisation. Thus for example, if “can” is interrupted after “ca. . .”, it is impossible to complete the word naturally, and so it will be re-uttered. Similarly, if a phrase is interrupted it is usually easier to re-start the phrase than to attempt to continue it from the point of interruption. All such instances of repeats in the immediate aftermath of a shock were omitted from the scores. Tapes were transcribed and scored according to the above measures, and the number of stutters scored was divided by the number of syllables uttered then multiplied by 1000 to give the number of stutters per 1000 syllables. The syllable was chosen as the reference unit in preference to the word since it is relatively invariant in length, thereby giving a more stable basis for comparison. The proportions thus obtained were submitted to a square root transformation before analysis of variance, since there were some scores of zero, and also because the scores were expressed as proportions (Alder and Roessler, 1968). The data were further examined in relation to the hypothesis that a stutter is a false alarm response, and should therefore show an appropriate distribution when compared with hits and misses. Phonetic errors (distortions, omissions, and additions of speech sounds, and misplaced word stresses) were therefore counted and categorised as either hits (corrected errors) or misses (uncorrected errors). These scores were treated in the same way as the stutter scores.
Stutters The analysis of variance on stutters showed significant effects of masking (F=14.5, df 1,23: p>-O1) and of task (i.e. base or divided attention task) (F=16-61, df 1,23; p < 401). There was no significant effect of anxiety (F=0.46, df I$). Table 2 shows that masking reduced stutters, and that the divided attention (maze) task increased stutters.
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BRITISH JOURNAL OF DISORDERS OF COMMUNICATION
Table 2 Mean stutters per 1000 syllables. F
=
with auditory feedback, M
ANXIETY
F M
-
X
Base 3.2 2.0
Maze
2,6
CONTROL
5.0
Base 2.2
4.2
0.5
Maze 6.3 2.4
4.6
1.3
4.3
-
x
with masking
-
X 4.2 2.2
Table 3 Mean proportion of hits to misses per 1000 syllables. F M = with masking
F M
=
=
with auditory feedback,
-
ANXIETY
CONTROL
X
0.9 0.6
0.3 0.2
0.6 0.4
0.7
0.3
Hits and misses Analysis of variance on these scores showed significant effects of anxiety (F= 20.58, df 1,55; p
Psychology Department, University of Durham*
Summary It is hypothesised that a stutter is a false alarm, or superfluous correction response to speech feedback signals, such that the overt behavioural response is the repetition of a speech unit. The experiment attempts to elicit stuttering in normals by setting up two conditions, anxiety and divided attention, in which false alarms in any perceptual-motor task would be predicted. The divided attention condition does increase stutters. The anxiety condition does not increase stutters, but increases the probability of a speech error being corrected, indicating that anxiety directs attention to speech feedback. Auditory masking reduces stutters in all conditions, demonstrating the crucial role of auditory feedback in normal as well as in pathological speech disfuencies. The distribution of elicited stutters is analogous to the distribution of false alarms in orthodox signal detection experiments. Introduction Stutterers can speak fluently under many conditions, such as for example when whispering, singing, speaking in time with a metronome, with masking noise: with delayed auditory feedback, speaking with an assumed accent or manner, shadowing another speaker, speaking in chorus, and so on almost ad infinitum. This characteristic condition-dependency of stuttering is sometimes cited as evidence that the speech mechanism itself is intact in the disorder, and that stuttering must have its cause in some other generalised but speech-related function such as perception (Cherry, Sayers and Marland, 1955). It is important to be aware that this argument is fallacious, as has been pointed out by Hamre (1972); for mere intermittency or condition-dependency in a disorder does not preclude the possibility of a localised organic cause, as examples such as epilepsy and Parkinsonism demonstrate. Nevertheless, it is impressive that no evidence has yet come to light of any defect of the speech or hearing mechanism in stutterers, despite a multitude of “physical” theories which have been put forward in the past (see Fransella, 1971), and which continue to be put forward (e.g. Tomatis, 1956; Shearer, 1964; Wyke, 1971). The evidence available to date suggests then that the cause of stuttering, if there is a unitary cause, is not to be sought in organic defects. If not organic, the disorder must be explained in terms of psychological variables, and it should be possible, once the relevant variables have been identified, to induce stuttering experimentally in normal subjects. The experiment reported here is such an attempt. There are two psychological variables strongly associated with stuttering, one as a result of empirical research, the other by virtue of accumulated clinical observation. These are auditory feedback and anxiety. The importance of auditory feedback is *Now at Psychology Department, Edinburgh University. 83
84
BRITISH JOURNAL OF DISORDERS OF COMMUNICATION
known from experiments showing that stuttering is largely eliminated if the speaker is prevented from hearing his own voice (Maraist and Hutton, 1957; Burke, 1969; Murray, 1969) and less directly from demonstrations of the delayed auditory feedback phenomenon (delayed auditory feedback is brought about in magnetic recording by interspersing a delay between an utterance and its playback to the speaker during ongoing speech, and this frequently produced a speech pattern strikingly similar to that of stuttering, with multiple repetitions, prolongations and blocks). The second variable, anxiety, is clinically observed to be a frequent concomitant of stuttering (in the form of both trait and state anxiety). State anxiety has also been shown to precipitate speech disturbances, including stutter-like disfluencies, in normals (Kanfer, 1958 ; Kasl and Mahl, 1965; Cook, 1969). Consideration of possible mechanisms whereby anxiety may produce stuttered speech leads to the postulation of a third variable, narrowed attention. Anxiety is a state of high arousal, and research on the effects of high arousal on information-processing has shown repeatedly that it produces a narrowing of attention (Easterbrook, 1959; Callaway and Stone, 1960; Zaffy and Bruning, 1966; Wachtel, 1968; Hockey, 1970). Depending on the number and distribution of signals in a task, this narrowing of attention may be useful, or it may mean that signals or other stimuli which are not relevant to the task cannot be ignored, and therefore interfere with performance (Callaway and Band, 1958; Broadbent, 1970). The possible role of narrowed attention in stuttering is suggested by considering that the one common factor uniting the otherwise bewildering variety of conditions which will temporarily suppress stuttering is that of not attending to one’s own normal voice. All of these conditions provide a novel external stimulus. This in turn suggests that attention may be habitually narrowed to some disruptive stimulus during stuttering. Since the only stimulus critically absent during noise masking of the voice (which, as was pointed out earlier, largely eliminates stuttering) is auditory feedback, this variable would seem to be the candidate for the disruptive stimulus. Undue attention to feedback can have two related consequences. Firstly, in the case of a skilled, automatised behaviour (of which speech is a paradigm example) anxious attention to feedback can disrupt smooth performance. This is an effect familiar to athletes and musicians (Posner and Keele, 1968) which has been explained as a variety of cognitive inhibition (Kimble and Perlmuter, 1970). Secondly, and more specifically, an anxious, over-aroused performer of a skill is likely to over-correct his performance (Davis, cited by Broadbent, 1958). Davis and Tune (1970) cite evidence that overaroused performers are likely to make a high proportion of false alarm responses. False alarm responses are responses to erroneous perceptions of signals. In any situation, all responses to the presence or absence of signals are constrained by the possibilities set out in Table 1.
Table 1 Possible responses to presence or absence of signals
present
Response Yes No HIT MISS
Signal absent
FALSE ALARM
CORRECT DENIAL
STUTTERING AS “FALSE ALARM” RESPONDING
85
It is possible therefore to interpret the “over-correction” of anxious performers as a tendency to make false alarm responses to performance feedback signals, i.e. to attempt to “correct” performance in response to erroneously perceived “error” signals. In the case of speech, feedback monitoring is held to consist of a vigilance for error signals (Fairbanks, 1954). A false alarm in this context will therefore be a response to a supposed speech error, when no error had in fact occurred. Now, the “correction” of an item erroneously supposed to be incorrect can only result in a repetition of that item. If the item is a phoneme, the effect will be (for example) “y. .. yes”. If the response unit concerned is a higher one, the false alarm may issue in a repeated word or a repeated phrase. All such types of repetition have been included within the symptomatology of stuttering. The false alarm concept, as used here, is derived from signal detection theory, and it is appropriate to consider the extent to which it can properly be applied outside the context of signal detection theory experimentation. The concepts of the theory have already been applied in quite diverse fields: motor skill, (Davis and Tune, 1970), attention (Moray and O’Brien, 1967), and stuttering (Martin, 1970), though with quite a different approach to the present study in this latter case. When translating a model from one field to another, the propriety of doing so may be judged in terms of the number of assumptions which are required or implied in making the translation. In the present case, there are two major assumptions: (i) That speech feedback error signals are responded to in a manner analogous to the responses to externally produced signals such as are used in signal detection theory experiments. This assumption a t least has the virtue of parsimony; it assumes that self-generated, “maintenance” activities of the subject do not need a separate psychology for their description. (ii) That a response which is orientated toward potential “correction” shares the essential features of a “detection” response. In fact, it is not necessary to postulate an extra “correction” process here, over and above “detection”. The correction process is the response to signals, analogous to pressing a key in an orthodox signal detection theory experiment. However, there is a diflerence, which is that the response postulated in the case of stuttering is a response to self-generated, rather than external, signals. The ultimate justification for translating the signal detection model must be the degree of success achieved in describing and accounting for the new phenomenon; the proof of the pudding will be in the eating. We have so far postulated three variables in association with stuttering:- auditory feedback, anxiety, and narrowed attention. In strict terms, the latter two variables are postulated as mediators of another variable, which is the probability of false alarm responses. However, supposing that an experiment which applied these three variables to normal subjects did produce stuttered speech, then the identity inferred between stutters and false alarm responses would not have been subjected to test. In the experiment to be reported now, it was therefore decided to include a control condition which has been found to increase the probability of false alarms, but which has not previously been associated with stuttering. This is a divided attention condition, in which the subject performs two tasks simultaneously (Wiener, Poock and Steele, 1964; Mackworth, 1970). In order to be able to attribute any stutters elicited to the presence of auditory feedback, it was also necessary to include a control condition of masked auditory feedback. It was hypothesised that (i) anxiety would increase stutters (ii) divided attention would increase stutters (iii) stutters in all conditions would be reduced by auditory masking,
86
BRITISH JOURNAL OF DISORDERS OF COMMUNICATION
and (iv) the distribution of stutters would match that expected for false alarms in an analogous orthodox signal detection experiment. Method Subjects Twelve subjects were obtained by advertising in the university, and they were paid for taking part in the experiment, which was described as an investigation of the effects of anxiety on speech. Eight were male, and four female. The mean age was 22&,the age range was 19-30. Subjects were assigned randomly to groups. Materials Four texts were selected from a book on modern history, ranging in length from 532 to 569 words. For presentation to subjects, each text was typed on two foolscap sheets. Apparatus Shocks were used in the anxiety condition, and were generated from a 6-volt accumulator via an inductance coil. In the masked auditory feedback condition, noise was generated from a Linear Diatonic generator equipped with a special “speech” setting for masking use, selecting noise of equal energy per cycle up to 1,000 Hz, thereafter falling off at 12dB per octave. This noise was relayed to subjects through Koss Pro4A headphones, together with a pure sine wave tone from a Feedback function generator. Noise and tone were mixed by a Uher Stereo Mix Unit for binaural presentation. A stylus maze was used in the divided attention condition. The anxiety group received shocks when certain points in the maze were contacted, triggering micro-switches which activated the shock apparatus. In order to prevent learning of this maze, it was hidden from subjects’ view by a box placed over it, which had an open front through which the subject’s right arm passed. Speech was recorded from a free-standing Uher microphone onto a Tandberg 1221X tape recorder. Design and procedure All subjects performed the same four tasks. These were: 1. A “base” task, in which a text was simply read out, and which provided a sample of subjects’ ordinary oral reading speech. (All texts were presented in random order within subjects.) 2. A divided attention task, in which subjects read out a text while at the same time following the stylus maze with their right hand (all subjects were right-handed). 3. A masked “base” task, identical with task 1, except that auditory feedback was masked with noise and a pure tone. In masked conditions, the experimenter adjusted the intensity of the noise and the intensity and frequency of the tone until the subject was certain that he could not hear his own voice. 4. A masked divided attention task, which was the same as task 2, with the addition of masking. Half of each group (anxiety group and non-anxiety group) performed the tasks with masking first, and this order was reversed for the other half. The base and maze tasks were done alternately throughout by each subject. Since two maze tasks with the same maze were thus required of each subject, a further precaution to prevent learning
STUTTERING AS “FALSE ALARM” RESPONDING
87
of the maze or of the positions of the shock micro-switches was to change the direction of the maze by turning it through 90 degrees. Anxiety group subjects had electrodes attached to the back of the left hand from the start of the experiment, when an individual shock level was set for each subject. This was done by giving successively greater shocks until the subject gave distinct signs of discomfort. Voltage on the coil could be varied from 1.3 to 6-5volts. Subjects were not informed when to expect shocks (which were in fact only applied during the maze condition), and so the threat of shock was present throughout for this group. Subjects were instructed to speak at a steady rate throughout, and were tapped on the arm in accordance with a pre-arranged signal if their rate of speech increased.
Results Treatment of results A stutter was defined as a part-word repeat, word repeat, within-word hesitation, or phrase repeat (two or more words were considered a phrase). All measures were independent. In the anxiety condition, it was observed that sometimes the effect of a shock was to abruptly halt the ongoing utterance of a word or phrase, which would then be re-started once the subject began speaking again, resulting in some repetition. It seemed clear that these were instances of linguistic units which could not easily be continued after an interruption, for reasons of breath-group, prosody, or syllable organisation. Thus for example, if “can” is interrupted after “ca. . .”, it is impossible to complete the word naturally, and so it will be re-uttered. Similarly, if a phrase is interrupted it is usually easier to re-start the phrase than to attempt to continue it from the point of interruption. All such instances of repeats in the immediate aftermath of a shock were omitted from the scores. Tapes were transcribed and scored according to the above measures, and the number of stutters scored was divided by the number of syllables uttered then multiplied by 1000 to give the number of stutters per 1000 syllables. The syllable was chosen as the reference unit in preference to the word since it is relatively invariant in length, thereby giving a more stable basis for comparison. The proportions thus obtained were submitted to a square root transformation before analysis of variance, since there were some scores of zero, and also because the scores were expressed as proportions (Alder and Roessler, 1968). The data were further examined in relation to the hypothesis that a stutter is a false alarm response, and should therefore show an appropriate distribution when compared with hits and misses. Phonetic errors (distortions, omissions, and additions of speech sounds, and misplaced word stresses) were therefore counted and categorised as either hits (corrected errors) or misses (uncorrected errors). These scores were treated in the same way as the stutter scores.
Stutters The analysis of variance on stutters showed significant effects of masking (F=14.5, df 1,23: p>-O1) and of task (i.e. base or divided attention task) (F=16-61, df 1,23; p < 401). There was no significant effect of anxiety (F=0.46, df I$). Table 2 shows that masking reduced stutters, and that the divided attention (maze) task increased stutters.
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BRITISH JOURNAL OF DISORDERS OF COMMUNICATION
Table 2 Mean stutters per 1000 syllables. F
=
with auditory feedback, M
ANXIETY
F M
-
X
Base 3.2 2.0
Maze
2,6
CONTROL
5.0
Base 2.2
4.2
0.5
Maze 6.3 2.4
4.6
1.3
4.3
-
x
with masking
-
X 4.2 2.2
Table 3 Mean proportion of hits to misses per 1000 syllables. F M = with masking
F M
=
=
with auditory feedback,
-
ANXIETY
CONTROL
X
0.9 0.6
0.3 0.2
0.6 0.4
0.7
0.3
Hits and misses Analysis of variance on these scores showed significant effects of anxiety (F= 20.58, df 1,55; p
