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Speech-Motor Control and Interhemispheric Relations in Recovered and Persistent Stuttering David C. Forster & William G. Webster Published online: 08 Jun 2010.

To cite this article: David C. Forster & William G. Webster (2001) Speech-Motor Control and Interhemispheric Relations in Recovered and Persistent Stuttering, Developmental Neuropsychology, 19:2, 125-145, DOI: 10.1207/S15326942DN1902_1 To link to this article: http://dx.doi.org/10.1207/S15326942DN1902_1

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DEVELOPMENTAL NEUROPSYCHOLOGY, 19(2), 125–145 Copyright © 2001, Lawrence Erlbaum Associates, Inc.

Speech–Motor Control and Interhemispheric Relations in Recovered and Persistent Stuttering David C. Forster Department of Psychology Carleton University

William G. Webster Department of Psychology Brock University

The neurological basis of stuttering is associated with anomalies of interhemispheric relations and of the neural mechanisms of speech–motor control, specifically those involving the supplementary motor area (SMA). Stuttering typically develops through childhood and adolescence but many children will recover without formal treatment or intervention. The hypothesis that such spontaneous recovery is related to a maturation of the SMA is explored. Four experimental tasks were performed by adults whose stuttering has persisted, adults who reported having stuttered as children, and a control group of adults who reported never having stuttered. A Sequence Reproduction Finger Tapping task (Webster, 1986) and a Bimanual Crank Turning task (Preilowski, 1972) examined the functioning of the SMA, and 2 Divided Visual Field tasks examined asymmetries of hemispheric activation. The overall pattern of results for persistent stutterers compared to nonstutterers was consistent with motor–perceptual anomalies previously reported in the literature. The Bimanual Crank Turning task revealed additionally that the bimanual coordination deficits reported in adults who stutter are kinesthetically based and mediated through anterior callosal systems, including the SMA. Ex-stutterers were similar to nonstutterers in their performance of the motor control tasks, but similar to persistent stutterers in perceptual asymmetries associated with Divided Visual Field tasks. Taken together, the results from the four experimental tasks support the general hypothesis that an anomaly Requests for reprints should be sent to William G. Webster, Dean of Social Sciences, Department of Psychology, Brock University, St. Catharines, Ontario, Canada L2S 3A1. E-mail: [email protected]

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in interhemispheric relations and a deficit in the mechanisms of speech–motor control are each a necessary but not sufficient condition for stuttering and that recovery from childhood stuttering reflects a maturation of the mechanisms of speech–motor control.

The neurological basis of stuttering is associated with anomalies of interhemispheric relations and of the neural mechanisms of speech–motor control, specifically those involving the supplementary motor area (SMA). Although there is normal structural lateralization of speech and language mechanisms in people who stutter (Andrews, Quinn, & Sorby, 1972; Luessenhop, Boggs, Laborwit, & Walle, 1973; Webster, 1985, 1997), an anomalous pattern of hemispheric activation becomes apparent when people who stutter engage in speech- or language-related tasks (Braun et al., 1997; De Nil & Kroll, 1999; Kroll, De Nil, Kapur, & Houle, 1997; Moore, 1984; Moore, Craven, & Faber, 1982; Moore & Haynes, 1980; Rastatter & Dell, 1987b). This anomaly is reflected in attenuated and variable perceptual asymmetries (Blood, 1985; Blood & Blood, 1989; Curry & Gregory, 1969; Hand & Haynes, 1983; Moore, 1976, 1984; Quinn, 1972; Rosenfield & Goodglass, 1980; Sommers, Brady, & Moore, 1975). Webster (1990a) and Forster and Webster (1991) have suggested that the mechanism underlying this anomaly is a labile system of hemispheric activation that leads to inappropriate right hemisphere overactivation. This overactivation may have its effects on speech fluency through interhemispheric interference with the left hemisphere mechanisms of speech–motor control (Webster, 1990b, 1993, 1997). Behavioral studies of complex sequential movements by stutterers (Caruso, Abbs, & Gracco, 1988; Webster, 1988) and neuroimaging research findings (Fox et al., 1996; Pool, Devous, Freeman, Watson, & Finitzo, 1991; Van Lieshout, Hulstijn, & Peters, 1996; Watson et al., 1994) have indicated an association between stuttering and SMA mechanisms that Webster (1997) suggested is fragile or susceptible to interference from concurrent neural activity. Forster and Webster (1991) argued that each anomaly (an SMA susceptible to interference and a labile pattern of hemispheric activation that leads to the right hemisphere being one prime source of interference) is a necessary but not sufficient condition for the development of stuttering. Implicit in this idea is that recovery from childhood stuttering, which occurs in the majority of cases (Andrews et al., 1983; Andrews & Harris, 1964) and without formal treatment or intervention (Curlee & Yairi, 1997), results from the maturation of one or both of the underlying neural systems. Factors that influence recovery or persistence of stuttering are not understood (Bloodstein, 1995), but it is now clear that a genetic factor is associated with both the onset of stuttering and its persistence into adulthood (Ambrose, Cox, & Yairi, 1997). In our framework, that genetic factor relates to central nervous system maturation. A consideration of the literature on maturation leads us to suspect that the relevant maturational processes underlying recovery from stuttering relate more to

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the SMA factor than to the factor of labile hemispheric activation, which we view as being developmentally more invariate. In this study we explored this maturation hypothesis. Four experimental tasks were performed by adults who stutter, adults who reported having stuttered as children, and a control group of adults who reported never having stuttered. A Sequence Reproduction Finger Tapping task (Webster, 1986) and a Bimanual Crank Turning task (Preilowski, 1972) were included as tasks sensitive to SMA functioning and on which adult stutterers have been shown to be impaired. Two tachistoscopic Divided Visual Field tasks, a Lexical Decision task and a Dot Enumeration task, were intended to examine the hypothesized anomaly of interhemispheric relations.

METHOD Participants The participants were recruited from the university and broader community and comprised three groups of 24 adults each (16 men and 8 women in each group, ranging in age from 19 to 59 years): (a) self-reported stutterers (Group ST), (b) fluent speakers who reported never having stuttered (nonstutterers, Group NS), and (c) individuals who reported having stuttered in childhood but who no longer regard themselves as people who stutter (ex-stutterers, Group ES). All participants included in the analyses were right-handed as indicated by positive laterality quotients on the Edinburgh Handedness Inventory (Oldfield, 1971) administered at the time of experimental testing. The mean (and standard deviation) laterality quotients scores for Groups NS, ST, and ES were 94.1 (10.8), 91.5 (18.3), and 94.4 (1.5), respectively, and were not statistically different,F(2, 69) = 0.31. Family history of stuttering was assessed by interview as described by Poulos and Webster (1991). The percentage of participants in Groups NS, ST, and ES reporting at least one family member who stuttered was 8.3%, 66.7%, and 29.2%, respectively—the discrepancy between Groups ST and ES perhaps reflecting the influence of the genetic factor thought to be of significance for stuttering persistence (Ambrose et al., 1997). In categorizing ex-stutterers into Group ES, ideally we would have wanted to confirm retrospective self-report of the speech status of each participant with interviews of parents and former teachers, but the age range of participants and other practical considerations made this unfeasible. However, Finn’s (1997) study validating self-report of recovered stutterers does provide a strong basis for confidence in the approach adopted. Because the inclusion of nonstutterers into a grouping of ex-stutterers could arise from the relatively common occurrence of brief periods of speech disfluency in preschool children (Andrews et al., 1983), the selection criterion used in our study excluded as participants those whose age of

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stuttering onset and recovery from stuttering was too young to distinguish them with confidence from simple developmental disfluency. For this reason, the mean reported age of onset of stuttering, 5.6 years for participants in both Groups ST and ES (SD = 2.2 and 2.9 years, respectively), and the mean age (and standard deviation) at which participants in Group ES considered themselves to have recovered from stuttering, 14.6 years (SD = 3.7), are higher than the mean ages typically reported in unselected samples (Andrews et al., 1983; Andrews & Harris, 1964). As one means of validation of participant classification, participants were administered two versions of the Perception of Stuttering Inventory (PSI; Wolff, 1967), which has three 20-item subscales related to struggle, expectancy, and avoidance behavior in speech and social interactions. The first version was the standard PSI that focused on current speech. The second version was modified such that the individual was asked to answer the same questions but with respect to recollections of speech and social interactions in preadolescence. Additionally, participants in Groups ST and ES were asked to rate the severity of their stuttering using a 7-point scale ranging from 1 (very mild) to 7 (very severe). The mean (and standard deviation) of ratings of stuttering severity in childhood was 4.7 (1.5) and 4.8 (1.5) for Groups ST and ES, respectively, and the mean (and standard deviation) severity of stuttering reported by participants in Group ST for the present time was 3.5 (1.7). Sequence Reproduction Finger Tapping Task On each trial, participants observed on a computer monitor a different sequence of illuminated circles (three to five elements in length) following which, on the sound of a tone, they were to tap the sequence repeatedly on telegraph keys for 5 sec. The task was designed to be functionally similar to that described by Webster (1986), although a computer was used to control stimulus presentation and to record and analyze accuracy and response speed. Thirty-six trials were presented, and the participants responded on all trials with the right hand. Of the 36 trials, 12 were with three unique elements each (e.g., 1-3-2, in which 1 represents the index finger, 3 the ring finger, and 2 the middle finger); 12 were with four unique elements each (e.g., 1-3-4-2); and 12 were with five elements each (e.g., 2-1-3-4-2), one of which necessarily was repeated. The four-element sequences were selected from the 24 possible orders of such length by eliminating sequences beginning with either the little finger or ring finger. The 12 three-element sequences were generated by removing the last element from the 12 four-element sequences. The 12 five-element sequences (out of a possible 96 such sequences) were generated by adding the ring finger to 6 of the four-element sequences and the little finger to the other 6 four-element sequences. The 36 different trials were presented in the same randomly generated order for all participants. The dependent measures of interest were (a) response initiation time (RIT), computed as the time from the onset of the signal tone to the onset of the first key

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tap; (b) sequence execution time (SET) for the first three elements of the first sequence tapped, computed as the time from the onset of the first tap to the onset of the fourth tap; and (c) the proportion of first sequences correctly tapped.

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Bimanual Crank Turning Task This task was administered using a computer-based version of Preilowski’s (1972) X–Y recorder apparatus similar in principle to an “Etch-a-Sketch” child’s toy. The main portion of the apparatus was comprised of two balanced cranks, mounted side by side on a metal chassis. The cranks were interfaced with the inner workings of a MicrosoftÒ bus mouse that was connected to an IBM PC-compatible computer. Turning the cranks thus moved a cursor on a flat-screen monitor, and movement of the cursor left a trace of the cursor’s path. Prior to each trial, the monitor displayed a track consisting of two parallel lines, 15 cm in length and located 2 cm apart, presented at one of two different angles, 26.5° or 63.5°. A dot was then displayed 1 cm below the entrance to the track. The participant’s task was to turn the crank handles (simultaneously rather than in an alternating manner) to move the dot through the track as quickly as possible but staying within the lines. When the track angle was 26.5°, successful performance required the right hand to turn twice as quickly as the left; when the angle was 63.5°, it required the left hand to turn twice as quickly as the right. Following the terminology of Peters (1987) and Webster (1990b), the two conditions are referred to as the right lead hand and left lead hand condition, respectively. Each participant received four blocks of four trials each: Blocks 1 and 3 were with one lead hand condition and Blocks 2 and 4 with the other lead hand condition, the order being counterbalanced across participants within each group. Testing commenced without practice trials. Trials on which the dot moved outside the track lines were repeated. Following 16 successful trials as described, participants performed 4 additional trials with each track orientation (i.e., with each lead hand condition) in which visual feedback was removed after the cursor had moved halfway through the track. This was accomplished by taping a piece of paper over the upper half of the track. Participants were instructed to continue turning the cranks in the same manner in which they had been turning them before the cursor disappeared from view. Each trial ended when the cursor crossed the plane defined by the ends of the two lines that formed the track. Participants received 4 trials with one lead hand condition and then 4 with the other lead hand condition, the order of conditions being counterbalanced across participants within each group. Three dependent measures were used: (a) The first measure of accuracy, referred to as integrated error, was computed as the total area between the trace and the optimal straight line through the center of the track; (b) a second measure of accuracy that reflects inappropriate directional reversals of crank turning, by either hand, was the percent excess output (PEO), calculated as the ratio of the actual out-

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put from each crank over the minimal output that the response required, times 100; and (c) the third measure was the time taken by the participant to move the cursor from the entrance to the end of the track.

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Lexical Decision Task Procedures for the Lexical Decision task were based on those described by Mondor and Bryden (1992). The stimuli consisted of 40 three-letter words and 40 pronounceable three-letter nonwords, which were presented on a white background on the computer monitor. Participants were asked to identify on each trial whether the stimulus had been a word or a nonword by pressing one response key or the other. They were asked to do this as quickly and as accurately as possible but not to sacrifice accuracy for the sake of speed. Participants were tested under both a cued condition, wherein a precue (blue dot) was presented 67 msec before the target stimulus in the visual field into which the target stimulus would appear, and a noncued condition wherein the precue and target stimuli were presented simultaneously. In both conditions, the blue dot remained on the screen until the target stimulus was extinguished. Following 32 practice trials, each of the 80 different test stimuli was presented once to each visual field for a total of 160 trials. Order of presentation of the stimuli was randomized with the constraint that stimuli were never presented to the same visual field on more than 4 consecutive trials. The dependent variables were (a) d¢ , a measure of accuracy derived from signal detection theory (Eagan, 1975) together with percentage correct; and (b) response latency (speed). The within-group independent variables were visual field (left vs. right) and cuing (noncued vs. cued). Dot Enumeration Task This second Divided Visual Field task used the same equipment and similar procedures as described for the Lexical Decision task. The stimuli consisted of 160 different arrangements of from three to six dots (diameter = 2 mm) presented inside a 2 cm × 2 cm line frame that served as the precue. The task of the participant was to indicate, using the response keys, whether there had been an even number or odd number of dots. The independent and dependent variables were the same as for the Lexical Decision task. Summary of Approach The experimental session began with an interview and questionnaires designed to gather data on and assess personal and family history of stuttering, handedness, and possible exclusionary factors related to neurological history, musical instrument

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training, or visual or motor-skill disabilities. All participants were tested on the four tasks in the same order: (a) Lexical Decision task, (b) Sequence Reproduction Finger Tapping task, (c) Bimanual Crank Turning task (visual feedback condition first followed by no visual feedback condition), and (d) Dot Enumeration task. The two motor tasks were intended to probe the speech–motor mechanisms believed to be fragile in people who stutter. The two Divided Visual Field tasks were intended to probe the anomaly in interhemispheric relations in people who stutter. Group ST was expected to differ from the nonstutterer control group (Group NS) in ways predictable from the literature. The pattern of performance similarities and differences between Group ES and the other two groups was of primary interest in assessing the recovery hypothesis. RESULTS Participants Figure 1 illustrates the mean number (and standard errors) of items checked by the three groups on the three 20-item subscales (Struggle, Expectancy, and Avoidance)

FIGURE 1 Mean number (and standard error) of items checked (maximum = 20) by Groups NS, ST, and ES on the Struggle, Expectancy, and Avoidance subscales of the two versions of the Perception of Stuttering Inventory (Wolff, 1967).

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of the PSI (Wolff, 1967). The right side of the figure shows the data for speech at the present time and the left side relates to speech as recollected from preadolescence. Group NS checked off the fewest items as applying to them on both versions of the PSI, with the majority of checked items relating to avoidance behavior. Group ST checked off the most items. The scores for Group ES were similar to those of Group ST in the childhood recollection version, but an analysis of variance (ANOVA) of the scores on the three subscales for the two groups indicated a significant Group × Subscale interaction, F(2, 92) = 4.75, p = .05, indicating a differentiation of the two groups on the Avoidance scale. Although the adult version responses of Group ES were more similar to those of Group NS than to Group ST, significantly more items were checked off than by Group NS, F(1, 46) = 15.36, p < .0005. However, the significant Group × Subscale interaction, F(2, 92) = 12.28, p < .0005, indicated that Groups ES and NS were most similar on the avoidance scale, F(1, 46) = 1.81, ns, with significant differences between these two groups for the Struggle and Expectancy subscales, F(1, 46) = 12.84, p < .005; F(1, 46) = 26.42, p < .0005, respectively.

Overview of Analyses The primary analysis of the data from the four tasks involved, for each measure, two orthogonal planned comparison contrasts based on the hypotheses tested. Following the planned comparisons, a set of more general ANOVAs was undertaken for each task to explore the relation between the grouping variable and within-subject independent variables of secondary interest. In conducting the multivariate analyses of variance (MANOVAs) and ANOVAs, design complexity and the large number of statistical tests conducted were of concern with respect to the experiment-wise error rate as well as the likelihood of obtaining complex higher order effects of little interest to the conceptual issues being explored. To address these concerns, univariate tests have, in general, been reported only when the corresponding multivariate test was statistically significant. In addition, the MANOVAs reported here include only independent variables of specific relevance to the main conceptual issues. In some cases data have been collapsed across conditions such as trials or blocks that, although necessary methodologically, are of limited conceptual interest.

Sequence Reproduction Finger Tapping Task Figure 2 illustrates the mean and standard error for the three groups (NS, ST, ES) on RIT and SET collapsed across sequence length (three, four, and five elements). The mean RIT for Group ST was significantly longer than that for Groups NS and ES

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FIGURE 2 Mean (and standard error) times (msec) for response initiation time and sequence execution time on the Sequence Reproduction Finger Tapping task by Groups NS, ST, and ES.

combined, t(69) = 2.31, p < .02, and Group NS did not differ significantly from Group ES, t(69) = 1.41. Group ST also had slower overall SETs than Groups NS and ES combined, t(69) = 2.96, p < .005, with Group NS not differing from Group ES, t(69) = 0.49. With respect to accuracy, Group ST had mean probability of 0.81 for a correct first tapped sequence. This was not different from the mean probability (0.84) of the other two groups combined, t(69) = 1.28, p = .10, or from that (0.86) of Group NS alone, t(69) = 0.99. In summary, Group ST showed slower RITs and SETs on the Sequence Reproduction Finger Tapping task compared to Groups ES and NS combined, and Group NS did not differ reliably from Group ES on these measures. Bimanual Crank Turning Task Figures 3 and 4 illustrate the mean and standard error for the three groups for the dependent measures of integrated error (the deviation of the cursor movement from the optimal path, a straight line through the middle of the track) and PEO (reflecting

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FIGURE 3 Mean (and standard error) integrated error (cm2) on the Bimanual Crank Turning task by Groups NS, ST, and ES under the with visual feedback and no visual feedback conditions.

FIGURE 4 Mean (and standard error) percent excess output on the Bimanual Crank Turning task by Groups NS, ST, and ES under the with visual feedback and no visual feedback conditions.

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the degree to which either crank was turned in the wrong direction), respectively, collapsed across lead-hand conditions (which hand was turning the crank at the faster speed). Figure 5 shows the corresponding data for response time (RT; the time from when the cursor first moved until it crossed the top of the track) collapsed across lead-hand conditions. The left side of each figure shows the data obtained under the with visual feedback (WVF) condition and the right side the data obtained under the no visual feedback (NVF) condition for the upper track only (the portion of the track out of view). Separate primary analyses were carried out for each of the two visual feedback conditions rather than treating visual feedback as a variable in a consolidated analysis. There were several reasons for doing so. First, in the WVF condition participants were required to stay within the track, but this requirement could not be maintained in the NVF condition. Second, there were twice as many trials (16 vs. 8) for WVF as for the NVF condition. Third, it was necessary in the NVF condition to compute the dependent measures based only on the second half (the NVF portion) of the track rather than the entire track. Finally, there was considerably more variation in the NVF than the WVF condition. Time scores were natural log transformed because of positive skewness in the distribution. Furthermore, to minimize the number of dependent variables and to control for possible differential speed–accuracy trade-offs, the natural logarithm of time [ln(Time)] was treated as a “varying” covariate (i.e., there was a separate covariate for each level of each within-subject factor) in the subsequent planned

FIGURE 5 Mean (and standard error) time(s) required by participants in Groups NS, ST, and ES to move the cursor from the start to finish of the track on the Bimanual Crank Turning task under the with visual feedback and no visual feedback conditions. (Note: The two conditions are not directly comparable because of the different length of track.)

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comparisons and ANOVA on the accuracy measures. The ln(Time) covariate was left in the analysis if it was statistically significant for any of the main effects or interactions. The hypotheses and corresponding planned comparisons for the Bimanual Crank Turning task were the same as those for the Sequence Reproduction Finger Tapping tasks. The first comparison contrasted Group ST with Groups NS and ES combined, and the second contrasted Group NS with Group ES. When visual feedback was available, Group ST did not generate less accurate traces (integrated error measure) than Groups NS and ES combined, t(68) = 1.16. Similarly, Group ES did not differ significantly from Group NS, t(68) = 0.89. On the second accuracy measure, PEO, Group ST generated more excess output (i.e., more crank movement in the wrong direction) than did Groups NS and ES combined, t(69) = 2.81, p < .005, but Group NS did not differ from Group ES, t(69) = 1.14. As illustrated on the right side of Figures 3 and 4, when visual feedback was not available, Group ST was both less accurate and generated more excess output than Groups NS and ES, t(69) = 3.46, p < .0005, and t(69) = 2.43, p < .01, respectively, but Groups NS and ES did not differ on either measure, t(69) = 0.19 and t(69) = 1.14, respectively. As a secondary analysis, a multivariate analysis of covariance (ANCOVA) was carried out on the data with visual feedback included as a factor (by doubling the NVF scores to simplify comparisons). Consistent with the results described previously, there was a statistically significant interaction between group and visual feedback, F(2, 68) = 3.47, p < .01. A similar Group × Visual Feedback interaction was evident in the univariate integrated error ANCOVA, F(2, 68) = 5.24, p < .01, and reflected the fact that the group difference in accuracy was evident only when visual feedback was unavailable. In summary, Group ST was impaired relative to Groups NS and ES on the Bimanual Crank Turning task overall. When visual feedback was not available, the traces for Group ST were 40% less accurate than for the other groups, but when visual feedback was available, the groups were similar in accuracy. Furthermore, under both visual feedback conditions, Group ST generated 40% more excess output than the other two groups combined, indicating a higher frequency of directionally reversed movements. Lexical Decision Task The analysis of Lexical Decision task performance involved the independent variables of group (NS, ST, ES), visual field (left [LVF] vs. right [RVF]), and cuing (noncued vs. cued). The dependent measures were RT and d¢ , a measure that reflects the discrepancy between the proportion of “hits” (responding “word” when the stimulus target is in fact a word) and “false alarms” (responding “word” when the stimulus target is in fact a nonword). Both measures have been included in the

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tables, but only d¢ was used in the statistical analysis. As described for the Bimanual Crank Turning task, RT was treated as a varying covariate and left in the analysis if it was significant for any of the main effects or interactions. Two planned comparisons tested whether, using RVF–LVF difference scores, Group NS would have a stronger RVF advantage than the other two groups. The mean values for percentage correct, d¢ and RT for the three groups, are presented in Table 1 as a function of visual field. Mean (right–left) difference scores for the three variables are presented in Table 2. As is evident by the positive values TABLE 1 Lexical Decision Task Performance by Groups NS, ST, and ES for Stimuli Presented to the Left Visual Field and Right Visual Field Group NS Dependent Measure

LVF

Percent correct M 73.1 SD 1.86 d¢ M 1.35 SD 0.58 Response time (in milliseconds) M 942 SD 197

Group ST

RVF

LVF

Group ES

RVF

LVF

RVF

79.1 1.64

73.3 2.33

74.6 1.58

75.9 1.91

78.6 1.47

1.85 0.70

1.44 0.78

1.55 0.62

1.68 0.83

1.82 0.67

2,917 180

1,209 377

1,190 339

1,031 266

985 215

Note. The data are collapsed across cued and noncued testing conditions. NS = nonstutterers; ST = stutterers; ES = ex-stutterers; LVF = left visual field; RVF = right visual field. TABLE 2 RVF–LVF Performance Asymmetries by Groups NS, ST, and ES for the Lexical Decision Task Performance Measures Dependent Measure Percent correct M SD d¢ M SD Response Time (in milliseconds) M SD

Group NS

Group ST

Group ES

5.94 7.21

1.30 9.06

2.66 7.43

0.496* 0.565

0.112 0.563

0.137 0.732

26 112

19 128

47 102

Note. Data are collapsed across the cued and noncued testing conditions. RVF = right visual field; LVF = left visual field; NS = nonstutterers; ST = stutterers; ES = ex-stutterers. *p < .0005.

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in the latter table, all three groups performed more accurately when the stimulus words were presented in the RVF than in the LVF. Group NS had a significantly larger RVF advantage than Groups ST and ES combined, t(69), = 2.66, p < .005, and Group ST did not differ significantly from Group ES, t(69) = 0.19. In a secondary analysis, a three-way (Group × Visual Field × Cuing condition) ANCOVA of d¢ scores, with RT as the covariate, indicated main effects for cuing, F(1, 68) = 6.19, p < .05, and visual field, F(1, 68) = 6.74, p < .05, and as illustrated in Figure 6, a Group × Visual Field interaction, F(1, 68) = 3.56, p < .05. Although all three groups displayed greater accuracy with stimuli presented in the RVF, the asymmetry was substantially larger for Group NS than for the other two groups. In fact, an analysis of simple effects revealed that the asymmetry was statistically significant only in Group NS, F(1, 68) = 13.65, p < .0005. Neither Group ST nor Group ES showed a statistically significant RVF advantage, F(1, 68) = 0.36, and F(1, 68) = 0.10, respectively. The percentage of individual participants in each group showing an RVF and LVF advantage was calculated. There was a more frequent anomalous LVF advantage in Groups ST and ES combined (21/48, or 44%) than in Group NS (4/24, or 17%), c 2(1, N = 72) = 5.18, p < .05. The frequency in Group ST (11/24, or 46%) did not differ significantly from that in Group ES (10/24, or 42%), c 2(1, N = 48) = 0.085. Also, an LVF advantage was more frequent in Group ST (11/24, or 46%) than in Group NS (4/24, or 17%), c 2(1, N = 48) = 4.75, p < .05. In summary, Group NS demonstrated the usual RVF advantage on this Lexical Decision task, but Groups ST and ES were similar to one another in showing no asymmetry.

FIGURE 6 Mean accuracy scores (d¢ ; and standard error) on Lexical Decision task as a function of group and visual field.

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FIGURE 7 Mean (and standard error) visual field asymmetry in accuracy scores (d¢ ) on the Dot Enumeration task as a function of group and cuing condition (LVF = Left Visual Field; RVF = Right Visual Field).

Dot Enumeration Task The same performance measures and analyses used for the Lexical Decision task were used for the Dot Enumeration task. Unfortunately, the overall performance on this task (particularly in the noncued condition) was sufficiently poor as to make the reliability of the results questionable and the interpretation of asymmetries problematic. When the analyses were limited to the cued condition and including only participants with performance levels above chance (56.25%, based on the binomial distribution with a one-tailed probability of 0.05), the results were similar to those of the Lexical Decision task. As illustrated in Figure 7, Group NS showed an RVF advantage, F(1, 58) = 9.60, p < .005, but Groups ST and ES did not show a visual field asymmetry in either direction, F(1, 58) = 0.18 and F(1, 58) = 0.54, respectively.

DISCUSSION The purpose of this study was to explore a hypothesis that would associate spontaneous recovery from childhood stuttering with the maturation of speech–motor

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control mechanisms (the SMA in particular). Accordingly, the focus of this study was on the performance of ex-stutterers (Group ES) compared to persistent stutterers (Group ST) and nonstutterers (Group NS). However, a consideration of these data requires first a consideration of the performance of Group ST compared to that of Group NS on the various tasks. On the Sequence Reproduction Finger Tapping task, the slower response initiation times for Group ST replicated the findings of Webster (1986) that had constituted part of the argument (Webster, 1988, 1997) for a role of the SMA in stuttering. The significant difference between Group ST and Group NS on sequence execution times, which was not found by Webster (1986), may reflect the inclusion of shorter sequence lengths in the analysis. The second motor task, Bimanual Crank Turning, was adapted from Preilowski (1972) specifically with a view to generalizing bimanual handwriting deficits (Greiner & Fitzgerald, 1992; Greiner, Fitzgerald, & Cooke, 1986; Webster, 1988) to a task with more specific and well-documented demands on brain mechanisms related to bimanual coordination. One important and new result of the present experiment with respect to stuttering is the evidence that the bimanual coordination deficit in people who stutter persistently is kinesthetically based. Under conditions of visual feedback, the groups did not differ significantly on the integrated error variable (a measure of accuracy of timing of bimanual coordination), whereas under conditions of no visual feedback, Group ST demonstrated 40% more integrated error than Group NS. Considered in concert with reports of difficulties by stutterers in the kinesthetic control of tongue (De Nil & Abbs, 1991) and jaw (Archibald & De Nil, 1999) movements, the data are consistent with the idea that the motor control deficit in stutterers is related to the kinesthetically guided medial motor system (Goldberg, 1985) and to the SMA in particular (Caruso et al., 1988; Webster, 1988, 1997). Under both feedback conditions, Group ST showed 40% more excess output than Group NS, indicating an increased tendency for stutterers to make directional reversals in crank movements. This mirror image response tendency, which had been reported earlier in bimanual handwriting performance by stutterers (Greiner et al., 1986; Webster, 1988), is of particular interest in light of mirror reversal responding being associated with an immature callosum in children (Jeeves, Silver, & Milne, 1988; Lazarus & Todor, 1987) and with SMA damage in adult humans and nonhuman primates (Brinkman, 1981, 1984; Chan & Ross, 1988). Previous studies have indicated that people who stutter as adults demonstrate either attenuated or reversed perceptual asymmetries in linguistic divided visual field and dichotic listening tasks (Blood, 1985; Blood & Blood, 1989; Hand & Haynes, 1983; Quinn, 1972; Rosenfield & Goodglass, 1980; Sommers et al., 1975). The results of the Lexical Decision task in this study are in agreement with those findings, both in terms of the relative size of the asymmetry in Groups ST and NS and in terms of the proportion of participants in the two groups who demonstrated reversed asymmetries. Group NS showed the expected (e.g., Bradshaw & Gates, 1978; Bradshaw,

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Hicks, & Rose, 1979; Faust, Kravetz, & Babkoff, 1993; Rastatter & Dell, 1987b; Wey, Cook, Landis, & Regard, 1993) RVF advantage on this linguistic task. In contrast, Group ST (and Group ES, which will be returned to) did not show performance asymmetries favoring either visual field. The slower overall reaction times for Group ST were also in agreement with the studies of Hand and Haynes (1983) and Rastatter and Dell (1987a) that employed similar tasks. Previous studies of persistent stutterers using a divided visual field paradigm have without exception used linguistic stimuli. The Dot Enumeration task was included in our experiment to determine whether the anomalous perceptual asymmetries reported in the literature are limited to the verbal–linguistic realm or would also be observed with nonverbal stimuli. Unfortunately, the task proved difficult for participants, particularly under the noncued testing condition, and this made interpretation of the data problematic at best. However, the fact that the analysis of the cued condition data indicated that Group NS showed an RVF advantage, whereas Group ST did not show any asymmetry, suggests that the divided visual field anomalies in stutterers are not limited to the verbal–linguistic domain but involve more general lateralized mechanisms.

Recovery From Stuttering The pattern of results for Group ES relative to the other two groups was task dependent and was in general consistent with the hypothesis linking recovery from childhood stuttering with maturation of speech–motor control mechanisms (the SMA in particular) while retaining an anomaly in hemispheric activation. On the two motoric tasks, Group ES performed in a manner similar to that of Group NS and different from that of Group ST. On the two Divided Visual Field tasks, the general pattern was reversed in that Group ES performed in a manner similar to Group ST and different from that of Group NS. Consequently, apart from the issue of recovery from stuttering, which will be discussed following, these data are important in that they indicate that an anomaly in speech–motor control does not necessarily go hand in hand with an anomaly in hemispheric activation. They also indicate that, although the two anomalies may be necessary conditions for persistent stuttering (Webster, 1997), neither is sufficient. Our hypothesis is based on the assumption that hemispheric activation asymmetries are developmentally invariant, whereas the neural mechanisms underlying speech–motor control undergo maturation throughout childhood. This leads to the hypothesis that although ex-stutterers will be differentiated from nonstutterers on tasks sensitive to hemispheric activation, ex-stutterers will be differentiated from persistent stutterers on tasks sensitive to speech–motor mechanisms. On both the sequence reproduction and bimanual coordination tasks, the results provided support for the hypotheses. Group ST was impaired relative to both

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Group NS and Group ES, and the performance of Group ES was not distinguishable statistically from that of Group NS. One potentially troubling aspect of the sequence reproduction data was that, although the difference between Groups NS and ES on RITs was not statistically significant, it was nonetheless substantial in magnitude. Our view is that the interpretation of the group differences on this particular measure should be tempered pending successful replication. The similarity of Groups NS and ES on the SETs was clearer. The results of the Divided Visual Field tasks were also consistent with the perspective. Although Group NS showed the expected RVF advantage on the Lexical Decision task, neither Group ST nor Group ES showed a perceptual asymmetry. The results of the Dot Enumeration task were less definitive, but even on this task the performance of Groups ES and ST was similar in that both failed to show the perceptual asymmetry that was evidenced by Group NS. Taken together, the results from the four experimental tasks support the general hypothesis that an anomaly in hemispheric activation and a deficit in the mechanisms of speech–motor control are each a necessary but not sufficient condition for stuttering and that recovery from childhood stuttering reflects a maturation of the mechanisms of speech–motor control. ACKNOWLEDGMENTS This article is based on a doctoral dissertation submitted by David C. Forster to Carleton University, Ottawa, Canada, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. The research was supported, in part, by operating Grant OGP0399 from the Natural Sciences and Engineering Research Council of Canada awarded to William G. Webster. We express our appreciation to the anonymous reviewers for their helpful suggestions and comments in regard to the article.

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