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This review provides a summary and appraisal commentary on the treatment review by Nye, C., Vanryckeghem, M., Schwartz, J.B., Herder, C., Turner III, H.M., and Howard, C. (2013). Behavioral stuttering interventions for children and adolescents: A systematic review and meta-analysis. Journal of Speech, Language, and Hearing Research, 56, 921–932.Source of funding and declarations of conflicts of interest: not reported

Recent brain imaging investigations of developmental stuttering show considerable disagreement regarding which regions are related to stuttering. These divergent findings have been mainly derived from group studies. To investigate functional neurophysiology with improved precision, an individual-participant approach ( N = 4) using event-related functional magnetic resonance imaging and test–retest reliability measures was performed while participants produced fluent and stuttered single words during two separate occasions. A parallel investigation required participants to imagine stuttering or not stuttering on single words. The overt and covert production tasks produced considerable within-subject agreement of activated voxels across occasions, but little within-subject agreement between overt and covert task activations. However, across-subject agreement for regions activated by the overt and covert tasks was minimal. These results suggest that reliable effects of stuttering are participant-specific, an implication that might correspond to individual differences in stuttering severity and functional compensation due to related structural abnormalities.

Purpose Brain imaging and brain stimulation procedures have now been used for more than two decades to investigate the neural systems that contribute to the occurrence of stuttering in adults, and to identify processes that might enhance recovery from stuttering. The purpose of this paper is to review the extent to which these dual lines of research with adults who stutter have intersected and whether they are contributing towards the alleviation of this impairment. Method Several areas of research are reviewed in order to determine whether research on the neurology of stuttering is showing any potential for advancing the treatment of this communication disorder: (a) attempts to discover the neurology of stuttering, (b) neural changes associated with treated recovery, and (c) direct neural intervention. Results and conclusions Although much has been learned about the neural underpinnings of stuttering, little research in any of the reviewed areas has thus far contributed to the advancement of stuttering treatment. Much of the research on the neurology of stuttering that does have therapy potential has been largely driven by a speech-motor model that is designed to account for the efficacy of fluency-inducing strategies and strategies that have been shown to yield therapy benefits. Investigations on methods that will induce neuroplasticity are overdue. Strategies profitable with other disorders have only occasionally been employed. However, there are signs that investigations on the neurology of adults who have recovered from stuttering are slowly being recognized for their potential in this regard.

Purpose It is proposed that stuttering treatment, particularly for adults and adolescents who stutter, may benefit from more inventive and extensive use of functional measurement—measures that are also treatment agents. Such measures can be tailored to produce more personally significant and evidence-based treatment benefits. They may be especially useful when employed in conjunction with partial self-management and performance-contingent procedures. Method Previous approaches to the definition of stuttering treatment goals and the measurement of stuttering treatment outcomes are critically reviewed. Suggestions for improvements are presented within the framework of an evidence-based and relatively standardized stuttering treatment. Results and Conclusion Results from a review of existing literature and from 2 case studies show that 2 specific personally significant problems, saying one’s name and addressing large audiences, were improved by implementing these strategies in treatment. Functional measures directly connected to treatment, and partially self-managed performance-contingent schedules, merit further research as methodologies that are suitable for conducting personally significant and evidence-based treatments with adults and adolescents who stutter.

High-density ERPs were recorded in eight adults with persistent developmental stuttering (PERS) and eight matched normally fluent (CONT) control volunteers while participants either repeatedly uttered the vowel 'ah' or listened to their own previously recorded vocalizations. The fronto-central N1 auditory wave was reduced in response to spoken vowels relative to heard vowels (auditory-vocal gating), but no difference in the extent of such modulation was found in the PERS group. Abnormalities in the PERS group were restricted to the LISTEN condition, in the form of early N1 and late N3 amplitude changes. Voltage of the N1 wave was significantly reduced over right inferior temporo-occipital scalp in the PERS group. A laterality index derived from N1 voltage moderately correlated with the PERS group's assessed pre-experiment stuttering frequency. Source localization with sLORETA (Pascual-Marqui, R. D. (2002). Standardized low-resolution brain electromagnetic tomography (sLORETA): Technical details. Methods & Findings in Experimental & Clinical Pharmacology, 24, 5-12.) revealed that at the peak of the N1 the PERS group displayed significantly greater current density in right primary motor cortex than the CONT group, suggesting abnormal early speech-motor activation. Finally, the late N3 wave was reduced in amplitude over inferior temporo-occipital scalp, more so over the right hemisphere. sLORETA revealed that in the time window of the N3 the PERS group showed significantly less current density in right secondary auditory cortex than the CONT group, suggesting abnormal speech sound perception. These results point to a deficit in auditory processing of speech sounds in persistent developmental stuttering, stemming from early increased activation of right rolandic area and late reduced activation in right auditory cortex.

Previous investigations of cerebral anatomy in persistent developmental stutterers have reported bilateral anomalies in the perisylvian region and atypical patterns of cerebral asymmetry. In this study, perisylvian sulcal patterns were analyzed to compare subjects with persistent developmental stuttering (PDS) and an age-, hand-, and gender-matched control group. This analysis was accomplished using software designed for 3-dimensional sulcal identification and extraction. Patterns of cerebral asymmetry were also investigated with standard planimetric measurements. PDS subjects showed a small but significant increase in both the number of sulci connecting with the second segment of the right Sylvian fissure and in the number of suprasylvian gyral banks (of sulci) along this segment. No differences were seen in the left perisylvian region for either sulcal number or gyral bank number. Measurements of asymmetry revealed typical patterns of cerebral asymmetry in both groups with no significant differences in frontal and occipital width asymmetry, frontal and occipital pole asymmetry, or planum temporale and Sylvian fissure asymmetries. The subtle difference in cortical folding of the right perisylvian region observed in PDS subjects may correlate with functional imaging studies that have reported increased right-hemisphere activity during stuttered speech.

Q Are the outcomes generated by the Smooth Speech treatment program for stuttering, administered by supervised student clinicians, comparable to the positive results produced by similar programs reported in the literature? 1 Abstracted from: Block, S., Onslow, M., Packman, A., Gray, B., & Dacakis, G. (2005). Treatment of chronic stuttering: Outcomes from a student training clinic. International Journal of Language and Communication Disorders, 40, 455–466.

Purpose This study compared a new adult stuttering treatment program (Modifying Phonation Intervals, or MPI) with the standard of care for reducing stuttered speech in adults (prolonged speech). Method Twenty-seven adults who stutter were assigned to either MPI or prolonged speech treatment, both of which used similar infrastructures. Speech and related variables were assessed in 3 within-clinic and 3 beyond-clinic speaking situations for participants who successfully completed all treatment phases. Results At transfer, maintenance, and follow-up, the speech of 14 participants who successfully completed treatment was similar to that of normally fluent adults. Successful participants also showed increased self-identification as a “normal speaker,” decreased self-identification as a “stutterer,” reduced short intervals of phonation, and some increased use of longer duration phonation intervals. Eleven successful participants received the MPI treatment, and 3 received the prolonged speech treatment. Conclusions Outcomes for successful participants were very similar for the 2 treatments. The much larger proportion of successful participants in the MPI group, however, combined with the predictive value of specific changes in PI durations suggest that MPI treatment was relatively more effective at assisting clients to identify and change the specific speech behaviors that are associated with successful treatment of stuttered speech in adults.

Aims Developmental stuttering is now generally considered to arise from genetic determinants interacting with neurologic function. Changes within speech-motor white matter (WM) connections may also be implicated. These connections can now be studied in great detail by high-angular-resolution diffusion magnetic resonance imaging. Therefore, diffusion spectrum imaging was used to reconstruct streamlines to examine white matter connections in people who stutter (PWS) and in people who do not stutter (PWNS). Method WM morphology of the entire brain was assayed in 8 right-handed male PWS and 8 similarly aged right-handed male PWNS. WM was exhaustively searched using a deterministic algorithm that identifies missing or largely misshapen tracts. To be abnormal, a tract (defined as all streamlines connecting a pair of gray matter regions) was required to be at least one 3rd missing, in 7 out of 8 subjects in one group and not in the other group. Results Large portions of bilateral arcuate fasciculi, a heavily researched speech pathway, were abnormal in PWS. Conversely, all PWS had a prominent connection in the left temporo-striatal tract connecting frontal and temporal cortex that was not observed in PWNS. Conclusion These previously unseen structural differences of WM morphology in classical speech-language circuits may underlie developmental stuttering.

This study reports an activation likelihood estimation (ALE) meta-analysis of imaging studies of chronic developmental stuttering in adults. Two parallel meta-analyses were carried out: (1) stuttered production in the stutterers; (2) fluent production in the control subjects. The control subjects' data replicated previous analyses of single-word reading, identifying activation in primary motor cortex, premotor cortex, supplementary motor area, Rolandic operculum, lateral cerebellum, and auditory areas, among others. The stuttering subjects' analysis showed that similar brain areas are involved in stuttered speech as in fluent speech, but with some important differences. Motor areas were over-activated in stuttering, including primary motor cortex, supplementary motor area, cingulate motor area, and cere- bellar vermis. Frontal operculum, Rolandic operculum, and anterior insula showed anomalous right- laterality in stutterers. Auditory activations, due to hearing one's own speech, were essentially undetect- able in stutterers. The phenomenon of efference copy is proposed as a unifying account of the pattern activation revealed within this ALE meta-analysis. This provides the basis for a stuttering system model that is testable and should help to advance the understanding and treatment of this disorder. Hum Brain Mapp 25:105-117, 2005. © 2005 Wiley-Liss, Inc.

The philosophy guiding evidence-based treatment and its application to decision-making in stuttering treatment is described. Limitations to the use of evidence-based treatment principles to guide stuttering treatment, namely the lack of a substantial treatment research literature that can serve as the basis for meta-analyses and systematic reviews of effective treatment studies, are bemoaned. Guidelines are provided to aid clinicians in their own conduct of critical evaluations of treatment research. Educational objectives: The reader will learn about and be able to (1) describe the four steps that characterize evidence-based practice, (2) evaluate previous efforts to develop practice guidelines for stuttering, (3) assess the importance of an evidence-based approach to stuttering treatment, and (4) discuss the need for more research that will contribute to the evidence base.

Four adult right-handed chronic stutterers and four age-matched controls completed H(2)(15)O PET scans involving overt and imagined oral reading tasks. During overt stuttered speech prominent activations occurred in SMA (medial), BA 46 (right), anterior insula (bilateral), and cerebellum (bilateral) plus deactivations in right A2 (BA 21/22). These activations and deactivations also occurred when the same stutterers imagined they were stuttering. Some parietal regions were significantly activated during imagined stuttering, but not during overt stuttering. Most regional activations changed in the same direction when overt stuttering ceased (during chorus reading) and when subjects imagined that they were not stuttering (also during chorus reading). Controls displayed fewer similarities between regional activations and deactivations during actual and imagined oral reading. Thus overt stuttering appears not to be a prerequisite for the prominent regional activations and deactivations associated with stuttering.

To distinguish the neural systems of normal speech from those of stuttering, PET images of brain blood flow were probed (correlated voxel-wise) with per-trial speech-behaviour scores obtained during PET imaging. Two cohorts were studied: 10 right-handed men who stuttered and 10 right-handed, age- and sex-matched non-stuttering controls. Ninety PET blood flow images were obtained in each cohort (nine per subject as three trials of each of three conditions) from which r-value statistical parametric images (SPI¿r¿) were computed. Brain correlates of stutter rate and syllable rate showed striking differences in both laterality and sign (i.e. positive or negative correlations). Stutter-rate correlates, both positive and negative, were strongly lateralized to the right cerebral and left cerebellar hemispheres. Syllable correlates in both cohorts were bilateral, with a bias towards the left cerebral and right cerebellar hemispheres, in keeping with the left-cerebral dominance for language and motor skills typical of right-handed subjects. For both stutters and syllables, the brain regions that were correlated positively were those of speech production: the mouth representation in the primary motor cortex; the supplementary motor area; the inferior lateral premotor cortex (Broca's area); the anterior insula; and the cerebellum. The principal difference between syllable-rate and stutter-rate positive correlates was hemispheric laterality. A notable exception to this rule was that cerebellar positive correlates for syllable rate were far more extensive in the stuttering cohort than in the control cohort, which suggests a specific role for the cerebellum in enabling fluent utterances in persons who stutter. Stutters were negatively correlated with right-cerebral regions (superior and middle temporal gyrus) associated with auditory perception and processing, regions which were positively correlated with syllables in both the stuttering and control cohorts. These findings support long-held theories that the brain correlates of stuttering are the speech-motor regions of the non-dominant (right) cerebral hemisphere, and extend this theory to include the non-dominant (left) cerebellar hemisphere. The present findings also indicate a specific role of the cerebellum in the fluent utterances of persons who stutter. Support is also offered for theories that implicate auditory processing problems in stuttering.

The pressing need to document treatment efficacy for young children who stutter is discussed. Guidelines for such documentation are suggested and illustrated. Measures for verifying treatment effects in four realms are delineated: (a) conditions of documentation, (b) dependent variables to be measured, (c) establishment of treatment integrity, and (d) verification of the relationship between treatment and outcome. Illustrations of the application of the suggested guidelines are presented for 2 children.

A series of single-subject experiments evaluated the effects of frequency-altered auditory feedback (FAF) on the speech performance of four adult males who stutter. Using alterations of plus or minus one octave, FAF was compared with normal auditory feedback (NAF) in oral reading and spontaneous speech with measurements made of stuttered intervals, stutter-free speech rate, and speech naturalness. The effects of extended FAF conditions on spontaneous speech were also evaluated for two subjects who demonstrated a positive response to FAF. Results showed no consistencies across subjects in responses to FAF: One subject showed no response, another produced an initial temporary response, a third showed a deterioration in speech quality with minimal reductions in stuttering, and a fourth displayed substantial and sustained improvements in speech performance. Some implications of these findings for current research and theory about the relationship between stuttering and FAF are discussed.

Developmental stuttering is known to be associated with aberrant brain activity, but there is no evidence that this knowledge has benefited stuttering treatment. This study investigated whether brain activity could predict progress during stuttering treatment for 21 dextral adults who stutter (AWS). They received one of two treatment programs that included periodic H2(15)O PET scanning (during oral reading, monologue, and eyes-closed rest conditions). All participants successfully completed an initial treatment phase and then entered a phase designed to transfer treatment gains; 9/21 failed to complete this latter phase. The 12 pass and 9 fail participants were similar on speech and neural system variables before treatment, and similar in speech performance after the initial phase of their treatment. At the end of the initial treatment phase, however, decreased activation within a single region, L. putamen, in all 3 scanning conditions was highly predictive of successful treatment progress.

THE cause of stuttering is unknown1. Failure to develop left-hemispheric dominance for speech is a long-standing theory1 although others implicate the motor system more broadly2, often postulating hyperactivity of the right (language non-dominant) cerebral hemisphere3. As knowledge of motor circuitry has advanced4, theories of stuttering have become more anatomically specific, postulating hyperactivity of premotor cortex, either directly5 or through connectivity with the thalamus and basal ganglia6. Alternative theories target the auditory7 and speech production8,9 systems. By contrasting stuttering with fluent speech using positron emission tomography combined with chorus reading to induce fluency, we found support for each of these hypotheses. Stuttering induced widespread over-activations of the motor system in both cerebrum and cerebellum, with right cerebral dominance. Stuttered reading lacked left-lateralized activations of the auditory system, which are thought to support the self-monitoring of speech, and selectively deactivated a frontal–temporal system implicated in speech production. Induced fluency decreased or eliminated the overactivity in most motor areas, and largely reversed the auditory-system underactivations and the deactivation of the speech production system. Thus stuttering is a disorder affecting the multiple neural systems used for speaking.

This study was designed to investigate the apparent contradiction between recent reports of physiological and interpersonal research on stuttering that claim or imply high agreement levels, and studies of stuttering judgment agreement itself that report much lower agreement levels. Four experienced stuttering researchers in one university department used laser videodisks of spontaneous speech, from persons whose stuttering could be described as mild to severe, to locate the precise onset and offset of individual stuttering events. Results showed a series of interjudge disagreements that raise serious questions about the reliability and validity of stuttering event onset and offset judgments. These results highlight the potentially poor reliability of a measurement procedure that is currently widespread in stuttering research. At the same time, they have isolated some few highly agreed stuttering events that might serve as the basis for the further development of either event-based or interval-based judgment procedures.

Many differences in brain activity have been reported between persons who stutter (PWS) and typically fluent controls during oral reading tasks. An earlier meta-analysis of imaging studies identified stutter-related regions, but recent studies report less agreement with those regions. A PET study on adult dextral PWS ( n = 18) and matched fluent controls (CONT, n = 12) is reported that used both oral reading and monologue tasks. After correcting for speech rate differences between the groups the task-activation differences were surprisingly small. For both analyses only some regions previously considered stutter-related were more activated in the PWS group than in the CONT group, and these were also activated during eyes-closed rest (ECR). In the PWS group, stuttering frequency was correlated with cortico–striatal–thalamic circuit activity in both speaking tasks. The neuroimaging findings for the PWS group, relative to the CONT group, appear consistent with neuroanatomic abnormalities being increasingly reported among PWS.

Several diffusion tensor imaging (DTI) studies have reported fractional anisotropy (FA) reductions within the left perisylvian white matter (WM) of persistent developmental stutterers (PSs). However, these studies have not reached the same conclusions in regard to the presence, spatial distribution (focal/ diffuse), and directionality (elevated/reduced) of FA differences outside of the left perisylvian region. In addition, supplemental DTI measures (axial and radial diffusivities, diffusion trace) have yet to be utilized to examine the potential etiology of these FA reductions. Therefore, the present study sought to reexamine earlier findings through a sex- and age-controlled replication analysis and then to extend these findings with the aforementioned non-FA measures. The replication analysis showed that robust FA reductions in PSs were largely focal, left hemispheric, and within late-myelinating associative and commissural fibers (division III of the left superior longitudinal fasciculus, callosal body, forceps minor of the corpus callosum). Additional DTI measures revealed that these FA reductions were attributable to an increase in diffusion perpendicular to the affected fiber tracts (elevated radial diffusivity). These findings suggest a hypothesis that will be testable in future studies: that myelogenesis may be abnormal in PSs within left-hemispheric fiber tracts that begin a prolonged course of myelination in the first postnatal year.

This article reports a gender replication study of the P. T. Fox et al. (2000) performance correlation analysis of neural systems that distinguish between normal and stuttered speech in adult males. Positron-emission tomographic (PET) images of cerebral blood flow (CBF) were correlated with speech behavior scores obtained during PET imaging for 10 dextral female stuttering speakers and 10 dextral, age- and sex-matched normally fluent controls. Gender comparisons were made between the total number of voxels per region significantly correlated with speech performance (as in P. T. Fox et al., 2000) plus total voxels per region that were significantly correlated with stutter rate and not with syllable rate. Stutter-rate regional correlates were generally right-sided in males, but bilateral in the females. For both sexes the positive regional correlates for stuttering were in right (R) anterior insula and the negative correlates were in R Brodmann area 21/22 and an area within left (L) inferior frontal gyrus. The female stuttering speakers displayed additional positive correlates in L anterior insula and in basal ganglia (L globus pallidus, R caudate), plus extensive right hemisphere negative correlates in the prefrontal area and the limbic and parietal lobes. The male stuttering speakers were distinguished by positive correlates in L medial occipital lobe and R medial cerebellum. Regions that positively correlated with syllable rate (essentially stutter-free speech) in stuttering speakers and controls were very similar for both sexes. The findings strengthen claims that chronic developmental stuttering is functionally related to abnormal speech-motor and auditory region interactions. The gender differences may be related to differences between the genders with respect to susceptibility (males predominate) and recovery from chronic stuttering (females show higher recovery rates during childhood).

This paper overviews recent developments in an ongoing program of brain imaging research on developmental stuttering that is being conducted at the University of Texas Health Science Center, San Antonio. This program has primarily used H 2 15 O PET imaging of different speaking tasks by right-handed adult male and female persistent stutterers, recovered stutterers and controls in order to isolate the neural regions that are functionally associated with stuttered speech. The principal findings have emerged from studies using condition contrasts and performance correlation techniques. The emerging findings from these studies are reviewed and referenced to a neural model of normal speech production recently proposed by Jurgens [Neurosci. Biobehav. Rev. 26 (2002) 235]. This paper will report (1) the reconfiguration of previous findings within the Jurgens Model; (2) preliminary findings of an investigation with late recovered stutterers; (3) an investigation of neural activations during a treatment procedure designed to produce a sustained improvement in fluency; and (4) an across-studies comparison that seeks to isolate neural regions within the Jurgens Model that are consistently associated with stuttering. Two regions appear to meet this criterion: right anterior insula (activated) and anterior middle and superior temporal gyri (deactivated) mainly in right hemisphere. The implications of these findings and the direction of future imaging investigations are discussed. Educational objectives: The reader will learn about (1) recent uses of H 2 15 O PET imaging in stuttering research; (2) the use of a new neurological model of speech production in imaging research on stuttering; and (3) initial findings from PET imaging investigations of treated and recovered stutterers.

This paper reports the results of an efficacy study of a stuttering treatment program known as Modifying Phonation Intervals (MPI), which trains stuttering speakers to reduce the frequency of relatively short phonation intervals (PIs) during connected speech across speaking tasks and situations. Five young adult male stuttering speakers were treated in this computer-based program that systematically trains speakers to reduce selected short PIs found to functionally control stuttering. The treatment process was evaluated using multiple-baseline designs. Treatment was largely self-managed and based on a performance-contingent schedule of within-clinic speaking tasks (Establishment), beyond-clinic speaking tasks (Transfer), and systematic decreases in assessment occasions (Maintenance). Assessments were made at regular intervals before, during, and after treatment. All speakers achieved stutter-free and natural-sounding speech during within- and beyond-clinic speaking tasks at the completion of Mainte-nance. All were tested 12 months after completion of Maintenance, and all maintained the results. The findings from this study suggest that this procedure may make a significant contribution to stuttering treatment practice.

Positron emission tomographic (PET) H 2 15 O measurements of resting-state regional cerebral blood flow (CBF) were obtained in 29 right-handed men, 10 of whom stuttered and 19 of whom did not. PET images were analyzed by sampling 74 regions of interest (ROIs), 37 per hemisphere. ROI placement was guided both physiologically and anatomically. Physiological ROI placement was based on speech motor activations. Anatomical ROIs were positioned by reference to a stereotactic, neurosurgical atlas with positions confirmed and finely adjusted by co-registered magnetic-resonance images (MRIs). For all subjects, PET and MR images were normal to visual inspection. Highly significant ( p < 0.0001) between-region and between-hemisphere effects were found for both groups, as have been previously reported for normal subjects, but no significant between-group differences were found for any regional CBF values. Analysis by a laterality index found a weakly significant between-groups effect ( p =0.04) that was isolated to five regions, four of which are implicated in speech or hearing. However, these regional laterality effects showed no consistent directionality, nor did these regions have absolute differences in regional blood flow between groups. Present findings do not support recent suggestions that developmental stuttering is associated with abnormalities of brain blood flow at rest. Rather, our findings indicate an essentially normal functional brain terrain with a small number of minor differences in hemispheric symmetry.

In response to the recognized need for a valid and reliable way to measure stuttering, this study investigates a measurement methodology based on time-interval analyses of stuttering event judgments. Three groups of judges, differing in stuttering judgment experience, identified stuttering events in 12 repeated presentations of five 1-min speech samples. Fixed time intervals ranging from 0.2 sec to 7.0 sec were then superimposed on the event judgments by a data analysis program. Inter- and intrajudge interval-by-interval agreement, and agreement for total numbers of intervals containing stuttering event judgments, were calculated for each judge group. Results showed that agreement was superior among more experienced judges and in longer interval lengths. Agreement varied across speech samples but not across the repeated judgment opportunities. Agreement was maximized at greater than chance levels for an interval of approximately 3.0 sec, but even this best agreement did not exceed a mean of approximately 60% for experienced judges.

Using single-subject experiments with 3 adult stutterers, this study evaluated the effects of instructions to stutterers to rate and modify how natural their speech sounds on experimenters' ratings of speech naturalness, stuttering frequency, and speaking rate. The study also included an investigation of the reliability of stutterers' and listeners' naturalness ratings. The stutterers were partway through a therapy program using prolonged speech or rate control. Results showed the stutterers could modify their speech so that their naturalness ratings increased or decreased. These changes were independent of stuttering or speaking rate. Experimenter ratings of speech naturalness were unchanged in conditions where stutterers judged their speech to sound more natural, but paralleled the stutterers' ratings when they judged their speech to sound more unnatural. An attempt to see if stutterers differed in their ratings of how natural their speech sounded or felt showed differences for one stutterer. Reratings of randomized session recordings by experimenters and independent judges showed that their ratings were highly reliable. When the same randomized session recordings were rerated by the stutterers (1 and 3 months after the experiment), their judgments of changes in their speech naturalness, which were not found in the experimenters' ratings, remained consistent and reliable.

To distinguish the neural systems of normal speech from those of stuttering, PET images of brain blood flow were probed (correlated voxel-wise) with per-trial speech-behaviour scores obtained during PET imaging. Two cohorts were studied: 10 right-handed men who stuttered and 10 right-handed, age- and sex-matched non-stuttering controls. Ninety PET blood flow images were obtained in each cohort (nine per subject as three trials of each of three conditions) from which r-value statistical parametric images (SPI?r?) were computed. Brain correlates of stutter rate and syllable rate showed striking differences in both laterality and sign (i.e. positive or negative correlations). Stutter-rate correlates, both positive and negative, were strongly lateralized to the right cerebral and left cerebellar hemispheres. Syllable correlates in both cohorts were bilateral, with a bias towards the left cerebral and right cerebellar hemispheres, in keeping with the left-cerebral dominance for language and motor skills typical of right-handed subjects. For both stutters and syllables, the brain regions that were correlated positively were those of speech production: the mouth representation in the primary motor cortex; the supplementary motor area; the inferior lateral premotor cortex (Broca's area); the anterior insula; and the cerebellum. The principal difference between syllable-rate and stutter-rate positive correlates was hemispheric laterality. A notable exception to this rule was that cerebellar positive correlates for syllable rate were far more extensive in the stuttering cohort than in the control cohort, which suggests a specific role for the cerebellum in enabling fluent utterances in persons who stutter. Stutters were negatively correlated with right-cerebral regions (superior and middle temporal gyrus) associated with auditory perception and processing, regions which were positively correlated with syllables in both the stuttering and control cohorts. These findings support long-held theories that the brain correlates of stuttering are the speech-motor regions of the non-dominant (right) cerebral hemisphere, and extend this theory to include the non-dominant (left) cerebellar hemisphere. The present findings also indicate a specific role of the cerebellum in the fluent utterances of persons who stutter. Support is also offered for theories that implicate auditory processing problems in stuttering.