Speech-motor control and interhemispheric relations in recovered and persistent stuttering.

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 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.

Concurrent language and motor performance in bilinguals: a test of the age of acquisition hypothesis.

The objective of the present study was to test the hypothesis that the age at which a second language is acquired influences the pattern of cerebral lateralization associated with that language. Subjects who differed in terms of the age at which they had acquired their second language (English or French) were tested on a concurrent task paradigm involving motor and language performance. Hemispheric processing was inferred from the pattern of lateralized and generalized interference between the tasks. No support was found for the age-of-acquisition hypothesis. Instead, the data indicated a language-specific effect. Regardless of age of acquisition and of whether the first language was English or French, bilingual subjects showed lateralized interference effects consistent with left-hemisphere processing when reading in English and translating from French into English, but no lateralized interference when reading in French and translating from English into French. Whether this effect reflects characteristics of the two languages or the influence of social factors in subject-experimenter interaction is considered.

Concurrent task interference in stutterers: dissociating hemispheric specialization and activation.

Using all four combinations of hand and foot, adult stutterers and nonstutterers performed a unimanual sequential finger tapping task together with a stimulus-contingent foot responding task. The two groups were similar in that both demonstrated more dual-task interference when using the ipsilateral than contralateral limb pairs. Contrary to a prediction based on a hypothesis that attributes stuttering to neural interference due to an ungated or unregulated activity flow between the hemispheres, stutterers did not show more interference than nonstutterers when using the contralateral limb pairs. The groups did differ, however, in terms of finger tapping interference associated with right- versus left-foot concurrent task responding. Nonstutterers showed more interference when using the left than right foot. This was interpreted in terms of the attentional demands that the foot responding task placed on a system with an inherent left-hemisphere attentional bias. The lack of a reliable difference in stutterers with respect to interference by the two feet is consistent with indications from other research that stutterers have a relatively labile system of hemispheric attention or activation. The results of the study are placed into the context of a general model of the brain mechanisms associated with stuttering.

Task complexity and manual reaction times in people who stutter.

The study was designed to test the hypothesis that manual reaction time differences between people who stutter and those who do not reflect the information processing complexity of the task. The study focused on decision complexity in a reaction time paradigm. One manipulation involved increasing the number of response alternatives. The second involved the spatial contiguity of signal and response locus. Twenty-four adult stutterers and 24 nonstutterers were compared with respect to response initiation and completion times on the various task conditions. Contrary to the hypothesis, there was no significant Group x Complexity interaction in the analysis of either response measure for either complexity manipulation. Stutterers were slower than nonstutterers overall, but with increasing decision complexity, the group response times paralleled one another. It is concluded that whatever response planning and organization deficit there may be in people who stutter, it is independent of decision complexity but may be evident in manipulations of response complexity defined in terms of spatial and temporal coordination.

Family history as a basis for subgrouping people who stutter.

Of a clinical population of 169 adult and adolescent stutterers, 112 members (66%) reported a family history of stuttering. Only 3 (2.4%) of these reported any birth or early childhood factors or events that were thought to be associated with stuttering onset or that potentially might have precipitated stuttering. In contrast, 21 (37%) of the 57 members without a family history of stuttering reported such a factor or event. On the basis of this retrospective investigation of family history, the stutterers could be segregated in ways that may be informative relative to etiology and underlying mechanisms despite being apparently similar with respect to time of stuttering onset, dysfluency characteristics, and emotional concomitants. It is suggested that these data are consistent with a hypothesis that within the clinical population of adults presenting as developmental stutterers there are really two subgroups. One subgroup is thought to consist of individual with a genetically inherited predisposition for stuttering, and the second of individuals without such a predisposition but who may have sustained some form of early brain damage. The incidence of false negatives in the 36 individuals classified as having no family history and no known early physical trauma remains to be ascertained. Implications for research on both brain mechanisms of stuttering and responsiveness to clinical treatment and fluency maintenance are discussed.

Motor performance of stutterers: a search for mechanisms.

A continuing program of research, which is concerned with identifying brain mechanisms underlying stuttering through an analysis of manual motor control, is described. Clear evidence has been found that the neural mechanisms associated with sequential responding (and, by implication, with speech) are lateralized in stutterers as they are in nonstutterers. Although no gross or general incoordination has been found in motor performance by most stutterers, their left hemisphere mechanisms appear to be inefficient for organizing and initiating new sequences of responses and vulnerable to interference from other neural activities. Results of research on bimanual coordination in stutterers are consistent with a model that attributes the interference, in part, to interhemispheric processes, possibly involving the supplementary motor area. One implication of the research is that the disfluency of stuttering is only one manifestation of a more general disfunction in motor and cognitive organization and planning.

Evidence in bimanual finger-tapping of an attentional component to stuttering.

The performance of right- and left-handed male and female stutters was compared with that of non-stutters on a bimanual coordination task that involved tapping a key twice with one hand for each single tap of a key by the other hand. Right-handed non-stutters performed this 2:1 tapping better when it was the right hand that tapped twice (R2/L1 condition) rather than the left hand (L2/R1 condition), but among left-handers performance was similar under the two conditions. This replicated previous findings of Peters10 that were interpreted as indicating the role of attentional mechanisms in the expression of handedness. The performance of the stutters differed in two major respects. First, overall bimanual tapping rates were significantly slower than those of the non-stutters. Second, right-handed stutters did not show the asymmetry in performance between the R2/L1 and L2/R1 conditions. The results are interpreted within the framework of current neuropsychological research that relates stuttering to anomalous mechanisms of interhemispheric communication.

Concurrent cognitive processing and letter sequence transcription deficits in stutterers.

Previous research has indicated that men who stutter transcribe rapidly presented sequences of letters more slowly and less accurately than nonstutterer controls. Experiment 1 demonstrated that the transcription deficit is not limited to task conditions that demand concurrent monitoring and responding. This was evidenced by comparable deficits on a successive response condition that required subjects to write letters after the presentation was complete. The results of Experiment 2 indicated that the deficit is not due to a difficulty by stutterers in parsing streams of stimulus information internally. Their performance did not differentially improve when letters were grouped with brief pauses, nor with experience in transcribing preparsed letter sequences. This experiment also demonstrated that the phenomenon is generalizable to women. In related testing, stutterers were slower than controls in writing internally generated sequences of letters, those of the alphabet forwards and backwards, but not in writing the same two letters, A and B, repetitively nor in the cognitively more demanding task of writing numbers backwards by three's. These results parallel those obtained with finger tapping of same versus unique sequences by stutterers and were interpreted as being consistent with the idea that while stutterers are not generally slower motorically than nonstutterers, they experience difficulty when required to organize and carry out tasks with new multiple response transitions. The two experiments have replicated and extended, under different conditions, the earlier findings of a letter sequence transcription deficit in stutterers, but the nature of the interference still remains to be clarified.

Stuttering: current status of theory and therapy.

When people become concerned about stuttering, either their own or their child's, they typically seek help from their family physician or pediatrician. Characteristics and effects of stuttering can help the physician determine whether the child's speech patterns are within normal range or could be early warning signs of stuttering. Referral to a speech-language pathologist should be considered when any of the following are noted: excessive part-word repetitions or sound prolongations; signs of tension, such as lip tremors, facial grimaces, or breath holding associated with speech; indications of embarrassment or apprehension; and a tendency to withdraw from speech opportunities. The authors briefly review theories on the causes of stuttering, describe current therapy, and identify some specialized treatment centres.

Bimanual handedness in adults who stutter.

25 adult stutterers and 29 nonstutterers who were right-handed as defined by a positive Laterality Quotient on the Edinburgh Handedness Inventory were compared with respect to their pattern of hand use in performing seven common tasks that involve bimanual cooperation. Among the stutterers was a higher proportion of participants who showed anomalies in how they carried out the two tasks that required synchronous manipulation by the two hands. On two other tasks that required speeded performance, the groups were similar in dealing playing cards with the right or left hands, but the stutterers were slower than nonstutterers and did not show a right-hand advantage on a task requiring removal of a nut from a bolt. The results were interpreted as indicating difficulty by stutterers in carrying out synchronously different response elements of motoric tasks.

Sequence initiation performance by stutterers under conditions of response competition.

Male stutterers and nonstutterers were compared on their performance of a task that required tapping keys as rapidly and accurately as possible to reproduce different sequences of finger movements as soon as they had been demonstrated on a visual display panel. Testing using the right hand alone confirmed previous findings that stutterers are significantly slower than nonstutterers in initiating new movement sequences and make significantly more errors on initial sequences. Subjects were also tested on this sequence reproduction task while they performed concurrently a manual task with the left hand that required turning a knob in response to signal tones. The hypothesis underlying the study was that associated with stuttering is a dysfunction in the mechanisms regulating interhemispheric communication, and it was predicted that the concurrent task would interfere more with the sequence reproduction task performance of stutterers than with that of nonstutterers. A joint analysis of the speed and accuracy aspects of performance of the two tasks confirmed the prediction. Additional research required to clarify whether the effects are in fact due to the hypothesized interhemispheric mechanisms or to a generally greater susceptibility of stutterers to interference from any concurrent motor or cognitive activity is discussed.

Neural mechanisms underlying stuttering: evidence from bimanual handwriting performance.

Left- and right-handed male and female stutterers were compared with fluent speakers on a bimanual handwriting task. On each trial four words were read to the subject. After repeating the words, subjects had to write the initial letters as quickly as possible using the two hands simultaneously and without visual guidance. As a group, stutterers were slower, made more mirror-reversed letters, and formed letters of poorer quality than fluent speakers. The effects were the same for males and females, and the data for left- and right-handers were mirror-symmetric with respect to left and right hands. Evidence was found for two subgroups of stutterers with respect to scores on the dependent variables. It was suggested that the overall pattern of results implicates the supplementary motor area in the mediation of stuttering, possibly through relatively ungated callosal pathways.

Handedness distributions among adults who stutter.

Handedness testing carried out in the course of clinical treatment and research with stutterers has provided no support for an association between left-handedness and stuttering. Stutterers had neither a greater incidence of left-handedness than is usually reported for the general population, nor demonstrated less consistency than fluent speakers in hand preference.

Rapid letter transcription performance by stutterers.

To test the idea that associated with stuttering is a cognitive difficulty related to the organization of sequences of responses, stutterers and fluent speakers were compared on a task that required the transcription to paper of rapidly presented letter sequences ranging in length from 3 to 10 letters. Compared to fluent speakers, stutterers transcribed significantly fewer sequences correctly. A number of possible hypotheses to account for the results are discussed.

Neuropsychological models of stuttering--II. Interhemispheric interference.

The performance of male adult stutterers and fluent speakers was compared on repetitive sequential finger tapping and index finger tapping with one hand while carrying out concurrent paced tasks with the other hand. For the theoretically meaningful condition of right-hand sequential tapping and left-hand concurrent task performance, there was more interference on the tapping task among stutterers than fluent speakers. The data were interpreted as being consistent with a neuropsychological model of stuttering that includes as an element enhanced left-hemisphere vulnerability to interference by concurrent right-hemisphere activity.

Response sequence organization and reproduction by stutterers.

Male stutterers and fluent speakers were compared on their performance of a task requiring tapping keys as rapidly and as accurately as possible to reproduce different finger movement sequences demonstrated on a visual display panel. Although overall finger tapping rate was the same in the two groups, indicating no difficulty by stutterers in performing simple motor movements, stutterers achieved fewer correct sequences and made more errors than fluent speakers. In addition, their response initiation times were slower. Once a correct response was initiated, however, the time to execute the sequence was similar to that of fluent speakers. Replicating earlier work, the two groups were not found to differ on a repetitive sequential finger tapping task with respect to correct sequences or total presses, although the probability of error was greater for the stutterers. The data were interpreted as indicating that in stutterers sequential response mechanisms are lateralized normally as they are in fluent speakers; these left-hemisphere sequential response mechanisms in stutterers appear unusually susceptible to interference, possibly from on-going right-hemisphere activity; stutterers have special difficulty in organizing and/or initiating new response sequences, but once the sequence is initiated, they can perform the sequence as rapidly (but with greater probability of error) as do fluent speakers; and stuttering reflects not a simple motor problem per se, but a higher level organizational problem of a cognitive nature.

Neuropsychological models of stuttering--I. Representation of sequential response mechanisms.

Male stutterers and fluent speakers were compared on index finger tapping and sequential finger tapping tasks. For both groups performance was better with the right than the left hand, and was better under conditions of visualization than under conditions of no visualization of the hand. In addition, participants in both groups showed similar variations in performance on various sequences. The pattern of results suggests that stuttering does not result from a general problem in sequencing and timing of behaviour, and is consistent with a neuropsychologial model of stuttering that would propose normal lateralization of neural mechanisms associated with sequential processing including speech.

Morphological asymmetries of the cat brain.

The fissure patterns of 112 cat brains were examined and 45% were found to be asymmetric with respect to Otsuka and Hassler sulcus pattern types. As well, 22% were asymmetric with respect to the presence of a postcrutiate sulcus. No relationship was found between the presence or asymmetry of specific cortical features and paw preference, suggesting that these asymmetries are not associated with a functional asymmetry in visual discrimination performance previously demonstrated in the cat.