Left Hemisphere Speech Research Articles

The foreign accent syndrome: A perspective

Although there are now a fair number of reported case studies of the foreign accent syndrome, there is little consensus about it. We provide a perspective on the foreign accent syndrome focusing on three areas for which there is as yet disagreement in the literature. These include whether the foreign accent syndrome is indeed a syndrome in its own right, whether the features of the disorder can be explicated in terms of a single underlying mechanism, and whether there is a common neural substrate that gives rise to this disorder. Based on a review of the literature and our own work, we propose that the foreign accent syndrome is properly considered a syndrome and that it is distinct in both its characteristics and underlying mechanism from an apraxia of speech, a dysarthria, and an aphasic speech output disorder. We hypothesize that a deficit in linguistic prosody underlies the foreign accent syndrome. And finally, we argue that the foreign accent syndrome emerges as a consequence of damage to the dominant language (usually left hemisphere) speech output motor system affecting the primary motor cortex and either its cortico-cortical connections or its cortico-subcortical projections.

Features of cerebral support of verbal processes in children with dysgraphia and dyslexia

Comprehensive neuropsychological and electroencephalographic examination of children with dysgraphia and dyslexia demonstrated local EEG anomalies in various zones of the cortex in both hemispheres, mainly in the posterotemporal and/or anterior regions of the left and the posterotemporal regions of the right hemisphere. The character of speech disturbances depended considerably on the localization of the baseline EEG anomalies: disturbances of motor components of writing prevailed when these anomalies were localized in the anterior parts of the left hemisphere, while phonological and morphological speech disturbances were noted in children with local anomalies in the temporal regions of both hemispheres and in posterior association regions, mainly in the right hemisphere. Spectral and correlation analyses of the EEG demonstrated the features of the cerebral organization of verbal activity in children with dysgraphia and dyslexia, namely, weakened intra- and interhemispheric integration of electrical activity of different regions of the brain cortex, particularly temporal and posterior association regions, and predominance of integration based on low-frequency EEG components (θ and α1), unlike children without learning disabilities, in whom functional integration of cortical zones involved high-frequency rhythms (α2 and β). On the whole, the results of the study testify to an important role of systemic interactions between the regions of the cortex of both hemispheres in the formation of verbal activity.

Cortical plasticity associated with stuttering therapy

Neuroimaging studies have indicated that persistent developmental stuttering (PDS) may be associated both with an abnormality in white matter of left-hemispheric speech areas and a right-hemispheric hyperactivity. The latter may compensate for the deficient structural connectivity in the left hemisphere. To investigate the effects of stuttering therapy on brain activity nine male adults with PDS underwent functional magnetic resonance imaging (fMRI) before and within 12 weeks after fluency shaping therapy. Brain response differences during overt sentence reading before and after therapy were assessed by utilizing random effects analyses. After therapy, a more widespread activation was observed in frontal speech and language regions and temporal areas of both hemispheres, particularly and more pronounced on the left side. Interestingly, distinct posttreatment left-sided activation increases were located directly adjacent to a recently detected area of white matter anomaly [M. Sommer, M.A. Koch, W. Paulus, C. Weiller, C. Büchel (2002). Disconnection of speech-relevant brain areas in persistent developmental stuttering. The Lancet, 360, 380–383] suggesting that fluency shaping techniques reorganize neuronal communication between left-sided speech motor planning, motor execution, and temporal areas. Hence, a therapeutic mechanism can be assumed to remodel brain circuitry close to the source of the dysfunction instead of reinforcing compensation via homologous contralateral brain networks. Educational objectives: The reader will learn about and be able to: (1) describe brain activation changes detected shortly after fluency-shaping therapy; (2) identify left-hemispheric regions where a (re)functionalization after fluency-shaping therapy seems to occur adjacent to a recently described abnormal white matter region in PDS subjects; and (3) discuss how an effective cerebral compensation mechanism for stuttering could work.

Preliminary results of a functional MRI study of brain activation patterns in stuttering and nonstuttering speakers during a lexical access task

An fMRI study examining lexical access and lexical generation in nine nonstuttering and seven stuttering speakers is presented. Lexical access was examined during a word description task that was presented auditorily while subjects “silently” thought of the target words. Participants alternated between four 30-s rest blocks and four 30-s “active” blocks. Activation patterns were assessed utilizing a standard subtraction paradigm, where the activation during the rest blocks was subtracted from the activation during the active blocks. High levels of variability characterized activation patterns within both speaker groups. Group comparisons using random effects statistical analyses did not identify significant differences between the groups when corrected for multiple comparisons. Analyses were subsequently conducted by comparing the trends in the group activation patterns between the speaker groups using fixed (corrected) and random effects (uncorrected) analyses. Nonstuttering control speakers activated primarily left hemisphere cortical speech and language areas while the stuttering speakers appeared to produce more bilateral activation. Discussion of these results focuses on the specific within- and between-hemispheric activation patterns and possible interpretations of these patterns. Educational objectives: The reader will learn about: (1) issues related to interpreting brain activation findings in stuttering speakers; (2) the role and neurological substrates of lexical access during speech production in nonstuttering and stuttering speakers; (3) the basics of functional MRI; and (4) the brain activation areas involved during a silent lexical retrieval task in nonstuttering and stuttering speakers.

Atypical speech is rare in individuals with normal developmental histories.

The prevalence of atypical (right, bilateral) speech lateralization is unknown in normal populations. The authors investigated this by studying people with normal developmental histories but a later, specific adult neurologic event leading to intractable epilepsy. Fifty of 836 people receiving intracarotid amobarbital procedures (IAPs) met criteria of normal neurologic histories through age 15 years, with later head trauma or cerebral infection as probable cause of subsequent epilepsy. All 50 patients had left hemispheric speech on IAP. Atypical speech lateralization is rare unless there is also a positive neurologic history.

Ictal urinary urge: further evidence for lateralization to the nondominant hemisphere.

To determine the lateralizing value of ictal urinary urgency. A retrospective database search was performed for patients with ictal urinary urgency admitted to the Epilepsy Monitoring Unit at the Cleveland Clinic between 1994 and 2001. Six patients were identified; intracarotid amytal test demonstrated left hemispheric speech dominance in five cases. The sixth patient continued to speak during right temporal seizures. EEG and imaging data supported right temporal or frontotemporal epilepsy in all six cases. Two patients were seizure free after focal right hemispheric resection. Ictal urinary urge appears to be a lateralizing sign for nondominant temporal lobe epilepsy.

LEFT HEMISPHERE DYSFUNCTION AFFECTS DICHOTIC LISTENING IN PATIENTS WITH TEMPORAL LOBE EPILEPSY

This study evaluated the relative effect of left hemisphere dysfunction and side of seizure onset on dichotic listening performance in patients with temporal lobe epilepsy and left hemisphere speech dominance. Seventeen patients were divided into groups based on side of seizure onset and based on scores on a composite measure revealing left hemisphere dysfunction. The group with left hemisphere dysfunction had more correct responses from the left ear, and a left ear advantage, on dichotic listening. The group with normal left hemisphere function showed the expected right ear advantage. Side of seizure onset did not affect dichotic listening performance significantly.­­

Ipsilateral Reorganization of Language in Early-Onset Left Temporal Lobe Epilepsy

Purpose. Decline in confrontation naming ability occurs in a subset of temporal lobe epilepsy (TLE) patients following left (dominant) anterior temporal lobectomy (ATL). Patients with late age of onset of seizures are most vulnerable to such decline. In addition, object names typically acquired later in language development are the words most likely to be inaccessible after ATL. Early-onset left TLE patients may be at lower risk for post-ATL dysnomia either because they have a limited preoperative lexicon that does not include most late-age-of-acquistion names or they undergo early ipsilateral language reorganization, which results in a lexicon similar to that of late-onset TLE patients but offers protection from post-ATL naming decline.Methods. Sixty-five left hemisphere speech dominant left TLE patients who had undergone ATL were assessed pre- and postoperatively on the Boston Naming Test (BNT).Results. The early- and late-onset groups performed similarly across three BNT age-of-acquisition categories at the preoperative assessment. Words acquired relatively later in life were most likely to become inaccessible postoperatively for both groups, but the early-onset patients showed significantly less overall postoperative decline in naming ability compared with the late-onset group.Conclusions. The more stable pre- to postoperative naming performance exhibited by early-onset patients cannot be attributed to lack of acquisition of the words shown to be most vulnerable to postoperative decline (i.e., late-age-of-acquisition words). Their object naming stability suggests that early-onset left TLE patients undergo intrahemispheric reorganization of language early in life that provides protective benefits.

Effects of domain-specific interference on brain activation associated with verbal working memory task performance.

Previous neuroimaging studies have identified brain regions that underlie verbal working memory in humans. According to these studies a phonological store is located in the left inferior parietal cortex, and a complementary subvocal rehearsal mechanism is implemented by mostly left-hemispheric speech areas. In the present functional magnetic resonance imaging study, classical interfering and non-interfering dual-task situations were used to investigate further the neural correlates of verbal working memory. Verbal working memory performance under non-interfering conditions activated Broca's area, the left premotor cortex, the cortex along the left intraparietal sulcus and the right cerebellum, thus replicating the results from previous studies. By contrast, no significant memory- related activation was found in these areas when silent articulatory suppression prevented the subjects from rehearsal. Instead, this non-articulatory maintenance of phonological information was associated with enhanced activity in several other, particularly anterior prefrontal and inferior parietal, brain areas. These results suggest that phonological storage may be a function of a complex prefronto-parietal network, and not localized in only one, parietal brain region. Further possible implications for the functional organization of human working memory are discussed.

Neural correlates of duplex perception: a whole-head magnetencephalography study.

Simultaneous experience of the same acoustic stimulus in two distinct phenomenological modes, e.g. as a speech-like and as a non-speech event, is referred to as duplex perception (DP). The most widely investigated DP paradigm splits each of the stop consonant-vowel (CV) syllables /ga/ and /da/ into an isolated formant transient (chirp) and the remaining sound structure (base). The present study recorded mismatch fields in response to a series of dichotically applied base and chirp components using whole-head magnetencephalography (MEG). Preattentive mismatch fields showed larger amplitudes in response to contralateral deviants. During attention to the fused percept /da/, the left ear deviants chirps elicited an enhanced and posteriorly shifted dipole field over the ipsilateral hemisphere. These data provide first neurophysiological evidence that the integration of acoustic stimulus elements into a coherent syllable representation constitutes a distinct stage of left-hemisphere speech sound encoding.

Heart rate does not decrease after right hemispheric amobarbital injection

Ten years ago it was reported that the heart rate of 25 patients with different focal epilepsies changed following unilateral hemispheric inactivation by intracarotid amobarbital injection (Wada Test).1 Using mean amobarbital dosages of 126 and 128 mg, the authors showed heart rate increases after left hemisphere inactivation and decreases following right hemisphere inactivation. It was concluded that the results are consistent with differential left/right cerebral hemispheric effects on autonomic function. Within 9 years after publication, this study was cited by 59 publications as reported by the Science Citation Index. However, only one of these studies was aimed at reproducing the results.2 In this study, 32 patients with temporal lobe epilepsy (TLE) and with left hemisphere speech dominance were investigated using 200 mg amobarbital. The patients demonstrated an increase of heart rate immediately after each injection but …

Comparing the Fused Dichotic Words Test and the Intracarotid Amobarbital Procedure in children with epilepsy

The validity of the Fused Dichotic Words Test (FDWT) in predicting the nature of speech representation, as determined by the Intracarotid Amobarbital Procedure (IAP), was examined in a sample of 28 children with epilepsy. Various methods of analysis (difference score, λ, and λ ∗), for the FDWT data were calculated and compared. Results showed the validity of the FDWT did not change depending on the method of analysis. The difference scores showed that 18 of the 19 patients with left hemisphere speech obtained right-ear advantages, while the patient with right hemisphere speech showed a left-ear advantage. As a group, patients with left-hemisphere speech obtained a statistically significant right-ear advantage for the λ and λ ∗ indices, while the patient with right-hemisphere speech showed a left-ear advantage that was also significant for both λ measures. Patients with bilateral speech, as a group, displayed a non-significant ear advantage. Some of the scores from the left-hemisphere group overlapped with those from patients with bilateral speech representation. Controlling for stimulus dominance effects using the λ ∗ measure did not improve classification accuracy for nature of speech representation based on FDWT scores. Finally, comparison of our data using the laterality index from a similar study, showed scores smaller in magnitude than that found in adults with epilepsy.

Effects of Preferred Hand and Sex on the Perception of Tactile Simultaneity

The effects of handedness, sex and the influence of hand placement in extrapersonal space on temporal information processing was investigated by measuring thresholds for perceiving the simultaneity of pairs of tactile stimuli. Simultaneity thresholds of preferred right handed and left handed university students with left hemisphere speech representation were compared using unimanual and bimanual stimulation at three hand placements (midline, lateral and crossed). In unimanual conditions two fingers of one hand were stimulated (single hemisphere), whereas in the bimanual conditions one finger of each hand was stimulated (cross hemispheres). Bimanual minus unimanual thresholds provided an estimate of interhemisphere transmission time (IHTT) regardless of hand placement. The effects of hemispace varied with the type of stimulation. With unimanual stimulation, overall thresholds were longer at the midline placement, however, with bimanual stimulation, thresholds were longer when the hands were spatially separated (crossed and/or uncrossed). Left handers' IHTTs were 8 ms faster than those of right handers. IHTTs in males were faster than females with hands placed in lateral (by 10.8 ms) or crossed (by 9.8 ms) but not midline positions. It was concluded that the cerebral hemispheres are equally capable of discriminating temporal intervals, but that the left hemisphere predominates when there is uncertainty about location of stimulation.

Preoperative FDG-PET temporal lobe hypometabolism and verbal memory after temporal lobectomy.

To examine the relationship of preoperative fluorodeoxyglucose (FDG)-PET asymmetry in temporal lobe metabolism and memory outcome after anterior temporal lobectomy (ATL). In a university-based epilepsy surgery center, 60 ATL patients (27 left, 33 right) were divided into two groups: no/mild (n = 21) or moderate/ severe (n = 39) asymmetry in temporal lobe hypometabolism as determined by FDG-PET. All patients were nonretarded, at least 18 years of age, left-hemisphere speech dominant, without MRI abnormalities other than hippocampal atrophy, and with unilateral temporal lobe origin of intractable complex partial seizures. Neuropsychological measures of intelligence and verbal and visual memory function were assessed preoperatively and 6 months postoperatively. Left ATL patients with no/mild asymmetry in FDG-PET temporal lobe metabolism exhibited significantly greater verbal memory decline compared with left ATL patients with moderate/severe hypometabolism. There was no significant relationship between PET asymmetry and pre- to postsurgical IQ change. No significant relationship was observed between extent of PET hypometabolism and memory outcome for right ATL patients. FDG-PET asymmetry can be added to the preoperative clinical markers that appear useful in predicting verbal memory decline after left ATL.

Cerebral Specialization for Speech Production in Persons with Down Syndrome

The study of cerebral specialization in persons with Down syndrome (DS) has revealed an anomalous pattern of organization. Specifically, dichotic listening studies (e.g., Elliott & Weeks, 1993) have suggested a left ear/right hemisphere dominance for speech perception for persons with DS. In the current investigation, the cerebral dominance for speech production was examined using the mouth asymmetry technique. In right-handed, nonhandicapped subjects, mouth asymmetry methodology has shown that during speech, the right side of the mouth opens sooner and to a larger degree then the left side (Graves, Goodglass, & Landis, 1982). The phenomenon of right mouth asymmetry (RMA) is believed to reflect the direct access that the musculature on the right side of the face has to the left hemisphere's speech production systems. This direct access may facilitate the transfer of innervatory patterns to the muscles on the right side of the face. In the present study, the lateralization for speech production was investigated in 10 right-handed participants with DS and 10 nonhandicapped subjects. A RMA at the initiation and end of speech production occurred for subjects in both groups. Surprisingly, the degree of asymmetry between groups did not differ, suggesting that the lateralization of speech production is similar for persons with and persons without DS. These results support the biological dissociation model (Elliott, Weeks, & Elliott, 1987), which holds that persons with DS display a unique dissociation between speech perception (right hemisphere) and speech production (left hemisphere).

Morphology of the planum temporale and corpus callosum in left handers with evidence of left and right hemisphere speech representation.

In the present study we investigated planum temporale asymmetry and corpus callosum morphology in a sample of young adult left-handed males, using MRI. Two subgroups of left-handed males were identified on the basis of their differing speech lateralization patterns, which were inferred from results of the Fused Dichotic Words Test. These individuals then underwent MRI in order to obtain area measurements of the left and right planum temporale and the midsagittal corpus callosum. Comparisons between these left-handed males and an archival sample of age-matched right-handed males were also performed. Results demonstrated a strong leftward asymmetry in the planum temporale among subjects with left-hemisphere speech representation, regardless of handedness, but no consistent planum temporale asymmetry among subjects with right hemisphere speech representation. The results suggest that reversed speech lateralization is not necessarily accompanied by a concomitant reversal of planum temporale asymmetry. Examination of callosal areas revealed that left-handed subjects with left hemisphere speech functions had a larger corpus callosum than either left-handed subjects with right hemisphere speech functions or right-handed subjects. Increased interhemispheric communication may be required when the neural systems underlying speech and handedness are represented in opposite hemispheres.

Neuroimaging analyses of human working memory.

We review a program of research that uses neuroimaging techniques to determine the functional and neural architecture of human working memory. A first set of studies indicates that verbal working memory includes a storage component, which is implemented neurally by areas in the left-hemisphere posterior parietal cortex, and a subvocal rehearsal component, which is implemented by left-hemisphere speech areas, including Broca's area as well as the premotor and supplementary motor areas. We provide a number of neuroimaging dissociations between the storage and rehearsal areas. A second set of studies focuses on spatial working memory and indicates that it is mediated by a network of predominantly right-hemisphere regions that include areas in posterior parietal, occipital, and frontal cortex. We provide some suggestive evidence that these areas, too, divide into storage and rehearsal regions, with right-hemisphere posterior parietal and premotor regions subserving spatial rehearsal. In a final set of studies, we turn to "executive processes," metaprocesses that regulate the processing of working-memory contents. We focus on the executive process of inhibition as it is used in verbal working memory. We provide evidence that such inhibition is mediated by the left-hemisphere prefrontal region and that it can be dissociated from verbal storage and rehearsal processes.

Prefrontal asymmetric interictal glucose hypometabolism and cognitive impairment in patients with temporal lobe epilepsy.

Depressions of regional cerebral metabolism beyond the epileptogenic zone have been demonstrated in patients with intractable temporal lobe epilepsy. However, their clinical relevance, and the causes of prefrontal metabolic asymmetries are less well understood. We investigated 96 temporal lobe epilepsy patients by FDG-PET and neuropsychological assessment who had a corresponding unilateral temporal hypometabolism, left hemisphere speech dominance, full scale IQ of > 70 and no extratemporal lesion in MRIs. The regional glucose metabolism was determined in each patient in homologous regions including prefrontal cortex, and normalized to whole brain metabolism. Regional differences of > 10% were regarded as asymmetrical. Prefrontal metabolic asymmetries were more frequent in patients with left temporal lobe epilepsy (21 left, six right) and a history of secondarily generalized seizures. A multivariate analysis of variance revealed a main effect for prefrontal metabolic asymmetry on neuropsychological 'frontal lobe measures', including verbal and performance intelligence measures. Prefrontal metabolic asymmetry was not related to 'measures of episodic memory', presence of psychiatric symptoms or frontal interictal epileptiform discharges. We conclude that prefrontal metabolic asymmetry is associated with cognitive impairment. Patients with temporal lobe epilepsy of the left speech dominant hemisphere and a history of secondarily generalized seizures are at considerable risk of developing prefrontal metabolic asymmetry.

The Relation of Planum Temporale Asymmetry and Morphology of the Corpus Callosum to Handedness, Gender, and Dyslexia: A Review of the Evidence

Asymmetry of the planum temporale in relation to handedness, gender, and dyslexia is reviewed. The frequency of rightward asymmetry is rather higher than are estimates of the proportion of right hemisphere speech representation in the general population. Conversely, the frequency of leftward asymmetry is lower than the proportion of the population with left hemisphere speech. Neuro-anatomic asymmetry may relate more to handedness than to language lateralization. There are suggestions that neuroanatomic asymmetry is reduced in females compared to males but the data are inconclusive. Reports concerning handedness and gender differences in callosal structure are conflicting but, as with planum asymmetry, any effect of handedness is as likely to relate to degree as to direction of handedness. It has been reported that the plana are more often symmetrical in size or larger on the right side among dyslexics than controls but this has not always been found. However, greater frequency of atypical (a)symmetry of the planum in dyslexia would be consistent with the absence of a factor which, when present, biases the distribution of planum asymmetry toward the left (and handedness towards the right) as hypothesized by Annett (1985). Studies of the size of the corpus callosum in dyslexia have produced conflicting findings.

Word-finding deficits persist after left anterotemporal lobectomy.

To determine the incidence and extent of exacerbation of word-finding difficulty following anterotemporal lobectomy and to identify predictors of change. Case-series study of qualitative and quantitative changes in confrontation naming ability before surgery and 1 year after surgery. Stepwise multiple regression analysis of predictors of postoperative naming change. A university epilepsy surgery program. Fifty-nine consecutive patients. Standard, en bloc anterotemporal lobectomy. Raw scores and types of errors on the 85-item Boston Naming Test. A significant exacerbation of word-finding difficulty was noted that persisted at least 1 year after surgery in 25% of patients with left, speech-dominant anterotemporal lobectomy (Laterality x Time interaction [F = 24.5; P < .0005]). "Tip-of-the-tongue"-type errors were most frequent (F = 54.66; P < .001), as opposed to paraphasic-type errors seen more frequently among patients with aphasia or dementia. Word-finding decline was worse among patients who underwent a left anterotemporal lobectomy and who were left-hemisphere speech dominant, older, or evaluated earlier in the postoperative course. Significant, persisting worsening of word-finding difficulties is not a rare consequence of a left anterotemporal lobectomy, as suggested by previous studies. Differences between these results and those of previous studies may reflect differences in extent of surgical resection and/or differential sensitivity of psychometric measures of naming to word-finding problems. Information regarding extent, frequency, and predictors of word-finding declines may be used in counseling surgical candidates about the potential cognitive side effects of anterotemporal lobectomy.