Cerebral Lateralization For Language Research Articles

Speech disorder in schizophrenia: review of the literature and exploration of its relation to the uniquely human capacity for language.

The language capacity of modern humans is thought by some to be clearly distinct from that of nonhuman primates (Bickerton 1990). Crow (1997, 1998a) has proposed that a disturbance in the uniquely human aspects of language is central to the genetic etiology of schizophrenia. A review of the literature on language disorder in schizophrenia provides evidence for widespread deficits in comprehension, production, attention, and cerebral lateralization of language. We focused here on those anomalies that are uniquely human aspects of language. Bickerton's five distinctly human language devices were examined in patients with schizophrenia and their families by using a structured scoring format on oral soliloquies. The chronic patients showed reduced use of clausal embedding and used fewer words than first episode patients or well family members. The amount of sentence complexity was found to be familial and to cosegregate with schizophrenia within families. These data are consistent with previous literature and additionally show a familial component to these measures, thus suggesting that deficits in specifically human aspects of language may be related to the genetics of schizophrenia.

Salivary testosterone concentrations in left-handers: An association with cerebral language lateralization.

The level of testosterone exposure in early brain development may influence the direction or degree of cerebral language lateralization. Possible links between individual differences in testosterone levels and patterns of speech representation were investigated in 180 healthy young adults (97 left handed, 83 right handed) using the Fused Dichotic Words Test (T. Halwes, 1991). Among left-handed participants, significantly higher testosterone concentrations were observed in individuals with a left-ear advantage on dichotic listening than in individuals with a right-ear advantage. Among right-handed participants, the pattern of group differences in testosterone tended to be reversed, resulting in a statistically significant interaction. Results extend prior findings by S. D. Moffat and E. Hampson (1996a) and raise the possibility that higher testosterone is associated with patterns of brain organization in which speech and praxic functions are lateralized to the same hemisphere.

Salivary testosterone concentrations in left-handers: An association with cerebral language lateralization.

Salivary testosterone concentrations in left-handers: An association with cerebral language lateralization.

Language lateralization in healthy right-handers.

Our knowledge about the variability of cerebral language lateralization is derived from studies of patients with brain lesions and thus possible secondary reorganization of cerebral functions. In healthy right-handed subjects 'atypical', i.e. right hemisphere language dominance, has generally been assumed to be exceedingly rare. To test this assumption we measured language lateralization in 188 healthy subjects with moderate and strong right-handedness (59% females) by a new non-invasive, quantitative technique previously validated by direct comparison with the intracarotid amobarbital procedure. During a word generation task the averaged hemispheric perfusion differences within the territories of the middle cerebral arteries were determined. (i) The natural distribution of language lateralization was found to occur along a bimodal continuum. (ii) Lateralization was equivalent in men and women. (iii) Right hemisphere dominance was found in 7.5% of subjects. These findings indicate that atypical language dominance in healthy right-handed subjects of either sex is considerably more common than previously suspected.

Cerebral lateralization of language in normal left-handed people studied by functional MRI.

To use functional MRI (fMRI) to further define the occurrence of left-hemisphere, bilateral, and right-hemisphere language in a normal left-handed population. A total of 100 healthy volunteers, consisting of 50 left-handed subjects and a reference group of 50 right-handed subjects, were studied by fMRI of the frontal cortex during silent word generation. Ninety-six percent of right-handed subjects showed fMRI changes lateralized to the left hemisphere, whereas 4% showed a bilateral activation pattern. In contrast, left-hemisphere lateralization occurred in 76% of left-handers, bilateral activation in 14%, and right-hemisphere lateralization in the remaining 10%. The predominance of right-hemisphere activation, however, was weak in these cases; only a single left-handed subject (2%) showed complete right-hemisphere lateralization. Silent word generation lateralizes to the left cerebral hemisphere in both handedness groups, but right-hemisphere participation is frequent in normal left-handed subjects. Exclusive right-hemisphere activation rarely occurred in the frontal lobe region studied.

Computational studies of lateralization of phoneme sequence generation.

The mechanisms underlying cerebral lateralization of language are poorly understood. Asymmetries in the size of hemispheric regions and other factors have been suggested as possible underlying causal factors, and the corpus callosum (interhemispheric connections) has also been postulated to play a role. To examine these issues, we created a neural model consisting of paired cerebral hemispheric regions interacting via the corpus callosum. The model was trained to generate the correct sequence of phonemes for 50 monosyllabic words (simulated reading aloud) under a variety of assumptions about hemispheric asymmetries and callosal effects. After training, the ability of the full model and each hemisphere acting alone to perform this task was measured. Lateralization occurred readily toward the side having larger size, higher excitability, or higher-learning-rate parameter. Lateralization appeared most readily and intensely with strongly inhibitory callosal connections, supporting past arguments that the effective functionality of the corpus callosum is inhibitory. Many of the results are interpretable as the outcome of a "race to learn" between the model's two hemispheric regions, leading to the concept that asymmetric hemispheric plasticity is a critical common causative factor in lateralization. To our knowledge, this is the first computational model to demonstrate spontaneous lateralization of function, and it suggests that such models can be useful for understanding the mechanisms of cerebral lateralization.

Reproducibility of functional transcranial Doppler sonography in determining hemispheric language lateralization.

Since functional transcranial Doppler ultrasonography (fTCD) allows convenient and fully automated quantification of language lateralization, it seems ideal for longitudinal studies of perfusion changes during deterioration as well as recovery of language functions. However, during serial examinations, the technical, stochastic, and physiological variabilities of cerebral blood flow velocities (CBFV) have to be considered. Therefore, before fTCD is accepted as a tool for evaluation of changes in lateralization in the diseased state, its reliability in healthy subjects needs to be determined. We performed fTCD during a word generation task based on a previously validated technique with automated calculation of the averaged CBFV differences in the middle cerebral arteries providing an index of lateralization (LI). (1) The accuracy of the LI as assessed by the confidence interval was better than 1% of the mean hemispheric difference. (2) On repeated examination, LIs obtained from 10 subjects showed a high test-retest reproducibility (Pearson product moment correlation coefficient r = 0.95, P < 0.0001). (3) On 10 repeated assessments of LI in the same subject, no practice effects were detected. Functional TCD is a suitable and very robust tool for the longitudinal quantitative measurement of cerebral language lateralization.

Effect of Wada memory stimulus type in discriminating lateralized temporal lobe impairment.

To examine the effects of memory stimulus type on Wada memory performance. Ninety-six patients (left, 47; right, 49) from four epilepsy centers who were candidates for anterior temporal lobectomy (ATL) and who have subsequently undergone surgery were studied. Patients with atypical cerebral language lateralization or with evidence on magnetic resonance imaging (MRI) to suggest a lesion other than hippocampal sclerosis were excluded. Wada memory performance was obtained by using both real objects and line drawings as memory stimuli. Wada memory laterality scores with either real objects or line drawings as memory stimuli discriminated left from right-ATL groups. However, objects were superior to line drawings in making this differentiation. Further, objects were superior to line drawings in individual patient classification of candidates for left ATL, with no difference in the classification rates using either objects or line drawings in candidates for right ATL. Type of memory stimuli is an important factor affecting memory results during the Wada test.

Cerebral lateralization for language revealed in neuronal responses of the human brain cortex

Neuronal impulse activity (NIA) of the cortex and subcortical nuclei in the thalamus and striopallidar complex was studied during performance of the word recognition test in patients with long-term intracerebral electrodes implanted for diagnostics and therapy. In NIA of the left hemisphere cortex, endogenous responses were found to prevail in the interval between expositions of two stimuli, whereas NIA of the right cortex exhibited more responses during perception of stimuli. Asymmetry in neuronal responses of subcortical nuclei in the thalamus and striopallidar complex appeared to differ from that observed in the cerebral cortex.

Cerebral language lateralization: Evidence from intracarotid amobarbital testing

Cerebral language lateralization was investigated in 103 patients undergoing intracarotid amobarbital testing as part of their diagnostic work-up for epilepsy surgery. Inclusion criteria included adequate bilateral intracarotid amobarbital studies and no radiologic lesion in areas other than the temporal lobe. Language was evaluated with respect to strict presence or absence of language representation, in which a patient was considered to have bilateral language despite potentially having asymmetric language representation, and with respect to forced relative hemispheric dominance, in which a single side could be considered dominant despite bilateral language representation. Seventy-nine patients displayed exclusive left hemisphere language representation, two patients showed exclusive right hemisphere language representation, and 22 patients had language represented in each hemisphere. In the 22 patients with bilateral language, an asymmetry was present in 17 cases (13 L > R, 4 R > L). These data indicate that language restricted only to the right hemisphere is rare, and that in the absence of purely left hemisphere language, most patients exhibit bilateral representation. Previously reported incidence of exclusive right hemisphere language may be an artifact of dichotomizing a continuous variable.

Cerebral lateralization of language in deaf and hearing people

In Experiment 1 neither hearing nor prelingually deaf signing adolescents showed marked lateralization for lexical decision but, unlike the hearing, the deaf were not impaired by the introduction of pseudohomophones. In Experiment 2 semantic categorization produced a left hemisphere advantage in the hearing for words but not pictures whereas in the deaf words and signs but not pictures showed a right hemisphere advantage. In Experiment 3 the lexical decision and semantic categorization findings were confirmed and both groups showed a right hemisphere advantage for a face/nonface decision task. The possible effect of initial language acquisition on the development of hemispheric lateralization for language is discussed.

Handedness and language among the mentally retarded: Implications for the model of pathological left-handedness and gender differences in hemispheric specialization

The relationship between handedness and language was examined among 238 mentally retarded subjects representing the four DSM III classification ranges of mental retardation. Language ability was found to be significantly related to handedness, with an increased prevalence of left-handedness among those individuals with language deficits. In addition, both expressive and receptive language ability interacted with gender, with the main effects of each language variable being stronger for females than males. Although no main effects were found for severity of retardation, a significant interaction was found between severity and gender. Results support the model of pathological left-handedness and are discussed within the context of this theory, as well as with regard to gender differences in cerebral lateralization of language and cognitive functioning.

Right Hemisphere Processing and Foreign Language Aptitude

In this paper we attempt to link three research areas in the field of second language acquisition : cerebral lateralization for language, language aptitude, and success in inductive versus deductive approaches to language teaching. Based on findings from these three areas, we argue that right as well as left hemisphere type processing may play a role in language acquisition at any age. Furthermore, individuals vary in their ability to use the characteristic processing modes of the two hemispheres and these individual differences may partially underly differences in performance on language aptitude measures as well as success in different language teaching approaches. A study of English-speaking adults is reported. The study tests the hypothesis that different components of language aptitude, as measured by subtests II and IV of the Modern Language Aptitude Test (Carroll and Sapon 1959) may represent functions which are differentially lateralized. Specifically, subtest II, Phonetic Coding, is hypothesized to require at least some right hemisphere processing, whereas subtest IV, Words in Sentences, is hypothesized to rely more on left hemisphere processing. MLAT subtest scores are correlated with scores on an English dichotic listening test. Evidence is found to support the hypothesis that right hemisphere linked abilities may underly some aspects of language aptitude. These findings are discussed in the light of other studies relating MLAT scores and hemisphere preference to success in deductive versus inductive second language courses.

Dichotic listening with the communicatively impaired: results from trials of a short British-English dichotic word test

The construction and testing of a new, short British-English dichotic word test, designed specifically for the communicatively impaired, is described. Sixteen normal, 6 aphasic and 6 stammering subjects were administered the test and a copy of the American-English original. Results show that the British-English version gives reliable ear preference scores, indicative of cerebral lateralization for language, and is suitable for use with subjects who have severe communication problems.

Basic research in speech and lateralization of language: Some implications for reading disability

Basic research in speech and the lateralization of language is shown to illuminate the problems of reading and some of its disabilities. First, it is pointed out how speech, or language for the ear, differs markedly from reading, or language for the eye. Though the sounds of speech are a very complex code and the optical shapes of written language are a simple cipher or alphabet on the phonemes, we all perceive speech easily but read only with difficulty. Perceiving speech is easy because, as members of the human race, we all have access to a special physiological apparatus that decodes the complex speech signal and recovers the segmentation of the linguistic message. Reading is hard because the phonemic segmentation, which is automatic and intuitive in the case of speech, must be made fully conscious and explicit. The syllabic method supplemented by phonics (used with certain reservations) is suggested for remediation of segmentation problems. Second, it is posited that since the sounds of speech are processed differently from non-speech sounds, the two should not be diagnosed and remediated interchangeably. Third, it is shown that the relationships among cerebral lateralization for language, handedness and poor reading can now be studied more meaningfully because of the recent development of new techniques. A truism often heard in the opening lecture of graduate classes in education is that we have few answers to the problems that beset us, only questions. In the field of reading, the difficulty may be owing at least in part to our impatient attempts to find immediate solutions for the teacher and the student in the classroom, and our consequent neglect of basic research. I should like to suggest today how knowledge of basic research in related disciplines may lead to clues for improving beginning reading instruction and the lot of the disabled reader—if only by affording us a deeper understanding of the reading process.