Lateralization of cognitive processes in the brain

The lateralization of cognitive processes in the brain is discussed. The traditional view of a language-visuo/spatial dichotomy of function between the hemispheres has been replaced by more subtle distinctions. The use of magnetic resonance imaging (MRI) to study brain morphology has resulted in a renewed focus on the relationship between structural and functional asymmetry. Focus has been on the role played by the planum temporale area in the posterior part of the superior temporal gyrus for language asymmetry, and the possible significance of the larger left planum. The dichotic listening technique is used to illustrate the difference between bottom-up, or stimulus-driven laterality versus top-down, or instruction-driven laterality. It is suggested that the hemispheric dominance observed at any time is the sum result of the dynamic interaction between bottom-up and top-down processing tendencies. Stimulus-driven laterality dominance is always monitored and modulated through top-down cognitive processes, like shifting of attention and changes in arousal. A model of top-down modulation of bottom-up laterality is presented with special reference to the understanding of psychiatric disorders.     

Total surface of temporoparietal intrasylvian cortex: diverging left-right asymmetries

Measurements of the total convoluted surface area of cortex buried in the temporoparietal transition of the Sylvian fissure were performed using "three-dimensional" magnetic resonance morphometry in 10 randomly selected human cadaver brains. The measurements comprised the planum temporale, which covers the superior temporal gyrus posterior to Heschl's first transverse gyrus, and the caudally adjacent walls of the descending and ascending rami of the Sylvian fissure. Also, the total convoluted surface of the planum temporale was compared with its superficially exposed surface as determined by photographical planimetry after brain section, so that the index of cortical folding of the planum temporale could be estimated. The following results were obtained: Cortical folding of the planum temporale did not differ significantly between left and right hemispheres (p greater than .6). The total and the superficially exposed surface of the planum temporale showed significant leftward asymmetry (p less than or equal to .0078). In contrast, the total surface of cortex buried in the caudal segments of the Sylvian fissure posterior to the planum exhibited significant rightward asymmetry (p less than or equal to .016). The combined cortical surface area buried in the whole posterior Sylvian fissure caudal to the first transverse gyrus did not show significant left-right asymmetry (p greater than 0.3). The data suggest divergent lateral asymmetries in the posterior intrasylvian region with excesses of superior temporal cortex in left hemispheres and of supramarginal cortex in right hemispheres.     

Structural imaging in dyslexia: the planum temporale

The search for a neurobiological substrate for dyslexia has focused on anomalous planum symmetry. The results of imaging studies of the planum have been inconsistent, perhaps due to diagnostic uncertainty, technical differences in measurement criteria, and inadequate control of handedness, sex, and cognitive ability. Although structural imaging studies have not clarified the neurobiology of reading disability, converging evidence suggests that variation in asymmetry of the planum temporale does have functional significance. Studies in a variety of populations have shown a significant association between planar asymmetry, the strength of hand preference, and general verbal skills such as vocabulary and comprehension. Future structural imaging studies of dyslexia should match participants on hand preference and general verbal ability in order to determine the relationship between brain structure and written and oral language.     

Child Age and Planum Temporale Asymmetry

Investigations using in vivo magnetic resonance (MR) morphometry have shown that left-right asymmetry of the planum temporale (PT) is a structural correlate of hemispheric functional asymmetries in adult humans (e.g., handedness, language representation). Postmortem studies of brains of fetuses and newborns have demonstrated that PT asymmetry becomes visible as early as in the last gestational trimester. The same studies could not clarify when the full (adult) degree of PT asymmetry is reached during brain development and whether this process may be influenced by functional specialization during childhood. We examined 61 neuropsychiatrically normal right-handed children aged 3 to 14 years (mean age +/-SD, 8.4 +/- 2. 7 years; cross-sectional study). MR morphometry showed no change in PT or planum parietale asymmetry with increasing age or brain volume. An unexpected gender difference of unknown significance emerged, with girls displaying a stronger leftward PT asymmetry, independently of age. For the age range studied, the results suggest that functional differentiation follows a structural asymmetry that is already "preset." Copyright 1999 Academic Press.     

MRI evaluation of the size and symmetry of the planum temporale in adolescents with developmental dyslexia

MRI technique was used to examine the size and symmetry of the plana temporale in 19 dyslexic students in grade 8 and in carefully matched control subjects. The results demonstrated a high frequency of planum symmetry among the dyslexics (70%) whereas symmetry was observed in only 30% of the control subjects. It was not possible to demonstrate any clear association between symmetry/asymmetry of planum temporale and handedness. Word-reading strategies among the dyslexics and control subjects were investigated with computerized tasks where accuracy and naming latency were recorded. All subjects with pure phonological deficits in reading had symmetrical plana temporale indicating a possible neuroanatomical basis for a characteristic symptom of linguistic processing deficiency in developmental dyslexia.     

Dyslexia, Neurolinguistic Ability, and Anatomical Variation of the Planum Temporale

This article addresses the relationship between patterns of planum temporale symmetry/asymmetry and dyslexia and neurolinguistic abilities. Considerable research indicates that dyslexic individuals typically do not display the predominant pattern of leftward planum temporale asymmetry. Variable findings on the structural basis of symmetry are due partially to measurement issues, which are examined in some detail in this critical review. The physiological basis of symmetry may be reduced neuronal elimination in the right planum, although other alternatives are offered. Theories are offered to explain how symmetrical plana are related to dyslexia, and it is evident that symmetrical plana are not sufficient to produce dyslexia. However, some evidence suggests that nonleftward plana asymmetry is associated with deficits in verbal comprehension, phonological decoding, and expressive language. It is concluded that nonleftward asymmetry is associated with linguistic deficits, but that explanatory theories need to be further developed. Among the many issues that need to be addressed, future research needs to determine whether the relationship between patterns of planum temporale symmetry/asymmetry and linguistic ability is specific to dyslexia or if asymmetry covaries lawfully with linguistic abilities in nondyslexic populations.     

Variability in perisylvian brain anatomy in healthy adults

Gray matter volumes of Heschl's gyrus (HG), planum temporale (PT), pars triangularis (PTR), and pars opercularis were measured on MRI in 48 healthy right-handers. There was the expected leftward PT asymmetry in 70.8%, and leftward PTR asymmetry in 64.6% of the sample. When asymmetry patterns within individuals were examined, there was not one typical pattern, rather several typical configurations were found. In addition, some combinations of asymmetry did not exist in our sample suggesting that some perisylvian anatomical configurations may provide a more suitable neural substrate for the development of language than others. There were also sex differences in HG. Men had rightward asymmetry and women demonstrated leftward asymmetry, due to women having smaller right HG, compared to men.     

Volumetric asymmetries of the human brain: intellectual correlates

Volumetric measures of the brain and ventricles were derived from CT films and related to intellectual variables from the Wechsler Adult Intelligence Scale (WAIS). Subjects were patients referred for neurological examination for headache or somatic complaints, sometimes accompanied by anxiety or dysphoric affect (N = 41), for whom a comprehensive neurological work-up revealed no evidence of abnormality. The asymmetry of hemispheric volume (left minus right over total, times 100) was correlated (r = .57, p less than .001) with Verbal IQ minus Performance IQ within subjects. No relationship was observed between total brain or hemispheric volumes and IQ scores. Brain and ventricular volumes were larger for the left hemisphere than the right.     

A single optimum degree of hemispheric specialisation in two tasks, in two UK national birth cohorts

How differences between the two sides of the brain (or 'laterality') relate to level of function are important components of theories of the origin and purpose of hemispheric asymmetry, although different measures show different relationships, and this heterogeneity makes discerning any underlying relationships a difficult task. There are some exceptions, for example it has been concluded that increasing lateralization (eg of hand skill or planum temporale area) occurs at the expense of the non-dominant hemisphere. However, we have previously demonstrated this latter relationship to be an artefact: a consequence of plotting two variables against each other, that are not independent of each other [Leask, S. J., & Crow, T. J. (1997) How far does the brain lateralize? An unbiased method for determining the optimum degree of hemispheric specialisation. Neuropsychologia, 36, 1275-1282; Mazoyer, B. M., & Tzourio-Mazoyer, N. G. (2004). Title Planum temporale asymmetry and models of dominance for language: a reappraisal. Neuroreport, 15, 1057-1059]. Two approaches to discerning any underlying relationships are presented in data from over 20,000 10- and 11-year olds from the 1958 and 1970 UK national cohort studies. These demonstrate that maximal performance, both cognitive and hand function, is found in association with one particular degree of functional lateralization.     

Variability in the anatomy of the planum temporale and posterior ascending ramus: do right- and left handers differ?

The anatomy of the planum temporale (PT) and posterior ascending ramus (PAR) was studied in vivo in 67 healthy right- and left-handed adults using MRI-based morphometry. The left PT was significantly larger than the right, and there was a weakly significant effect of the right PAR larger than the left. A leftward PT asymmetry was found in 72%, and a rightward PAR asymmetry was found in 64% of the sample. The "typical" configuration of a larger left PT and larger right PAR co-occurred in 56% of the subjects studied, which was only slightly more often than predicted by chance. Eight of 67 subjects had "reversed" PT and PAR asymmetries, with consistent left and mixed handers over-represented in this group. Right PAR size was the only variable that predicted writing hand, and left PT size was the only measure that differed by sex. The left PT was expanded relative to the left PAR in 93% of the sample, suggesting that this configuration may be developmentally regulated and may be a critical substrate for the development of language. These findings demonstrate that important relationships exist between hand preference, and the anatomy of posterior cortical language areas.     

Smoking reduces language lateralization: a dichotic listening study with control participants and schizophrenia patients

Schizophrenia has been associated with deficits in functional brain lateralization. According to some authors, the reduction of asymmetry could even promote this psychosis. At the same time, schizophrenia is accompanied by a high prevalence of nicotine dependency compared to any other population. This association is very interesting, because sex-dependent effects of smoking in auditory language asymmetries have been reported recently, and the verbal domain is also one major focus in cognitive deficit studies of schizophrenia. Thus, the altered laterality pattern in schizophrenia could, at least in part, result from secondary artefacts due to smoking rather than being a pure cause of the disease itself. To test this hypothesis, the present study examined auditory language lateralization in 67 schizophrenia patients and in 72 healthy controls in a phonemic and an emotional dichotic listening task. Our findings replicate previous research, in that smoking reduces language lateralization in men in phonemic dichotic listening. In addition, we show that smoking also reduces laterality in women in the emotional dichotic listening task. Thus, smoking alters phonemic and emotional language asymmetries differentially for men and women, with a stronger effect for men in the left hemisphere phonemic task, and a stronger effect for women in the right hemisphere emotional task. Together, these findings point towards an effect of smoking which is possibly independent of sex and hemisphere. Importantly, by testing equal numbers of smoking and non-smoking patients and controls, we found no schizophrenia-associated asymmetry effect. Possible neurobiological mechanisms with which smoking may alter auditory microcircuits and thereby diminish left-right differences are discussed.     

Reorganization of the cerebro-cerebellar network of language production in patients with congenital left-hemispheric brain lesions

Patients with congenital lesions of the left cerebral hemisphere may reorganize language functions into the right hemisphere. In these patients, language production is represented homotopically to the left-hemispheric language areas. We studied cerebellar activation in five patients with congenital lesions of the left cerebral hemisphere to assess if the language network is reorganized completely in these patients, i.e. including also cerebellar language functions. As compared to a group of controls matched for age, sex, and verbal IQ, the patients recruited an area not in the right but in the left cerebellar hemisphere. The extent of laterality of the cerebellar activation correlated significantly with the laterality of the frontal activation. We suggest that the developing brain reacts to early focal lesions in the left hemisphere with a mirror-image organization of the entire cerebro-cerebellar network engaged in speech production.     

Dyslexia and brain morphology

Although the neurological basis of dyslexia has long been assumed, little direct evidence documents a relation between deviations in brain morphology and behavioral correlates of dyslexia. This article reviews two sources of evidence. Results of CT/MRI studies suggest that in the brains of dyslexics there is an increased incidence of symmetry in the region of the planum temporale and parietooccipital cortex that may be associated with language delay and handedness. Postmortem/cytoarchitectonic studies document symmetry of the plana, provide evidence of thalamic involvement, and chart widely distributed focal dysplasias preferentially involving the left frontal, left temporal, and right frontal regions. Methodological deficiencies characterize this literature, however, particularly regarding the diagnosis of dyslexia, appraisal of handedness and neurolinguistic deficits, and a failure to provide evidence that this pattern of involvement is unique to the dyslexic syndrome. These findings are discussed as they relate to neurobiological theory.     

绝对音高加工的认知神经机制

绝对音高(absolute pitch,AP)是一种比较罕见的音高加工能力,具有特殊的认知和神经机制。事件相关电位研究表明AP音乐家进行音高命名时,工作记忆参与较少但涉及多个认知策略。功能神经成像研究发现左侧额叶背侧后部和左侧颞叶平面对AP音乐家非常重要,而准AP音乐家(quasi-AP)的某些右侧脑区的参与则反映其增加的音高加工负荷和难度。结构神经成像研究发现AP音乐家具有特殊的灰质结构形态及白质连接。未来研究有待将AP能力进一步分为"具有相对音高能力"与"没有相对音高能力"两类并观察相应的认知神经机制,并通过影像基因组学来探索基因多态性对AP能力的影响,以及有必要观察以声调语言为母语的音乐家进行音高加工的神经机制。     

音乐绝对音高信息加工的脑机制

音乐绝对音高(absolute pitch, AP)一直是音乐心理学者感兴趣的问题。AP能力是一种特殊的音乐能力。ERP研究表明具有AP能力的群体只需较少的认知资源进行音高加工, 同时不同AP能力的加工机制也不同; 功能神经成像研究表明大脑左侧额叶背侧后部和左侧颞叶平面对AP加工发挥重要作用; 脑结构研究发现AP群体的某些脑区皮层厚度低于没有绝对音高能力(Non-AP)群体, 这可能与AP特殊的加工机制有关。AP能力形成需要先天遗传和后天环境的共同作用, 其相互作用的脑机制需结合行为遗传学的研究方法及成果。     

Brain activity varies with modulation of dynamic pitch variance in sentence melody

Fourteen native speakers of German heard normal sentences, sentences which were either lacking dynamic pitch variation (flattened speech), or comprised of intonation contour exclusively (degraded speech). Participants were to listen carefully to the sentences and to perform a rehearsal task. Passive listening to flattened speech compared to normal speech produced strong brain responses in right cortical areas, particularly in the posterior superior temporal gyrus (pSTG). Passive listening to degraded speech compared to either normal or flattened speech particularly involved fronto-opercular and subcortical (Putamen, Caudate Nucleus) regions bilaterally. Additionally the Rolandic operculum (premotor cortex) in the right hemisphere subserved processing of neat sentence intonation. As a function of explicit rehearsing sentence intonation we found several activation foci in the left inferior frontal gyrus (Broca's area), the left inferior precentral sulcus, and the left Rolandic fissure. The data allow several suggestions: First, both flattened and degraded speech evoked differential brain responses in the pSTG, particularly in the planum temporale (PT) bilaterally indicating that this region mediates integration of slowly and rapidly changing acoustic cues during comprehension of spoken language. Second, the bilateral circuit active whilst participants receive degraded speech reflects general effort allocation. Third, the differential finding for passive perception and explicit rehearsal of intonation contour suggests a right fronto-lateral network for processing and a left fronto-lateral network for producing prosodic information. Finally, it appears that brain areas which subserve speech (frontal operculum) and premotor functions (Rolandic operculum) coincidently support the processing of intonation contour in spoken sentence comprehension.     

Crossed aphasia in Chinese: a clinical survey.

According to our clinical observations from various aspects of stroke patients, such as the total incidence of aphasia, the incidence of aphasia after left brain damage of the dextrals, the aphasia that occurs in patients without hemiplegia, and the types of aphasia, a much higher incidence of crossed aphasia is seen among the stroke patients of the Han (the largest ethnic group in China) as compared with the Uighur-Kazaks (U-K) in China and the Occidentals documented in the literature. Motor aphasia is most common and pure sensory or posterior aphasia is rarely seen in Han patients. The distinct features of the Chinese language is a possible explanation for this difference. We suspect that language function of the Han is not localized in the left brain but in the right or both hemispheres. There is no definite Wernicke's area in the left brain of the Chinese people and the neural pathway of the language function in the brain of the Chinese people is not similar to people who speak phonetic languages. Consequently the universal applicability of the theories of cerebral laterality of the language function and dominant hemisphere established by Dax and Broca are questioned in this paper.     

Laterality of lesions and trait-anxiety on working memory performance

An asymmetry of anterior cerebral activation favoring the right hemisphere has been associated with dispositional negative affect including trait-anxiety, while the opposite appears true of cerebral asymmetry favoring the left hemisphere. It was hypothesized that an asymmetry of cerebral activation, as defined by scores on a measure of trait-anxiety, ipsilateral to the side of an anterior brain lesion would be associated with less efficient cognitive processing than greater activation in the hemisphere contralateral to the lesion. Patients with anterior left (n = 16) or right (n = 15) hemisphere lesions completed the State-Trait Anxiety Inventory and several neurocognitive tasks. Of the abilities tested, only Digit Span scores showed an interaction between side of lesion and presumed activation asymmetry. Patients with right- but not with left-hemisphere damage showed significant differences in working memory performance depending on the presumed direction of asymmetry of the two hemispheres, supporting the dual roles of the right hemisphere in affective processing and directed attention.     

From the left to the right: How the brain compensates progressive loss of language function

In normal right-handed subjects language production usually is a function oft the left brain hemisphere. Patients with aphasia following brain damage to the left hemisphere have a considerable potential to compensate for the loss of this function. Sometimes, but not always, areas of the right hemisphere which are homologous to language areas of the left hemisphere in normal subjects are successfully employed for compensation but this integration process may need time to develop. We investigated right-handed patients with left hemisphere brain tumors as a model of continuously progressive brain damage to left hemisphere language areas using functional neuroimaging and transcranial magnetic stimulation (TMS) to identify factors which determine successful compensation of lost language function. Only patients with slowly progressing brain lesions recovered right-sided language function as detected by TMS. In patients with rapidly progressive lesions no right-sided language function was found and language performance was linearly correlated with the lateralization of language related brain activation to the left hemisphere. It can thus be concluded that time is the factor which determines successful integration of the right hemisphere into the language network for compensation of lost left hemisphere language function.     

3D mapping of language networks in clinical and pre-clinical Alzheimer's disease

We investigated the associations between Boston naming and the animal fluency tests and cortical atrophy in 19 probable AD and 5 multiple domain amnestic mild cognitive impairment patients who later converted to AD. We applied a surface-based computational anatomy technique to MRI scans of the brain and then used linear regression models to detect associations between animal fluency and Boston Naming Test (BNT) performance and cortical atrophy. The global permutation-corrected significance for the maps associating BNT performance with cortical atrophy was p=.0124 for the left and p=.0196 for the right hemisphere and for the animal fluency maps p=.055 for the left and p=.073 for the right hemisphere. The degree of language impairment correlated with cortical atrophy in the left temporal and parietal lobes (BA 20, 21, 37, 39, 40, and 7), bilateral frontal lobes (BA 8, 9, and 44) and the right temporal pole (BA 38). Using a novel 3D mapping technique, we demonstrated that in AD language abilities are strongly influenced by the integrity of the perisylvian cortical regions.