Crossed aphasia and related anomalies of cerebral organization: case reports and a genetic hypothesis

[Crossed aphasia in neurosurgical practice: case report and literature review]

BACKGROUND

Crossed aphasia in dexstral (CAD) is an extremely rare disorder. To date, about 200 cases have been described in the literature.

MATERIAL AND METHODS

The authors report 4 cases of CAD. Luria's neuropsychological examination was carried out. Functional asymmetry in the arm was analyzed using the Annett questionnaire. MRI and intraoperative findings confirmed localization of lesion in the right hemisphere in all patients.

RESULTS

A 59-year-old right-handed man developed frontotemporal aphasia after previous resection of right-sided glioblastoma of fronto-insular-temporal region. In a 31-year-old right-handed man, crossed aphasia occurred after extensive hemorrhage from arteriovenous malformation into the right frontotemporoparietal region. A 39-year-old right-handed man developed severe combined aphasia after resection of diffuse glioma of the right insular-temporal region. A 10-year-old right-handed boy developed aphasia with word forgetting after resection of a large tumor of the right temporal lobe. All patients had impaired dynamic praxis. Other cognitive functions were intact.

CONCLUSION

The authors discuss possible combinations of functions in one hemisphere for both right-handers and left-handers and emphasize diverse types of functional asymmetries described in the literature. Different localizations of functions in right-handers and left-handers indicate that local zones with different methods of information processing (successive or simultaneous) can coexist in one hemisphere. Therefore, the functions requiring different processing of information (for example, speech with praxis or spatial functions) can develop in one hemisphere. Cognitive impairment depends not only on the type of functional asymmetry, but also on localization of lesion. Crossed aphasia in right-handers indicates the rarest normal type of functional brain asymmetry with localization of speech functions in the right hemisphere. Possible aphasia in right-handers following damage to homologues of speech zones in the right hemisphere can require additional examination of these patients.

Manual praxis and language-production networks, and their links to handedness

While Liepmann was one of the first researchers to consider a relationship between skilled manual actions (praxis) and language for tasks performed "freely from memory", his primary focus was on the relations between the organization of praxis and left-hemisphere dominance. Subsequent attempts to apply his apraxia model to all cases he studied - including his first patient, a "non-pure right-hander" treated as an exception - left the praxis-handedness issue unresolved. Modern neuropsychological and recent neuroimaging evidence either showed closer associations of praxis and language, than between handedness and any of these two functions, or focused on their dissociations. Yet, present-day developments in neuroimaging and statistics allow us to overcome the limitations of the earlier work on praxis-language-handedness links, and to better quantify their interrelationships. Using functional magnetic resonance imaging (fMRI), we studied tool use pantomimes and subvocal word generation in 125 participants, including righthanders (N_RH = 52), ambidextrous individuals (mixedhanders; N_MH = 31), and lefthanders (N_LH = 42). Laterality indices were calculated both in two critical cytoarchitectonic maps, and 180 multi-modal parcellations of the human cerebral cortex, using voxel count and signal intensity, and the most relevant regions of interest and their networks were further analyzed. We found that atypical organization of praxis was present in all handedness groups (RH = 25.0%, MH = 22.6%; LH = 45.2%), and was about two and a half times as common as atypical organization of language (RH = 3.8%; MH = 6.5%; LH = 26.2%), contingent on ROI selection/LI-calculation method. Despite strong associations of praxis and language, regardless of handedness and typicality, dissociations of atypically represented praxis from typical left-lateralized language were common (~20% of cases), whereas the inverse dissociations of atypically represented language from typical left-lateralized praxis were very rare (in ~2.5% of all cases). The consequences of the existence of such different phenotypes for theoretical accounts of manual praxis, and its links to language and handedness are modeled and discussed.

High Frequency of Crossed Aphasia in Dextral in an Italian Cohort of Patients with Logopenic Primary Progressive Aphasia

BACKGROUND

Primary progressive aphasia (PPA) has been described as a neurodegenerative language disorder mainly affecting the left hemisphere. Few cases of right hemisphere damage in right-handed PPA subjects have been reported. This condition, named crossed aphasia in dextral (CAD), is relatively rare and probably related to an alteration during neurodevelopment of language networks.

OBJECTIVE

To explore the prevalence of CAD in an Italian cohort of 68 PPA patients, in order to evaluate whether right hemisphere language lateralization could be a risk factor for PPA.

METHODS

Clinical-demographic and cerebral [18F]-fluorodeoxyglucose positron emission tomography ([18F]FDG-PET) scan were analyzed, resulting in 23 logopenic variant (lvPPA) patients, 26 non-fluent variant (nfvPPA) patients, and 19 semantic variant (svPPA) patients. SPM single subject routine was performed for diagnostic purposes in order to identify the hypometabolic pattern of each patient. Based on brain metabolic profile, PPA patients were divided in right and left lvPPA, nfvPPA, and svPPA. [18F]FDG-PET group analyses were performed with SPM two-sample t-test routine.

RESULTS

26% of lvPPA cases were identified as CAD based on right hypometabolic pattern. CAD patients did not differ from left lvPPA regarding demographic features and general cognitive performance; however, they performed better in specific working memory tasks and showed brain hypometabolism limited to the superior, middle, and supramarginal temporal gyri.

CONCLUSION

Atypical lateralization of language function could determine a vulnerability of the phonological language loop and in that way could be a risk factor for lvPPA.

Typical and atypical language brain organization based on intrinsic connectivity and multitask functional asymmetries

Based on the joint investigation in 287 healthy volunteers (150 left-Handers (LH)) of language task-induced asymmetries and intrinsic connectivity strength of the sentence-processing supramodal network, we show that individuals with atypical rightward language lateralization (N = 30, 25 LH) do not rely on an organization that simply mirrors that of typical leftward lateralized individuals. Actually, the resting-state organization in the atypicals showed that their sentence processing was underpinned by left and right networks both wired for language processing and highly interacting by strong interhemispheric intrinsic connectivity and larger corpus callosum volume. Such a loose hemispheric specialization for language permits the hosting of language in either the left and/or right hemisphere as assessed by a very high incidence of dissociations across various language task-induced asymmetries in this group.

A case report on crossed aphasia in dextrals: Consideration about clinical features and neural network

RATIONALE

The term crossed aphasia in dextrals (CAD) describes aphasia following a right hemisphere lesion in right-handed subjects. The diagnostic criteria for CAD, defined on the basis of clinical cases observed over the years, are aphasia; lesion in right hemisphere; strong preference for right hand use without familial history of left handedness; structural integrity of left hemisphere; and absence of brain damage in childhood. The studies of CAD have mainly been focused on the neurobiological mechanisms underlying the functional neurocognitive lateralization and organization of the brain, such as a dissociation between language and handedness, language and praxis, or other cognitive functions.Patient concerns: We described a case of a patient affected by an aphasic syndrome following cerebral hemorrhage located in right hemisphere.

DIAGNOSIS

Considering the correlation between clinical data and instrumental investigations such as magnetic resonance imaging, we diagnose the patient with non-fluent aphasia. Specifically, the patient came to our attention showing a trans-cortical mixed aphasia that, later, developed in a trans-cortical motor aphasia. Contrary to most cases of CAD, our patient does not show apraxia and visuo-spatial neglect. Interventions language and visual attention when latter functions are related to right hemisphere.

INTERVENTIONS

The rehabilitation program consisted in exercises stimulating verbal fluency, comprehension, reading, and writing.

OUTCOMES

After 5 months of rehabilitation patient showed significant improvement in comprehension and absence of echolalia.

LESSONS

At present there is no agreement about pathogenesis of CAD and neural mechanism is still unclear. Considering the clinical symptomatology, we can argue that we observed a non-fluent aphasia. However, a more large sample should be studied to asses the role of brain circuits.

How right hemisphere damage after stroke can impair speech comprehension

Acquired language disorders after stroke are strongly associated with left hemisphere damage. When language difficulties are observed in the context of right hemisphere strokes, patients are usually considered to have atypical functional anatomy. By systematically integrating behavioural and lesion data from brain damaged patients with functional MRI data from neurologically normal participants, we investigated when and why right hemisphere strokes cause language disorders. Experiment 1 studied right-handed patients with unilateral strokes that damaged the right (n = 109) or left (n = 369) hemispheres. The most frequently impaired language task was: auditory sentence-to-picture matching after right hemisphere strokes; and spoken picture description after left hemisphere strokes. For those with auditory sentence-to-picture matching impairments after right hemisphere strokes, the majority (n = 9) had normal performance on tests of perceptual (visual or auditory) and linguistic (semantic, phonological or syntactic) processing. Experiment 2 found that these nine patients had significantly more damage to dorsal parts of the superior longitudinal fasciculus and the right inferior frontal sulcus compared to 75 other patients who also had right hemisphere strokes but were not impaired on the auditory sentence-to-picture matching task. Damage to these right hemisphere regions caused long-term speech comprehension difficulties in 67% of patients. Experiments 3 and 4 used functional MRI in two groups of 25 neurologically normal individuals to show that within the regions identified by Experiment 2, the right inferior frontal sulcus was normally activated by (i) auditory sentence-to-picture matching; and (ii) one-back matching when the demands on linguistic and non-linguistic working memory were high. Together, these experiments demonstrate that the right inferior frontal cortex contributes to linguistic and non-linguistic working memory capacity (executive function) that is needed for normal speech comprehension. Our results link previously unrelated literatures on the role of the right inferior frontal cortex in executive processing and the role of executive processing in sentence comprehension; which in turn helps to explain why right inferior frontal activity has previously been reported to increase during recovery of language function after left hemisphere stroke. The clinical relevance of our findings is that the detrimental effect of right hemisphere strokes on language is (i) much greater than expected; (ii) frequently observed after damage to the right inferior frontal sulcus; (iii) task dependent; (iv) different to the type of impairments observed after left hemisphere strokes; and (v) can result in long-lasting deficits that are (vi) not the consequence of atypical language lateralization.

Crossed Aphasia as a Manifestation of Glioblastoma

Language and speech function is commonly accepted to be a heavily lateralized function. Greater than 95% of right-handed individuals have left hemispheric dominance for language, and reports in the literature of crossed aphasia (language deficits in a right-handed individual from right-sided pathology) are scant. We report the case of a 52-year-old woman presenting with crossed aphasia from a right temporal glioblastoma. We then expand on a discussion of crossed aphasia in the setting of brain tumors.

Dissociated Crossed Speech Areas in a Tumour Patient

In the past, the eloquent areas could be deliberately localised by the invasive Wada test. The very rare cases of dissociated crossed speech areas were accidentally found based on the clinical symptomatology. Today functional magnetic resonance imaging (fMRI)-based imaging can be employed to non-invasively localise the eloquent areas in brain tumour patients for therapy planning. A 41-year-old, left-handed man with a low-grade glioma in the left frontal operculum extending to the insular cortex, tension headaches, and anomic aphasia over 5 months underwent a pre-operative speech area localisation fMRI measurement, which revealed the evidence of the transhemispheric disposition, where the dominant Wernicke speech area is located on the left and the Broca's area is strongly lateralised to the right hemisphere. The outcome of the Wada test and the intraoperative cortico-subcortical stimulation mapping were congruent with this finding. After tumour removal, language area function was fully preserved. Upon the occurrence of brain tumours with a risk of impaired speech function, the rare dissociate crossed speech areas disposition may gain a clinically relevant meaning by allowing for more extended tumour removal. Hence, for its identification, diagnostics which take into account both brain hemispheres, such as fMRI, are recommended.

Facets of Pantomime

OBJECTIVES

Exploring the nature of defective pantomime in apraxia.

METHODS

Critical review of behavioral associations and dissociations between defective pantomime, imitation of gestures, and real tool use. Analysis of congruencies between crucial lesions for pantomime, imitation, and tool use.

RESULTS

There are behavioral double dissociations between pantomime and imitation, and their cerebral substrates show very little overlap. Whereas defective pantomime is bound to temporal and inferior frontal lesions, imitation is mainly affected by parietal lesions. Pantomime usually replicates the motor actions of real use but on scrutiny there are important differences between the movements of real use and of pantomime that cast doubt on the assumption that pantomime is produced by the same motor programs as actual use. A more plausible proposal posits that pantomime is a communicative gesture that uses manual actions for conveying information about objects and their use. The manual actions are constructed by selection and combination of distinctive features of tools and actions. They frequently include replications of characteristic motor actions of real use, but the main criterion for selection and modification of features is the comprehensibility of the gestures rather than the accurate replication of the motor actions of real use.

CONCLUSIONS

Pantomime of tool use is a communicative gesture rather than a replication of the motor actions of real use. (JINS, 2017, 23, 121-127).

A case of expressive-vocal amusia in a right-handed patient with left hemispheric cerebral infarction

A 53-year-old right-handed woman had an extensive lesion in the left hemisphere due to an infarction caused by vasospasm secondary to subarachnoid bleeding. She exhibited persistent expressive-vocal amusia with no symptoms of aphasia. Evaluation of the patient's musical competence using the Montreal Battery for Evaluation of Amusia, rhythm reproduction tests, acoustic analysis of pitch upon singing familiar music, Japanese standard language tests, and other detailed clinical examinations revealed that her amusia was more dominantly related to pitch production. The intactness of her speech provided strong evidence that the right hemisphere played a major role in her linguistic processing. Data from functional magnetic resonance imaging while she was singing a familiar song, a scale, and reciting lyrics indicated that perilesional residual activation in the left hemisphere was associated with poor pitch production, while right hemispheric activation was involved in linguistic processing. The localization of infarction more anterior to the left Sylvian fissure might be related to the dominant deficits in expressive aspects of the singing of the patient. Compromised motor programming producing a single tone may have made a major contribution to her poor singing. Imperfect auditory feedback due to borderline perceptual ability or improper audio-motor associations might also have played a role.

Choosing words: left hemisphere, right hemisphere, or both? Perspective on the lateralization of word retrieval

Language is considered to be one of the most lateralized human brain functions. Left hemisphere dominance for language has been consistently confirmed in clinical and experimental settings and constitutes one of the main axioms of neurology and neuroscience. However, functional neuroimaging studies are finding that the right hemisphere also plays a role in diverse language functions. Critically, the right hemisphere may also compensate for the loss or degradation of language functions following extensive stroke-induced damage to the left hemisphere. Here, we review studies that focus on our ability to choose words as we speak. Although fluidly performed in individuals with intact language, this process is routinely compromised in aphasic patients. We suggest that parceling word retrieval into its subprocesses-lexical activation and lexical selection-and examining which of these can be compensated for after left hemisphere stroke can advance the understanding of the lateralization of word retrieval in speech production. In particular, the domain-general nature of the brain regions associated with each process may be a helpful indicator of the right hemisphere's propensity for compensation.

Quantifying cerebral asymmetries for language in dextrals and adextrals with random-effects meta analysis

Speech and language-related functions tend to depend on the left hemisphere more than the right in most right-handed (dextral) participants. This relationship is less clear in non-right handed (adextral) people, resulting in surprisingly polarized opinion on whether or not they are as lateralized as right handers. The present analysis investigates this issue by largely ignoring methodological differences between the different neuroscientific approaches to language lateralization, as well as discrepancies in how dextral and adextral participants were recruited or defined. Here we evaluate the tendency for dextrals to be more left hemisphere dominant than adextrals, using random effects meta analyses. In spite of several limitations, including sample size (in the adextrals in particular), missing details on proportions of groups who show directional effects in many experiments, and so on, the different paradigms all point to proportionally increased left hemispheric dominance in the dextrals. These results are analyzed in light of the theoretical importance of these subtle differences for understanding the cognitive neuroscience of language, as well as the unusual asymmetry in most adextrals.

Repeating with the right hemisphere: reduced interactions between phonological and lexical-semantic systems in crossed aphasia?

Knowledge on the patterns of repetition amongst individuals who develop language deficits in association with right hemisphere lesions (crossed aphasia) is very limited. Available data indicate that repetition in some crossed aphasics experiencing phonological processing deficits is not heavily influenced by lexical-semantic variables (lexicality, imageability, and frequency) as is regularly reported in phonologically-impaired cases with left hemisphere damage. Moreover, in view of the fact that crossed aphasia is rare, information on the role of right cortical areas and white matter tracts underpinning language repetition deficits is scarce. In this study, repetition performance was assessed in two patients with crossed conduction aphasia and striatal/capsular vascular lesions encompassing the right arcuate fasciculus (AF) and inferior frontal-occipital fasciculus (IFOF), the temporal stem and the white matter underneath the supramarginal gyrus. Both patients showed lexicality effects repeating better words than non-words, but manipulation of other lexical-semantic variables exerted less influence on repetition performance. Imageability and frequency effects, production of meaning-based paraphrases during sentence repetition, or better performance on repeating novel sentences than overlearned clichés were hardly ever observed in these two patients. In one patient, diffusion tensor imaging disclosed damage to the right long direct segment of the AF and IFOF with relative sparing of the anterior indirect and posterior segments of the AF, together with fully developed left perisylvian white matter pathways. These findings suggest that striatal/capsular lesions extending into the right AF and IFOF in some individuals with right hemisphere language dominance are associated with atypical repetition patterns which might reflect reduced interactions between phonological and lexical-semantic processes.

Laterality of seizure onset and the simple reaction time: revamping the Poffenberger's paradigm for seizure surgery

BACKGROUND

Crossed-uncrossed differentials (CUDs) are viewed as surrogates for interhemispheric transfer time (IHTT). Not uncommonly CUDs assume statistically significant negative values (inverted CUDs). This raises doubts of the accepted interpretation of CUDs, i.e. intra- and inter-hemispheric routings of signals in uncrossed and crossed responses, respectively.

METHOD

Based on the evidence supporting directionality in callosal traffic, data are provided indicating that callosal transfers exclusively involve non-dominant responses and such transfers are modality non-specific. The evidence also indicates that neural handedness corresponds to behavioral only in a statistical manner and the former remains unchanged regardless of the subject's life experience.

RESULTS

The neurally dominant side is the side that is directly connected to the major hemisphere (command center). The connection of the non-dominant side to the command center is via the corpus callosum; therefore, a delay occurs in the reaction time of all non-dominant effectors, corresponding to IHTT. Accordingly, negative CUDs indicate a mismatch of neural and behavioral (avowed) handedness of the subject. This group comprises a minority of 15-20% of the population.

CONCLUSION

Comparing the response time of symmetrically located effector is a robust way of lateralizing a person's major hemisphere. The latter is also the site of initiation of seizures, as the minor hemisphere is bereft of independent motor activity. Sensory signals arising from the nondominant side of the body traverse the callosum before reaching the major hemisphere. Searching for ipsilateral somatosensory evoked potentials provides another approach in lateralizing the non-dominant side of the body (ipsilateral to the major hemisphere). Practical uses of a conceptually revamped Poffenberger paradigm in neurosurgery are briefly reviewed.

Left- and right-hemisphere forms of phonological alexia

We studied the ability of patients with lesions arising from operation for an anterior or posterior (left or right) brain tumour to read a set of words and pronounceable nonwords. In line with previous works, we observed that damage to the left posterior or left anterior cortex can give rise to phonological alexia, where the reading performance of nonwords is affected more than that of words. More surprisingly, similar effects were found in the right posterior group. However, there were significant differences in the error types, for both complex and positional errors, between phonological alexic patients in the three location groups. The findings present difficulties for the position held by theorists of the triangle model that phonological alexia arises from impairments in the language production system or in a general-purpose orthographic-phonological translation system. They also pose new questions about the possible role of the right posterior cortex in letter sequence representation.

Lateralization of cognitive functions in aphasia after right brain damage

PURPOSE

The lateralization of cognitive functions in crossed aphasia in dextrals (CAD) has been explored and compared mainly with cases of aphasia with left hemisphere damage. However, comparing the neuropsychological aspects of CAD and aphasia after right brain damage in left-handers (ARL) could potentially provide more insights into the effect of a shift in the laterality of handedness or language on other cognitive organization. Thus, this case study compared two cases of CAD and one case of ARL.

MATERIALS AND METHODS

The following neuropsychological measures were obtained from three aphasic patients with right brain damage (two cases of CAD and one case of ARL); language, oral and limb praxis, and nonverbal cognitive functions (visuospatial neglect and visuospatial construction).

RESULTS

All three patients showed impaired visuoconstructional abilities, whereas each patient showed a different level of performances for oral and limb praxis, and visuospatial neglect.

CONCLUSION

Based on the analysis of these three aphasic patients' performances, we highlighted the lateralization of language, handedness, oral and limb praxis, visuospatial neglect and visuospatial constructive ability in aphasic patients with right brain damage.

Atypical conduction aphasia and the right hemisphere: Cross-hemispheric plasticity of phonology in a developmentally dyslexic and dysgraphic patient with early left frontal damage

We report the rare case of a patient, JNR, with history of mixed handedness, developmental dyslexia, dysgraphia, and attentional deficits associated with a Klippel-Trenaunay syndrome and a small subcortical frontal lesion involving the left arcuate fasciculus. In adulthood, he suffered a large right perisylvian stroke and developed atypical conduction aphasia with deficits in input and output phonological processing and poor auditory-verbal short-term memory. Lexical-semantic processing for single words was intact, but he was unable to access meaning in sentence comprehension and repetition. Reading and writing deficits worsened after the stroke and he presented a combination of developmental and acquired dysgraphia and dyslexia with mixed lexical and phonological processing deficits. This case suggest that a small lesion sustained prenatally or early in life could induce a selective rightward shift of phonology sparing the standard left hemisphere lateralisation of lexical-semantic functions.

Crossed Transcortical Motor Aphasia, Left Spatial Neglect, and Limb and Magnetic Apraxia Due to Right Anterior Cerebral Artery Infarction

Crossed aphasia refers to language disturbance due to right-hemisphere lesions in right-handed individuals, while magnetic apraxia is described as 'forced grasping and groping' caused by lesions in the contralateral frontal lobe. This is a case report of a 70-year-old right handed woman who suffered from crossed transcortical motor aphasia and left hand magnetic apraxia due to right anterior cerebral artery infarction. The definite mechanism of this disorder is not yet understood, but neurophysiological observations suggest that affected supplementary motor areas may be responsible for this phenomenon.

Crossed aphasia elicited by intraoperative cortical and subcortical stimulation in awake patients

Object

Crossed aphasia (aphasia resulting from a right hemispheric lesion among right-handed patients) is rare. The authors describe for the first time transient crossed aphasia elicited by intraoperative electrostimulation of both cortex and white matter pathways in awake patients.

Methods

Three right-handed adults underwent surgery for a right-sided glioma. Because slight language disorders occurred during partial seizures or were identified on preoperative cognitive assessment, with right activations detected on language functional MR imaging in 1 patient, awake craniotomy was performed using intraoperative cortico-subcortical electrical functional mapping.

Results

Transient language disturbances were elicited by stimulating discrete cortical areas (inferior frontal gyrus and posterior part of the superior temporal gyrus) and white matter pathways (inferior frontooccipital fasciculus and arcuate fasciculus). A subtotal resection was achieved in all cases, according to functional boundaries. Postoperatively, 1 patient experienced a transient dysphasia, which resolved after speech rehabilitation, with no permanent deficit.

Conclusions

These original findings highlight the possibility of finding crucial cortico-subcortical language networks in the right hemisphere in a subgroup of atypical right-handed patients. These findings provide new insights into the neural basis of language, by underlining the role of the right inferior occipitofrontal fasciculus in semantics and that of the right arcuate fasciculus in phonology, and by supporting the hypothesis of a mirror organization between the right and left hemispheres. The authors suggest that, in right-handed patients, if language disturbances are detected during seizures or on presurgical neuropsychological assessment, especially when right activations are observed on language functional MR imaging, awake craniotomy with intraoperative language mapping should be considered.

Intuition, insight, and the right hemisphere: Emergence of higher sociocognitive functions

Intuition is the ability to understand immediately without conscious reasoning and is sometimes explained as a 'gut feeling' about the rightness or wrongness of a person, place, situation, temporal episode or object. In contrast, insight is the capacity to gain accurate and a deep understanding of a problem and it is often associated with movement beyond existing paradigms. Examples include Darwin, Einstein and Freud's theories of natural selection, relativity, or the unconscious; respectively. Many cultures name these concepts and acknowledge their value, and insight is recognized as particularly characteristic of eminent achievements in the arts, sciences and politics. Considerable data suggests that these two concepts are more related than distinct, and that a more distributed intuitive network may feed into a predominately right hemispheric insight-based functional neuronal architecture. The preparation and incubation stages of insight may rely on the incorporation of domain-specific automatized expertise schema associated with intuition. In this manuscript the neural networks associated with intuition and insight are reviewed. Case studies of anomalous subjects with ability-achievement discrepancies are summarized. This theoretical review proposes the prospect that atypical localization of cognitive modules may enhance intuitive and insightful functions and thereby explain individual achievement beyond that expected by conventionally measured intelligence tests. A model and theory of intuition and insight's neuroanatomical basis is proposed which could be used as a starting point for future research and better understanding of the nature of these two distinctly human and highly complex poorly understood abilities.

Cerebral localization of functions and the neurology of language: fact versus fiction or is it something else?

Over the last 15 years there has been a burgeoning number of publications using functional brain imaging (>40,000 articles based on an ISI/Web of Science search) to localize behavioral and cognitive processes to specific areas in the human brain that are often not confirmed by traditional, lesion-based studies. Thus, there is a need to reassess what cerebral localization of functions is and is not. Otherwise, there is no rational way to interpret the escalating claims of localization in the functional imaging literature that is taking on the appearance of neurophysiologic "phrenology". This article will present arguments to suggest that functional localization in the brain is a robust but very dynamic, four-dimensional process. It is a learned phenomenon driven over time by large-scale, spatially distributed, neural networks seeking to efficiently maximize the processing, storage, and manipulation of information for cognitive and behavioral operations. Because of historical considerations and space limitations, the main focus will be on localization of language-related functions whose theoretical neurological basis can be generalized for any complex cognitive-behavioral function.

Lateralized genetic and environmental influences on human brain morphology of 8-year-old twins

It has been increasing rapidly interest in understanding genetic effects on brain structure and function in recent years. In this study, we examined the genetic and environmental influences on the variation in cortical thickness and specific tissue volumes in a large cohort of 8-year-old healthy twins. The present study can provide a better estimation of the genetic and environmental effects by virtue of the homogeneously aged pediatric twin pairs with a similar growing environment. We found that common environmental factors contributed significantly to the variations of the right lateral ventricle (36%) and corpus callosum (36%) volumes while genetic factors accounted for most of the phenotypic variance in other brain tissue volumes. In the case of cortical thickness, several regions in the left hemisphere showed statistically significant additive genetic factors, including the middle and inferior frontal gyri, lateral fronto-orbital and occipitotemporal gyri, pars opercularis, planum temporale, precentral and parahippocampal gyri and the medial region of the primary somatosensory cortex. Relatively high common environmental influence (>50%) was observed in the right anterior cingulate cortex and insula. Our findings indicate that the genetic and common environmental influences on individual human brain structural differences are lateralized, with the language-dominant left cerebral cortex under stronger genetic control than the right.

Acute aphasia after right hemisphere stroke

Right hemispheric stroke aphasia (RHSA) rarely occurs in right- or left-handed patients with their language representation in right hemisphere (RH). For right-handers, the term crossed aphasia is used. Single cases, multiple cases reports, and reviews suggest more variable anatomo-clinical correlations. We included retrospectively from our stroke data bank 16 patients (right- and left-handed, and ambidextrous) with aphasia after a single first-ever ischemic RH stroke. A control group was composed of 25 successive patients with left hemispheric stroke and aphasia (LHSA). For each patient, we analyzed four modalities of language (spontaneous fluency, naming, repetition, and comprehension) and recorded eventual impairment: (1) on admission (hyperacute) and (2) between day 3 and 14 (acute). Lesion volume and location as measured on computed tomography (CT) and magnetic resonance imaging (MRI) were transformed into Talairach stereotaxic space. Nonparametric statistics were used to compare impaired/nonimpaired patients. Comprehension and repetition were less frequently impaired after RHSA (respectively, 56% and 50%) than after LHSA (respectively, 84% and 80%, P = 0.05 and 0.04) only at hyperacute phase. Among RHSA, fewer left-handers/ambidextrous than right-handers had comprehension disorders at second evaluation (P = 0.013). Mean infarct size was similar in RHSA and LHSA with less posterior RHSA lesions (caudal to the posterior commissure). Comprehension and repetition impairments were more often associated with anterior lesions in RHSA (Fisher's exact test, P < 0.05). Despite the small size of the cohort, our findings suggest increased atypical anatomo-functional correlations of RH language representation, particularly in non-right-handed patients.

[Psychopathology of Ganser's syndrome. Literature review and case report]

The key feature of Ganser's syndrome includes approximate answers to simple questions. The cause of this rare syndrome remains uncertain. Current classification systems categorise it as a dissociative disorder, the symptoms of which are judged as psychogenic in origin. Our review of the literature (n=151) demonstrates however that Ganser's syndrome is frequently associated with brain injury, although detailed imaging, neuropsychological, and neurological data of this for the most part do not exist. We describe a right-handed patient with Ganser's syndrome after a large left-hemispheric middle cerebral artery infarction. Detailed neuropsychological examination showed atypical lateralisation of cognitive functions with so-called crossed nonaphasia and pronounced frontal-executive dysfunctions. Regarding both psychiatric and neuropsychological aspects, we discuss how the key feature of approximate answers may be associated with frontal-executive cerebral dysfunctions.

Paradoxical recovery in a bilingual patient with aphasia after right capsuloputaminal infarction

We report the case of a bilingual dextral patient, who presented with an uncommon pattern of aphasic deficit following a right capsulo-putaminal infarction. In this patient, the linguistic deficit concerned the use of her mother tongue (Galician, L1) much more than the lesser practised second language (Spanish, L2). Our patient presented spontaneous fluent speech in L2 but not in L1, automatic translation into L2, and impaired repetition in L1, whereas comprehension was spared in both L1 and L2. Reading and writing were less valuable due to educational interference (reduced schooling). Spontaneous speech 16 months after the stroke showed the stability of the impairment. This is the first reporting of a crossed subcortical aphasia in a bilingual patient.

Does functional neuroimaging solve the questions of neurolinguistics?

Neurolinguistic research has been engaged in evaluating models of language using measures from brain structure and function, and/or in investigating brain structure and function with respect to language representation using proposed models of language. While the aphasiological strategy, which classifies aphasias based on performance modality and a few linguistic variables, has been the most stable, cognitive neurolinguistics has had less success in reliably associating more elaborately proposed levels and units of language models with brain structure. Functional imaging emerged at this stage of neurolinguistic research. In this review article, it is proposed that the often-inconsistent superfluity of outcomes arising from functional imaging studies of language awaits adjustment at both "ends" of the process: model and data. Assumptions that our current language models consistently and reliably represent implicit knowledge within human cerebral processing are in line for major revision; and the promise of functional brain imaging to reveal any such knowledge structures must incorporate stable correlates of the imaging signal as dependent variable.

Atypical hemispheric dominance for attention: functional MRI topography

The right hemisphere is predominantly involved in tasks associated with spatial attention. However, left hemispheric dominance for spatial attention can be found in healthy individuals, and both spatial attention and language can be lateralized to the same hemisphere. Little is known about the underlying regional distribution of neural activation in these 'atypical' individuals. Previously a large number of healthy subjects were screened for hemispheric dominance of visuospatial attention and language, using functional Doppler ultrasonography. From this group, subjects were chosen who were 'atypical' for hemispheric dominance of visuospatial attention and language, and their pattern of brain activation was studied with functional magnetic resonance imaging during a task probing spatial attention. Right-handed subjects with the 'typical' pattern of brain organization served as control subjects. It was found that subjects with an inverted lateralization of language and spatial attention (language right, attention left) recruited left-hemispheric areas in the attention task, homotopic to those recruited by control subjects in the right hemisphere. Subjects with lateralization of both language and attention to the right hemisphere activated an attentional network in the right hemisphere that was comparable to control subjects. The present findings suggest that not the hemispheric side, but the intrahemispheric pattern of activation is the distinct feature for the neural processes underlying language and attention.

Crossed right hemisphere syndrome following left thalamic stroke

In most right-handed people, language and motor functions are more reliant on systems of the left hemisphere while several non-linguistic visuo-spatial and attentional processes depend more on the right hemisphere. The rare exceptions to these rules provide important clues as to what functions co-lateralise, and are thus crucial for models of cerebral specialization. Here we report on the case of a patient, who, after a lesion restricted to the left thalamic region, showed signs normally associated with right hemispheric injury including motor impersistence, visuo-spatial dysfunction and poor comprehension of facial expression. Language abilities were spared and no signs of apraxia were present, in spite of his right hand, foot and eye preference, a pattern normally associated with conventional cerebral dominance. In spite of his other right hemispheric signs, the patient showed no signs of hemi-spatial neglect. The patient's pattern of spared and impaired abilities is compared and contrasted with other rare cases of crossed right hemisphere syndrome.

Hemispheric lateralization of spatial attention in right- and left-hemispheric language dominance

Hemispheric lateralization of the neural systems supporting language and spatial attention most commonly dissociate in healthy individuals. However, the reverse pattern with association of language and attention within the right hemispheres has also been observed. We investigated in 75 healthy volunteers (37 right-handed, 38 left-handed) if language and spatial attention may associate not only in individuals with an atypical pattern of language lateralization, but also in subjects showing the standard, i.e. left-hemispheric dominance for language. Hemispheric lateralization of cerebral perfusion was determined with functional transcranial Doppler ultrasonography during a visuospatial attention, and a word generation task. We found that language and visuospatial attention associated within the left hemisphere in five subjects and within the right hemisphere in eight subjects. We conclude that all combinations of cerebral lateralization for language and attention may exist in the healthy brain.

Determining the hemispheric dominance of spatial attention: a comparison between fTCD and fMRI

Human brain mapping allows the systematic assessment of interindividual differences in functional brain anatomy. Functional transcranial Doppler sonography (fTCD) is an imaging tool that allows for fast and mobile assessment of hemispheric lateralization of task-related brain activation. It is ideal to screen large cohorts of subjects. The goal of the present study was to investigate whether fTCD and functional magnetic resonance imaging (fMRI) determine hemispheric lateralization of brain activation related to visuospatial attention concordantly. Used together, fMRI and fTCD may then open up a wide range of potential applications in neuroscience. Fifteen subjects were examined both with fTCD and fMRI while they judged accuracy of line bisections (Landmark task). For fTCD, the maximal mean difference in stimulus-related relative cerebral blood flow velocity changes in the left and right middle cerebral arteries was assessed as the lateralization index LI(fTCD). For fMRI, two approaches were used to determine hemispheric dominance. First, we measured brain activity as the extent of the activated region, i.e., the number of activated voxels above a statistical threshold. Second, we calculated the magnitude of the fMRI signal change between the activation and the control task within a region of interest. Results of fTCD and fMRI were concordant in every single case. Therefore, scanning large cohorts with fTCD for hemispheric dominance during Landmark task will provide results consistent with fMRI. FMRI can then be used for in depth assessment of the specific patterns of activation.

Transient crossed aphasia evidenced by functional brain imagery

Crossed aphasia refers to language deficits induced by unilateral right hemisphere injuries in right-handed people who had no previous history of brain damage. One of the intriguing questions concerning crossed aphasia is the atypical language representation in the brain. In this respect, fMRI is a valuable tool for understanding the neural basis of crossed aphasia. Here, we used neuropsychological and fMRI language tasks in a right-handed subject who presented a crossed aphasia due to a right frontal meningioma. fMRI maps from two language tasks showed bilateral patterns of activation. In the light of previous studies reporting much frequent bilateral than exclusive right hemisphere representations, we hypothesise that some crossed aphasia cases could occur in subjects with bilateral language representation.

Crossed aphasia: an analysis of the symptoms, their frequency, and a comparison with left-hemisphere aphasia symptomatology

This study presents a thorough analysis of published crossed aphasia (CA) cases, including for the first time the cases published in Japanese. The frequency of specific symptoms was determined, and symptomatology differences based on gender, familial sinistrality, and CA subtype were investigated. Results suggested that the CA population is comparable to the left-hemisphere patient population. However, male were significantly more likely than female CA subjects to show a positive history of familial sinistrality. Typical right-hemisphere (i.e., nonlanguage-dominant) symptoms were frequent but rarely carefully reported or assessed. Results are compared with previous CA reviews and left-hemisphere aphasia. Suggestions for a more systematic assessment of the CA symptomatology are presented.

The epigenesis of planum temporale asymmetry in twins

Variation in hemispheric asymmetry of the planum temporale (PT) has been related to verbal ability. The degree to which genetic and environmental factors mediate PT asymmetry is not known. This study examined the heritability for planar asymmetry in 12 dizygotic (DZ) and 27 monozygotic (MZ) male twin pairs who were between 6 and 16 years of age. There was weak but positive evidence for heritability of planar asymmetry. Co-twin similarity for planar asymmetry and Sylvian fissure morphology increased when excluding twins discordant for writing hand and when excluding twins exhibiting birth weight differences >20% from the analyses. Birth weight differences were also related to twin differences in total cerebral volume, but not central sulcus asymmetry. These results suggest that exogenous perinatal factors affect the epigenesis of planar asymmetry development.

Heritability of lobar brain volumes in twins supports genetic models of cerebral laterality and handedness

Although the left and right human cerebral hemispheres differ both functionally and anatomically, little is known about the environmental or genetic factors that govern central nervous system asymmetry. Nevertheless, cerebral asymmetry is strongly correlated with handedness, and handedness does have a significant genetic component. To explore the relative contribution of environmental and genetic influences on cerebral asymmetry, we examined the volumes of left and right cerebral cortex in a large cohort of aging identical and fraternal twins and explored their relationship to handedness. Cerebral lobar volumes had a major genetic component, indicating that genes play a large role in changes in brain volume that occur with aging. Shared environment, which likely represents in utero events, had about twice the effect on the left hemisphere as on the right, consistent with less genetic control over the left hemisphere. To test the major genetic models of handedness and cerebral asymmetry, twin pairs were divided into those with two right handers and those with at least one left hander (nonright handers). Genetic factors contributed twice the influence to left and right cerebral hemispheric volumes in right-handed twin pairs, suggesting a large decrement in genetic control of cerebral volumes in the nonright-handed twin pairs. This loss of genetic determination of the left and right cerebral hemispheres in the nonright-handed twin pairs is consistent with models postulating a right-hand/left-hemisphere-biasing genetic influence, a "right-shift" genotype that is lost in nonright handers, resulting in decreased cerebral asymmetry.

Acquired crossed aphasia in dextral children revisited

In contrast to the estimated low incidence of crossed aphasia in dextral adults (among 1%), crossed aphasia in children has been considered a common finding for almost a century. However, reviewing the literature on crossed aphasia in dextrals (CAD) and its related topics from 1975 onward, we encountered only 5 children in a corpus of 180 cases (2.7%). Critical analysis rendered three of the reported cases ambiguous and hence not suitable to draw potentially relevant conclusions. In this review, the neurobehavioral manifestations of the two representative childhood CAD cases are analyzed and compared with adult CAD and acquired childhood aphasia (ACA). In the light of our findings, which support the position of innate cerebral specialization for language, the long-standing controversy as to whether lateralized hemispheric specialization for language is innate or develops progressively during maturation is briefly discussed.

Crossed Nonaphasia in a Dextral With Left Hemispheric Lesions

Background — General conclusions concerning mechanisms of cerebral lateralization may be learned from the investigation of functional brain organization in patients with anomalous lateralization.

Case Description — The functional organization of language, attention, and motor performance was investigated in a 42-year-old patient with crossed nonaphasia by means of functional MRI. The strongly right-handed man experienced a left middle cerebral artery infarction documented by MRI without exhibition of aphasia. However, the left hemispheric stroke was accompanied by visuospatial impairment, right-sided slight sensory and motor paresis, and right homonymous hemianopia. No history of familial sinistrality or prior neurological illness was present. Functional MR language mapping revealed strong right hemispheric activation in inferior frontal and superior temporal cortices. Finger tapping with the right hand recruited ipsilateral premotor and motor areas as well as supplementary motor cortex. A Stroop task, usually strongly associated with left-sided inferior frontal activation in dextrals, resulted in strong right hemispheric frontal activation.

Conclusions — From our data there is clear evidence that different modalities, such as language perception and production, attention, and motor performance, are processed exclusively by 1 hemisphere when atypical cerebral dominance is present.

The many faces of crossed aphasia in dextrals: report of nine cases and review of the literature

A substantial body of the aphasia literature has been devoted to the topic of crossed aphasia in dextrals (CAD) over the past century but still no theory exists that explains the anomalous organization of neurocognitive functions in this population. However, if strict selection criteria are applied only some cases of vascular CAD are reported in which the correlation between neurocognitive disturbances and the locus of the brain lesion is studied. This study describes nine new cases of vascular CAD who underwent in-depth neurolinguistic and neuropsychological investigations. Our analysis shows the semiological variability of CAD phenomena and the many faces of the lesion-behaviour relationships of this exceptional neurobiological condition.