Hemispheric asymmetry of hand and tool perception in left- and right-handers with known language dominance.

Regions in the brain that are selective for images of hands and tools have been suggested to be lateralised to the left hemisphere of right-handed individuals. In left-handers, many functions related to tool use or tool pantomime may also depend more on the left hemisphere. This result seems surprising, given that the dominant hand of these individuals is controlled by the right hemisphere. One explanation is that the left hemisphere is dominant for speech and language in the majority of left-handers, suggesting a supraordinate control system for complex motor sequencing that is required for skilled tool use, as well as for speech. In the present study, we examine if this left-hemispheric specialisation extends to perception of hands and tools in left- and right-handed individuals. We, crucially, also include a group of left-handers with right-hemispheric language dominance to examine their asymmetry biases. The results suggest that tools lateralise to the left hemisphere in most right-handed individuals with left-hemispheric language dominance. Tools also lateralise to the language dominant hemisphere in right-hemispheric language dominant left-handers, but the result for left-hemispheric language dominant left-handers are more varied, and no clear bias towards one hemisphere is found. Hands did not show a group-level asymmetry pattern in any of the groups. These results suggest a more complex picture regarding hemispheric overlap of hand and tool representations, and that visual appearance of tools may be driven in part by both language dominance and the hemisphere which controls the motor-dominant hand.

Assessment of language lateralization in epilepsy patients using the super-selective Wada test.

BACKGROUND: The classical Wada test (cWada), performed by injecting a short-acting anesthetic through the intracarotid route, helps determine language dominance. In the cWada, adverse effects are observed in 10-30% of trials, hindering accurate assessments. In this study, we assessed the effectiveness of the super-selective Wada test (ssWada), a more selective approach for anesthetic infusion into the middle cerebral artery (MCA). METHODS: We retrospectively examined the data of 17 patients with epilepsy who underwent ssWada via anesthetic injection into one M1 segment of the MCA and at least one contralateral trial. RESULTS: The ssWada identified 12 patients with left language dominance, 3 with right language dominance, and 2 with bilateral language distribution. Nine trials on the language dominant side resulted in global aphasia for patients with left- or right language dominance. Of the 13 trials conducted on the non-dominant language side, 12 revealed intact language function and one resulted in confusion. Among these, the outcomes of global aphasia or no language impairment were confirmed in the contralateral trials. Among the 22 trials of unilateral M1 injections in patients with unilateral language dominance, 21 (95.5%) showed either global aphasia or no language impairment, indicating language dominance. CONCLUSIONS: The ssWada yields clear results, with a high rate of over 90% in determining the language dominant hemisphere with few side effects.

Functional Transcranial Doppler (fTCD) investigation of brain lateralization following visual stimuli.

This preliminary investigation of the local cerebral perfusion evaluated by Transcranial Doppler (TCD) monitoring of the Posterior Cerebral Arteries shows that selective stimulation of visual hemifields evokes significantly different vascular responses. TCD can therefore allow for functional evaluation of lateralized enhancements in cerebral metabolism.Clinical Relevance-The cerebral lateralization evaluated with the functional TCD can be a useful and low-cost approach to evaluate the effectiveness of the rehabilitation therapy in post stroke subjects experiencing hemianopia or to assess cerebral reorganization after cerebrovascular accidents.

Cerebral dominance representation of directed connectivity within and between left-right hemispheres and frontal-posterior lobes in mild cognitive impairment.

Electroencephalography can assess connectivity between brain hemispheres, potentially influencing cognitive functions. Much of the existing electroencephalography research primarily focuses on undirected connectivity, leaving uncertainties about directed connectivity alterations between left-right brain hemispheres or frontal-posterior lobes in mild cognitive impairment. We analyzed resting-state electroencephalography data from 34 mild cognitive impairment individuals and 23 normal controls using directed transfer function and graph theory for directed network analysis. Concerning the dominance within left-right hemispheres or frontal-posterior lobes, the mild cognitive impairment group exhibited decreased connectivity within the frontal compared with posterior brain regions in the delta and theta bands. Regarding the dominance between the brain hemispheres or lobes, the mild cognitive impairment group showed reduced connectivity from the posterior to the frontal regions versus the reverse direction in the same bands. Among all participants, the intra-lobe frontal-posterior dominance correlated positively with executive function in the delta and alpha bands. Inter-lobe dominance between frontal and posterior regions also positively correlated with executive function, attention, and language in the delta band. Additionally, interhemispheric dominance between the left and right hemispheres positively correlated with attention in delta and theta bands. These findings suggest altered cerebral dominance in mild cognitive impairment, potentially serving as electrophysiological markers for neurocognitive disorders.

Unlocking the symmetric transfer of irrelevant information: gene-environment interplay and enhanced interhemispheric cross-talk.

Hemispheric specialization influences stimulus processing and behavioural control, affecting responses to relevant stimuli. However, most sensory input is irrelevant and must be filtered out to prevent interference with task-relevant behaviour, a process known as habituation. Despite habituation's vital role, little is known about hemispheric specialization for this brain function. We conducted an experiment with domestic chicks, an elite animal model to study lateralization. They were exposed to distracting visual stimuli while feeding when using binocular or monocular vision. Switching the viewing eye after habituation, we examined if habituation was confined to the stimulated hemisphere or shared across hemispheres. We found that both hemispheres learned equally to ignore distracting stimuli. However, embryonic light stimulation, influencing hemispheric specialization, revealed an asymmetry in interhemispheric transfer of the irrelevant information discarded via habituation. Unstimulated chicks exhibited a directional bias, with the right hemisphere failing to transfer distracting stimulus information to the left hemisphere, while transfer from left to right was possible. Nevertheless, embryonic light stimulation counteracted this asymmetry, enhancing communication from the right to the left hemisphere and reducing the pre-existing imbalance. This sharing extends beyond hemisphere-specific functions and encompasses a broader representation of irrelevant events.

MFCSC: Novel method to calculate mismatch between functional and structural brain connectomes, and its application for detecting hemispheric functional specialisations.

We introduce a novel connectomics method, MFCSC, that integrates information on structural connectivity (SC) from diffusion MRI tractography and functional connectivity (FC) from functional MRI, at individual subject level. The MFCSC method is based on the fact that SC only broadly predicts FC, and for each connection in the brain, the method calculates a value that quantifies the mismatch that often still exists between the two modalities. To capture underlying physiological properties, MFCSC minimises biases in SC and addresses challenges with the multimodal analysis, including by using a data-driven normalisation approach. We ran MFCSC on data from the Human Connectome Project and used the output to detect pairs of left and right unilateral connections that have distinct relationship between structure and function in each hemisphere; we suggest that this reflects cases of hemispheric functional specialisation. In conclusion, the MFCSC method provides new information on brain organisation that may not be inferred from an analysis that considers SC and FC separately.

Face distortions in prosopometamorphopsia provide new insights into the organization of face perception.

Prosopometamorphopsia (PMO) is a striking condition of visual perception in which facial features appear distorted, for example drooping, swelling, or twisting. Although numerous cases have been reported, few of those investigations have carried out formal testing motivated by theories of face perception. However, because PMO involves conscious visual distortions to faces which participants can report, it can be used to probe fundamental questions about face representations. Here we review cases of PMO that address theoretical questions in visual neuroscience including face specificity, inverted face processing, the importance of the vertical midline, dissociable representations for each half of the face, hemispheric specialization, the relationship between face recognition and conscious face perception, and the reference frames that face representations are embedded within. Finally, we list and touch upon eighteen open questions that make clear how much is left to learn about PMO and the potential it has to provide important advances in face perception.

Attention and Interhemispheric Communication: Implications for Language Dominance.

Dominance of the left hemisphere for language processing is a prominent feature of brain organisation. Whereas structural models clarify the functional asymmetry due to direct access to local language circuits, dynamic models propose functional states of intrahemispheric activation and interhemispheric inhibition that are coupled with attentional processes. Real word settings often require modulations of lateralised neural processing and further express individual heterogeneity. In this research, we tested left- and right-handers, and used a behavioural paradigm with presentation of lateralised cue-target pairs to the same or opposite visual field. We observed that handedness distinctly affected word processing in the left hemisphere following contralateral cueing. Moreover, left-hemispheric dominance strengthened for right-handers vs abolished for left-handers, influencing behavioural efficiency. In combination with eye dominance recordings, these data suggest that attentional biases guided the processing strategies of both groups and in turn their achievements. Therefore, hand and eye dominance are both essential factors with a functional role in directing the communication of visual information between both hemispheres. Overall, the findings underline the importance of interacting hand-eye control systems in contributing to interhemispheric patterns in the context of language processing.

Abnormalities of hemispheric specialization in drug-naïve and drug-receiving self-limited epilepsy with centrotemporal spikes.

PURPOSE: Self-limited epilepsy with centrotemporal spikes (SLECTS) is a pediatric benign epilepsy but often accompanied by subsequent (in adulthood) functional changes such as language, which are thought to have distinct areas of hemispheric lateralization and functional differentiation. This study aimed to explore hemispheric specialization measured by resting-state functional magnetic resonance imaging (rs-fMRI) functional connectivity in drug-naïve and drug-receiving SLECTS. METHODS: Hemispheric specialization was quantified in three groups of children, including 21 drug-naïve patients (DNP) with SLECTS, 34 drug-receiving patients (DRP) with SLECTS and 36 demographically matched typical development (TD). RESULTS: Compared with the TD group, both the DNP and DRP groups exhibited significantly higher specialization in the left superior temporal gyrus, right parahippocampus, left putamen, and right caudate. The DNP group exhibited significantly higher hemispheric specialization in the right precentral gyrus and right inferior temporal gyrus, while the DRP group demonstrated significantly higher hemispheric specialization in the left postcentral gyrus and right hippocampus than the TD group. Furthermore, bilateral cerebellum_6 showed opposing hemispheric specialization trends in the two patient groups. Further meta-analytical mapping demonstrated that hemispheric specialization-related differential brain regions are primarily involved in language processing. CONCLUSION: Our findings showed that children with SLECTS had altered hemispheric specialization, mainly in language processing regions, suggesting both abnormal intrahemispheric segregation and interhemispheric integration in these children.

Reconsidering the Relationship Between Hand Preference and Cerebral Vascular Dominance: A Computed Tomography (CT) Angiography Study.

Background: To date, it has been suggested that there may be many genetic, environmental, and vascular factors that affect hand preference. In previous studies evaluating the relationship between cerebral dominance and hand preference, carotid and vertebral artery (VA) Doppler ultrasonography (USG) was generally preferred; and these studies only measured VA diameters. Unlike other studies, we aimed to reevaluate the relationship between hand preference and cerebral vascular dominance by measuring VA and internal carotid artery (ICA) diameters. In addition, we used carotid and VA computed tomography (CT) angiography instead of Doppler USG. Methods and Material: A total of 345 participants were included in the study. The results of carotid and VA CT angiography taken during hospitalization were retrospectively evaluated by two radiologists, and the Edinburgh Hand Preference Questionnaire was applied to these patients. Results: In right-handed patients, the diameter of the left VA was significantly larger than the diameter of the right VA (p = 0.005). In left-handed patients, the diameter of the left ICA was larger than the diameter of the right ICA, but the difference was not statistically significant (p = 0.055). There was no significant difference between the diameter of the right and left ICA in right-handed patients (p = 0.771). Conclusions: In our study, we found a correlation between the dominant hemisphere VA diameter and hand preference. Using CT angiography, we were able to eliminate many challenges of ultrasonography that make radiological evaluation difficult, such as differences of opinion between radiologists, and technical and anatomical reasons.

Exploring brain activity for positive and negative emotions by means of EEG microstates.

Microstate analysis applied to electroencephalographic signals (EEG) allows both temporal and spatial imaging exploration and represents the activity across the scalp. Despite its potential usefulness in understanding brain activity during a specific task, it has been mostly exploited at rest. We extracted EEG microstates during the presentation of emotional expressions, presented either unilaterally (a face in one visual hemifield) or bilaterally (two faces, one in each hemifield). Results revealed four specific microstate's topographies: (i) M1 involves the temporal areas, mainly in the right hemisphere, with a higher occurrence for stimuli presented in the left than in the right visual field; (ii) M2 is localized in the left temporal cortex, with higher occurrence and coverage for unilateral than bilateral presentations; (iii) M3, with a bilateral temporo-parietal localization, shows higher coverage for bilateral than unilateral presentation; (iv) M4, mainly localized in the right fronto-parietal areas and possibly representing the hemispheric specialization for the peculiar stimulus category, shows higher occurrence and coverage for unilateral stimuli presented in the left than in the right visual field. These results suggest that microstate analysis is a valid tool to explore the cerebral response to emotions and can add new insights on the cerebral functioning, with respect to other EEG markers.

Structural and functional brain asymmetries in the early phases of life: a scoping review.

Asymmetry characterizes the brain in both structure and function. Anatomical asymmetries explain only a fraction of functional variability in lateralization, with structural and functional asymmetries developing at different periods of life and in different ways. In this work, we perform a scoping review of the cerebral asymmetries in the first brain development phases. We included all English-written studies providing direct evidence of hemispheric asymmetries in full-term neonates, foetuses, and premature infants, both at term post-conception and before. The final analysis included 57 studies. The reviewed literature shows large variability in the used techniques and methodological procedures. Most structural studies investigated the temporal lobe, showing a temporal planum more pronounced on the left than on the right (although not all data agree), a morphological asymmetry already present from the 29th week of gestation. Other brain structures have been poorly investigated, and the results are even more discordant. Unlike data on structural asymmetries, functional data agree with each other, identifying a leftward dominance for speech stimuli and an overall dominance of the right hemisphere in all other functional conditions. This generalized dominance of the right hemisphere for all conditions (except linguistic stimuli) is in line with theories stating that the right hemisphere develops earlier and that its development is less subject to external influences because it sustains functions necessary to survive.

Clinical Speech fMRI in Children and Adolescents : Development of an Optimal Protocol and Analysis Algorithm.

PURPOSE: In patients with drug-resistant focal epilepsy, surgical resection is often the only treatment option to achieve long-term seizure control. Prior to brain surgery involving potential language areas, identification of hemispheric language dominance is crucial. Our group developed and validated a functional magnetic resonance imaging (fMRI) battery of four pediatric language tasks. The present study aimed at optimizing fMRI data acquisition and analysis using these tasks. METHODS: We retrospectively analyzed speech fMRI examinations of 114 neuropediatric patients (age range 5.8-17.8 years) who were examined prior to possible epilepsy surgery. In order to evaluate hemispheric language dominance, 1-4 language tasks (vowel identification task VIT, word-chain task WCT, beep-story task BST, synonym task SYT) were measured. RESULTS: Language dominance was classified using fMRI activation in the 13 validly lateralizing ROIs (VLR) in frontal, temporal and parietal lobes and cerebellum of the recent validation study from our group: 47/114 patients were classified as left-dominant, 34/114 as bilateral and 6/114 as right-dominant. In an attempt to enlarge the set of VLR, we then compared for each task agreement of these ROI activations with the classified language dominance. We found four additional task-specific ROIs showing concordant activation and activation in ≥ 10 sessions, which we termed validly lateralizing (VLRnew). The new VLRs were: for VIT the temporal language area and for SYT the middle frontal gyrus, the intraparietal sulcus and cerebellum. Finally, in order to find the optimal sequence of measuring the different tasks, we analyzed the success rates of single tasks and all possible task combinations. The sequence 1) VIT 2) WCT 3) BST 4) SYT was identified as the optimal sequence, yielding the highest chance to obtain reliable results even when the fMRI examination has to be stopped, e.g., due to lack of cooperation. CONCLUSION: Our suggested task order together with the enlarged set of VLRnew may contribute to optimize pediatric speech fMRI in a clinical setting.

Speech-related auditory salience detection in the posterior superior temporal region.

Processing auditory human speech requires both detection (early and transient) and analysis (sustained). We analyzed high gamma (70-110 Hz) activity of intracranial electroencephalography waveforms acquired during an auditory task that paired forward speech, reverse speech, and signal correlated noise. We identified widespread superior temporal sites with sustained activity responding only to forward and reverse speech regardless of paired order. More localized superior temporal auditory onset sites responded to all stimulus types when presented first in a pair and responded in recurrent fashion to the second paired stimulus in select conditions even in the absence of interstimulus silence; a novel finding. Auditory onset activity to a second paired sound recurred according to relative salience, with evidence of partial suppression during linguistic processing. We propose that temporal lobe auditory onset sites facilitate a salience detector function with hysteresis of 200 ms and are influenced by cortico-cortical feedback loops involving linguistic processing and articulation.

Cerebral dominance in an unusual case of Landau-Kleffner syndrome.

Landau-Kleffner syndrome (LKS) is described by the International Classification of Epileptic Syndromes since 1985 as a constellation of clinical and electrographic signs, including acquired aphasia, regression of language milestones and seizures, along with sleep-activated paroxysms on electroencephalogram which can progress to electrographic status epilepticus of sleep. In this case, a 7-year-old boy presented with an atypical history of new-onset aphasia and regression of language milestones with rare seizures. However, there was an electrographic mismatch in the form of right-sided epileptiform activity and continuous spike and wave of sleep pattern. Detailed speech analysis and perusal of the history revealed a possibly ambidextrous child with right hemispheric language dominance, and he was diagnosed with LKS and treated. This report illustrates the many pitfalls in the diagnosis and treatment of this rare epileptic syndrome.

Story reading with incidental comprehension and memory: left hemisphere dominance.

BACKGROUND: There are no studies on adults with unilateral brain lesions regarding story reading with incidental/implicit comprehension and memory, in which memory is only assessed through delayed recall. There is a need for validation of cerebral laterality in this type of verbal recall, which includes spontaneous performance (free or uncued condition (UC)), and induced-through-question performance regarding the forgotten units (cued condition (CC)). OBJECTIVES: To explore the effects of unilateral brain lesions, of oral reading with expression (RE) and comprehension (RC) on delayed recall of a story, as either UC or CC; and to validate the ability of UC and CC to discriminate the side of brain injury. METHODS: Data were obtained from 200 right-handed volunteers, among whom 42 had left-hemisphere injury (LHI), 49 had right-hemisphere injury (RHI) and 109 were demographically-matched healthy participants (HP). Patients who were unable to read, understand or speak were excluded. RESULTS: LHI individuals presented impairment of both UC and CC, in relation to the other two groups (non-LHI) with sensitivity and specificity above 70%. LHI and RHI individuals were not significantly different in RE and RC, but they were both different from HP in all the assessments except CC, in which RHI individuals resembled HP. Despite this lack of abnormality in RHI individuals during CC, about half of this group showed impairment in UC. Additionally, whereas RE had a significant effect on UC, the moral of the story (RC) had a significant effect on both UC and CC. CONCLUSIONS: The left hemisphere was dominant for this memory task involving implicit processing.

Story reading with incidental comprehension and memory: left hemisphere dominance / Lectura de una historia con comprensión y memoria incidentales: dominancia del hemisferio izquierdo

ABSTRACT Background: There are no studies on adults with unilateral brain lesions regarding story reading with incidental/implicit comprehension and memory, in which memory is only assessed through delayed recall. There is a need for validation of cerebral laterality in this type of verbal recall, which includes spontaneous performance (free or uncued condition (UC)), and induced-through-question performance regarding the forgotten units (cued condition (CC)). Objectives: To explore the effects of unilateral brain lesions, of oral reading with expression (RE) and comprehension (RC) on delayed recall of a story, as either UC or CC; and to validate the ability of UC and CC to discriminate the side of brain injury. Methods: Data were obtained from 200 right-handed volunteers, among whom 42 had left-hemisphere injury (LHI), 49 had right-hemisphere injury (RHI) and 109 were demographically-matched healthy participants (HP). Patients who were unable to read, understand or speak were excluded. Results: LHI individuals presented impairment of both UC and CC, in relation to the other two groups (non-LHI) with sensitivity and specificity above 70%. LHI and RHI individuals were not significantly different in RE and RC, but they were both different from HP in all the assessments except CC, in which RHI individuals resembled HP. Despite this lack of abnormality in RHI individuals during CC, about half of this group showed impairment in UC. Additionally, whereas RE had a significant effect on UC, the moral of the story (RC) had a significant effect on both UC and CC. Conclusions: The left hemisphere was dominant for this memory task involving implicit processing.

RESUMEN Antecedentes: No existen estudios en adultos con lesiones cerebrales unilaterales sobre la lectura de una historia, con comprensión y memoria incidental/implícita, midiendo sólo el recuerdo diferido. Se necesita validar la lateralidad cerebral en este tipo de recuerdo verbal que incluye: desempeño espontáneo (condición sin claves (CS) o libre) y desempeño inducido con preguntas sobre las unidades olvidadas (condición con claves (CC)). Objetivos: Explorar el efecto de las lesiones cerebrales unilaterales, la lectura expresiva (LE) y la comprensiva (LC), sobre el recuerdo diferido de una historia, sea CS o CC. Validar CS y CC en su capacidad para discriminar el lado de la lesión cerebral. Métodos: Los datos se obtuvieron de 200 voluntarios diestros, 42 con lesiones del hemisferio izquierdo (LHI), 49 con lesiones del hemisferio derecho (LHD), y 109 participantes sanos (PS), equiparados demográficamente. Se excluyeron los pacientes que no pudieron leer, entender o hablar. Resultados: LHI resultó perjudicado respecto de los otros dos grupos (no-LHI) en CS y CC, con una sensibilidad y especificidad superior al 70%. LHI y LHD no se diferenciaron significativamente en LE ni LC, pero ambos fueron diferentes de los PS en todas las evaluaciones excepto CC, en donde LHD se asemejó a los PS. A pesar de esta ausencia de anomalía en LHD durante CC, aproximadamente la mitad de LHD mostró deterioro en CS. Además, mientras que LE tuvo un efecto significativo en CS, la moraleja de la historia (LC) tuvo un efecto significativo tanto en CS como en CC. Conclusiones: El hemisferio izquierdo fue dominante para esta tarea de memoria que involucró procesamiento implícito.