Variability in lateralised blood flow response to language is associated with language development in children aged 1-5 years

Patterns of language and visuospatial functional lateralization and cognitive ability: a systematic review

For most individuals, language is predominately localized to the left hemisphere of the brain and visuospatial processing to the right. This is the typical pattern of functional lateralization. Evolutionary theories of lateralization suggest that the typical pattern is most common as it delivers a cognitive advantage. In contrast, deviations from the typical pattern may lead to poorer cognitive abilities. The aim of this systematic review was to assess the evidence for an association between patterns of language and visuospatial lateralization and measures of cognitive ability. We screened 9,122 studies, retrieved from PsycINFO, EMBASE, MEDLINE, PubMed, and Web of Science. The 17 studies that met our selection criteria revealed little evidence for an advantage of typical compared to atypical patterns of lateralization, although atypical lateralization patterns were related to worse language comprehension, spatial ability, and reading, but further research is needed to confirm this. We conclude with recommendations that future researchers recruit larger samples of atypical participants, and consider strength of lateraliation and bilaterality when analysing functional lateralization patterns.

Exploring cerebral laterality of writing and the relationship to handedness: a functional transcranial Doppler ultrasound investigation

Cerebral lateralization of oral language has been investigated in a plethora of studies and it is well established that the left hemisphere is dominant for production tasks in the majority of individuals. However, few studies have focused on written language and even fewer have sampled left-handers. Writing comprises language and motor components, both of which contribute to cerebral activation, yet previous research has not disentangled. The aim of this study was to disentangle the language and motor components of writing lateralization. This was achieved through the comparison of cerebral activation during (i) written word generation and (ii) letter copying, as assessed by functional Transcranial Doppler (fTCD) ultrasound. We further assessed cerebral laterality of oral language. The sample was balanced for handedness. We preregistered the hypotheses that (i) cerebral lateralization of the linguistic component of writing would be weaker in left-handers compared to right-handers and (ii) oral language and the linguistic component of written language would not be correlated in terms of cerebral lateralization. No compelling evidence for either of our hypotheses was found. Findings highlight the complexity of the processes subserving written and oral language as well as the methodological challenges to isolate the linguistic component of writing.

Cerebral laterality of writing in right- and left- handers: A functional transcranial Doppler ultrasound study

The cerebral lateralization of written language has received very limited research attention in comparison to the wealth of studies on the cerebral lateralization of oral language. The purpose of the present study was to further our understanding of written language lateralization, by elucidating the relative contribution of language and motor functions. We compared written word generation with a task that has equivalent visuomotor demands but does not include language: the repeated drawing of symbols. We assessed cerebral laterality using functional transcranial Doppler ultrasound (fTCD), a non-invasive, perfusion-sensitive neuroimaging technique in 23 left- and 31 right-handed participants. Findings suggest that the linguistic aspect of written word generation recruited more left-hemispheric areas during writing, in right-handers compared to left-handers. This difference could be explained by greater variability in cerebral laterality patterns within left-handers or the possibility that the areas subserving language in left-handers are broader than in right-handers. Another explanation is that the attentional demands of the more novel symbol copying task (compared to writing) contributed more right-hemispheric activation in right-handers, but this could not be captured in left-handers due to ceiling effects. Future work could investigate such attentional demands using both simple and complex stimuli in the copying condition.

Finding hidden treasures: A child-friendly neural test of task-following in individuals using functional Transcranial Doppler ultrasound

Despite growing interest in the mental life of individuals who cannot communicate verbally, objective and non-invasive tests of covert cognition are still sparse. In this study, we assessed the ability of neurotypical children to understand and follow task instructions by measuring neural responses through functional transcranial Doppler ultrasound (fTCD). We recorded blood flow velocity for the two brain hemispheres of twenty children (aged 9 to 12) while they performed either a language task or a visuospatial memory task, on identical visual stimuli. We extracted measures of neural lateralisation for the two tasks separately to investigate lateralisation, and we compared the left-minus-right pattern of activation across tasks to assess task-following. At the group level, we found that neural responses were left-lateralised when children performed the language task, and not when they performed the visuospatial task. However, with statistically robust analyses and controlled paradigms, significant lateralisation in individual children was less frequent than expected from the literature. Nonetheless, the pattern of hemispheric activation for the two tasks allowed us to confirm task-following in the group of participants, as well as in over half of the individuals. This provides a promising avenue for a covert and inexpensive test of children's ability to follow task instructions and perform different mental tasks on identical stimuli.

Cognitive consequences of the left-right asymmetry of atrophy in semantic dementia

Semantic dementia (SD) is a condition in which atrophy to the anterior temporal lobes (ATL) produces a selective deterioration of conceptual knowledge. As this atrophy is always bilateral but usually asymmetrical, differences in performance of the two SD subgroups-with left > right (L > R) versus right > left (R > L) atrophy-constitute a major source of evidence regarding the roles of the left and right sides of this region. We explored this issue using large scale case-series methodology, with a pool of 216 observations of neuropsychological data from 72 patients with SD. Anomia was significantly more severe in the L > R subgroup, even when cases from the two subgroups were matched on severity of comprehension deficits. For subgroups matched on the degree of anomia, we show that asymmetry of atrophy also affected both the nature of the naming errors produced, and the degree of a semantic category effect (living things vs artefacts). A comparison across tasks varying in their loading on verbal and visual processing revealed a greater deficit in object naming for L > R cases and in a picture-based semantic association test for R > L cases; this held true whether severity across subgroups was controlled using pairwise matching or statistically via principal components analysis. Importantly, the size of our sample allowed us to demonstrate considerable individual variation within each of the L > R and R > L subgroups, with consequent overlap between them. Our results paint a clear picture of how asymmetry of atrophy affects cognitive performance in SD, and we discuss the results in terms of two mechanisms that could contribute to these differences: variation in the information involved in semantic representations in the left and right ATL, and preferential connectivity between each ATL and other more modality specific intra-hemispheric regions.

What Box: A task for assessing language lateralization in young children

The assessment of active language lateralization in infants and toddlers is challenging. It requires an imaging tool that is unintimidating, quick to setup, and robust to movement, in addition to an engaging and cognitively simple language processing task. Functional Transcranial Doppler Ultrasound (fTCD) offers a suitable technique and here we report on a suitable method to elicit active language production in young children. The 34-second "What Box" trial presents an animated face "searching" for an object. The face "finds" a box that opens to reveal a to-be-labelled object. In a sample of 95 children (1 to 5 years of age), 81% completed the task-32% with ≥10 trials. The task was validated (ρ = 0.4) against the gold standard Word Generation task in a group of older adults (n = 65, 60-85 years of age), though was less likely to categorize lateralization as left or right, indicative of greater measurement variability. Existing methods for active language production have been used with 2-year-old children while passive listening has been conducted with sleeping 6-month-olds. This is the first active method to be successfully employed with infants through to pre-schoolers, forming a useful tool for populations in which complex instructions are problematic.

Network dynamics in dyslexia: Review and implications for remediation

Extant neurobiological theories of dyslexia appear fractional in focusing on isolated brain regions, mechanisms, and functional pathways. A synthesis of current research shows support for an Interactive Specialization (IS) model of dyslexia involving the dysfunctional orchestration of a widely-distributed, attentionally-controlled, hierarchical, and interhemispheric circuit of intercommunicating neuronal networks. This circuitry is comprised principally of the frontostriatal-parietal cognitive control system of networks, the posterior corpus callosum, and the left arcuate fasciculus. During development, the coalescence of these functionally specialized regions, acting together, may be essential to preventing the core phonemic and phonological processing deficits defining the dyslexic phenotype. Research demonstrating an association of each with processing phonology presents the foundational outline for a comprehensive, integrative theory of dyslexia and suggests the importance of inclusive remedial efforts aimed at promoting interactions among all three networking territories.

Hemispheric speech lateralisation in the developing brain is related to motor praxis ability

Commonly displayed functional asymmetries such as hand dominance and hemispheric speech lateralisation are well researched in adults. However there is debate about when such functions become lateralised in the typically developing brain. This study examined whether patterns of speech laterality and hand dominance were related and whether they varied with age in typically developing children. 148 children aged 3-10 years performed an electronic pegboard task to determine hand dominance; a subset of 38 of these children also underwent functional Transcranial Doppler (fTCD) imaging to derive a lateralisation index (LI) for hemispheric activation during speech production using an animation description paradigm. There was no main effect of age in the speech laterality scores, however, younger children showed a greater difference in performance between their hands on the motor task. Furthermore, this between-hand performance difference significantly interacted with direction of speech laterality, with a smaller between-hand difference relating to increased left hemisphere activation. This data shows that both handedness and speech lateralisation appear relatively determined by age 3, but that atypical cerebral lateralisation is linked to greater performance differences in hand skill, irrespective of age. Results are discussed in terms of the common neural systems underpinning handedness and speech lateralisation.