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

The development of a brief Functional Magnetic Resonance Imaging test battery for neurofunctional investigation of language, memory encoding and motor function in normal children

Several patient groups (e.g., intractable epilepsy, intracerebral tumours) require neurosurgical treatment. The goals of neurosurgery are to resect affected tissue, and to avoid morbidity of function. Previous studies have validated the ability of single fMRI tasks for the preoperative investigation of the localization and lateralization of functions such as language, memory and motor functioning, with a notable paucity of fMRI studies conducted in children. The limitations of using single-tasks for preoperative evaluations have been identified, including discrepancies with ‘gold-standard’ techniques such as the Wada Test and Cortical Stimulation Mapping in terms of lateralization of functions, particularly for language. However, few studies have attempted to use a battery of tasks to combat these issues. Of the few existing studies employing fMRI task batteries, most have been conducted in adults, and have tended to focus on language functions. The current study developed and evaluated the use of a brief fMRI task battery for neurofunctional investigation of language, memory encoding and motor functioning in a sample of 22 healthy children (aged 6.4-16.6 years); that can be used with children of various ages and abilities, and thus it is envisaged that the task battery can be applied to clinical paediatric populations for the purposes of preoperative localization and lateralization of these functions. The study employed two tasks well-reported in existing adult and paediatric fMRI literature (noun-verb generation, NVG; and finger opposition, FO), and two ‘novel’ tasks for use in children (an auditory word presentation task of parametric design, WA; and a picture encoding task, PE). As hypothesized, the tasks showed robust activation in the predicted regions (inferior and middle frontal regions for NVG; M1, S1, SMA and cerebellum for the FO task; superior temporal gyrus and primary auditory association cortex for the WA task; and anterior medial temporal lobe for the PE task). Lateralization indices were also conducted for the NVG, WA and PE tasks. The limitations and considerations of use of fMRI in clinical paediatric populations are also discussed, including the use of mock scanners with paediatric fMRI studies, and the lack of a paediatric atlas equivalent to existing adult atlases for analysis of fMRI data. This study concludes that the proposed fMRI battery provides a brief, valid means for neurofunctional investigation of language, memory encoding and motor functioning in healthy children; which has potential for use in preoperative investigation of these functions in children with neuropathology, in conjunction with other assessment methods such as the Wada Test, Cortical Stimulation Mapping, and neuropsychological assessment

The diminishing dominance of the dominant hemisphere: Language fMRI in focal epilepsy

“Which is the dominant hemisphere?” is a question that arises frequently in patients considered for neurosurgery. The concept of the dominant hemisphere implies uniformity of language lateralisation throughout the brain. It is increasingly recognised that this is not the case in the healthy control brain, and it is especially not so in neurological diseases such as epilepsy. In the present work we adapt our published objective lateralisation method (based on the construction of laterality curves) for use with sub-lobar cortical, subcortical and cerebellar regions of interest (ROIs). We apply this method to investigate regional lateralisation of language activation in 12 healthy controls and 18 focal epilepsy patients, using three different block design language fMRI paradigms, each tapping different aspects of language processing. We compared lateralisation within each ROI across tasks, and investigated how the quantity of data collected affected the ability to robustly estimate laterality across ROIs. In controls, lateralisation was stronger, and the variance across individuals smaller, in cortical ROIs, particularly in the Inferior Frontal (Broca) region. Lateralisation within temporal ROIs was dependent on the nature of the language task employed. One of the healthy controls was left lateralised anteriorly and right lateralised posteriorly. Consistent with previous work, departures from normality occurred in ~15–50% of focal epilepsy patients across the different ROIs, with atypicality most common in the Lateral Temporal (Wernicke) region. Across tasks and ROIs the absolute magnitude of the laterality estimate increased and its across participant variance decreased as more cycles of task and rest were included, stabilising at ~4 cycles (~4 min of data collection). Our data highlight the importance of considering language as a complex task where lateralisation varies at the subhemispheric scale. This is especially important for presurgical planning for focal resections where the concept of ‘hemispheric dominance’ may be misleading. This is a precision medicine approach that enables objective evaluation of language dominance within specific brain regions and can reveal surprising and unexpected anomalies that may be clinically important for individual cases