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

Abstract

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.

One hand or the other? Effector selection biases in right and left handers

“…Cumming (2012; argues that using meta analysis to provide confidence intervals around an estimate provided a more accurate and interpretable picture then comparing separate studies which do or do not result in a statistically significant differences. Given the discrepancies between our results and those of others on hand choice, we decided that a random effects meta analysis might allow for an across-study estimate of the size of any difference between right handers and left handers on hand choice tasks, and would allow for 95% confidence interval construction based on between and within-study heterogeneity (see Carey & Johnstone, 2014, for a similar analysis on language laterality in left and right handers). Such approaches are useful for creating precise estimates of effects (and their likely range in the population; Cumming, 2012;Kline, 2004).…”

Stability of right visual field advantage in an international lateralized lexical decision task irrespective of participants’ sex, handedness or bilingualism

“…63) and picture naming (r = .77). Such recent studies revived the notion that results from behavioural half-field paradigms should be taken seriously in the theoretical and clinical domain (Carey & Johnstone, 2014;Hugdahl, 2011;Van der Haegen, Cai, Seurinck, & Brysbaert, 2011; see also Van der Haegen, Westerhausen, Hugdahl, & Brysbaert, 2013, for validation evidence with respect to dichotic listening).…”

How to get a left‐ear advantage: A technical review of assessing brain asymmetry with dichotic listening

“…However, the direction of the perceptual preference can differ between individuals. In a typical group of right‐handers about 20% of individuals exhibit a left‐ear advantage when tested with a standard verbal dichotic‐listening paradigm (Bryden, ; and reanalysis of Carey & Johnstone, , meta‐analysis only including dichotic listening, personal communication, D. Carey, April 2017). For example, as presented in Fig…”

“…However, as argued by Bryden (), dichotic‐listening studies show greater prevalence of left‐ear advantage in population than the prevalence of “atypical” right‐hemispheric specialization in population as suggested by assessment using more direct measures. For example, in the Wada (sodium amobarbital) test to assess hemispheric dominance, about 13% of the right‐handed patients show atypical right‐hemispheric dominance according to a recent meta‐analysis (Carey & Johnstone, ). Comparable prevalence estimates can also be obtained using functional magnetic‐resonance imaging (fMRI) on healthy individuals, indicating that between 5% (Badzakova‐Trajkov, Häberling, Roberts & Corballis, ) and 12% (Mazoyer, Zago, Jobard et al ., ) of the right‐handed individuals show right‐hemispheric dominance.…”