Discordant cerebral lateralisation for verbal fluency is not an artefact of attention: evidence from MzHd twins.

The current study investigated how the brain structure-function relationships between covert and overt verbal fluency in monozygotic handedness discordant (MzHd) twins relates to broader attentional measures. Evidence presented here shows that the structure-function correlation demonstrated between functional cerebral lateralisation and corpus callosum widths 22-39, contiguous with Broca's area and the middle frontal gyri (Gurd et al. in Brain Struct Funct 218:491-509, 2013), is not an artefact of attention. Twenty-five pairs of female MzHd twins performed a verbal switching task titrated for number of switches. The paradigm permits calculation of switch rates and costs. The switch rate and cost varied as a function of number of switches (1, 2, 3). There were no differences in any measure in relation to right-left handedness in twin pairs. This was supported by large and significant within-twin pair correlations. Atypical functional lateralisation of inferior and middle frontal lobes does not appear to be associated with better or worse performance on verbal task-switching. Discordant lateralisation for verbal fluency does not predict behavioural performance profiles in MzHd twins. This evidence is compatible with a view that attentional components of verbal fluency task performance may have significant heritable components. It does not indicate that neural correlates of frontal cerebral laterality for verbal fluency in MzHd twins are significantly accountable for by heritable components.