Further fMRI validation of the visual half field technique as an indicator of language laterality: a large-group analysis.

The best established lateralized cerebral function is speech production, with the majority of the population having left hemisphere dominance. An important question is how to best assess the laterality of this function. Neuroimaging techniques such as functional Magnetic Resonance Imaging (fMRI) are increasingly used in clinical settings to replace the invasive Wada-test. We evaluated the usefulness of behavioral visual half field (VHF) tasks for screening a large sample of healthy left-handers. Laterality indices (LIs) calculated on the basis of the latencies in a word and picture naming VHF task were compared to the brain activity measured in a silent word generation task in fMRI (pars opercularis/BA44 and pars triangularis/BA45). Results confirmed the usefulness of the VHF-tasks as a screening device. None of the left-handed participants with clear right visual field (RVF) advantages in the picture and word naming task showed right hemisphere dominance in the scanner. In contrast, 16/20 participants with a left visual field (LVF) advantage in both word and picture naming turned out to have atypical right brain dominance. Results were less clear for participants who failed to show clear VHF asymmetries (below 20 ms RVF advantage and below 60 ms LVF advantage) or who had inconsistent asymmetries in picture and word naming. These results indicate that the behavioral tasks can mainly provide useful information about the direction of speech dominance when both VHF differences clearly point in the same direction.