While it is well known that the left hemisphere is more efficient than the right in most tasks involving perception of speech stimuli, the neurophysiological pathways leading to these lateralised performance differences are as yet rather unclear.
To clarify this question, a team of researchers from the University of Dresden and the Biopsychology lab recorded event-related potentials (ERPs) during tachistoscopic presentation of horizontally or vertically presented verbal stimuli in the left (LVF) and the right visual field (RVF). On the behavioural level, participants showed stronger hemispheric asymmetries for horizontal, than for vertical stimulus presentation. In addition, ERP asymmetries were also modulated by stimulus presentation format.
Moreover, sLORETA revealed that ERP left-right asymmetries were mainly driven by the extrastriate cortex and reading-associated areas in the parietal cortex. Taken together, the present study shows electrophysiological support for the assumption that language lateralisation during speech perception arises from a left dominance for the processing of early perceptual stimulus aspects.

Selpien, H., Siebert, C., Genc, E., Beste, C., Faustmann, P.M., Güntürkün, O., Ocklenburg, S., Left dominance for language perception starts in the extrastriate cortex: An ERP and sLORETA study. Behavioural Brain Research, 2015, 291: 325-333.