Speeded right-to-left information transfer: the result of speeded transmission in right-hemisphere axons?

Both reaction time (RT) and evoked potential (EP) studies have shown that interhemispheric transfer is faster from the right to the left hemisphere than vice versa. This has been explained either in terms of an asymmetry of callosal fibres or as a result of hemispheric specialization. Here we suggest that it may be due to greater activity resulting from a greater number of fast-conducting, myelinated fibres in the right hemisphere than in the left. Interhemispheric transfer times (IHTTs) were measured in 13 males by comparing latencies and amplitudes of N160 EPs ipsilateral and contralateral to checkerboard stimuli presented to the left or right visual field. IHTT estimates were obtained from three homologous electrode pairs. The shorter IHTT from right-to-left was associated with a concomitant increase in N160 negativity in the right hemisphere. There was no evidence from RTs to stimuli in each visual field to suggest that the right hemisphere was dominant for this task, suggesting that the faster speed of transfer from the right-to-left hemisphere may depend on faster axonal conduction in the right hemisphere relative to the left.