INTERHEMISPHERIC-TRANSFER AND THE PROCESSING OF FOVEALLY PRESENTED STIMULI

Two arguments are commonly given in favor of a nasotemporal overlap al ong the vertical meridian of the visual field: anatomical findings and the existence of macular sparing in hemianopia. A review of the liter ature, however, points to the weakness of the evidence. The anatomical indications are exclusively based on horseradish peroxidase studies, which can not give an unequivocal answer to the amount of overlap in c entral vision, and which were not supported by a recent study that mad e use of the more direct [C-14]2-deoxy-D-glucose technique. The argume nt of macular sparing in hemianopia appears to be derived evidence tha t depends on the validity of the anatomical findings. In addition, beh avioral studies consistently failed to find functional confirmation of the overlap. To further test the possibility of bilateral representat ion in central vision, a new paradigm is proposed. It is argued that i f interhemispheric transfer is needed for the processing of foveally p resented stimuli, the word-beginning superiority effect should be larg er for subjects with left hemisphere dominance than for subjects with right hemisphere dominance. Results are in line with the hypothesis an d point to the fact that interhemispheric transfer of visual informati on may be involved in more processing than usually accepted. It is als o noted that transfer time seems to depend on the amount of informatio n that must be transferred, and is significantly shorter than the esti mates obtained in visual half field studies.