Corpus callosum has different channels for transmission of spatial frequency information

Many studies suggest that separate retinocortical channels with different conduction speeds transmit the information about high and low spatial frequencies (SF). Similarly, separate callosal channels may be responsible for the transfer of different SFs. To test this hypothesis, interhemispheric transfer time (IHTT) was estimated using visual evoked potentials (VEPs) elicited by reversal of different SF checkerboard patterns, which were presented either in the right or left visual hemifield. VEPs were recorded from homologous occipital and parietal leads in 11 subjects. The P100 latencies obtained from directly stimulated hemispheres were defined as retinocortical conduction times. The difference in P100 latencies obtained from directly and indirectly stimulated hemispheres was defined as IHTT (i.e., contralateral P100 latency values subtracted from the ipsilateral latency values). The results, showing faster retinocortical transfer for low SF than for high SF, and the shortest transfer time at parietal leads only for low SF, indicate that the presented stimuli are transferred on separate retinocortical channels. Concerning the interhemispheric transfer (VEP-IHTT), faster right-to-left than left-to-right transfer for both low and high SF stimuli was found, which is in congruence with previous studies. Most important however, the VEP-IHTT was faster for low SF than high SF. These different interhemispheric transfer speeds support the hypothesis that transferring low SF, compared with high SF information, relies on larger callosal fibers. Thus, we conclude that indeed distinct callosal channels respond selectively to the SF content of visual stimuli.