Transcallosal Pathway of Whisker Information Between Rat Primary Somatosensory Cortices

It has been suggested that rodent primary somatosensory cortex (S I) can be activated by ipsilateral whisker stimulation, while S I only receives the ascending input from contralateral whiskers. Previous anatomical research revealed two transcallosal pathways transferring whisker information between bilateral cortices: perigranular zone (PGZ) pathway and dysgranular zone (DZ) pathway. But which pathway plays more important role in transferring whisker information remains unknown. We used voltage-sensitive dye (VSD) imaging to visualize the S I activation by stimulating whiskers. We found that the contralateral whisker stimulation first activates the barrel (granular zone, GZ), then the activity forms a propagating wave and spread to the DZ outside the sub-barrel field cortex (BFC). In contrast, ipsilateral whisker stimulation first activates the DZ outside the BFC. The evoked activity in the DZ forms a propagating wave and spreads in to the BFC in S I. Inactivating opposite crotex blocks this ipsilateral whisker activation, Electrical stimulus to opposite cortex also first evokes DZ in the imaging cortex. Our results suggested that whisker stimulation first activated the barrel and then DZ in the opposite S I. After a delay of transcallosal transfer, the DZ of the other hemisphere (ipsilateral to the whisker being stimulated) was activated. The callosal fibers connecting the DZs on the two hemispheries play a major role in the S I activation by the ipsilateral whisker stimulation.