NON-MIRROR-SYMMETRICAL PATTERNS OF CALLOSAL LINKAGES IN AREA-17 AND AREA-18 IN CAT VISUAL-CORTEX

In the cat, callosal connections in area 17 are largely confined to a 5-6-mm-wide strip at the 17/18 border. It is commonly thought that cal losal fibers extending from between the 17/18 border regions interconn ect loci that are mirror-symmetric with respect to the midline of the brain, but this idea has not been tested experimentally. The present s tudy examined the organization of callosal linkages in the 17/18 borde r region of normal adult cats by analyzing the patterns of connections revealed in one hemisphere after small injections of different fluore scent tracers into the opposite 17/18 callosal region. The location of the injection sites within areas 17 and 18 was assessed by examining architectonic data and by inspecting the labeling pattern in the ipsil ateral visual thalamus. Area 17 and 18 were separated by a 1-1.5-mm-wi de zone of cytoarchitectonic transition rather than by a sharp border. The results show that, in general, callosal fibers interconnect loci that are not mirror-symmetric with respect to the midline. Thus, area 17 injections placed nearly 3 mm away from the 17/18 transition zone p roduced discrete labeled areas located preferentially within the contr alateral 17/18 transition zone. However,when the injection site was wi thin the 17/18 transition zone, labeled cells were found primarily med ial and lateral to, but not within, the 17/18 transition zone in the c ontralateral hemisphere. Previous studies have indicated that the 17/1 8 transition zone contains a representation of a strip of the ipsilate ral visual field. Comparison of the retinotopy of the 17/18 border reg ion with the mirror-reversed pattern of callosal linkages found in the present study suggests that callosal fibers Link points that are in r etinotopic correspondence in both hemispheres. (C) 1996 Wiley-Liss, In c.