Composition of the supraoptic decussation of the chick (Gallus gallus). A possible factor limiting interhemispheric transfer of visual information.

In the chick the supraoptic decussation consists of dorsal, ventral, and subventral subregions. The dorsal region contains about 580000 axons of which 24% are myelinated. The mean diameters of unmyelinated and myelinated fibers are 0.31 micron and 1.0 micron, respectively. The ventral region contains about 520000 axons of which only 0.53% are myelinated. The mean diameters of unmyelinated and myelinated fibers are 0.35 micron and 1.09 micron, respectively. A small proportion of unmyelinated fibers has regions of localized expansions. The subventral region, the smallest subregion, contains about 31000 fibers of which 20% are myelinated. The mean diameters of unmyelinated and myelinated axons are 0.31 micron and 1.15 micron, respectively. On the basis of the interhemispheric channels available to the visual system it is argued that visual information must be modified to some extent prior to its transfer.