Changes in the retinotopical fiber order along the horizontal and dorsoventral axes of the nasal retina in the monkey optic chiasm.

The fiber arrangement of the retinogeniculate pathways was investigated in the Japanese monkey chiasm by iontophoretical injections into the lateral geniculate nucleus of wheat germ agglutinin conjugated to horseradish peroxidase. The present data demonstrated that the horizontal and dorsoventral axes of the nasal retina were rearranged in gross dorsoventral and posteroanterior orders in the chiasm. Thus, foveal/parafoveal fibers passed across the dorsal chiasm in a cluster and midperipheral nasal fibers passed across the central and ventral chiasms. Far-peripheral nasal fibers progressed in the ventral chiasm. Chiasmal fibers from the dorsal and ventral nasal retinas took pathways exactly similar to those from the midperipheral and the far-peripheral nasal retinas, respectively. In the anteroposterior direction, foveal/parafoveal fibers crossed the chiasmal midline extensively. However, midperipheral nasal fibers and dorsal nasal fibers crossed the posterior chiasm, and far-peripheral nasal fibers and ventral nasal fibers crossed the anterior chiasm. The correspondence of the retinotopical order in the chiasm with the chronological order is discussed.