Retinogeniculate projection fibers in the monkey optic chiasm: a demonstration of the fiber arrangement by means of wheat germ agglutinin conjugated to horseradish peroxidase.

The fiber arrangement of the retinogeniculate pathway was investigated in the chiasm of Japanese monkeys (Macaca fuscata) by an iontophoretic injection of wheat germ agglutinin conjugated to horseradish peroxidase into the lateral geniculate nucleus (LGN). It has been claimed that there is a distinct retinotopy in the monkey chiasm, despite lack of any clear anatomical evidence. However, the present data indicate a rather gross retinotopy or almost no discernible retinotopy. Fibers from the foveal-to-peripheral axis of the temporal retina show substantially no retinotopy owing to a marked overlap of fibers in the anterolateral and the posterocentral parts of the ipsilateral hemichiasm. In contrast, the foveal-to-peripheral axis of the nasal retina is re-formed in a gross dorsoventral order in the chiasm. That is, nasal foveal-parafoveal fibers which arise from small cells (which are P beta mode) pass in the dorsal part of the chiasm adjacent to the brain. They widely overlap nasal perifoveal fibers which cross the chiasm more ventrally with very little contact with the brain. The nasal perifoveal fibers also widely overlap nasal peripheral fibers which cross the chiasm more ventrally. Furthermore, the nasal peripheral fibers overlap nasal far peripheral fibers which arise from large cells (including many of the P alpha mode) which run near the pial surface. Fibers from the dorsal and ventral nasal retina cross the midline of the posterior and anterior parts of the chiasm, respectively, and are finally positioned in the medioventral and ventrocentral parts in the tract. Consequently, the dorsoventral retinal axis is re-formed posteroanteriorly in the midline of the chiasm and in a roughly mediolateral direction in the tract. Furthermore, the present study shows that the nasal and temporal retinal fibers coming from the same eye are acutely segregated in the prechiasmal region and the anterior part of the hemichiasm.