The possibility of retinotopic organization in the optic nerve projections to the contralateral and ipsilateral diencephalon was studied by means of partial retinal lesions and staining for terminal degeneration by the Fink-Heimer technique. A retinotopic pattern of projection was observed in the nucleus of Bellonci, the corpus geniculatum thalamicum and the posterior thalamic nucleus. The temporal quadrant of the retina, and, to a lesser extent, the ventral quadrant projected to the ipsilateral side as well as to the contralateral side. In each diencephalic region noted above, the temporal and dorsal quadrants of the retina were represented more posteriorly (posteroventrally), and ventral and nasal quadrants projected more anteriorly (anterodorsally). The areas of representation for the temporal and ventral quadrants were located superior (superoposterior) to those for the dorsal and nasal quadrants. In their overall configuration and orientation, the retino-diencephalic maps show mirror-image reversal with respect to the retino-tectal projection. Since, in their areal extent, both the retino-diencephalic maps and the retino-tectal map are approximately parallel to the ventricular surface, their mirror-image reversal appears to indicate a reversal in the polarity of developmental processes across the di-mesencephalic junction.
The retinotopic organization within the optic tract in the diencephalon and tectum was also analyzed. In the optic tract, the quadrants of the retina are reassembled such that the dorsal and nasal quadrants are widely separated in, respectively, the ventral and dorsal edges of the tract; the temporal and ventral quadrants are systematically represented in intermediate levels in the tract, the temporal quadrant above the dorsal, and the ventral quadrant below the nasal. When the optic tract bifurcates to encircle the tectum, the fibers from the ventral and nasal quadrants enter the dorsomedial arm and the fibers from the temporal and dorsal quadrants enter the ventrolateral arm of the optic tract. The paths taken by optic fibers in traversing the tectum to reach their areas of termination were reconstructed. Many optic fibers show an alignment parallel to an anteroventral posterodorsal axis as they cross the surface of the tectum, but the OS vs IS characterization of the fibroarchitecture of the tectum appears to be an oversimplification.