Decussation

Abstract: The carbocyanine dye, DiI, has been used to study the retinal origin of the uncrossed retinofugal component of the mouse and to show the course taken by these fibres through the optic nerve and chiasm during development. Optic axons first arrive at the chiasm at embryonic day 13 (E13) but do not cross the midline until E14. After this stage, fibres taking an uncrossed course can be selectively labelled by unilateral tract implants of DiI. The earliest ipsilaterally projecting ganglion cells are located in the dorsal central retina. The first sign of the adult pattern of distribution of ganglion cells with uncrossed axons located mainly in the ventrotemporal retina is seen on embryonic day 16.5, thus showing that the adult line of decussation forms early in development. A small number of labelled cells continue to be found in nasal and dorsal retina at all later stages. At early stages (E14-15), retrogradely labelled uncrossed fibres are found in virtually all fascicles of the developing nerve, intermingling with crossed axons throughout the length of the nerve. At later stages of development (E16-17), although uncrossed fibres pass predominantly within the temporal part of the stalk, they remain intermingled with crossed axons. A significant number of uncrossed axons also lie within the nasal part of the optic stalk. The position of uncrossed fibres throughout the nerve in the later developmental stages is comparable to that seen in the adult rodent (Baker and Jeffery, 1989). The distribution of uncrossed axons thus indicates that positional cues are not sufficient to account for the choice made by axons when they reach the optic chiasm.

Abstract: The retinogeniculate pathways of several different genotypes of mink have been studied by the Nauta and Fink-Heimer methods. An abnormal retinogeniculate pathway has been found in all mink in which the retinal pigment is reduced. Most of the abnormally routed nerve fibers arise in the temporal retina and cross in the chiasm, instead of staying ipsilateral as is normal. Some abnormal fibers in some of the mink also appear to arise in the nasal retina close to the line of decussation, and these pass ipsilaterally instead of following their normal crossed pathway. A lack of pigment in the coat is not by itself associated with a pathway abnormality. In general, the size of the abnormal fiber component is related to the severity of the retinal pigment deficit. The abnormality is not related specifically to one gene or gene combination. We have found eight different gene combinations which produce a reduction of retinal pigment associated with a pathway abnormality.

Abstract: 1. If classical partial decussation exactly segregates the projections of right and left hemi-retinae on to the two optic tracts, the images of an object in central vision, nearer or further than the fixation point, should project to separate hemispheres. This would prevent the encoding of retinal disparity by binocularly driven neurones of the visual cortex.2. It is proposed that there is a central vertical strip of retina in each eye which is represented in both hemispheres. The angular width of this strip should be exactly one half the actual range of horizontal disparities of binocular receptive fields near the central vertical meridian.3. By recording from single neurones in the area 17/18 region in both hemispheres of a cat, it was found that there is such a strip of bilateral projection. The centres of receptive fields for units from the two hemispheres overlap in the middle of the visual field by about 1.5 degrees and the S.D. of the distribution is about 0.5 degrees .4. The horizontal disparities of the centres of binocular receptive fields were measured for samples of units representing different parts of the visual field. The range of horizontal disparity for fields near the area centralis is about 2.3 degrees , the S.D. of the distribution about 0.9 degrees . The proposed relationship between bilateral projection and disparity coding is thus confirmed.5. The origin of the bilateral projection is a matter of speculation, but in the cat some of it is almost certainly due to imprecision in the nature of the nasotemporal division of optic nerve fibres at the optic chiasma. A case can be made, however, that the overlap is partly due to connexions through the corpus callosum between the two occipital lobes.6. Evidence for the importance of the callosal pathway in man is drawn from the effects on stereopsis of section of the chiasma and the callosum.