Overlapping ipsilateral and contralateral retinal projections to the lateral geniculate nucleus and superior colliculus in the cat: a retrograde triple labelling study.

To analyze the relative proportion and distribution of retinal ganglion cells projecting ipsilaterally and contralaterally in the cat, large injections of the fluorescent tracers Fluoro Gold, Fast Blue, and Diamidino Yellow were made in the main layers of the lateral geniculate nucleus (LGN) and superior colliculus (SC). One tracer was injected in both the LGN and SC on one side, and the other two tracers were injected contralaterally, in the LGN and SC, respectively; labelled ganglion cells were charted on retinal whole mounts. Ganglion cells labelled from the LGN and SC were highly intermingled in both the ipsilateral and contralateral retinae. The adopted combinations of tracers allowed the detection of cells double labelled from the SC and LGN, supporting the occurrence of branched retino-thalamic axons to the SC. About one-fourth of the ganglion cells labelled from the LGN and SC was located in the eye ipsilateral to the injection. Retrograde labelling from the ipsilateral side was almost entirely confined to the temporal hemiretina. In the contralateral eye, labelled cells were mainly concentrated in the nasal hemiretina, but more than 10% were also detected in the temporal half of the retina. In the latter area, cells displaying the entire range of sizes of the retinal ganglion cells, labelled from the contralateral LGN and SC, were found throughout the entire hemiretina. However, more than 50% of such "wrong" projecting cells were grouped in a strip of 2 mm closest to the nasotemporal division. Control experiments, in which the tracers injections were restricted to the rostral and dorsal portions of the LGN to avoid optic tract contamination, consistently confirmed the occurrence and distribution of the "wrong" projecting cells in the temporal hemiretina. Thus, these latter cells are not grouped in a central strip, where ganglion cells would have the same chance of projecting to the same or to the opposite side, and sparsely distributed in the temporal periphery, as previously believed. Instead, the present findings indicate that the retinal ganglion cells of origin of contralateral projections are distributed more in a continuum, with a naso-temporal gradient of density, across the temporal hemiretina.