Large retinal ganglion cells in the rat: their distribution and laterality of projection

The distributions of the ipsilaterally and contralaterally projecting large ganglion cells in the retina of the rat were determined, using the retrograde transport of Horseradish peroxidase (HRP) following injections into one optic tract. Labelled large retinal ganglion cells occur throughout the contralateral retina and throughout the temporal crescent of the ipsilateral retina, but there is a noticeable decrease in their density in the contralateral retina's temporal crescent. This retinal region was identified in these same retinae by injecting a retrogradely transported fluorescent tracer into the optic tract opposite that receiving the HRP. The density of large retinal ganglion cells increases in both the contralateral retina and the ipsilateral temporal crescent in the upper temporal periphery such that, together, these two populations of large cells combine to produce a peak density centred on the retinal representation of the visual field's vertical midline. This peak density of large retinal ganglion cells must therefore be further peripheral than the peak density for the total population of retinal ganglion cells, since all evidence indicates that the latter is positioned nasal to the vertical midline's representation. This was verified in one rat, in which the density distribution of the total population of retinal ganglion cells was determined and compared with the distribution of the large cell population. The results suggest that the rat possesses a specialized retinal focus of large ganglion cells for viewing the visual field directly in front of the animal.