Nasal and temporal retinal ganglion cells projecting to the midbrain: implications for "blindsight".

We placed pellets of horseradish peroxidase in the superior colliculus of four macaque monkeys and retrogradely labelled the retinal ganglion cells of both eyes. The ratio of labelled cells in the contralateral nasal retina and the ipsilateral temporal retina was no different from the ratio found after implants in the optic nerve, which label the entire afferent pathway. Our finding therefore invalidates the proposal that prominent differences in the properties of "blindsight" in monocular nasal and temporal visual fields arise from differences in the projection from the nasal and temporal retina to the midbrain. We also measured the size of the soma and dendritic field of the labelled ganglion cells (mostly gamma cells) and compared them with those of alpha and beta cells that project to the dorsal lateral geniculate nucleus. Soma size was very close to that of beta cells at all eccentricities but was much smaller than that of alpha cells. Dendritic field size was significantly larger than that of beta cells but was smaller than that of alpha cells. The number of primary dendrites was counted for cells labelled from the midbrain and in samples of alpha and beta cells labelled from the optic nerve. At eccentricities of 3-7 mm there was a consistent and prominent difference between beta and gamma cells. The results show that at intermediate eccentricities even ganglion cells whose distal dendrites are too poorly labelled to reveal their morphological class can never the less be categorized as alpha, beta or gamma by using a combination of soma size and number of primary dendrites. This is particularly useful when attempting to classify retinal ganglion cells following microinjections into selected target nuclei of optic axons.