The retinal projection to the pregeniculate nucleus in normal and destriate monkeys

Taking advantage of the segregation of different classes of ganglion cell fibres in the optic tract, we investigated the ganglion cell class that projects to the pregeniculate nucleus (PGN) in the normal macaque monkey (Macaca mulatta and Macaca fascicularis) and following long-standing removal of striate cortex in one hemisphere. Confining small pellets of horseradish peroxidase and biocytin into ventral or dorsal parts of the tract unilaterally, or placing several pellets throughout the tract, we labelled the retina retrogradely and the PGN anterogradely. Classification of ganglion cells according to soma size and dendritic morphology showed that implants in the dorsal part of the tract labelled predominantly, and perhaps exclusively, P beta cells, and produced dense anterograde label in the parvocellular lateral geniculate nucleus as well in the PGN. Labelling of the PGN was sparse or absent following implants into the ventral aspect of the tract, which labelled the magnocellular geniculate nucleus anterogradely and chiefly P alpha and P gamma cells retrogradely. The finding of a projection to the PGN from P beta cells has implications for the permanent selective sparing of a subpopulation of these cells after removal of striate cortex and their contribution to wavelength processing in blindsight.