The Neural Control of Vision

D. The Lateral Geniculate Nucleus of the Thalamus

Figure 8 shows a coronal section through the lateral geniculate nucleus (LGN) of the rhesus monkey. This thalamic structure receives profuse projections from retinal ganglion cells. In central retina (up to about 17 degrees of eccentricity), this structure has 6 layers. The number of layers representing larger eccentricities reduces to 4. The individual layers are segregated according to left and right eye input and also according to cell types. Layers 6, 4 and 1 receive input from the contralateral eye, the rest from the ipsilateral eye. The upper four parvocellular layers receive input from the retinal midget ganglion cells whereas the lower two magnocellular layers that have larger cells, receive input from the retinal parasol ganglion cells. The layout in the LGN is orderly; the horizontal meridian is along the line shown, with the medial portion of the LGN representing the lower, and the lateral portion the upper visual field. Antero-posteriorly in this structure we move from peripheral to central representation of the visual field. In each LGN the contralateral visual hemifield is represented as already shown in Figure 1. The interlaminar layers of the LGN have very small cells of a different type that also receive input from the retina; at this time little is known about this rather heterogeneous group of cells. The receptive field properties of cells in the parvocellular layers are similar to those of the midget ganglion cells; the receptive field properties of the magnocellular layers similar to those of the parasol cells.