Laminar and retinotopic organization of the macaque lateral geniculate nucleus: magnocellular and parvocellular magnification functions.

The laminar morphology and electrophysiologically determined retinotopic organization of a single rhesus macaque lateral geniculate nucleus (LGN) were reconstructed on series of coronal, sagittal, and horizontal cuts through a three-dimensional computer representation of the nucleus. Neurons were counted in this same nucleus, allowing the magnification functions (cells/degree2 as functions of eccentricity) of magnocellular and parvocellular layers to be determined after eliminating the effects of nonuniform volume shrinkage. Parvocellular magnification was approximately 10,000 times higher in the foveola than in the far periphery. On average, magnocellular neurons made up 2.6% of the LGN in the central 2 degrees (but probably a smaller fraction in the central fovea). The magnocellular portion increased steadily with eccentricity to 27% in the far periphery. Thus the magno/parvo ratio increases from foveola to far periphery by a factor of at least 14. The parvocellular magnification function matches estimates of cortical magnification, whereas the density of magnocellular afferents to cortex increases monotonically with eccentricity. At the posterior pole of the nucleus, the numbers of layers are reduced through a fusion of two layers and the disappearance of one or two others, a feature that may be associated with the foveal ipsilateral hemifield representation.