The overrepresentation of the fovea and adjacent retina in the striate cortex and dorsal lateral geniculate nucleus of the macaque monkey.

The central part of the retina, which includes the fovea, is substantially overrepresented in the topographic map of the retina in the striate cortex. We tested whether this simply reflects the uneven distribution of ganglion cells in the retina in accordance with the "principle" of peripheral scaling, or whether there is additional expansion of the fovea and adjacent retina in the retinocortical projection. Wheatgerm agglutinated horseradish peroxidase was injected into the striate cortex of three rhesus macaque monkeys so as to surround the representation of the fovea at a mean eccentricity of 8.6 degrees, and the retinae were processed histochemically to stain the retrogradely and transneuronally labelled ganglion cells which projected topographically to the injection sites. This enabled regions of the striate cortex to be related precisely to corresponding regions of the dorsal lateral geniculate nucleus and retina. Mathematical models of the distribution of ganglion cells in the retina, clipped, three-dimensional computer reconstructions of the striate cortex and lateral geniculate nucleus, and counts of neurons in the latter, were used to calculate the proportion of neurons allocated to the marked perifoveal region at each stage of projection. This was used to calculate the relative allocation of neurons to the representation of the fovea and surrounding retina among the different stages of the visual pathway. The values obtained showed that the cortical representation of the perifovea was expanded two to three times more than could be accounted for on the basis of ganglion cell topography in the retina, and that the expansion occurred both between the retina and the thalamus, and between the thalamus and the cortex. These results are inconsistent with the idea that peripheral scaling is a general principle of sensory representation in the cortex. They could also explain why many visual thresholds, including hyperacuities, cannot be accounted for by peripheral factors such as ganglion cell density.