THE REPRESENTATION OF THE RETINA ON THE OPTIC TECTUM OF THE FROG. CORRELATION BETWEEN RETINOTECTAL MAGNIFICATION FACTOR AND RETINAL GANGLION CELL COUNT

¹ Physiology Department, University of Edinburgh

The projection of the retina to the optic tectum of Rana temporaria and Bufo vulgaris was mapped by recording action potentials evoked in the tectum by a small light in the visual field. The magnification factors were then calculated: that is, the number of microns of tectum representing 1° of retina measured radially from the optic axis. The retinal region which has the greatest magnification factor forms a horizontal band above the horizontal meridian. In this area the magnification factor is nearly three times that in the upper and lower peripheral retina.

Ganglion cells were counted in horizontal and vertical serial sections of the retina of Rana temporaria. The number of cells in a 10° segment in vertical sections of the retina is greatest 10-20° above the optic axis. From this region the number of ganglion cells diminishes by a factor of about 3 with retinal eccentricity towards the upper and lower margins of the retina. In horizontal sections of the retina, the greatest density of ganglion cells is 50-60° temporal to the optic axis, and the number diminishes by a factor of less than 1 with retinal eccentricity along the horizontal meridian.

There is a close correlation between the region of greatest magnification factor and the retinal area with the greatest density of ganglion cells. The number of ganglion cells per degree of retina and the magnification factor both vary in the same way with retinal eccentricity.

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