The magnification factor (M) of the retina is the linear extent of visual striate cortex to which each degree of the retina projects. It has been suggested that magnification factor is directly proportional to visual acuity, but magnification factor measured in monkeys was compared with visual acuity in man. Here we first describe calculation of the magnification factor in man, and then compare it to human visual acuity.
We calculated M for the first 30 degrees of the lower visual field by using infor mation provided by Brindley and Lewin (1968), who plotted the distribution of phosphenes evoked by stimulation of visual cortex in a human patient with electrodes implanted on the visual cortex. Since the inter-electrode distance was specified it was possible to calculate M for each of many pairs of electrodes by measuring the angular separation and mean eccentricity of the corresponding pairs of phosphenes. For the lower visual field, M was approximately 4 mm/degree at 2 degrees eccentricity and declined monotonically to 0.5 mm/degree at 25 degrees eccentricity.
The results indicated that the reciprocal of M is directly proportional to the minimum angle of resolution and, correspondingly, that the magnification factor is directly proportional to visual acuity in man.
By extrapolating this function for the whole of the visual field it was possible to estimate the area of striate cortex. The total extent of striate cortex estimated in this way agreed closely with previous direct measurements, suggesting that the measurements of M are accurate.