A difference in apparent size between opposite halves of the same visual meridian may be called a half-meridional difference (HMD). The HMD’s have been measured in six subjects for both the horizontal and vertical meridians, using the right eye, left eye, and both eyes. All six kinds of measurements were made on each subject for periods of from two to nine weeks. Significant differences were found between the HMD’s of the left and right eyes in both meridians. Thus an ocular factor contributes to HMD’s. This factor appears to be different visual angles for corresponding retinal points. Trends were sometimes found with respect to time, so there is also an unstable factor in the visual system which contributes to HMD’s. This factor varies independently along the horizontal and vertical meridians. With a given meridian, however, the HMD’s of the left and right eyes vary concomitantly. Thus the unstable factor is common to the two eyes and is probably located in the visual cortex. Since trends were found, apparent size in a given part of the visual field sometimes varies gradually but markedly with respect to time. This means that for a given locus of retinal stimulation the subjective directional value relative to a foveal stimulus varies in a similar manner with respect to time. Thus topological transformations of subjective directional values have been found to occur.
HMD’s produced by the ocular factor appear relatively stable and usually small, whereas the HMD’s of the left and right eyes are modulated concomitantly by the unstable factor, which sometimes causes large HMD’s. These findings indicate that the relation between visual angles for a pair of corresponding retinal points is relatively stable, but the subjective directional values of both members of the pair with respect to a foveal stimulus sometimes vary markedly and concomitantly as a function of time.
KENNETH T. BROWN, "Factors Affecting Differences in Apparent Size Between Opposite Halves of a Visual Meridian," J. Opt. Soc. Am. 43, 464-471 (1953)