Abstract

An experiment is described in which measurements were made of the difference between apparent size on the right and left sides of the fixation point. This measurement is called a half-meridional difference (HMD). A point of light was fixated, and two side points were adjusted by the subject to give apparent equality between the left and right distances. Monocular determinations of the HMD were made with the left and right eyes of three subjects over periods of from four to six weeks.

Changes in the HMD as a function of time occurred in all three subjects. In two subjects the changes were quite marked and progressive. This means that changes occurred in the relation between apparent size on the right and left sides of the fixation point. These changes were too large and too well coordinated in the left and right eyes to be attributed to changes in ocular dioptric factors. Thus they must be attributed to changes in the subjective directional values which result from stimulation of particular retinal points (retinal local signs). These changes in retinal directional values have therefore been demonstrated with maximally simple test stimuli, which cannot be considered illusory, and in the absence of any type of distorting optical device. It has also been shown that when the changes in HMD are measured for a time in only one eye, subsequent measurements show that concomitant changes were occurring in the other eye. This makes it very likely that the physiological process, which is responsible for the changes in retinal directional values, is located at least as far back in the visual system as the primary visual cortex.

© 1955 Optical Society of America

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