A psychophysical investigation of ocular expansion in human eyes.

PURPOSE To investigate spatial anisotropies in the peripheral visual field of axially myopic eyes, in an attempt to distinguish between models of isotropic and anisotropic ocular stretching. METHODS Stimuli consisted of two high-contrast Gaussian patches presented in one of four orientations (90 degrees, 180 degrees, 45 degrees, or 135 degrees). For each orientation, perceived separation was established relative to that for all other orientations. The experiment was conducted with central fixation and at 15 degrees in the nasal and inferior visual fields. Eleven myopes and nine emmetropes participated in the study. Biometric data were collected from all subjects. RESULTS For foveal fixation, the magnitude of the spatial anisotropy (approximately 5%) was consistent with the well-documented horizontal-vertical illusion (HVI), and unrelated to axial length. In the nasal visual field, much larger misperceptions were found (approximately 19%), the magnitude of which increased significantly with increasing axial length. Inferiorly, a reversal of the traditional HVI is found in most subjects (approximately 7%), with a tendency for a larger reversed illusion with increasing axial length. Differences between nasal and inferior misperceptions were significantly correlated with axial length. CONCLUSIONS Isotropic stretching, such as globe expansion, should preserve the aspect ratios of receptive fields, predicting a separation misperception which is independent of axial length. In contrast, the magnitude of the misperception is significantly correlated with axial length, supporting anisotropic stretching models of myopic growth.