Visual plasticity as revealed in the two-dimensional modulation transfer function of a meridional amblyope

The mammalian visual system is known to have considerable maturational plasticity, since the characteristics of early visual experience can have lasting effects on the organization and functioning of the visual cortex. One example which reveals this property is meridional amblyopia, an abnormal dependence of visual contrast sensitivity on orientation; it is demonstrably neural in origin and is presumed to reflect the redistribution of orientation-selective mechanisms in the visual cortex in response to anisotropic visual input during development. An unusual case of this abnormality was studied with two-dimensional (2D) Fourier techniques in a meridional amblyope having no history of significant astigmatism since the age of five. Modulation transfer function and point-spread surfaces were computed for comparison with a normal subject in both the 2D spatial frequency domain and the 2D space domain, based on contrast sensitivity measurements spanning the Fourier plane by radial and angular cross-sections as well as a 2D Cartesian sampling lattice. Contrary to the generalization that meridional amblyopia is only found in continuing astigmats, this case suggests the potency of transient infant astigmatism to induce permanent neural anisotropy.