Walsh & Hoyt: Differentiation by Visual Field

When the photoreceptors are affected, the defect in the visual field corresponds to the retinal defect in position, shape, extent, and intensity. When the ganglion cell or nerve fiber layer is damaged, however, the field defect does not conform to the size and shape of the lesion but rather to the field represented by the ganglion cells whose fibers are damaged. Thus, a small lesion situated close to the optic disc that damages only photoreceptors usually causes a small scotoma, whereas a lesion of the same size that damages the ganglion cells and nerve fibers in the same area may result in an extensive defect in the visual field. Conversely, a small lesion in the periphery that damages only a few fibers transmitting impulses from widely separated ganglion cells may produce such a small field defect that it may be difficult or even impossible to detect.