The differential light sensitivity (DLS) in healthy, but not glaucomatous eyes, is stable and unaffected by substantial loss of the ocular perfusion pressure resulting either from increased intraocular pressure (IOP) or from moderate to severe stenosis of the internal carotid arteries. Results of pulsatile ocular blood flow measurements provide evidence that this stability of vision in healthy eyes is dependent on autoregulation of blood flow to the retinal ganglion cell axons. The onset and progression of glaucoma is shown to be associated with instability of the DLS at discrete retinal points, which develops prior to visual loss, and with an abnormal sensitivity of the DLS to increased IOP. The abnormal fluctuation of the DLS in glaucomatous eyes and the loss of DLS induced by increased IOP and decreased ocular blood flow provides evidence of a microvascular impairment to discrete areas of the optic nerve.