A scotoma is an area of partial alteration in one's field of vision consisting of a partially diminished or entirely degenerated visual acuity which is surrounded by a field of normal - or relatively well-preserved - vision. Every normal mammalian eye has a scotoma in its field of vision, usually termed its blind spot. This is a location with no photoreceptor cells, where the retinal ganglion cell axons that comprise the optic nerve exit the retina. This location is called the optic disc. When both eyes are open, visual signals that are absent in the blind spot of one eye are provided from the opposite visual cortex for the other eye, even when the other eye is closed...