Tunnel vision occurs when one loses visual acuity in the peripheral visual fields while retaining visual acuity in the central regions. The vision can be considered to be constricted and concentrated in the central area, as when one is inside a tunnel looking out. Tunnel vision can be caused by any type of damage to the optic nerve, to the retina of the eye, or to areas in the brain responsible for processing of visual input. Loss of peripheral vision may be a symptom of some of the conditions that cause a generalized loss of vision. Glaucoma is a common cause of true tunnel vision.