A persistent vegetative state (PVS) is a condition of patients with severe brain damage in whom coma has progressed to a state of wakefulness without detectable awareness. Patients in a persistent vegetative state are usually considered to be unconscious and unaware. They are unresponsive to external stimuli, except, possibly, pain stimuli. Unlike coma, in which the patient s eyes are closed, patients in a vegetative state often open their eyes. Their eyes might be in a relatively fixed position, or track moving objects, or move in a disconjugate (i.e. completely unsynchronised) manner. They may experience sleep-wake cycles, or be in a state of chronic wakefulness. They may exhibit some behaviors that can be construed as arising from partial consciousness, such as grinding their teeth, swallowing, smiling, shedding tears, grunting, moaning, or screaming without any apparent external stimulus. http://en.wikipedia.org/wiki/Vegetative_state