A condition where the supply of oxygen to the brain is reduced. It has numerous causes, ranging from choking or smoke inhalation to shock or trauma. In mild cases, symptoms include difficulty in learning tasks and reduced short term memory. If it continues it can cause sensory distortions and hallucinations. This has been pointed to as a cause of some near-death experiences. However, it cannot explain the similarity of some of these experiences among people of all ages and cultures. Complete lack of oxygen going to the brain, known as cerebral anoxia, will cause irreversible brain damage within four to six minutes.