Parallel amnesia is amnesia in which forgetting occurs for things that happened after the event that caused the amnesia. The disorder differs from retrograde amnesia in that it is not forgetting what happened before the amnesic event. To a large extent, because the exact mechanism of memory is not well studied and scientists can only point out which part of the brain is at work, prograde amnesia is still a very mysterious disease. The disease is caused by two main causes: either due to certain drug-induced (benzodiazepines such as midazolam, flunitrazepam, temazepam, triazolam and nimetazepam have strong amnesic effects) or due to traumatic brain injury to the hippocampus or peripheral cortex of the brain. Amnesia can also occur in patients who have had parts of their brain removed to prevent more severe disease, or who have memory circuits such as the middle temporal lobe (MTL) memory system. Often, patients with epilepsy caused by MTL disorders have unilateral or bilateral brain tissue (one piece of tissue from each hemisphere) removed. In addition, patients with brain tumors who undergo surgery often injure these tissues. Damage to parts of the system, including the hippocampus and peripheral cortex, can cause memory loss. Heavy alcohol abuse is also a causative factor in cis-amnesia. This is often due to a blood alcohol concentration of more than 0.25% and is accompanied by a hangover.