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Anterograde amnesia is a form of amnesia, or memory loss, in which new events are not transferred to long-term memory. The disorder makes its sufferers unable to recall an event which occurred only moments earlier when their attention has shifted to something else.
Those who have theoretically pure anterograde amnesia are still able to access memories formed before its onset, but they exist in a transient world where anything beyond their immediate attention span disappears from their consciousness permanently. However, theoretically pure anterograde amnesia rarely surfaces: in reality, long-term cases nearly always occur with some degree of retrograde amnesia.
Anterograde amnesia is often informally called "short-term memory loss", conjuring the idea, as in the movie Memento, that the problem lies with the short-term memory. For this reason, formal (correct technical or scientific) usage demands the term anterograde amnesia, since the condition is a deficit not in short-term memory but in long-term encoding.
Anterograde amnesiacs suffer differing degrees of impairment to different types of memories. Patients can often learn and remember how to do a new physical skill (e.g., playing the guitar, learning the words to a new song and then singing them, etc.) but not remember when they learned it. Such "how-to", motor skill learning (procedural memory) and its attendant behavioural conditioning and priming are collectively known as non-declarative memory, which appears to be unaffected by anterograde amnesia.
However, the condition tends to impair both episodic memory (the memory of events) and semantic memory (the memory of facts and general knowledge). For the most part, patients are unable to make new semantic or episodic memories. Yet the research at this time conflicts enough that consensus on this point has not been reached: some patients appear able to create new semantic memories, and young children with anterograde amnesia seem to have semantic learning capabilities similar to non-amnesiacs.
Anterograde amnesia can result from damage to the hippocampus, fornix, or mammillary bodies, thus lending credence to the theory that these structures are primarily responsible for laying down long-term memories. However, the condition can also arise from damage to the basal forebrain (which produces acetylcholine) or a set of brain structures called the diencephalon.
"Traveler's amnesia" is a temporary form of anterograde amnesia in which victims may, for instance, realise they have changed planes during a memory gap or discover that they rented a car. This condition is caused by some medications, notably imidazopyridines and benzodiazepines, especially when they are used as sleep aids. Although medical researchers characterize this side effect as "less common",[1], the benzodiazepine triazolam (Halcion) apparently has the greatest chance of inducing traveler's amnesia, whether taken exactly as directed, varying the dosage (say, when coming off the drug too quickly), drinking alcohol, or not getting enough sleep.[1] However, benzodiazepines alprazolam (Xanax) and nitrazepam (Mogadon) are also more likely to be at fault, the former on its own and the latter when the victim is sleep-deprived or in some way changing the dose.[1]
Criminals may unfortunately use medications with anterograde amnesic effects for date rape. Unbeknownst to the victim, the perpetrator uses drugs such as flunitrazepam, temazepam, and other common substances, usually in a drink, to cause disorientation; incapacitation; unconsciousness; distortions in vision, time, sense, and identity; and an uninhibited state, the hallmark of which is anterograde amnesia. [2]
Amnesia automatism is usually induced by prescription drugs, often in association with moderate alcohol intake. Victims have memory gaps for a period shortly after taking the drug concerned, which causes embarrassment and fear for what might have happened. Disinhibited and uncharacteristic behaviour (sometimes together with carrying out quite complex tasks - e.g. cooking and serving a nice meal, but in the nude) is sometimes witnessed during such episodes, which adds further embarrassment and distress. Automatisms involve doing something "automatically" and not remembering afterwards how one did it or even that one did it.
The most famous case of anterograde amnesia is that of HM or Henry M. His brain lesions accidentally started the inquiry into the neurobiology of learning and memory.
Another notable patient is Clive Wearing, who was featured in the documentary The Man with the 7 Second Memory. Wearing fell ill with a variety of herpes simplex virus. The virus attacked his brain, doing greatest damage to the hippocampus, which is crucial for handling memory.
Oliver Sacks writes on two men with anterograde amnesia in two chapters of his book The Man Who Mistook His Wife for a Hat. Both suffer from Korsakoff's Syndrome, which causes an anterograde amnesia that is irreversible. "The Lost Mariner" chronicles the life of a patient who, since has forgotten everything that has happened since World War II, lives in complete certainty that it is 1945. Oblivious to his condition, he also believes he is decades younger. In "A Matter of Identity", Sacks profiles the other man, also unaware he suffers from amnesia. Rather than having a consistent false belief about his situation, he deals with his amnesia by constantly re-evaluating and re-explaining his situation. For instance, he greets whoever is with him in the room over and over again, each time with a different name.