Anterograde amnesia

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Anterograde amnesia is a form of amnesia, or memory loss, where new events are not transferred to long-term memory. After the onset of the disorder, the sufferer will not be able to recall events which occurred only moments earlier, once their attention has shifted to something else.

Symptoms

Those who suffer from theoretically pure anterograde amnesia will still be able to recall memories laid down before the onset of anterograde amnesia, but will exist in a transient world where anything beyond their immediate attention-span disappears permanently from their consciousness. In reality, long-term cases of anterograde amnesia are nearly always accompanied by some degree of retrograde amnesia.

This form of amnesia is often referred to as "short-term-memory loss" with the implication that data is lost from the short term memory, such as in the movie Memento. However, in technical writing anterograde amnesia is used, since the condition is not a deficit in short-term memory but in long-term encoding.

Different types of memories (e.g., of new physical skills, of new words, of the events of the day, facts of history, etc.) are affected to different degrees by anterograde amnesia. Patients with anterograde memory loss often can learn and remember a new physical skill (e.g., a musician learning how to play a new tune, or singing the words to a new song that you learned) and yet not remember when he or she had learned it. Such motor skill learning (procedural memory), along with behavioural conditioning, and priming, are collectively known as non-declarative memory and appear to be unaffected from amnesia.

However, memory of facts and general knowledge (semantic memory) and memory of events (episodic memory), tend to both be impaired in anterograde amnesia. For the most part patients are unable to make new memories of either of these types. Yet there is conflicting research, and some patients have appeared to be able to create new semantic memories, learn new facts. It has also been found that young children that develop anterograde amnesia have similar semantic learning capabilities to non-amnesics.

Causes

Damage to the hippocampus, fornix, or mammillary bodies can result in anterograde amnesia, which contributes further evidence to the theory that these are the structures primarily responsible for the process of laying down long-term memories. The occurrence of anterograde amnesia can also be due to damage to the Basal Forebrain which produces acetylcholine. The final cause of anterograde amnesia is damage to a set of structures which are located deep in the brain called the diencephalon.

A temporary form of anterograde amnesia, sometimes referred to as traveler's amnesia, is also induced by some medications. Benzodiazepines and imidazopyridines all have varying degrees of anterograde amnesic effects. This is utilised in the use of benzodiazepines such as temazepam and lorazepam as premedicatants. In some cases, for instance, victims may realise they have changed planes during their memory gap or discover that they rented a car.

Large intake of alcohol (see binge drinking) can cause anterograde amnesia commonly referred to as en bloc black outs. This is likely caused by alcohol's effect on GABA receptors, which play a role in memory formation in the hippocampus. [1] Because of the other symptoms of intoxication, it is usually not evident to observers when an intoxicated person is suffering from amnesia, and they themselves will only become aware of it later if they identify a period of missing time. [2]

A more sinister, criminal abuse is in date rape (chemical submission), in which the drugs flunitrazepam, GHB, temazepam, and other common substances are used, unbeknownst to the victim, to produce an uninhibited state, the hallmark of which is anterograde amnesia.

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.

Famous cases

The most famous case of anterograde amnesia is that of HM or Henry M., the man whose lesions accidentally started the inquiry into the neurobiology of learning and memory.

Clive Wearing was featured in the documentary, The Man with the 7 Second Memory. Wearing fell ill with a herpes simplex virus which attacked his brain. The affected area was the hippocampus, which has a major role in handling memory.

Oliver Sacks characterizes two individuals with anterograde amnesia in his book The Man Who Mistook His Wife for a Hat. Both suffer from Korsakoff's Syndrome, which causes irreversible anterograde amnesia. The story of one patient is called "The Lost Mariner": he began forgetting everything from the end of WWII and felt certain that the date was constantly 1945. He does not even know that he suffers from amnesia and believes himself to be decades younger than he is. The other story is called "A Matter of Identity". Rather than having a consistent false belief about his situation, this patient dealt with his amnesia by constantly re-evaluating and re-explaining his situation. He would greet whoever was in the room over and over again, each time with a different name. Much like "The Lost Mariner", he was unaware of his condition.

Steve Wozniak suffered from anterograde amnesia for approximately five weeks in 1981 following the crash of an ultralight plane he piloted ^{[citation needed]}.