Short-term memory, long-term memory, amnesia, dementia, Alzheimer’s – people often use these terms incorrectly. The reason is partly because memory is one of the most complex mental abilities. It involves experiencing the current moment through the five senses, holding that experience for a fraction of a second, filtering just a proportion of this through to the next stage, consciously holding this information for perhaps 90 seconds, and then either losing the experience or somehow successfully making enough of an impression in the mind that it can be evoked at a later time.
When people complain of having terrible short-term memory, they are usually referring to not being able to hold onto information for more than a couple of weeks. This difficulty is actually to do with the information not having been stored very efficiently in long-term memory, resulting in rapid forgetting. Real short-term memory is literally what you can hold onto in your consciousness. They can last for just seconds: you’re in a lift, for example, and remember that you’ve already hit the button to go to the floor you need. You won’t recall this for any longer than you need to.
The most famous memory-impaired person ever studied, Henry Molaison (referred to as “HM” in the medical literature) had a part of his brain called the hippocampi surgically removed to try and alleviate the devastating epilepsy he suffered from. Following the operation, Molaison did not create a single new long-term memory until he died 50 years later.
Despite this profound impairment, if Molaison was given a string of digits and asked to recall them in the same order, he would have remembered the same number (about seven) that you and I could, and that was because in “classical amnesia” short-term memory is fully intact.
The amnesia that Molaison suffered from is very different to the post-traumatic amnesia that can result from head trauma. This tends to be solely for a period of minutes or hours (sometimes days) following a blow to the head.
With post-traumatic amnesia, even if information is transferred successfully into long-term memory, it is still fragile and susceptible to forgetting if the brain’s normal “house-keeping” procedures are disrupted by a physically traumatic event.
Different still is a “psychogenic fugue”. This is when a person is unaware of their own identity. It’s as if their entire personal memory has been wiped clean. Despite this, they are perfectly able to create new memories.
This little-understood condition is probably caused by an emotionally-traumatic event which causes the personal “psyche” to do a “systems-shutdown”, making access to any personally-relevant information impossible. Examples of this include the “Piano Man” who was found on the Kent coast without any awareness of who he was, despite remembering how to play the piano, and the recent case of Edgar Latulip who hadn’t known who he was for 30 years but then suddenly seemed to recover his memories.
Given the current lack of understanding of the condition, a neuropsychologist or a neuropsychiatrist can have a difficult job trying to ascertain how genuine the amnesia is. This is especially the case when there is a suspicion that the person might be malingering or faking the memory loss for some sort of gain, for example financial compensation.
A famous example was that of John Darwin who had disappeared after going out canoeing on his own and had been presumed to have drowned. A number of years later, a man walked into a police station and said that he thought he might be John but he didn’t have any knowledge of who he was. While this could have been a case of psychogenic fugue, when recent photos emerged of him and his wife in Panama, it became obvious that his death had been staged for a big life insurance claim, allowing the couple to move to Panama.
Equally complex is dementia, which is a group of degenerative disorders of which Alzheimer’s is the best known. Each type of dementia has a unique impact on a person’s abilities. While the gradual erosion of both creating new memories and retrieving old ones is an aspect that people associate with dementia, this is simply because the brain abnormality will be affecting quite diverse regions. This will sometimes include areas that are involved in memory storage and retrieval.
However, the signature of each dementia will depend on where these other areas are and what those areas specialise in. For example, in frontotemporal dementia, memory is largely unaffected but the change is in personality and behaviour. But in semantic dementia, the problems are more linguistic and cause difficulty recognising family and friends.
Given the huge complexity of human memory, it is unsurprising that there is so much confusion among the public regarding different types of memory. Sometimes even the professionals get it wrong.