Memory disorders
Memory can be divided into:
- Implicit (Non-declarative) which are unconscious memory systems responsible for motor tasks like hitting a golf ball or playing a piece of music.
- Explicit (Declarative), which is consciously learned memory.
Explicit memory can be divided into:
Working memory
This is the amount of memory that can be held on line (such as reading a phone number and then dialling it). Information can only be held for a few seconds. It requires attention, and lapses of attention, for example forgetting why you opened the fridge door are common normal everyday occurrences. Conditions that affect attention, for example depression, anxiety, head injury and advancing age can produce deterioration in working memory.
Episodic memory (Autobiographical memory)
This is the event-based memory unique to each individual. Difficulties with acquisition of new event based memories, such as the inability to recall details of a television programme are associated with the amnestic syndrome. The amnestic syndrome is caused by pathology affecting the “limbic system” which is related to the temporal lobe. Bilateral pathology is usually required to cause a full-blown amnestic syndrome but unilateral pathology can result in selective deficits, which may be identified on neuro-psychiatric testing.
Retrograde memory (memory established prior to onset of the pathology) is usually better preserved than antero-grade memory (memory established after onset of the pathology). Retrograde memory for distant events is better preserved than that for recent events.
People with amnestic syndrome are unable to recall recent life events. They can usually repeat a name and address perfectly but forget after a few minutes.
Some people with amnestic syndrome, for example, Wernicke-Korsakoff syndrome, due to thiamine deficiency in alcohol abuse, or those with damage to the forebrain, have such poor memory, that they are unable to repeat something after a few seconds and have to confabulate, or invent new information to fill in the gaps in their memory. Roughly one quarter of people with Wernicke-Korsakoff syndrome recover fully with thiamine treatment and half do not improve.
In transient global amnesia, there is a sudden onset of severe amnesia that lasts for several hours before resolution. Afterwards the person is unable to recall events during the period of amnesia. Transient global amnesia usually occurs as a single episode with no repeat. Repeated episodes of short duration amnesia may be associated with epileptic seizures.
The causes of amnestic syndrome include:
- Transient - transient global amnesia, transient epileptic amnesia, post ECT or drugs/alcohol, post head injury (may be permanent)
- Permanent - dementia, encephalitis, stroke, brain surgery, head injury
Semantic memory
This is the brain’s knowledge store of, for example, objects, word meanings and facts such as Paris is the capital of France. Pathology affecting the temporal lobe (left lobe for words), caused by, for example, semantic dementia or post-encephalitis, is responsible for deficits in semantic memory. Loss of memory for words is the usual presentation, especially words that are used less often. People with Alzheimer’s disease have semantic memory loss but it is usually overshadowed by episodic memory loss.
Amended April 2008
