Spared Procedural Memory
Poor Declarative Memory vs Good Procedural Memory: A Case Study of H.M's Amnesia
One of the most striking findings in amnesia is that amnesics suffer a deficit of long-term memory while their other aspects of memory are intact. This explains why amnesic patients are usually able to carry on a fairly normal conversation, despite having virtually no recollection of any recent events.
Cohen and Squire (1980) argued that long-term memory is divided into two memory systems:
- Declarative memory, which is concerned with “knowing that”. For example, we know we had prepared lunch for the guests and that Paris is the capital of France.
- Procedural memory, in contrast which is concerned with “knowing how”; we know how to play various sports, play the piano, and so on.
If declarative memory and procedural memory belong to separate systems in different brain regions, some amnesics (or brain-damaged patients) might suffer problems with only one of the two systems. This is exactly what has been found.
There was a famous case study involving a man who is known by the initials “H.M.” in scientific literatures. He received brain surgery because he suffered from frequent epileptic seizures. After the operation, his declarative memory was very poor—he couldn’t remember most of the events and experiences he had had after the operation. However, his procedural memory was good. For example, he learned mirror drawing (tracing a figure seen only in mirror image) almost as rapidly as normal neurological individuals.
|A Case Study: The Amnesic who never grew older|
|In the 1953, a man known as “H.M.” (his actual name was Henry Molaison) sought medical help for his epileptic seizures. He had had to stop work because the seizures had become so frequent and severe, and they couldn’t be controlled with drugs. In his desperation, the surgeon removed the hippocampus from both brain hemispheres because this was the seat of his seizures. The operation did reduce his epilepsy but it also had a dramatic effect on his memory.
His personality and intellect remained the same, but his memory was severely affected. Some aspects of his memory were fairly intact: he could still talk and recall the skills he knew previously (semantic memory), he could form short-term memories, but was unable to form any new long-term declarative memories. For example, given the task of memorizing a number he could recall it 15 minutes later but, after being distracted, he had no recollection. He could read the same magazine repeatedly without realizing he had read it before.
H.M. moved house after his operation and had great difficulty learning his new route home. After 6 years he was finally able to find his way around the house. This shows he did have some memory capacity and, intellectually, he was quite ‘intact’ so had some awareness of his predicament.
For many years, he reported that the year was 1953 (when he had his operation) and he was 27 years old. As time went on he realized this could not be true and he started to guess a more appropriate answer.
Spiers et al. (2001) reviewed 147 cases of amnesia (severe problems with long-term memory). They all had poor declarative memory, but none had any problems with procedural memory. The procedural skills the amnesic patients had acquired included learning to play the piano and mirrow drawing. Thus there is an important distinction between declarative memory and procedural memory.