April 24, 2007

Well March Break has come and gone. Other than the pics left on your digital camera and maybe that tan, all that's probably left are the memories.

For most of you, when asked to recall what you did on your week off, these memories are likely retrieved almost instantaneously, but for a small percentage of the population, it's an impossible task.

Amnesia is the medical term for memory disorder. Amnesic patients are impaired at forming new memories, and at retrieving old memories.

One of the most famous amnesic patients is known as H.M. (his initials). H.M. suffered from epilepsy, a condition where electrical activity travels through the brain in away that is out of control, causing seizures in the patient.

To cure H.M.'s seizures,a surgeon removed the parts of his brain that were responsible for starting them. After the surgery, H.M. was cured of his epilepsy, but he had become completely amnesic.

The structure that was removed from H.M.'s brain is called the hippocampus. Hippocampus means "sea horse" in Latin, because its curved shape.People have two hippocampi, one on the left, and one on the right side of the brain.

Because of H.M., brain specialists began to understand that the hippocampus served an important role in human memory. Importantly, H.M.'s intelligence and his capacity to perceive the world were normal, and he could carry a normal conversation; he just could not remember that he had that conversation.

Did You Know?
Without the Hippocampus, you would not be able to remember your first kiss in detail.

Remembering the past...

Among other things, people with damaged hippocampus cannot remember events from their life very well: they have a deficit in autobiographical memory.

For example, some patients can remember their own prom only because they have pictures to remind them that they were there. They cannot sit back and imagine themselves going through an episode from their past; they cannot remember the temperature, what the room smelled like, how they felt and where people where sitting in relation to them, among other things.

Did You Know?
Without the hippocampus, you could not imagine what your next kiss would be like either.

...and imagining the future

A new scientific study has shown that patients with damage to the hippocampus are also very bad at imagining events that might happen in their future. A group of scientists from London asked amnesic patients to describe imaginary events in detail.

For example, patients were asked to imagine themselves at the beach, and to describe all that was going on around them.

Interestingly, these patients were not capable of describing the imaginary events very well. People with healthy brains could give a detailed description of the sound of the waves, the smell of the sea, the sensation of the sun on their skin, the hot sand underneath their feet, and all the things they imagined around them, like birds, trees, people, etc.

Amnesic patients were not capable of doing that. While they knew there was water and sand at the beach, they could not imagine themselves on that beach.

Did You Know?
The capacity to remember the past seems to be necessary in order to imagine the future. These findings suggest that the same brain areas are used to remember past events, and to imagine the future. Scientists believe that the capacity to imagine future events may be dependent on people's capacity to remember past events, so that pieces of memory from these past events may be reassembled to build new, imagined events.

If the system that allows memory of the past is damaged, then people may not be able to use past information in order to imagine the future. These new findings suggest that amnesic patients are not only cut off from their past, but they are also cut off from their future, which condemns them to live in the present.

Learn More!

Patients with hippocampal amnesia cannot imagine new experiences. PNAS, vol 104, 5, pp 1726 to 1731.

Wikipedia on HM patient

Wikipedia on the hippocampus

Softpedia on memory

Cosmos magazine on amnesia

BBC news on amnesia

Article written by Marie St. Laurent.

Photo credit: Julia Freeman-Woolpert

Article originally published April 24, 2007.

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