Neurological Amnesia

Forgetfulness is like taking out the trash. By doing so, you keep the house clean, but occasionally you throw away some useful information, like a receipt or a bill. Likewise, forgetting frees our brains of excessive information, but we sometimes mentally misplace people's names or where we put the remote control because our brains didn't encode the information as discussed earlier.

Now imagine what would happen if instead of picking up and throwing away trash by hand, you used a huge vacuum cleaner. It would suck up almost everything in its path, swallowing your magazines, photos, books and more. When it comes to memories, amnesia acts like that vacuum cleaner. It's an extreme form of forgetfulness that gets rid of more memories than the brain normally discards. The amount and type of memory eliminated depends on the cause of amnesia.

The kind of amnesia portrayed most often in television and movies is neurological, or organic, amnesia. Neurological amnesia is caused by damage to the areas of our brain that create memories: the cortex -- particularly in the temporal lobe -- and the hippocampus. These parts of the brain make those neural pathways that convert brief sensory memories to long-term ones, as mentioned earlier. When this kind of amnesia occurs, it's like cutting a telephone line. To the person at the end of the receiver, there is no information because there is no cord for it to pass through.

limbic system
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The limbic system within the brain. Damage to this part of your brain can cause neurological amnesia.


If there is no pathway for the information to travel across, the brain cannot form new memories or retrieve some old ones. The severity and specific location of the damage determine the extent and length of the amnesia. A brief loss of oxygen flow to the brain, for instance, may leave someone unable to remember only a few hours.

Learning from H.M.
The medical community owes much of its understanding of amnesia to a famous patient known only as H.M. To relieve his seizures, H.M. underwent surgery in 1953 that removed his amygdala and most of the hippocampus. His seizures went away, but he could not remember his recent past or make new memories. His short-term memory -- which lasts only 20 to 30 seconds -- was preserved, along with his pre-surgery level of intelligence. The research based on his amnesia was groundbreaking for our understanding of how the hippocampus processes sensory information to become long-term memories.

There are several causes of neurological amnesia, according to the Mayo Clinic, including:

Clive Wearing's extreme case of amnesia caused by herpes encephalitis is an example of neurological amnesia. The herpes encephalitis destroyed his hippocampus and parts of his temporal lobe in the brain's cortex. Think back to the telephone. The hippocampus basically acts like the cord and the synapses in the cortex are voicemail messages. Since the hippocampus, or telephone cord was destroyed, Wearing's brain couldn't consolidate new information, giving him an incredibly short memory span. The damage to his cortex was like erasing all of his oldest voicemail messages, affecting his long-term memory as well.

Next, we'll look at how traumatic events can lead to amnesia.