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 and books. 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.
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.
There are several causes of neurological amnesia, according to the Mayo Clinic, including:
- Head injury
- Stroke (when blood flow to the brain stops)
- Viral infection (such as herpes encephalitis)
- Brief loss of blood or oxygen flow to the brain
- Electroconvulsive therapy (ECT -- used to treat severe depression)
- Alcohol abuse causing a thiamine deficiency
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.