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Amnesia is a form of memory loss at a greater extent than simple everyday forgetting or absent-mindedness. Lots of times amnesia is associated with damage to the hippocampus. There are two main types of amnesia, anterograde amnesia and retrograde amnesia.
anterograde amnesia
The state of not being able to form new memories is known as anterograde amnesia. This can be temporarily caused by drug and alcohol use but can also be permanent after a serious brain injury damaging the hippocampus. People who suffer from anterograde amnesia usually only lose declarative memory and their procedural memory remains in tact. This is because procedural memory does not rely on the hippocampus.
retrograde amnesia
The state of losing existing memories is known as retrograde amnesia. Usually retrograde amnesia is a result of damaging parts of the brain other than the hippocampus, such as the temporal lobe or prefrontal cortex. Typically in this condition, recent memories are hard to recall and the oldest memories are least affected. Diseases such as dementia cause gradual retrograde amnesia.