Imagine this. You were riding on your bike. It was quiet and peaceful, with a light breeze blowing through your hair… BUT THEN BAM! You tumble to the ground, hitting your head hard on a rock. A couple hours later you realize, hmm, “I don’t remember what I had for breakfast today.”
You are most likely experiencing retrograde amnesia as a result of the beginning of damage. Retrograde amnesia is an interesting idea that is frequently coupled with anterograde amnesia. To summarize retrograde amnesia, it is simply the inability to recall memories prior to the start of amnesia or the injury/illness. Anterograde amnesia is defined as the inability to form new long-term memories.
Let’s begin with anterograde amnesia first! Anterograde amnesia can be caused by a wide variety of causes – it can be caused by age-related brain diseases (like Alzheimer’s disease) or brain injury (like the fall in the first paragraph). It can also be caused by stroke, epilepsy, seizures, brain aneurysms, brain tumors, and more. Anterograde amnesia truly has a wide range of causes.
Before we move into the neuroscience of anterograde amnesia, we should cover some of the symptoms of anterograde amnesia – after all, application is important! Anterograde amnesia can manifest in a myriad of ways, it can manifest as forgetting conversations, forgetting the names and faces of other people, confusion or disorientation about current events, headaches, issues with speaking, writing, reading, etc. While this is a more generalized list of symptoms, the symptoms may change depending on what is the cause.
Before we cover the actual neuroscience of anterograde amnesia, it’s important to cover how memory works. Memory can be stored in many different ways, but two important ways are explicit and implicit memory. Explicit memory can be stored as semantic memory and episodic memory. Semantic memory is just remembering a fact, but episodic memory is more so remembering the experience. Implicit memories are things like habits or basic life skills that the individual uses every day. Examples would be riding a bike, tying shoes, or swimming. Anterograde amnesia is associated with being unable to make long-term memories, not being unable to make memories at all! In some cases, memories are made, it is just that it is overwritten by whatever memory comes next. This process is called retroactive interference. Anterograde amnesia is caused by damage to the hippocampus as that is the memory center of the brain.
Next, let’s cover retrograde amnesia. Retrograde amnesia can be caused by traumatic brain injury, a thiamine deficiency, encephalitis, Alzheimer’s disease, stroke, seizures, and cardiac arrest. Mainly, retrograde amnesia affects the emotion and memory centers of the brain, the thalamus and hippocampus. Some symptoms of retrograde amnesia are not remembering previous events, forgetting information from before the onset of amnesia, remembering previously learned skills, and keeping older memories.
One important fact to note is that retrograde and anterograde amnesia rarely occur on their own – they mostly occur together! Research has shown that anterograde and retrograde amnesia are positively correlated – as one increases, so does the other. Another interesting fact that research has found is that anterograde amnesia needs to reach a severity threshold before retrograde amnesia begins to manifest. It is easier to disrupt learning ability than to disrupt already learned memories.
The distinction between retrograde and anterograde amnesia is critical in both clinical and scientific settings. Since they both occur after traumatic brain injury, not being able to remember key aspects of an individual’s life after these events could be emotionally distressing to the individual. It is important to remember that, while these two conditions are not chronic (most of the time), they can still have significant consequences.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3837701/#:~:text=Anterograde%20amnesia%20(AA)%20refers%20to,before%20the%20onset%20of%20amnesia.
https://www.brain-injury-law-center.com/blog/traumatic-brain-injury-and-anterograde-amnesia/#:~:text=Damage%20to%20the%20hippocampus%20seems%20to%20be%20most%20responsible%20for%20anterograde%20amnesia.&text=Traumatic%20brain%20injury%20is%20a,or%20more%20is%20often%20fatal.
https://www.osmosis.org/answers/hippocampus#:~:text=The%20hippocampus%20is%20involved%20in,to%20the%20objects%20around%20them.
https://www.healthline.com/health/amnesia/anterograde-amnesia#causes
https://www.healthline.com/health/retrograde-amnesia#types-and-symptoms
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