When I was around 7 or 8 years old my uncle got into an accident. He was riding his motorcycle when a car, that made a wrongful turn, hit him. He got a few scratches and a cut on his pulse that was not too serious but he also hit his head pretty hard causing a concussion. He was brought to the hospital and the doctors said he was fine. The only bad thing was that my uncle could not remember the accident at all. His memory (persistence of learning over time, through active encoding, storage, and retrieval of information), which was perfect before the accident, was not working at all.
My uncle could remember very vividly moments before the car hit him but he could not remember the exact moment he actually got hit. He could not recall or retrive the information that happened to him. What happened was that his brain did not encode the event of the accident. It happened, but as soon as he hit his head, that memory got wiped away because he did not encode it. Therefore, he could not remember what he did not encode. The term encoding failure is used to describe this trauma.
A few months later he could remember some things from the accident, but could never fully explain those things or name them. This is called a retrieval failure. For example, my uncle wanted to talk about something from the accident but he could not describe what he wanted to say as if he almost knew what it was but he could explain what it was. He just could not say the specific word or phrase to describe what happened. This failure happens when people can say things about the word or phrase/sentence but cannot retrieve it to fully remember it.
His memory was slightly damaged for a while and his declarative memory, in which it stores things that happened to a person and events, was damaged as well. Therefore, his hippocampus was also damaged because it is a neural center in the limbic system that processes explicit (declarative) memories.
Hippocampus:
It might also have been some sort of retrograde amnesia. One that only happened during that one event on his life because of the concussion he suffered instants after the car hit his motorcycle. Retrograde amnesia is a loss of memory-access to events that occurred or information that was learned, before an injury. The severity will depend on the extent of damage and, for my uncle it was not as severe as it could have been since he only can’t remember that one event.
My uncle actually went through something really similar. He was riding a motorcycle down in South Carolina without a helmet. He got in an accident and his a truck, which literally launched him off the bike. While that’s scary in itself, of course, the scariest part was that he couldn’t remember anything that happened. He also couldn’t recognize his wife, daughters or son. While this was initially terrifying, especially with the looming possibility that he’d never remember, thankfully he did recover. After a few days, he was more able to recognize familiar faces. While he still doesn’t remember the actual accident itself, he’s made what seems like a complete recovery in all other aspects. It’s scary, but it perfectly illustrates human memory and brain function.