National Geographic has a well written article about memory. It is a sprawling, enjoyable article which bounces around the subject of memory.
As illustrations, two extreme cases are discussed in detail.
There is a 41-year old woman “who remembers almost every day of her life since age 11. She remembers what happened on Murphy Brown on December 12, 1988. She remembers world events and trips to the grocery store, the weather and her emotions.” Mention a date to her and memories of that time come flooding back in extraordinary detail.
“When I’m blow-drying my hair in the morning, I’ll think of whatever day it is. And to pass the time, I’ll just run through that day in my head over the last 20-something years—like flipping through a Rolodex.”
Then there is an “85-year-old man, a retired lab technician called “EP,” who remembers only his most recent thought”. After his brain was nailed by a virus he lost the ability to make new memories. He can’t remember anything that has happened since 1960 – he doesn’t even remember that he has a memory problem.
The article describes meeting and interacting with the man. His existence is only the thoughts currently in his mind. As soon as he is distracted, what he was thinking about is lost forever. If you ask him to remember something, like a number, he can remember it as long as he is thinking about the number. The second he is distracted, he forgets the number, who you are, and even that you asked him to remember something.
But EP is still able to ‘learn’ unconscious memories. For instance over the years he has learned the route of his daily walk around the block – even though he thinks he is heading out for the first time every time he goes. He has even learned who the neighbors are and is comfortable introducing himself to them.