I had email from a college friend a few days ago who I did not remember one bit. He was memorable, just not by me. I sent him what is by now a standard explanation (and sort of apology, even though it's out of my control) for forgetting him.
I was treated successfully in 1998 for Hodgkin's Disease: six months of chemo and a few months later 20 days of radiation. That year remains a blur. "Chemo brain" is a known phenomenon that affects memory and cognition. Somehow, I managed to keep working that year, but I don't recall much of it specifically. Lynn and I went to see The Truman Show one weekend, and a few days later I told her I'd like to see the movie. Freaked her out. In my memory, it was as if I'd seen a bunch of preview scenes; I couldn't remember being in the theater. (The movie is a bit picaresque; maybe it's the movie's fault, too.)
I recovered from the memory weirdness, but over the following years I discovered I had lacunae. I have always had a phenomenal memory, nearly photographic at times when I was in my teen years and younger, and it continues to be great after chemo. I call my memory a "birth affect": it's a neurological issue that's out of my control.
What I found, rather than a holistically worse memory, was that holes had opened up. Picture driving down the road and seeing a large hole in the ground. You pull over and walk around the hole. You are sure there was a building there once, but you're not sure what it was. A donut shop? A house? A skyscraper? A swimming pool? Try as you might, you can't reconstruct the building from your memory palace, but you know that you don't know what you knew. (That is, you don't simply not remember; you remember that you once remembered. How the brain handles that, I have no idea, but I suspect déjà vu arises from some part of that.)
Or another metaphor. You're reading a book, and you find that some parts of some pages have been blanked out. The story still makes a semblance of sense, and you're aware that the story has pieces missing, but you can't reconstruct that narrative. That's what the past is like for me now. Some things, I'm sure I have entirely forgotten.
But when this friend I haven't seen in over 20 years emailed, I recognized his name, but couldn't construct any story around him from memory. I can remember quite a bit from college, yet when I stop to think of specific incidents, I find that there are definitely gaps. (I do remember a wonderful story I've told before. My beloved German teacher was trying to define schadenfreude for us years before it became as widely used in English as it is today. "You see your enemy walking across the street wearing a bowler hat. The hat blows off and a passing car crushes it. What you feel at that moment is Schadenfreude.")
In many cases, if someone can provide details around the missing gap, some of it closes up. Apparently, this is like gapping the spark plugs, and my neurons reconstruct missing engrams. It's bizarre, and I assume some of it, I'm making up, but it's also associated with particular information I can confirm. (Does this sound like a file-based hard disk recovery using templates? Yes. Yes, it does.)
In the case of my college friend, I told him this, he gave me a few more details, and, click, I remembered him, and some details of our interaction. If I don't remember you, I do. I've just lost the index.