I read a short story, sometime in the early 80s, I think. The plot is about a guy who's had a motorcycle accident and is suffering from anterograde amnesia, so he cannot form new memories, though he does remember his life before the accident.
The catch is, it's been like twelve years since, and in between, some powerful aliens have invaded the Earth. They don't want to destroy us. When they came, they showed their true power by retroactively erasing things (like countries, or animal species) from history (and all history books, films, pictures, etc), but not from the human minds. So people remember that such and such animal used to exist, or such and such country, though there's no trace of them anywhere. The aliens usually do not meddle with human affairs, but they show curiosity every now and then, and of course the world governments are extremely interested in knowing (and trying to understand) what appeals to them. And our protagonist is one of the things the aliens feel curious about, so the government has him in an apartment, paying all his expenses and taking care of him.
The story is narrated through a document that he finds each morning, that gradually introduces him to the events so far. First he awakens in a closed room, no mirrors or windows, but there's clothes and the document itself. When he reaches certain point (and he already knows that he's grown older), he's allowed to another room with a mirror, then another with windows (so he can see the changes to the city), etc, so each step is small enough that he doesn't feel too overwhelmed. The document is in fact written by himself, so if he feels something hasn't worked and should be done differently, he can just change it for tomorrow.
That's all I remember. I read it in some anthology, and I'm pretty sure is not by some obscure author, but a well-known one. Gene Wolf comes to mind, but perhaps because of the similarity to "Soldier of the Mist". I've long suspected John Crowley, too, but I haven't been able to find any story by him with that premise.
Any ideas? (Don't bother with film references, etc, I know anterograde amnesia has been used several times, like in "Memento". I'm really looking for the story. Thanks.)