it mostly happens when i think of periods in my life. each block of time has a kind of flavor, like when i think about it a flash of images roll through my psyche like places i went, people i knew, things i wore, songs i heard, but it's not just a succession of images, it's salient than just a few memories. perhaps i'm making no sense, and perhaps my wording makes it increasingly difficult to understand, but i've just always wondered.
also, sometimes before i sleep i get a brief image of a person. it's always a different person, but it's always someone i've never seen in my life, ever. they usually have some sort of notable feature, like piercing eyes or a receding hairline. i don't know what this means, but perhaps someone else experiences it as well...?