In autumn 2003, vacationing in Berkeley with wife and son, I was up early and late, in secret nooks of our hotel, reading E.B. Titchener's great 1600-page laboratory training manual of introspective psychology. (Security rousted me out of one corner where a woman must have thought I was a sexual predator lying in wait. Doubtless I was unshaven and uncombed.) The result was a more bleary-eyed interaction with my Bay Area in-laws, a vast string of confusions about conscious experience, and one essay that vanished into the void (as most essays do). Let me share one confusion.
Titchener seems to have assumed we have nearly constant afterimages. This follows naturally from his view that the eyes are constantly adapting to their surroundings, that they're constantly in motion, and that adaptation and de-adaptation are experienced in part as afterimages. Close your eyes and note that you do not experience perfect blackness but an array of afterimages. Now consider this question: Do you always experience afterimages like this when your eyes are closed -- for example as you lie down to sleep? The two most obvious possibilities: Yes, but you simply ignore and forget them most of the time, or no, but you can evoke them by thinking about them. For that matter, do you always experience blackness when your eyes are closed or do you sometimes (usually?) have no visual experience at all?
People will say different things. But how the heck can we figure out who's right?
Of course, your eyes needn't be closed to experience afterimages, so the question generalizes. You've been looking at your computer for a bit, I assume; now turn toward a blank wall. You can experience some (probably subtle) afterimage effects, if you attend properly. Anywhere you look, your eyes will start to adapt; then when they move somewhere else, their adaptation will start to change. Is there then a constant flux of subtle afterimages, generally lost and unattended against the much more vivid visual array? Or is visual experience relatively stable and simple? Though subtle, this question is fundamental to a full understanding of visual phenomenology. Yet it's not obvious how best to explore it.
Our visual experience is constant (or maybe only frequent). Yet retrospective reflection on it is tainted by our poor memory for what's unattended, and concurrent introspection risks inventing the very objects we seek. So it remains unknown, despite its proximity.
Titchener, by the way, claims there are are also afterimages of pressure, temperature, and movement. Do you ever experience those? Do you always experience them?