Wednesday, August 02, 2006

The Prevalence of Afterimages?

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?


Pete Mandik said...

Eric, the following questions strike me as importantly different and I wonder what you (and others) think of them:

1. Does sensory experience always involve afterimage effects?

2. Does sensory experience always involve conscious afterimage effects?

3. Does sensory experience always involve being conscious of afterimage effects?

For what it's worth, my answers are 1. yes, 2, no, and 3. no; but I wouldn't base my case solely on introspection of my phenomenology.

Eric Schwitzgebel said...

Thanks for the comment, Pete! I guess I've tended implicitly to assume that afterimages are necessarily conscious experiences; but likely there are adaptation effects of various sorts that impact visual processing without changing conscious experience (or without changing it in the relevant way). I could see a choosing to call certain types of such effects non-conscious afterimages, perhaps. So I think I can accept your 1-2 distinction, though not in the terminology I usually employ.

The 2-3 distinction I also accept: We often have conscious experiences we're not "conscious of" in the sense (if this is your intended sense!) of knowing about them, being aware of them, being epistemically attuned to them in the sense people often mean when they say they're "conscious of" something. But of course there's a hornets'-nest here of issues around higher-order theories of consciousness and such!

Graeme McElligott said...

Hi Eric. I was just searching for something else and came across this post of yours. I have never seen your blog before but this post intrigues me.

Actually... more to the point, I don't really understand it. I've seen some mention of "afterimages" before as though these are something remarkable and I simply don't understand why. I suppose I don't even know what you mean by afterimages - to me they would be an effect caused by a prior stimulation of my retinal cells by external stimulus, but I'm not sure that's what you are getting at. For example, a bright light will cause an afterimage when I close my eyes. However, I can close my eyes and see all sorts of visual artefacts which I assume to be caused by inner effects whereby my conscious visual experience is generated, perhaps by some degree of light on my retinal cells, or by some kind of residual firing of neurons in the visual processing system. I've never really thought about this before I guess.

Why are afterimages of such interest? And when you talk of afterimages of say pressure or temperature, are you talking about actual visual afterimages or simply residual perceptual effects? It's clear (well, sort of clear to me) that afterimages must be the same kind of thing as say the lingering sense of feeling caused by a sudden blow to my toe, or the ringing in my ears after a sudden sound (or the echoing memory of that sound that persists for several seconds).

I must really spend some time reading your blog, it looks very interesting!