Monday, October 30, 2006

Can You See the Insides of Your Eyelids?

Visitors to The Splintered Mind in August and September will know I have this weird fascination with the question of what we see with our eyes closed. Admittedly, maybe the issue is not quite as important as the nature and pursuit of happiness, which I wrote about Friday.

Setting aside issues about afterimages, "light chaos", visual imagery, etc., here's one possibility: we see the insides of our eyelids. What do you think?

If I close one eye and hold one hand about a foot before the other eye, it's clear that I see my hand, right? Now I bring it slowly closer to the open eye until it eventually covers it completely, blocking out all light (though my eye remains open). Is there a point at which I go from seeing the hand to not seeing the hand? Or, as I sit here hand over eye am I see seeing the hand, though no light whatsoever is reflecting off it or coming into my eye?

It seems to me slightly more natural to say that I see "nothing" than to say that I still see my hand. Maybe, then, we can say that when the hand stops reflecting light into my eye I stop seeing it? But reflecting light into the eye is a strange criterion for seeing, since it implies that I could never see anything that was absolutely black. And we don't want to say that: A good enough coat of black paint doesn't make things invisible -- just very black!

Maybe we can say that I see things as long as they would reflect light shining on them, into my eye, if they were not completely light-absorbent? No, that doesn't work either: Translucent things are visible, so reflecting light into my eye can't be a condition of seeing. And, indeed, my hand is partly translucent, as can be seen if I shine a flashlight through it, while sitting in the dark.

So say I do sit in the dark with a hand completely over an open eye and shine a light through that hand into the eye. Now am I seeing the hand? -- the redness of its blood, say? Or am I just seeing the light? Or both? And if I do see the hand in this case, do I also see it in the case when there is no detectable light coming through? Maybe we should say this, at least: I can see that something (mostly) opaque is covering my eye, even if I can't see the object itself?

All the same questions arise, of course, in the more normal case where one's eyelids are doing the occluding rather than one's hand.

What a magnificent tangle!


Anonymous said...

Hi Eric,
I vote for "nothing" too as answer of what we see when we put the hand over the eyes.

In connection to your last example, might be interesting to consider the case when one is wearing sunglasses of particular color.
While when they are on the table we can see the colored glass, when we put them on it doesn't seem right to say that we are seeing the glass.
Analogously in the case when we shine light through the hand, it doesn't seem right to say that "I see that something mostly opaque is covering our eye" , but rather that "I notice that something mostly opaque is covering my eye".

Anonymous said...

This is a very interesting blog. I have some ideas on how to approach the answer to some of your questions on this post. And of course more questions.

If I look (now there's an interesting expression) at this idea of vision from a purely physiological perspective the cones and rods in the eyes are going to fire when hit by light. So putting your hand over your eye blocks any light from activating those cells. And when they aren't activated I'm ?assuming? they send "nothing" to the visual cortex which your mind interprets as seeing "nothing" since there is no light at all in the scene. I'm sure some psychologist somewhere has a PET scanner coordinated with a light source to test something like this.

When shining the light through the hand the cells in the eye would pick up the frequencies of light that are not absorbed in tissues in the hand. I actually tried shining a light through my hand and maybe I don't have a powerful enough flashlight, but one thing I did observe that when shining through the outstretched, bound together finger there was the translucent red quality but if I put the flashlight on the palm of the hand away from the fingers nothing got through. Which makes me wonder if the light just somehow makes its way between the cracks of the tightly sealed fingers anyway, but give the impression in the mind that is that is is going through. Or maybe the fingers are just a lot less dense than the palm, though the thickness is pretty much the same.

On the question of seeing black when you look at a black car. When you see pure black maybe you see "nothing" just like with the hand covering the eye. So the light is absorbed completely by the black object and thus does not reflect any light in that part of your visual field and thus those cells don't fire and the visual cortex knows to register that as the color black because the no cell firing occurs in context of a larger visual field where other colors are present? So the mind could label black vs. nothing dependent on the context of the entire picture possibly. If "nothing" appears in the scene with other colors then "nothing" = black?

I performed an experiment in which I covered part of my eye with my hand (making sure it was covered good so no light got in the other side), closed the other eyelid, and then compared the "nothingness" of the part of the open eye covered by the hand to the black of say this computer's lcd frame. You can move your move your head from side to side to place the black object in the environment over the black nothingness space of the hand and compare. The "quality" when comparing one half of the visual field with the "black" to the part that is "nothing" show they are very similar though the black monitor frame has a little more to it, no doubt due to the light coming from the lcd crystals creating the three dimension view around the edges. If you have some black object that you can move into a dimly lit room though then the "nothing" (created by the hand) and black are pretty much the same the same.

You say and ask "here's one possibility: we see the insides of our eyelids. What do you think?"

From the above I could possibly say that the insides of my eyelids are black and seeing never stops even when we say we see "nothing". It is the fact that none of the cones or rods are being triggered by light that the mind labels "not seeing". But maybe when seeing black in the everyday working world, like the text of this computer screen, we could say our eyes don't see anything there. The object is not giving us any light and our mind infers its presence from the surroundings. But going down that road moves us squarely into the figure-ground ideas of Gestalt psychology.

You also asked if there is a point in which you go from seeing the hand to not seeing it. I would say that would be the point where the visual cortex is no longer receiving signals from the eye because no light is hitting it. But as they say in communications theory, you cannot, not communicate. Meaning that even if you say nothing it still sends meaning to the person you ignore or forget (think wife not getting anniversary card here).

So maybe the same applies to the eyes talking to the visual cortex and rest of the brain? Maybe saying that we are "not seeing" is incorrect period? Maybe we always "see"? Just as we always communicate.

What "sees" period of course is the bigger question :).

kboughan said...

Perhaps there is a certain reflexivity in sight, such that an intuition that we see is inseparable from the act of seeing X. The power of sight becomes (or tends to become) its own object.

So if X is null -- as perhaps it is with eyes closed -- the virtue or power of vision comprehends that nullity as the intellect comprehends the "null set" in set theory.

(I know of at least one complex scholastic question concerning whether it is necessary to posit a *sensus communis* ontologically distinct from *sensus exterior* [the virtue of sight, hearing, etc.] in order to account for sensing that one is sensing. In the case of eyes closed, we apprehend a null visual field; but the nullity objectified is not *nothing.*)

All of the foregoing may itself be a null object, but what the heck.

Eric Schwitzgebel said...

What a delightful group of comments -- thanks!

Tanasije: The comparison to tinted glasses (prior to accommodation to the tint) is a nice one, and I agree it does tend to tip the balance against saying we see our eyelids rather than seeing the sun (say) through our eyelids. But I'm not as sure about the hand with the flashlight shining through it. I still want to say that we see the redness of the tissue (contra Todd, I don't think it's light squeezing through the cracks, but rather diffusing through the flesh). Does that change when the hand is up against the eye?

Todd: That's a very interesting set of reflections. I especially like the idea of half-covering the eye and then comparing that with a black object! I'm also somewhat inclined to agree with your thought that not communicating is a way of communicating. One possible problem occurs to me in your comment that didn't occur to me in the original post: Presumably the insides of our eyelids are flesh-colored, not black (depending on your skin tone, this may be a large or small difference); that adds a level of strangeness to saying that we see them when our eyes are closed if the color we experience is blackness (or at least not flesh-color).

Kboughan: I should have guessed the medievals would have something weird and metaphysical to say about this! The idea of a sensus communis is intriguing. I recall the phrase from Descartes as well. Maybe you can recommend something to me on this?

One of the great pleasures and benefits of blogging is getting comments such as these that challenge and advance my thinking.

Anonymous said...

Nothing like more experiments. I tested the the flesh color theory by just shining a flashlight on my closed eyes. My guess is the reddish color is from the light bouncing off blood vessels.

I tested the finger issue further (I should have tried this the first time around) by placing a solid ruler over the crack between the pinkey and the the ring finger and pinkey still glows red. So it not the cracks, but the actual flesh of the finger it goes through.

As far as the inside of our eyelids. I don't know the color. I'm no opthamologist or biologist. My assumption is that the color of the insides doesn't matter when the eyelids do such a great job of sealing out most light. A blue object in a room devoid of light lacks "blueness". A flesh colored eyelid closed shut under normal light circumstances lacks color because the tissues of the eyelid are specially designed to block out light.

The last point I'm really guessing on, but with a quick search of the eyelid anatomy I found this

and it talks about one of the structures of the eyelid called the tarsi:

"The tarsi (tarsal plates) (Fig. 894) are two thin, elongated plates of dense connective tissue, about 2.5 cm. in length; one is placed in each eyelid, and contributes to its form and support."

"dense connective tissue" being the key words here.

So its highly probable that these tissues absorb a significant amount of normal external light making the eyelid an effective tool to keep any and all light from triggering the cones of the eye, which fire in response to color.

Another interesting perspective when you talk about "color" is that humans are one of the rare species to have color vision. Many animals only have rods and no cones in their eyes and thus never see color and always would, if they could talk, describe the world as shades of black and white or shades of nothings and somethings intermixed.

Eric Schwitzgebel said...

Very cool reflections, Todd. Especially: Why should I assume the insides of our eyelids are flesh-colored? Thanks!

Blinn Combs said...

In the original post, I'm not sure you're distinguishing sufficiently between the mechanism in virtue of which we "see," and the physical properties of objects in virtue of which they reflect (visible) light. The black paint example is hardly relevant to whether or not we are "seeing", since we pick out the black object by virtue of its distinction from other colored objects in the field of vision (not to mention the standard case, where the black paint usually does tend to reflect some light, usually visible as the "sheen" of the paint.

For some very early speculation on this topic, though, check out Aristotle's _De Sensu_ among his small essays collected under the title _Parva Naturalia_, especially Ch. 2.

Eric Schwitzgebel said...

Thanks! I recently reread De Sensu, but not with this issue in mind.

I'm still a bit puzzled by seeing black. Of course we see black things, but I think making sense of that can get a bit puzzling if you push a bit on intermediate cases.

Anonymous said...

How about this, you cant see hairs on eyelid . What is the reason ?

Eric Schwitzgebel said...

Anon: I'm inclined to think you can see them, in a blurry way, if you close your eyes almost entirely. With your eyes wide open, maybe they're too far from the pupil to interfere.