Thursday, April 16, 2009

Where Does It Look Like Your Nose Is?

Following the suggestion of H. Ono et al. in their weird and fascinating 1986 article on "Facial Vision" in Psychological Research, I drew two lines on a piece of cardboard, and you might want to do the same. The lines start at one edge, about 6 cm apart, and converge to a point at the other edge. (A piece of paper held the long way will work fine, as long as you can keep it rigid.) Hold the midpoint of the 6 cm separation at the bridge of your nose and converge your eyes on the intersection point. If you do it right, it should look like there are three or four lines, two on the sides (one going toward each ear) and one or two in the center, headed right for the bridge of your nose.

The weird thing of course is that there are no lines on the cardboard that aim toward your ears or terminate at the bridge of your nose. Ono et al. suggest that the explanation (of the nose part at least) is that from the perspective of each eye the nose appears to be at the location of the other eye, so that the line headed toward your left eye seems to your right eye to be headed toward your nose and the line headed toward your right eye seems to the left eye also to be headed toward the nose.

With that in mind, I remove the cardboard and close one eye. Where does it seem that my nose is? Well, at first I'm inclined to say my perception is veridical: To the open eye, the nose seems closer than does my closed eye (or my bodily map of where my closed eye should be). But now I open and shut each eye in alternation. It does seem that my nose jumps around, maybe an inch or two side to side when I do this. But maybe that's just because my assumed egocentric position changes, relative to my nose?

Ono et al. also suggest trying to locate your phosphenes with one closed eye. (I had a post on this some time ago.) Phosphenes are those little circles you can see when you press on your eye. I find them easiest to see when I press on the corner of a closed eye and attend to the opposite corner of that same eye, looking for a dark or bright circle. (It may take some trial and error to get this right.) As I noted in the old post, for me at least the phosphenes generated by pressing the outside corner of a closed eye, with the other eye open, appear to be spatially located inside or behind the nose. This seems to me to be the case no matter which part of my closed eye I press. At the time of that post, it didn't occur to me that this might be because my nose was subjectively located as co-positional with the closed eye. Holding my nose with two fingers and pressing my closed eye with another finger from the same hand, to throw some tactile feedback into the mix, doesn't seem to change anything.

7 comments:

Arnold Trehub said...

Eric,

You wrote:

"But now I open and shut each eye in alternation. It does seem that my nose jumps around, maybe an inch or two side to side when I do this. But maybe that's just because my assumed egocentric position changes, relative to my nose?"

This has nothing to do with a change in your egocentric position. It has everything to do with the difference between a *retinal* representation and a *retinoid* representation. Much confusion is caused by the notion that the retinal image gives us a direct visual percept. Actually, our visual perceptions are constructed in a dynamic post-retinal neuronal mechanism (the retinoid) which presents us with a sense of a volumetric space from a fixed egocentric perspective. You normally can't see your nose with both eyes open because your nose is too close to fuse in stereopsis, and binocular rivalry tends to cancel out each monocular image. If you close your right eye, however, your left eye looks right to see your nose so you see it slightly to the right of you. The opposite happens when you close your left eye and look at your nose with your right eye. In both cases your egocentric location remains constant, but the perceived location of your nose with respect to your fixed self-locus in retinoid space is shifted by the normal operating characteristics of the retinoid system.

Eric Schwitzgebel said...

Why do you say that I have fixed self-locus when I open and shut my eyes alternately? I see at least some phenomenological appeal in the idea that my visual egocentric point of view shifts a little bit. Do you not?

I don't need to close one eye to see my nose. I can't say how things seem when I'm not thinking about it (refrigerator light error!) but at least when I am thinking about seeing my nose, its position seems determined by my dominant eye. I might be unusual, though, in that I have a very dominant left eye. Maybe for others there's more canceling out?

Arnold Trehub said...

Eric,
you wrote:

"Why do you say that I have fixed self-locus when I open and shut my eyes alternately? I see at least some phenomenological appeal in the idea that my visual egocentric point of view shifts a little bit. Do you not?"

Wouldn't it be more accurate to say that your monocular *fovea-centered* point of view shifts according to which eye sees your nose? After all, with one eye your nose is seen somewhat to the right of you (your ego-center), while with the other eye your nose is seen somewhat to the left of you. Doesn't this indicate that your egocentric locus is the fixed origin of your visual perspective?

Eric Schwitzgebel said...

I don't have strong views about this. Does it seem (visually) like my point of view is moving or like my nose is moving, when I open and shut my eyes alternatingly? Is there a compelling reason to prefer one interpretation over the other? Or can we take a cue from Leibniz and say there's really no difference between these options?

Arnold Trehub said...

Eric,

You wrote:
"Does it seem (visually) like my point of view is moving or like my nose is moving, when I open and shut my eyes alternatingly? Is there a compelling reason to prefer one interpretation over the other? Or can we take a cue from Leibniz and say there's really no difference between these options?"

It seems to me (phenomenally) that my nose jumps from side to side when I open and close each eye alternately. If my point of view moved instead of my brain's representation of my nose in my egocentric space, my nose would seem stable instead of seeming to move side to side.

There is a critical difference between these options if we are interested in a causal explanation of how the biological mechanisms of the brain generate our phenomenal world.

Eric Schwitzgebel said...

I guess my first inclination is to agree with you on the phenomenology, Arnold (which is why I wrote the post the way I did). I agree that if so, there may be implications down the road for how we represent space, perspective, and the body.

Arnold Trehub said...

For my own take on how we represent space and our egocentric perspective on the world -- including the location of our body parts and sensations which are an intimate part of our phenomenal world -- you might be interested in looking at these papers:

http://eprints.assc.caltech.edu/355/

http://eprints.assc.caltech.edu/468/