Wednesday, August 23, 2006

Nasal Phosphenes and the Extent of the Visual Field

Of course you know this: If you press one corner of your eye, a bit of a spot will appear at the opposite end of your visual field, due to mechanical stimulation of the retina. (Or didn't you know that?) I find the effect clearest when I close one eye and press near the midline on the outside edge of the other eye. A phosphene -- seemingly a dark spot encircled by a bright ring -- appears right near the nasal edge of the visual field.

Now here's a challenge: See how far into the periphery you can get that nasal-side phosphene. Keep the other eye closed, half close the lid of the eye you're pressing, and rotate the pressure a bit toward the center of the eye. Can you see the phosphene go deeper in? At some point I find the phosphene disappears, but only well into the dark field -- not in the shade created by my nose but in what would, if the phosphene were located in space, be the interior region of my face. Who'd've thought we'd have retinal receptors that could refer stimuli there?!

Now if I open the other eye, I see that the phosphene is actually (barely) within the visual field created by both my eyes together. Well, that makes sense! Otherwise, there'd be some explaining to do about how the experienced visual field for phosphenes can extend beyond that for outward objects!

My question is this. When I close one eye, normally, does my visual field contract? My first impulse is to say yes. But now I wonder whether instead, my visual field might actually contain some of that dark region I've recently been exploring with the phosphene, the region inside my face -- a region I usually totally ignore, of course! If I turn my one open eye as far inward as it can go, I see my nose, and blackness farther in. But blackness, of course, isn't the end of the visual field. It's part of the visual field -- a part experienced as black. To see the contrast, rotate your eye as far outward as it can go. There's no blackness at the edge (or maybe just a little in the bottom half?) -- the visual field just stops.

So, when I stare straight ahead with one eye closed, does my visual field stop somewhere near my nose, like it stops at the outer edge, or does it include inward blackness, stretching over the entirety of the visual space that would normally be filled in by peripheral input from the other eye?

9 comments:

pete Mandik said...

Eric, when I first read the title, I though nasal phosphenes were going to turn out to be densziens of my olfactory, not visual, field.

On a more serious note, does the answer to your final question have to be a choice of one of those two disjuncts? I ask because I'm inclined to think that phenomenology is just neutral with respect to that and related sorts of questions, that visual phenomenology is oft populated with things that have no determinate shape or color.

Eric Schwitzgebel said...

Hm, yes, Pete, I suppose it's a misleading title in that way! I wonder if I can get olfactory stimulation by pressing my nose appropriately? -- or inappropriately, perhaps I should say! Actually it does seem that my olfactory experience changes a bit when I press on my nose -- though perhaps that's just from changes in air flow?

On your second point: The last question does sound a bit like an exhaustive disjunct. Did I mean it that way? You're right, of course, that that there are various intermediate possibilities.

Peter said...

It seems to me that closing an eye, or both eyes, simply fills the visual field (in part or whole) with black. After all if you hold a black sheet of paper in front of your eyes that part of the visual field doesn’t vanish, and if you bring the paper closer and closer the visual field becomes more and more dominated by black, but it doesn’t vanish. Eventually the paper touches the eye, which is equivalent to closing the eye. It doesn’t seem like there could be a point in this process where the visual field cuts out, so I conclude that the visual field must still exist and be filled with black. However, after the visual field has been black for some time we stop paying attention to it (or the unchanging black part of it), and thus, as I understand consciousness, it ceases to be conscious. I don’t see this as a problem, that the visual field, or part of the visual field, is at times only unconsciously sensed, since I don’t buy into the idea that all sensory inputs must be conscious, but I know that some would dispute it.

Eric Schwitzgebel said...

Thanks, Peter! I find an interesting diversity of opinion on what the visual field is like with the eyes closed (see here). Some say it's black, as you do; others disagree. Could it be that different people experience it differently? Or are some people misdescribing their experience?

Anibal said...

I´ve had the same very first impression when a read the title. Exploring the nuances of an olfactory phenomenology (what is like to smell?) with its corresponding metaphysics (what is a scent qua scent), language vocabulary for gradations, valence and intensity of odorants and how they impact cognition and behaviour (in such a strong manners still not much appreciated by current standards in philosophy always favouring other traditional senses) i think is a worthy philosophical task (or at least as legitimate and noble as others) to pursue.

Eric Schwitzgebel said...

Well, now that we have some interesting comments about the title, I'd better not change it, right? Apologies for being confusing!

I totally agree with you, Anibal. Philosophers (and psychologists) don't spend enough time thinking about the "lesser" senses -- olfaction, taste, and the various (interestingly various!) sorts of proprioception, tactile, and somatic sensations!

I've written a bit about human echolocation; maybe I should work up a little something on that for this blog!

Bargaining Sheep said...

Eric,

I don't get the phosphene phenomeonology, so I can't comment on the immediate question. But I think there's an interesting generalization of it: what are the boundaries of the visual field, when both eyes are wide open?

Here's an experiment: keeping your eyes fixed looking forward, imagine to yourself as vividly as possible a tiger behind you. Now ask yourself whether your visual phenomenology included a tiger behind you, outside what would normally be considered the visual field. (I'm assuming here that one effect of a certain kind of imagination can be to introduce a degree of visual phenomenology -- I'm not absolutely convinced that's right, but I think most people would accept it for "eyes closed" imaginings, and if it's accepted for "eyes closed" imaginings, I think it ought also to be accepted for "eyes open" imaginings.)

Here are some possible answers to the question posed by the thought experiment:
(1) One's visual field under the imagining includes a tiger 180 degrees removed from objects directly in front of one, showing that the visual field extends (or can be extended?) all the way around one.

(2) In the imagining, one imaginatively reorients the focus of one's gaze, so that the phenomenology includes a tiger representation which in fact appears "in front" of one, in the midst of the normal visual field, but is represented as appearing behind one. (Perhaps there is a sense of a "turning of the inner eye" in doing the imagining. One could try to imagine tigers all around one to avoid this, though.)

(3) There is a tiger phenomena in an "inner space" which is not spatially related to the external visual field, a secondary visual field which is made more readily apparent when the eyes are closed. (Perhaps it's like two concentric hemispheres?)

(4) There is no visual phenomenology at all in the tiger imagining, just a judgement that a tiger is behind one. Or perhaps there are tiny scraps of scattered phenomena associated with the judgement, and we are just very bad in introspection in recognizing what phenomenology there is and in drawing the intentionality/phenomenology line. (Maybe this is implausible in response to how things are for those why try the experiment? I find that I have at best very weak visual phenomenology in imagining under even ideal circumstances, so perhaps that makes this last more tempting than it ought to be.)

Given these four options (I'm sure I'm missing others), I think I gravitate toward #1. I have some temptation to think that visual phenomenology as a regular matter includes phenomenology of objects outside the visual field (as, as it were, "potentially viewable"). But then, I'm tempted to think that visual phenomenology includes lots of stuff that we wouldn't normally think of as phenomenological, such as modal facts about the possibility of motion. When I reflect on these kinds of cases, I find that they lead me to start wondering whether there's really such a thing as phenomenology at all, although I find difficult pinning down the inferential route from cases to skepticism.

kboughan said...

So we're saying there is no one-to-one correspondence of lines of sight to any part of the eye?

Does a person who is blind from birth have something that can be called a visual field nonetheless? Is there any way to determine that, and its "color"? What if the defect preventing apprehension of light is not in her eye or optic nerves, but farther "up" the organic visual apparatus?

Eric Schwitzgebel said...

Thanks for the thoughtful comment, bargaining sheep! You do a nice job of problematizing the notion of a boundary to the field of visual phenomenology. I normally think of visual imagery phenomenology as different in kind from visual sensory phenomenology -- but there's a long history of debate on this, going back to Hume and beyond.

I'm not unsympathetic to the idea that senses of expectation, completion, possibility, etc., are part of or are intimately entangled with visual phenomenology (cf. Husserl, Noe); but how exactly it's best to talk and think about such matters....

Kboughan, my hunch about people blind from birth is that they have no visual phenomenology at all -- not even of blackness -- like the lack of visual phenomenology (setting aside sheep's issues) behind our heads -- but how exactly to know for sure baffles me.

I'd be interested to hear if you know of any medieval treatments of this....