Friday, October 26, 2007

Do You Mostly See Double? (revisited)

Wednesday, my senior seminar surprised me. We were talking about depth perception and how your eyes converge slightly when you focus on something nearby, when one student casually remarked that when he focused on a nearby object a more distant object in front of him (a student sitting on the opposite side of the seminar table, as it happened) appeared double.

I find it easy to get a double image by holding one finger about four inches before my nose and focusing in the distance, but I've always found it more difficult to get doubling by the converse operation of focusing on something close and attending to an object in the distance -- though many early introspective psychologists claimed that the phenomenon of doubling in the distance is common or even pervasive (e.g., Reid, Purkinje, J. Mueller, Helmholtz, Stout, Sanford, Titchener). Helmholtz, for example, writes:

When a person's attention is directed for the first time to the double images in binocular vision, he is usually greatly astonished to think that he had never noticed them before, especially when he reflects that the only objects he has ever seen single were those few that happened at the moment to be about as far from his eyes as the point of fixation. The great majority of objects, comprising all those that were farther or nearer than this point, were all seen double (1910/1962, III.7; see also this post).

This has always seemed to me introspective psychology gone awry. In a 2006 essay (Do Things Look Flat?), I conjectured that the attribution of pervasive doubling in visual experience had something to do with the popularity of stereoscopes in the late 19th century and the analogy between binocular vision and stereoscopy; but recently, especially in light of the view's relatively early roots, I've been more inclined to think it has to do with overemphasis of the theory of the horopter in binocular vision.

With this in mind, I asked the eight students in my seminar to converge their eyes upon their fingers before the nose and report on whether the student across the table seemed to them to double. I went to the board to write down poll numbers, yes or no. No need to write the numbers down, though -- all the students immediately said yes! (Well, one was quiet, but when I specifically asked him, he agreed with the others.) Evidently, I was the only one who didn't find the effect.

One student then said that he has always been very aware of the persistent doubling of things in vision. He or another student then recommended that we look at the Julesz random dot stereogram on p. 112 of Dennett's (1991) Consciousness Explained (which we were reading). Several students claimed that they could "fuse" it and see the square pop out by allowing their vision to double (though I should say not all the alleged pop-outers initially agreed on the shape that popped out). Here's the stereogram:

Now to me the prospect of trying to merge those two images in my mind to get a three-dimensional pop-out effect seems utterly hopeless!

I'm sitting in my office trying to get that doubling in the distance. I put my finger before my nose and compare its position to that of a V8 bottle six feet away. I close one eye, then the other, and notice how my finger seems to change position relative to the bottle. This gives me a sense of how far apart, maybe, to expect to see the doubled bottles when I converge my eyes upon my finger. Then I do converge my eyes. Maybe that bottle doubles -- but I'm not sure. I try again, and now it seems clear that there is no doubling.

But I've always had an unusually dominant left eye (I had "lazy eye" as a kid), so maybe I'm the one who's unusual? Do most of us always see most things double (per Helmholtz et al.)? Or does it take an unusual effort? My confidence that the Helmholtz quote is a bit of madness with which few ordinary observers would agree has been shaken.


Justin said...

I definitely get that effect. It took me a minute because I was naively using a background object that was too big to notice the effect, but once I switched to things like bottles or lamps, it worked just fine.

Anonymous said...

Hi Eric,

I'm trying looking at my finger now few inches in front of my nose, in completely dark room, the only light coming from the monitor, in this comment posting page.

The verification word of the comment form appears on the both sides of the finger which I'm looking at, in a very salient way. I didn't have to concentrate or anything. I guess because of the salience of the verification word.

Eric, maybe you can try this kind of situation?

Eric Schwitzgebel said...

Thanks for the comments Justin and Tanasije! It might be worth mentioning that Reid highlights the effect by having the object in the distance be a candle flame -- both small (Justin) and salient (Tanasije). I have in fact tried it with a candle; and, Tanasije, I tried exactly what you suggested just now -- and frustratingly, I didn't seem to get it.

Not that I doubt you. I think I'm now sufficiently convinced that when people are *thinking* about doubling many of them can or simply do get doubling. But it's a separate -- and to my mind much more radical -- claim to say that objects off the horopter are always seen double!

For example, if they are, when you close your dominant eye, your finger should not move relative to the background, should it? Rather, the better way to put it would be that one of the doubled set of background figures disappears. (And this, by the way, is exactly what one student in my class said!)

Anonymous said...

Maybe you have superpower or something not to be subject to illusions :)

Anonymous said...

or perhaps the opposite superpower, of being incapable of seeing past persistent illusions. we can call you 'Common Sense Man'.

dan haybron said...

Hey Eric,

I get the doubling as well, and I still have a lazy (I prefer to call it independent-minded) eye. I kinda like the idea that we see mostly double.

Is it unusual to see different colors in each eye? I see slightly different shades in each, which leaves me wondering which is the "true" color. Could this be a low-grade real-life inverted earth? I don't remember the case very well tho...

nathan said...

Here is another data set for you. When I have taught Sensation and Perception in the past, during discussions of binocular vision when I have given the definition of the horopter and Panum's area, I've asked students the same question. (I should also admit, though, that since this is standard for S&P texts, and since the text I've used repeats the claim that vision outside of Panum's area is always doubled despite our lack of awareness of that fact, I've also made that claim as part of the discussion. I can't remember whether I typically make the claim before or after the demonstration, but given the way I enjoy surprising students with illusions, etc. I think probably I have made the claim only after the demonstration.) I cannot remember a single student who has reported not getting the effect.

Of course it is possible that students who don't get the effect don't bother to speak up (and it’s possible that there have been dissenting students that I am now forgetting). However in other discussions of other illusions and effects, frequently there are students that can't get the effect and who don't hesitate to say so. So I think it is significant that I can't remember any students not saying they get the doubling effect when they attend to things outside of Panum's area.

Eric Schwitzgebel said...

Thanks for all the comments, folks!

Dan: That's very interesting about the different colors with the two eyes! Worth thinking about / exploring more. Someone must have written on this....

Nathan: Wow! I'm in quite a state now. I still can't help but feel strong resistance to the idea that most of what we see most of the time is doubled! I've been walking around, thinking about the issue off and on, and it just doesn't seem to me that most things are doubled. It *could* be that I'm just unusual in this respect, but I'm reluctant to believe that.

What's the text you use? I don't find Palmer's _Vision Science_ completely unequivocal on the point. Palmer says "we seldom experience double vision" but also seems to imply that everything outside Panum's area will be doubled (both p. 209).

Eric Schwitzgebel said...

On second read, I think Palmer is with me on this one:

"For points outside of Panum's area, the disparity is normally experienced as depth. You *can* experience double images if you attend to disparity as "doubleness," however, or if the amount of disparity is great enough, as when you cross your eyes by focusing on your nose" (p. 209).

Alejandro said...

I have noticed this since I was a child (and used to worry if I was special in this too!) When I have my gaze fixed on any relatively close object (call it O) then objects further away, in the background, are doubled if I pay attention to them. Of course, normally I pay attention only to O, raising the typical question of whether I "really" "see" the background objects double when I don't notice it. Here's my attempt at describing the phenomenology:

The effect is very strong if O is very close (e.g. a finger lifted a few inches from my eyes) and gradually diminishes as O is farther away (also if it is bulkier than a finger). For O being thin like a finger up to a distance of about 1 meter the effect is strong enough that I am willing to say I really see the background double, even if I don't pay attention to it. When O is bulkier and at 2 meters or more I am more inclined to say I do not see background objects double unless I am trying specifically to pay attention to the effect.

I think this intuition has to do with the following difference: in first case (finger or other thin, close object) I can't be conscious of the background objects without seeing them as double. (Unless, of course, I focus the gaze on them, in which case it is the finger that is double.) In the second case (larger and farther objects) I can be conscious of what there is on the background (in the sense that I can tell you broadly what objects and colors there are, though obviously I can't give many details without taking the focus off O) without seeing the double effect, though I can also "generate" the double effect by paying attention to it.

Eric Schwitzgebel said...

Thanks, Alejandro, for your interesting and careful description!

Anonymous said...

Dear Eric Schwitzgebel:
(PS: this is what i mailed to you, if you have not received pls do me a favor to reply to my email add:, i would be appreciate your efforts and the opportnity that we talked about this very issue thanks)

Thanks God, today I happened to get to your blog concerning Doubled Images made by our two eyes.
and i was very interested to see that because i have been watching it for 2 years and therefore unfortunately got some problems with it , the vision.
i tried to email to you to this add to check if you could get it, so would you pls do me a favor to make sure
to reply to confirm to me of the availability of this add? thanks
sorry for forgetting my introductin, I am a Chinese unviersity student MA of economics and i paid attention to the phenomenon 2 years ago and then , unfortunately get some problems with it. and i am happy if you could go with me for research and encurement of this problem?
To give a brief introduction, generally our eyes could of course see the converged object with everything else doubled-whether you noticed it or not, to form 3-D image which was then transferred to our brain for understanding. therefore we wil of course see the farther or nearer doubled if you pay much attention to it.because of the distance of the eyes and the same image processing ceter in our brain behind our eyes- you could easily get it if you draw a picture showing that .
but the problem for me is : i could see my nose apart, a little bit far from each other,. not becasue of the coverge capability of two eyes, but naturally, there are there, the left image of the nose caused by the right eye while the right image caused by the left eye, with "two image of noses aparting from each other , 7-8CM like that"
yes if you could see my email and if you have friend of expert concerning that field, would you pls do me a favor to help me with that ? thanks
i have suffered from that for a long time. i appreciate very much your work if you could help me thanks.

Anonymous said...

It has to do with eye dominance. Over 95% of the population have one eye clearly dominant over the other.
The remaining are "ambiocular", meaning neither eye is dominant, thus the finger test will show 2 fingers, and they will be unable to perform the "eye dominance" tests out there.
There are many pros to being ambiocular, with few cons, mainly in targeting sports when trying to aim with both eyes open, though most ambiocular people can then aim equally well with either eye closed.

Larry said...

I do not have a dominant eye. As a hunter I can selectively choose to see cross hairs only within my scope with my right eye and overlay it onto the distant image seen by my left eye. In an instant, and with both eyes open, I can transfer my vision from my left eye to my right eye which is already looking through the scope. I can find and follow a target through a scope incredibly easy and fast. The background within the scope will be blackened as I choose to focus my left eye but will come alive as the transition takes place. After which the left eye will appear darkened. While experimenting I notice that I can transition back and forth on command. My wife says I'm weird. I am curious if this is common place for others who are Ambi-ocular?

FreshBrew said...

This is almost 9 years late, but I cannot see whatever " pops" out of the merged background squares either, Eric. Being right eye dominant ( also had lazy eye as a kid)I am unable to see 3D when tested during my annual Optometrist exam. Something to do with our optic nerves don't work simultaneously like people who are not one eye dominant. ( Please don't quote me there.I may be remembering my Eye MD s lay term explanation wrong). I thought this concept may be the functional reason some of us cannot see whatever object appears in the background distant squares.