Friday, August 13, 2010

The Spelunker Illusion

Ten years ago, when I visited the Luray Caverns in Virginia, the tour guide took us down to the deepest cave and turned off the lights. He told us to wave our hands in front of our faces and asked if we could see our hands waving. We could, faintly -- or so we thought. He then asked us to wave our hands in front of our friends' faces. Our friends' hands we couldn't see at all. When we thought we could see our own hands we were fooling ourselves, he said. Not a single photon penetrated that darkness; no light actually came from our hands into our eyes.

Call this the spelunker illusion. The existence of this illusion appears to be common lore among avid cave explorers. I have also confirmed it in more pedestrian lightless environments. Yet no psychologist discusses it in any of literature I've reviewed in writing my forthcoming chapter on visual experience in darkness. But surely someone must have written a treatment? If you know of any discussions, I'd appreciate the reference!

I see three possible explanations of the spelunker illusion:

(1.) The brain's motor output creates hints of visual experience in accord with that output.

(2.) Since you know you are moving your hand, your visual system interprets low-level sensory noise in conformity with your knowledge (much as you might see a meaningful shape in a random splash of line segments).

(3.) There is no visual experience of motion, but the spelunker mistakenly thinks there is such experience because she expects there to be.

There might be other explanations too. I can see how we might start to empirically tease apart the three explanations above. For example, (1) and (2) seem to come apart if you have your friend move your passive hand rather than actively moving your hand yourself. And (3) can come apart from (1) and (2) if we can quash the expectation of experience.

Is this a mere curious triviality? Maybe. But the three explanations above do bear somewhat differently on different theories of sensory experience and our knowledge of it. Like other illusions, this illusion promises to reveal something about the hidden operation of the visual system, if it can be properly understood.

10 comments:

beachjade said...

Hmmm. Seems like this could be related to the phenomenon of "blindsight" wherein persons who are physically blind still show remnants of vision. They will report for example that they do not see an object that is presented to them yet later they can identify that object by pointing to it in an array. (At least I think this is how it goes.) They don't think they can identify the object in the array either. They say they are guessing but...their guessing is above chance. So, could the same mechanisms that permit blindsight also be involved in visual experience in lightless environments? I don't know but it might give you some hints.

dejan said...

This is not quite what you want, but it might be helpful - or not.

http://www.bsos.umd.edu/psyc/mvsl/papers_posters/Hogendoorn_Neuropsychologia_2009.pdf

Dejan

Ken Marable said...
This comment has been removed by the author.
Ken Marable said...

This is really interesting and I can picture several more very simple experiments to test this out in addition to the ones you mention (might have to see about our visual lab has sometime this year to try this).

For example, you could try moving a noise-emitting object that the subject sees previously to determine if the auditory noise causes a visual sensation. A variation of that is if indeed it does, show a noise-emitting object moving in light, and then in the darkness only move the noise but not the object.

Also, if there does appear to be definite visual sensations under certain conditions, a far more elaborate experiment might be to involve fMRI and monitor V1 activity. That is, if someone can find subjects not only comfortable with the confines of a MRI unit, but a MRI unit in total darkness.

Oh, and to beachjade, the "blindsight" is due to damage to the primary visual cortex, but not to the eyes or alternate nerve pathways. So they still need light to "see", it is just that they do not have any conscious visual experience. The neural activations follow a different path that appears to be unconscious (but does still indirectly feed into other areas so they can detect objects and avoid them without every having a conscious visual experience).

Eric Schwitzgebel said...

Beachjade: That's an interesting thought. My guess is that it will be different, though. My understanding is that blindsight responses still depend upon input into the eyes, but running through different pathways than the main ones leading into primary visual cortex (which is damaged), pathways less associated with consciousness.

Dejan: Interesting link! I'll have to read that paper more carefully when I'm back in the office next week.

Ken: I really like the idea of trying it with a previously-seen noise-emitting object. That would help pull (1) and (2) apart even more effectively than the passive-hand variation I suggested. If you decide to tinker around with these ideas in your lab, I'd be very interested to hear what you find! And yes, in an fMRI would be great, too. Yuval Nir et al. (2006? 2008?) asked subjects about their visual experience in the dark while in an fMRI scanner, but the questions were so poorly phrased in my view that the results are uninterpretable. (See my discussion in Chapter 8.) It would be nice to try that sort of thing again, but with better introspective prompts.

Norman Nason said...

Eric, perhaps what is happening is a form of synesthesia, in which sensations of one's arm and hand movements are "leaking" over into the visual cortex.

Eric Schwitzgebel said...

Yes, Norm, that might be a way of thinking about it -- though of course it still leaves open the question of the mechanism of that synaesthesia, including at least possibilities 1 and 2 in the post. If we do think of it as synaesthesia, that implies that synaesthesia is not as rare as often assumed.

Robert said...

Cool post. I didn't know that this illusion had a name. I used to move my hand in front of my face in the dark before falling asleep at night as a child. I was sure that I could "see" my hand moving (even with my eyes closed) -- or at least just directional movement. I still experiment from time to time at night to see if the illusion persists. It does. Explanations (1) and (2) both seem pretty plausible.

A perhaps related illusion that I experienced as a child -- and can still experience -- was a faint tactile tingling between my eyes when I moved my finger toward the bridge of my nose with my eyes closed. Anyone else ever have this experience?

Anonymous said...

Not exactly blindsight, although the sensation is probably not dissimilar judging from various descriptions (Zeki, S., ffytche, D.H. (1998). The Riddoch syndrome: Insights into the neurobiology of conscious vision. Brain 121(1):25–45.). I'd guess the illusion is a consequence of the motor efference copy that's been hypothesized to be important for predicting the consequences of movement (Wolpert, D.M., Miall, R.C. (1996). Forward models for physiological motor control. Neural Networks 9(8):1265–1279.) (Schwoebel, J., Coslett, H.B. (2005). Evidence for multiple, distinct representations of the human body. Journal of Cognitive Neuroscience 17(4):543–553.) (Frith, C.D., Blakemore, S.J., Wolpert, D.M. (2000). Abnormalities in the awareness and control of action. Philosophical Transactions of the Royal Society of London. B. Biological Sciences 355(1404):1771–1788.). Given the tendency of higher brain centers' info to feedback to lower centers and alter primary perception that way (Hansen, T., Olkkonen, M., Walter, S., Gegenfurtner, K.R. (2006). Memory modulates color appearance. Nature Neuroscience 9(11):1367–1368.), it wouldn't surprise me if a motor efference copy signal combined with body schema info leaked back into the visual centers. Cool stuff.

Eric Schwitzgebel said...

Thanks for all the cool refs, Anon! Yes, I'm inclined to think some combination of motor efference copy and/or other contributions to the knowledge of one's own limb movements, plus interpretation of ambiguous noise. But it would be interesting to see something rigorous done that tests the various possible explanations.