Friday, November 01, 2013

Striking Confirmation of the Spelunker Illusion

In 2010, I worked up a post on what I dubbed The Spelunker Illusion (see also the last endnote of my 2011 book). Now, hot off the press at Psychological Science, Kevin Dieter and colleagues offer empirical confirmation.

The Spelunker Illusion, well-known among cave explorers, is this: In absolute darkness, you wave your hand before your eyes. Many people report seeing the motion of the hand, despite the absolute darkness. If a friend waves her hand in front of your face, you don't see it.

I see three possible explanations:

(1.) The brain's motor output and your own proprioceptive input create hints of visual experience of hand motion.

(2.) Since you know you are moving your hand, you interpret low-level sensory noise in conformity with your knowledge that your hand is in such-and-such a place, moving in such-and-such a way, much as you might see a meaningful shape in a random splash of line segments.

(3.) There is no visual experience of motion at all, but you mistakenly think there is such experience because you expect there to be. (Yes, I think you can be radically wrong about your own stream of sensory experience.)

Dieter and colleagues had participants wave their hands in front of their faces while blindfolded. About a third reported seeing motion. (None reported seeing motion when the experimenter waved his hand before the participants.) Dieter and colleagues add two interesting twists: One is that they add a condition in which participants wave a cardboard silhouette of a hand rather than the hand itself. Under these conditions the effect remains, almost as strong as when the hand itself is waved. The other twist is that they track participants' eye movements.

Eye movements tend to be jerky, jumping around the scene. One exception to this, however, is smooth pursuit, when one stabilizes ones gaze on a moving object. This is not under voluntary control: Without an object to track, most people cannot move their eyes smoothly even if they try. In 1997, Katsumi Watanabe and Shinsuke Shimojo found that although people had trouble smoothly moving their eyes in total darkness, they could do so if they were trying to track their ("invisible") hand motion in darkness. Dieter and colleagues confirmed smooth hand-tracking in blindfolded participants and, strikingly, found that participants who reported sensations of hand motion were able to move their eyes much more smoothly than those who reported no sensations of motion.

I'm a big fan of corroborating subjective reports about consciousness with behavioral measures that are difficult to fake, so I love this eye-tracking measure. I believe that it speaks pretty clearly against hypothesis (3) above.

Dieter and colleagues embrace hypothesis (1): Participants have actual visual experience of their hands, caused by some combination of proprioceptive inputs and efferent copies of their motor outputs. However, it's not clear to me that we should exclude hypothesis (2). And (1) and (2) are, I think, different. People's experience in darkness is not merely blank or pure black, but contains a certain amount (perhaps a lot) of noise. Hypothesis (2) is that the effect arises "top down", as it were, from one's high-level knowledge of the position of one's hand. This top-down knowledge then allows you to experience that noisy buzz as containing motion -- perhaps changing the buzz itself, or perhaps not. (As long as one can find a few pieces of motion in the noise to string together, one might even fairly smoothly track that motion with one's eyes.)

Here's one way to start to pull (1) apart from (2): Have someone else move your hand in front of your face, so that your hand motion is passive. Although this won't eliminate proprioceptive knowledge of one's hand position, it should eliminate the cues from motor output. If efferent copies of motor output drive the Spelunker Illusion, then the Spelunker Illusion should disappear in this condition.

Another possibility: Familiarize participants with a swinging pendulum synchronized with a sound, then suddenly darken the room. If hypothesis (2) is correct and the sound is suggestive enough of the pendulum's exact position, perhaps participants will report still visually experiencing that motion.

Update, April 28, 2014:

Leonard Rosgole and Miguel Roig point out to me that these phenomena were reported in the psychological literature in Hofstetter 1970, Brosgole and Neylon 1973, Brosgole and Roig 1983. If you're aware of earlier sources, I'd be curious to know.


Björn said...

Waving your hand while blindfolded seems rather different than doing it in complete darkness without a blindfold, in the sense that the blindfold is a very tangible reminder that you cannot possible see anything in front of you. The results indicate that the phenomenon is present despite of this, but I wonder wether it is even more pronounced in an actual cave (or a pitch black room).

Callan S. said...

How do they wave the cardboard cut outs in front of them? Their hand is down at one end, so it's really not in the same position at all if it were waving in front of the eyes?

I wonder if this ties in with people who have that condition where they think they have died (did I hear about it last on this blog or another one?). Perhaps their self modeling has been turned off somehow - someone who had the condition triggered by medication, when the condition was wearing off, was saying their arm definately wasn't theirs, for example.

Callan S. said...

Oh, and have they tried this with people who have artificial arms?

Eric Schwitzgebel said...

Bjorn: Yes, good point! Some very early research also suggested that the illusion might vary depending on intellectual knowledge of whether there is an occluder. I was a bit surprised to find Dieter and colleagues finding only a minority reporting experience of motion, since my impression in caves is that the majority experiences it. Perhaps knowledge of the occluder explains the difference.

Eric Schwitzgebel said...

Callan: It would be interesting to try this on amputees, but if Dieter and colleagues get it with the cut-out, then it might be a given that they would get it with an artificial arm. (Might be interesting to see if they would get it with a phantom limb.)

The actual motion that the person uses with the cut-out was not very clearly described (that I can see), perhaps partly because of the tight word limits at Psych Science. But my impression is that the hand is held low and the cut out juts up to wave before the eyes. I'm not sure where I got that impression, though -- I did have a couple of email exchanges with Dieter a couple years ago, and that might partly be the basis.

Will Leonard said...

Hi Eric. I've noticed a similar phenomenon. If three people mime jumping-rope (with two swinging and one jumping), then you'll seem to see the moving rope. The hallucinatory experience in this case seems to be caused by a combination of visual perception and interpretation of the scene, which sets up the expectation to see a rope swinging around. In the spelunker's illusion it looks like proprioception plays the same kind of role, setting up an expectation to see a hand pass in front of the face. I'm not sure if anyone has studied this "jump-rope effect" but perhaps they're related.

Callan S. said...

Thanks, Eric! :)

Can't help probing the idea...have they tried someone, instead of holding a cardboard cut out, holding another persons arm and waving it in front of their eyes?

Have they tried another person waving their hand in front of the person while the person also waves a hand (perhaps in sync with each other, as best can be managed), but then stops while the other person continues? Will they 'adopt' the other arm?

Does the other person stand in front of them or from behind, so as to wave a hand somewhat more naturalistically?

Also I think there have been studies of humans having slight echo location capacities, which comes to my mind at the moment. Perhaps the vision is actually an echo location virtual imaging? Possibly the person, when not waving their arm, also didn't activate their echo location capacity (it's more for self navigation, perhaps, than for detecting other creatures - so when you stand still, no navigation, so it turns off?)

Just asking cause I'm curious and really must ask! Not insisting on any answers! :)

Jennifer said...

In response to Will's comment - there are several laws of visual perception that cause similar illusions and/or pattern completion based on form, motion, expectancy, similarity, light and shadow effects, and spatial properties. Some of these are simply perceptual and inherent in our makeup - such as the law of good continuance, and others are experiential and likely include pattern completion based on expectancy and visual memory. A number of these properties have been known to cause illusory perception, and not just completion.

I'm not sure it would be relevant to pitch black environments, however.

Eric Schwitzgebel said...

Right, Jennifer!

Will and Callan -- terrific suggestions! Maybe they or someone else will do follow-ups along these lines. I've had a couple email exchanges with Dieter. I'll point him to this comments thread.

Callan: I've done a bit of work on echolocation myself, actually (most recently in chapter 4 of my 2011 book). Indeed, we can detect the presence of silent objects by noticing their effects on ambient surrounding noise. One striking example of this is if you are in a noisy environment and hold a book in one hand and then slowly move it toward one ear. My hunch is that in the spelunker cases there isn't enough usable echoic information for people who are inexpert in echolocation to use that as their informational basis for knowing the position of the hand -- but it seems worth doing a variation to try to rule that out, e.g., with earstops.

Callan S. said...

Eric: Maybe the echo location is fine tuned to sounds made by the individual? When they stand still, they cease making sounds? Even just moving your arms makes a little sound. If they moved their arms at their side while it's both tested with someone moving their hands in front and without, running the test multiple times and asking if the person thinks someone is waving a hand in front of them, there might be some indication of detection?

I must have heard about human echolocation here - sorry to be a bad commentator and missplace that!

Eric Schwitzgebel said...

Our understanding of human echolocation is still fairly limited. One view is that it's a bit easier to echolocate when you generate the sounds yourself but ambient noise can also work. Your variations all seem worth trying!

Anonymous said...

I have the same sort of visual experience when I wave my hands outside my field of vision as when I wave them before my eyes. When I'm in some really dark place, that is. Maybe I'm alone in this, but if others turn out to have the same kind of experience, that would suggest hypothesis 1 rather than 2. Right?

Jeremy said...

Why think this is an illusion? Why not think that the subjects are actually seeing their hands? Who says we need light to see?

Eric Schwitzgebel said...

It does raise the question of what counts as an illusion. I do think this, at least: In pure darkness we can see the absence of light; so light entering the eyes is not strictly necessary for vision.

Unknown said...

Maybe Dieter and colleagues weren’t only testing cavers. Based on your experience, an experiment with cavers only as participants would be very interesting.