# Seeing shapes in two different ways: how and when it happens

A Necker cube is bi-stable figure, meaning that it can be perceived as two different three-dimensional objects, depending on how you look at it:

Cube A is ambiguous -- the true Necker cube. Cube B and cube C show the two ways you can perceive the Necker cube: either the bottom of the cube is in front, or the top is in front. What's interesting about figures such as the Necker cube is that once you're aware of its bi-stable nature, it's impossible to see it only one way. Don't believe me? Then take this challenge. Play the movie below (it lasts for two minutes). The figure will flash every couple of seconds, but otherwise there are no tricks involved -- the identical, ambiguous figure is portrayed the whole time.

You can choose whether you'd like to see it as bottom-in-front (like cube B) or top-in-front (like cube A). But you won't be able to maintain that perception for the entire time you watch. Just make sure you're perceiving it as a cube, and not just a two-dimensional group of lines.

Stop the movie once you see the reverse, and let us know how long you lasted in this poll:

Researchers have found even when they're trying to avoid a reversal, people switch at least once every fifteen seconds. The flashing makes it more difficult to avoid reversing, but even non-flashing objects are always reversed. People who say they never reversed are lying (with one possible exception, which I'll get to later).

JÃ¼rgen Kornmeier, Christine Maira Hein, and Michael Bach showed ten volunteers movies similar to the one I presented above, asking them to press a key every time they saw a reversal. (This turned out to be agonizingly difficult after repeated trials, so eventually the viewers were given breaks and asked to return several weeks later to complete the experiment). The viewers were sometimes told to try to keep the image stable, sometimes to just relax and let it reverse as often as it would naturally, and sometimes to try to reverse as often as possible. Here are the results:

The three strategies resulted in three different results: Viewers were able to reverse the cube significantly less often when they were consciously trying to do so compared to when they just relaxed and let it happen. And deliberately trying to reverse resulted in more reversals.

But clearly reversal isn't completely under our control: we can never stop reversing entirely, and changing to a flashing display led to significantly more reversals, whether viewers were trying to reverse the cube or not. Flashing the cube had a similar effect on every strategy the viewers used to reverse (or prevent reversals).

There may be one exception to the rule that everyone reverses Necker cubes: One study found that Buddhist monks were able to prolong the stability of an ambiguous figure after meditation. But it's unclear whether even they would be able to manage this indefinitely.

J KORNMEIER, C HEIN, M BACH (2008). Multistable perception: When bottom-up and top-down coincide Brain and Cognition DOI: 10.1016/j.bandc.2008.06.005

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Hi,
I'm not a buddhist monk and I don't really meditate, but I like optical illusions and I trained to keep it one way or another. Usually I manage to keep one view for at least 3 min. Longer is too boring for testing :-)
Took me something like 3-4 h (over several days) to get there, but the ability acquired is stable, the last 20 years.

Flashing disturbs me too much to keep a particular point of view, I scored less than 6 sec.
On the other hand, reversal is quite easy for me and goes over 60 Hz, flashing or not; needs a slight motion of the eyes.

it is simple to view it non-reversing.
simply focus on where the top front horz.
line intersects with the right rear verticle
line and it will never reverse.

I learned long ago that if I relaxed then I could fully control the appearance of these illusions. Given that the exception that you cite is an example where generalized mental training allowed control over binocular rivalry, I'd go so far as to suggest that a more specialized training to deal with this phenomenon would be even easier than meditation training since instant feedback is provided and the targeted effect is so narrow; so much easier, in fact, that I doubt it would be hard for others to train themselves as I or the above commenter, oldcola, did.

I spend quite a bit of my time pouring over satellite images of the Earth. Quite often I get a Necker cube like effect when looking at mountainous regions. Deep valleys suddenly become sharp mountain ridges or a crater becomes a flat-topped dome. Unlike the Necker cube, however, it is next to impossible to get the valleys back "into the page" even though you know they should. It takes quite a bit of effort, but once done the image stays "fixed" and it becomes difficult to see it the way it was before. The eye-relaxing technique doesn't seem to work in this insatance.

It's also possible to see it as a (positive or negative) pyramid-like structure. I once spent a while conditioning my brain to perceive it this way first, in an effort to throw any hypothetical psychological tester a curve ball.

jfrancis:

Actually the original experiment required the viewers focus on the center of the object -- there was a fixation cross there. Give that a shot and you'll find it's much more difficult to prevent reversals.

It's a little rich to say that participants who don't go along with your view of the world "are lying". Do you or the authors of this study have some heretofore unknown magical way of directly accessing participants' subjective experience?

By Dirk Beer (not verified) on 20 Nov 2008 #permalink

Dirk:

Yeah, a little. But it's a likely explanation for a discrepancy found in an online poll that's not supported by laboratory research.

But if there are participants who don't see reversals, they would be the most interesting ones, so why take the chance of dismissing them?

The distribution of reversal frequencies in your poll looks like what one might expect if there is a continuous range of "Necker stability ability (tm)", with a few rare people who perceive it as continuously stable.