The Mirror Illusion

The brain is a wonderful organ. It provides us with a mostly accurate representation of the world around us, by making approximations based on sensory input and past experience. This has been mostly sufficient for our ancestors and their environment in which our brains have evolved. There are many examples of how the brain can be fooled, ranging from optical illusions, stereoscopy, to multi-sensory illusions such as virtual hands, phantom acupuncture, and the famous McGurk ba-ga experiment. What I will discuss here is something fundamentally different, which is how your brain can fool you by how it constructs the reality you perceive; and in particular, how difficult it can be to perceive it differently, even if you know the bias.

Look inside a mirror. You will see an image of yourself with a lateral but not vertical inversion, i.e., the left/right seems swapped but not up/down. A watch on your left hand shows up on the image’s right hand, but a shoe does not show up on the image’s head.

Think about it for a minute. That makes absolutely no sense, as the mirror is a piece of reflective glass, and should not discriminate between left/right and up/down. Most people have not thought through this apparent paradox. If you have not, I encourage you to take some time to think about it, and you will find that all of the obvious explanations that come to mind are in fact, incorrect. When first presented with the problem, I found myself experimenting with a mirror, closing one eye, in various orientations, and imagining different scenarios without gravity, to no avail.

What makes this problem so difficult? Simply put, we are looking in the wrong places. We look in the wrong places because what the brain constructs (and what we perceive) feels so real, we take for granted that it is real, and automatically exclude it from closer examination.

The first, and rather difficult step, is to realize that the mirror doesn’t care about direction.  You care about direction, and it is your brain that comes up with the representation, not the mirror. To illustrate this, point to the right, and the image will point in the same direction. Point up, same thing. However, point towards the mirror, and the images points back at you, in the opposite direction. The key observation is that the mirror inverts not in the left/right or up/down, but the front/back direction.

The second step, is to realize what you are actually seeing in the mirror. Imagine a cone pointed towards a wall. As you push the cone into the wall, imagine a cone growing on the other side of the wall, growing as you push, in the opposite direction. You end up with a cone, pointing towards you, on the other side of the wall. Put a red dot on the left side of the cone and a blue dot on the right side of the cone, and do the same thing. An inverted cone emerges on the other side of the wall, with a red dot and a blue dot, on the same corresponding side. Now, imagine a human face being pushed through the wall nose first, just like the cone, with the colored dots as eyes. You end up with an image of a face on the other side, inverted. The left eye is still the left eye, just flipped inside out. Although convoluted and highly counterintuitive, it is the correct interpretation of the image in the mirror.

Still having problems visualizing it? Take a latex glove and put it on your right hand. Now, take off the glove by inverting it, so it is inside out. The inverted right-hand glove is the analog of the image in the mirror, even though it looks like a left-hand glove.

The question now becomes, why do we so instinctively see a person swapped in the left/right direction, to the point where you cannot help but see it that way? The reason, simply put, is that it requires the least work from the brain. The correct interpretation (inverting), requires an incredible amount of work, as evidenced by the effort it takes simply to imagine it. There is no existing brain circuitry to do an inversion, because there was no need to do so when the brain evolved. It is far easier for the brain to represent the image as “someone” facing you rather than an inverted meaningless image. The agent detection circuitry in your brain is where the problem is, not the mirror.

Once the brain treats it as a “person”, it needs to orient the “person” in space to make sense of it. There are two main ways to mentally turn objects around in space; around a vertical axis (turning around), or around a horizontal axis (think foosball). Technically speaking, both ways are equally valid (as are any diagonal axes). Our brain will use the existing evolved circuitry, which is to turn around the vertical axis and spin the “person” around to face you, for that is what it encounters day-to-day. Interestingly enough, if one were to mentally flip around the horizontal axis, foosball-style, one would see the image as up/down inverted but not left/right inverted, further proving that the mirror does not discriminate, and the problem arises from a hardwired preference in your brain.

This example shows that something seemingly so real and right, is no more than an erroneous representation concocted by the brain. The explanation is counterintuitive, but readily verifiable, and probably enough to change your mind. Of course, this is a trivial question with a not so trivial answer, with no vested interest or emotional investment.

It makes me wonder though. I feel quite confident and passionate about many issues, far more complicated and nuanced than a piece of reflective glass. How many of those could I be completely wrong about, simply because it “feels” right? How many wrong trees could I be barking up, blissfully ignorant of the squirrel squarely perched on my back?

I wonder.


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