Seeing Isn’t Believing

Quick – which of these lines is longer?

If you're reading this, you're taking too long to answer the question.

If you’re reading this, you’re taking too long to answer the question.

If you said the one on the right, congratulations! You’re wrong, of course, but your brain is working perfectly well. This is an example of an optical illusion, an image specifically created to trick your brain. You probably saw a lot of these as a kid, and you may have even thought they were fun. I know I did. I remember finding optical illusions in books, learning about the trick, and then quizzing classmates about them (I was a strange kid). I got satisfaction from knowing the trick to the picture, especially when others couldn’t see the illusion.

Once I got through graduate school, though, I realized that there are a lot of times where knowing the trick just brings up more questions. For example, let’s look at those lines again:

If you're reading this, you're taking too long to answer the question.

Yup, still lines.

The right one still looks longer, right? But we absolutely know that these lines are the same length! That’s why, to me, the big question is this:

Why does the illusion still work, even after we know the trick?

What You See Is (Part Of) What You Get

Optical illusions like this one reveal some really interesting things about how your brain interprets what you see. As it turns out, light entering your eyes and stimulating your retinas is just the first step in a long process of interpretation. As that information filters through your brain circuits, different parts of your brain add their own input and interpretations onto what you see. Mostly, this additional interpretation is based on prior experience, even if it may not seem like it. An easy example is these words. You’re really just seeing black markings on a white background, but based on your previous experience with reading, your brain knows that this is supposed to be words that you can interpret as language. Contrast that with a picture of a horse made out of words, where information contained in the words is secondary to the shape they make:


A horse! A horse! My neurobiology for a horse! (With apologies to Shakespeare)

What does this mean for illusions? Essentially, your brain is making some decisions about how you see, and perhaps more importantly, how you interpret what you see. The reason why you know the difference between words and pictures is because your brain is making that call. Most of the time, this is very necessary. The eyes see what they see, and the brain has to decide what to make of that information. But how does it figure out what that visual information means?

A Wrong Turn At The Occipital Lobe

Pretty much all of this processing takes place in the occipital lobe, at the very back of your brain. Once the visual information gets there, it gets integrated with your previous experiences and intuition – a kind of “database” in your brain that helps you know, for example, that a person walking behind a tree should come out the other side, and gives you certain expectations for how long that should take. In fact, even babies have some expectations along these lines, so at some level this kind of visual information is probably encoded right in the structure of the brain.

Another way to think about vision is like this: All your eyes get is the information that the person is walking, and that there is a tree, while your brain’s database contains information about depth, the speed of walking, and the general sedentary-ness of trees. It’s only by combining what you see and what your brain has already stored that your brain can predict how this person and the tree will interact.

Generally, not like this.

Generally, not like this.

Your brain makes these assumptions for a simple reason: efficiency. It doesn’t want to waste time and resources figuring out every single situation as you experience it. By making assumptions and inferences about the world quickly, without waiting for visual confirmation, your brain is saving an incredible amount of time. If it waited for you to catch up, precious milliseconds might have passed, and sometimes those seconds can be the difference between life and death.

Agreeing To Disagree

So, your eyes relay information to your occipital lobe, in your brain, which combines that real-time information with previous knowledge to figure out the world around you. The majority of the time, these assumptions that your brain makes are just fine. Objects that are smaller are usually farther away, the horizon is usually flat, and objects in shadow usually have more muted colors than objects in direct light. These are reasonable assumptions for a brain to make, and they are assumptions that are reinforced by almost every situation you encounter. But occasionally, the information from your eyes does not agree with what your brain’s experience tells you should be going on. This is when something really strange, and kind of amazing, happens: Your brain ignores your eyes. Take this example, called the cafe wall illusion:

Honestly, looking at this for too long starts to hurt.

Honestly, looking at this for too long starts to hurt.

Looking at it, your brain convinces itself that the lines between the tiles must be slanting back and forth, or possibly curving towards the ends. But if you hold a ruler up to your computer, you’ll find that all of these lines are not only straight, but they are all parallel! That’s a neat little trick, but here’s the best part: Knowing that these lines are all parallel doesn’t stop the illusion from happening! Your occipital lobe is so convinced that this pattern of blocks can only happen if the edges are slanted that it will override the explicit instructions from your conscious brain that this is not the case.

So now we know the trick. The occipital lobe does a lot of work in vision, processing and making sense of the information that comes in from the eyes. More than that, the brain is so powerful here that it can override the direct input from the eyes – mostly because that input is only a small fraction of what you’re seeing. However, like all good answers, this one brings up another interesting question: How does this happen? More specifically, how is it that the brain can override the eyes, creating interpretations and filling in the gaps in the data without us even noticing? In some ways, that’s the best part: We don’t really know yet! Some illusions have explanations, like how humans are “primed” to see faces in things like toast and patterns in the stars, but for the most part we aren’t sure why an illusion continues to work.

Science works in these “edge cases,” where the limits of our understanding are tested. Most of the time, a lot of really interesting science comes out of studying cases like this. Even something as on the edge of seeming stupidity and superfluousness as finding bacteria in a hot spring can lead to massive changes in the way science is done (Google Thermus aquaticus if you don’t believe me). As Isaac Asimov once said, “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’ but ‘That’s funny…’” So it is with brain science. You can go outside right now and look at a person next to a tree, and it won’t tell you anything new about how brains interpret visual information. But if someone were to walk behind that tree and never come out again? That’s when curiosity and science happen.




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