Why Don’t We Get Dizzy When Moving Our Eyes?

Have you ever heard that the eye is like a camera? Generally that’s true. But look through a camera and move it from side to side. Notice the churning sensation in your gut and how it feels like you’re watching the car chase scene from The Bourne Supremacy all over again. Now move your eyes from side to side. Immediately you’ll notice a significant difference. We normally don’t loose our last meal every time we look around with our eyes.

suppression.pngThe brain is very good at minimizing sensory input so we don’t get over stimulated. (Autism is where the brain doesn’t do it’s job and allows too much stimulation, making it extremely difficult to pay attention to anything.) A camera, on the other hand, can’t filter out motion.

Experiment #1

Ok, ok, enough suspense. Let’s find out how the brain filters motion. To demonstrate this phenomenon, let’s set up an experiment, which I actually did in optometry school. An observer is seated in front of a large section of construction paper at eye level bend into a semicircle. Points A and B are just marks on the paper. Directly in front of the observer is a narrow cut-out section through which a light is seen. (Here’s the impressive part.) If the observer looks at point A then quickly looks at point B (or vice versa) the light is not seen. During this quick movement, we know at one point the eyes were briefly pointed directly at the light. Why didn’t we see the light? The phenomenon here is that actually the eye DID see the light, but the brain didn’t.

Experiment #2

Just about everyone has already done this experiment. Simply sit in a rotating chair and spin yourself really fast. Did you get dizzy? Ready for a second chance to taste that spinach salad with walnuts you had for lunch? The spinning simulated quick eye movements just like shifting our eyes from points A to B, but why did we get dizzy this time?

The Brain Categorizes the Type of Motion

To filter the visual sensation of motion, the brain considers not only the motion the retina sees but also if the eye is in motion at the time and if the motion was initiated by choice (voluntary) or forced (involuntary). Imagine (don’t try this yourself) numbing up the eye, then someone gently grabbing the white part of your eye with tweezers and having that person moving your eye side to side for you. (We actually do this in the exam room on certain patients to determine neurological dysfunctions.) You would get dizzy because your brain isn’t filtering out the motion.

Wow! That’s Cool!

Let’s put this all together now. Here’s a play-by-play of what happened in our experiment #1 that caused the brain to filter.

  1. The observer looks at point A.
  2. The observer starts to move their eye away from point A.
  3. The retina, sensing an image moving across it, sends impulses to the brain. (But, as we saw from experiment #2 and the tweezer test, this isn’t enough for the brain to filter.)
  4. Nerve impulses are sent to the brain from the eye muscles that initiated the movement. This combination of retina and muscle input tells the brain that the observer made a voluntary eye movement.
  5. The brain shuts down the visual system for a split second during the eye movement. This is called visual suppression. We’ve been using the layperson term “filtering” in this post.
  6. Once the eye reaches point B, the retina and muscles send impulses again to the brain telling it the eye movement has stopped.
  7. The brain turns the visual system on again. That explains why we don’t see the light in experiment #1.

You, being the bright person that you are, would now ask, “Why don’t we see black when the brain turns the visual system off?” This is quite a complex process, but suffice it to say, the brain has the amazing ability to cover up visual interruptions. Our world doesn’t go black each time we blink either. I may go into more detail on this in a future post.

By |December 19th, 2007|Illusions, Physiology|0 Comments

The Amazing Moon Illusion

One of my favorite illusions involves the moon. It’s common knowledge that the moon looks huge when close to the horizon, but appears smaller when higher in the sky. Many theories have attempted to explain this phenomenon. (Feel free to read about them on Wikipedia. However, the article wasn’t written in layman’s terms and can be confusing.)

a-blue-moon_small.jpgThe moon is actually the same size regardless of where it is in the sky. To prove this just take a picture of it at the horizon and at other positions and compare the size in the pictures. It is not an illusion caused by the atmosphere, as commonly believed. The illusion occurs at the brain level and deals with the brain’s inability to judge large distance differences.

To understand this you have to realize that the brain manipulates image sizes in an attempt to match up the image with the relative distance from the viewer. Here’s an example. Have someone lay down on their back on the floor. Take a picture of them by aiming the camera so that their feet are in the foreground and their head is in the background. Notice how small the person’s head is in the picture. Now assume the same position you were in to take the picture, but this time without the camera. Notice that the person’s head doesn’t look as small.

Your brain realizes that their head isn’t that far away from their feet, so it “enlarges” the head to match the perceived distance. The camera, on the other hand, doesn’t realize the head is only about 5 feet away from their feet and fails to match up the head size with the short distance. Now imagine the horizon is the “feet” and the moon is the “head”. Your brain doesn’t realize that the moon is actually 200,000 miles away. It sees it as being just beyond the horizon. Therefore, it “enlarges” the image of the moon just like it did with the person’s head.

Truly amazing!

By |November 21st, 2007|Illusions|0 Comments

This Will Be Fun

Welcome to my new blog! Return often to discover interesting stuff about your eyes. I became an optometrist for many reasons. The funnest (most fun?) reason is that the eye is one amazing organ! Yah sure, you could say the heart is also amazing. It beats over 100,000 times per day and can do so almost flawlessly for over 75 years. But it ain’t got nothing on the eyes. Trust me on this one.

Most of the posts will deal with:

  • Amazing things the eye can do
  • Optical illusions
  • Important or interesting news regarding vision
  • New technologies for vision

I will do my best to keep my postings interesting and understandable. This will be fun.

By |November 16th, 2007|Uncategorized|0 Comments