Introduction
We all know that our eyes rotate horizontally (from side-to-side) and vertically (up and down) as we look around. However, it may surprise you to learn that your eyes also move in a third direction; they rotate about the line of sight as well.
In this activity, we will conduct a simple demonstration that should allow you to observe your own torsional eye movements.
Demonstration, Part 1
For this demonstration you will need a mirror in a well-lit room. You will be tilting your head and body and watching the compensatory reactions your eyes make during the process.
First, stare at the iris of either your right or left eye in the mirror and tilt your head to the left and right. Keep your body as still as possible. As you tilt your head, try to keep the eye you are fixating on at the center of the rotation so that the movements of your eyeball are easier to keep track of. If you have been studying hard for this class and have slightly bloodshot eyes, keep track of the position of one of the veins in your eye as you rotate your head to the left and right.
What do you see? When you have a guess as to what is going on, read Demonstration, Part 2.
Demonstration, Part 2
At least some of you—especially those of you with lighter irises or bloodshot eyes—should be able to see your eye rotating about the line of sight. The direction of the eye rotation is opposite to that of your head tilt such that the eye rotation partially compensates for your head movements. This rotation of your eyeball is called a torsional eye movement.
Next, try to keep your head stationary relative to your shoulders and bend from your waist to the left and right. By doing this, your head does not move relative to the rest of your body but is tilting with respect to gravity. Do you still observe torsional eye movements under these circumstances?
Read Part 3 after you have tried this.
Demonstration, Part 3
When bending from your waist instead of your neck, you should still be able to observe torsional eye movements. Why?
The reason is that torsional eye movements are executed in response to signals from your vestibular system, not in response to movements from your body per se. As your vestibular system senses changes in head orientation away from vertical, it causes your eyes to roll about the x-axis (see Figure 12.3 of the textbook) to compensate. Torsional eye movements can only compensate for a few degrees of roll tilt, so you may have noticed that there was a limit to how much your eyes rotated when compensating for your body movements.