Introduction
Click here. (Upon clicking, a blue dot shows up on the screen to the left, and an immediate response is for the eyes to look at the dot.) What did you just do? Most likely, you moved your eyes to the left to see the circle that just appeared. You execute such eye movements thousands of times every day, and you almost never consciously think about them. But in reality, your visual system has to do some fairly complicated computations to cope with the changes in visual information that accompany each shift of your gaze.
More specifically, the visual system has to distinguish between motion on the retina caused by eye movements versus motion on the retina caused by moving objects. This activity is designed to help you understand why this is such an impressive feat. The text is written as if you are performing the activity – important concepts and takeaways from the activity are summarized.
Voluntary Eye Movements
(A rectangular surface filled with colorful squares is shown. There is a small white dot in the center of the rectangular surface.) Focus your gaze on the small white dot in the center of the colored grid at left. Then click on the grid and follow the dot with your eyes as it leaps from point to point across the grid. (The dot jumps to the right side of the grid, back to the center, then to the left side of the grid, then back to the center, and so on.) The discrete jumps your eyes are making are called saccades. Saccades are the most common form of voluntary eye movements and are typically used to examine different portions of stationary objects (e.g., when you are admiring your favorite painting at the local art museum).
Click on the grid again and the fixation point will shift more smoothly back and forth across the screen. (The dot moves continuously, without jumping, from left to right, back and forth.) Tracking a moving target like this involves smooth pursuit eye movements. Interestingly, it is impossible to execute smooth pursuit eye movements when viewing a static image. For example, shift your gaze to the upper right corner of your computer monitor, then move your eyes down slowly until you reach the bottom of the monitor. If you pay close attention to your eyes (or even better, if you watch someone else’s eyes as they do this), you will realize that you followed the edge of the monitor with a series of discrete saccades, whereas you followed the moving white dot with smooth eye movements. Apparently, feedback from a moving object is needed to keep the eyes moving smoothly.
Now click the grid again to stop it, then fixate on the dot again and slowly move your head towards the monitor, keeping the dot in focus the whole time. To keep your eyes on the dot as you get closer and closer to the monitor, you must execute a vergence eye movement, rotating each eye in the opposite direction (when moving towards the screen the eyes will converge inwards toward your nose; when you move your head back and keep the dot fixated, the eyes will diverge outwards toward your temples). Shifting focus from a far-away object to a closer one (or vice versa) would also require a vergence eye movement (in this case without any movement of the head).
Object Movements
Focus on the small white dot again and click on the grid. (The white dot stays in the same place but the grid jumps from left to middle to right to middle, in a loop.) Now you see the object (the grid) jumping back and forth on the screen, while your eyes remain fixed. Think about how the image moves across your retina as you keep your eyes stationary.
Now click on the grid again to switch back to moving the fixation dot while keeping the object steady. (Now the grid is stationary, but the white dot is jumping to the left, middle, right, middle, and so on.) Follow the dot with your eyes, as you did in the previous section of the activity. Do you see the problem that the visual system faces in distinguishing these two circumstances?
Top Views
Here you see a schematic drawing of the situation from above. When everything is stationary, the image of the grid is projected such that its center falls on the fovea of each eyeball. (Two eyeballs are fixated on the center of the grid, as seen from above.)
Click the image to see what happens when the grid moves from side to side while the gaze (dashed lines) stays fixed on one point. Naturally, the retinal images (solid lines) move from one side of the fovea to the other along with the grid. (As the grid moves from left to right, the image of the grid shifts on the backs of the eyes.)
Now click the image again to see what happens when the fixation dot moves while the grid remains stationary. As the eyes rotate to track the dot, the image of the grid moves back and forth on the retina in exactly the same way as it did when the object moved. If you look carefully you will note that the direction of retinal movement is exactly opposite in the two cases. When the eyes move left, the grid image moves to the left on the retina. When the grid moves left, its image moves right. So, an eye movement to the left produces the same change on the retina as an object movement to the right (and vice versa).
Distinguishing Eye from Object Movements
To distinguish between retinal motion caused by eye movements and retinal motion caused by object movements, the brain’s motor system sends a special signal to the visual system every time it executes an eye movement. The visual system then takes this information into account as shown in the following table:
|
Retinal Movement? |
Eye Movement? |
Object Movement? |
|
No |
No |
No |
|
Yes |
Yes |
No |
|
Yes |
No |
Yes |
|
No |
Yes |
Yes |
We will now examine these four possibilities in turn.
- If there is neither retinal movement nor eye movement, the object is stationary.
- If the visual system detects retinal movement but also receives a signal from the motor system telling it that the eyes are moving, the retinal movement is probably caused by the eye movement, so the object must be stationary.
- If there is retinal movement but no eye movement, the object is moving.
- Finally, if there is no retinal movement but the motor system is claiming that the eyes are moving, then we must be tracking the moving object with a smooth pursuit eye movement.
There are various ways to confuse these signals. The easiest is to close one eye, then carefully “jiggle” the other eyeball with your finger, by pressing lightly on the lower eyelid. When you do this, the visual system detects retinal movement without an eye movement signal from the motor system (technically, the motor system is telling your finger to move the eye, but the visual system is not set up to anticipate this odd behavior). As a result, you perceive every object in the visual field as moving. If you are lucky, this will be the closest you ever come to experiencing the sensation of being in an earthquake!
Involuntary Eye Movements
Although we call them voluntary, you generally do not consciously think about making saccades, pursuit, and vergence eye movements, though if you did, you could make each one of these eye movements on command. Certain portions of your brain (particularly the superior colliculus) are continuously firing away in the background to plan and execute these eye movements, but all you are typically concerned with is the visual information received once the eyes get to their final destinations.
However, even if you try to keep your eyes perfectly still, your gaze continues to jitter a little. These miniscule tremors of the eye muscles are involuntary eye movements, and you normally do not even notice them.
To become aware of your involuntary eye movements for a brief moment, do the following: Stare at the fixation dot in the center of the grid at left for 10–20 seconds, without moving your eyes. Then click on the grid (without looking away!) and stare at the white rectangle that replaces the grid. You should see an afterimage of the grid (you learned about afterimages in Chapter 5). If you try to keep your eyes fixed in one place, you will still notice that the afterimage drifts around slightly—no matter how hard you try, you will not be able to keep it completely still. The drifting is caused by involuntary eye movements!
What possible function could these eye tremors serve? Some fascinating experiments have revealed that when an image is kept perfectly stationary on the retina (counteracting the effect of these involuntary eye movements), the image actually disappears after a few seconds! It is not known exactly why this happens, but the result implies that constant retinal motion is necessary for the visual system to function properly.