When an object passes in front of us and we wish to continue looking at it, or if we just wish to look from one place to another there are potentially three mechanisms available.
We can rotate our entire body keeping our eyes and head still, we can rotate just our head, or we can move just our eyes. Here we consider the latter.
We move our eyes using the extra ocular muscles. As seen in the diagram, there are three pairs of muscles which control the direction of eye movements.
In the following exercises you can more easily see the eye movements if you watch another person's eyes while they perform the task.
Conjugate eye movements are those that preserve the angular relationship between the right and left eyes. For example, when you move both eyes left and then right, a conjugate movement is made. Up and down movements and combinations of vertical and lateral movements also fall into the conjugate category.
Vergence eye movements are ones where the angle between they eyes changes. For example, hold your index finger in front of your nose. Now move your finger towards and away from your face, all the while fixating on your finger tip. As you move your finger closer to your face both eyes will converge as you move your finger away from your face your eyes will diverge.
Saccades or saccadic eye movements are very fast jumps from one eye position to another. The velocity of saccades can be as large as 1000 deg/sec. To make a saccade or a series of saccades pick two objects as some distance from each other and look first at one then at the other. Because saccades are so very fast it may be difficult to see the eye movements as discrete jumps.
Smooth pursuit movements are just as their name implies. The eyes move smoothly instead of in jumps. They are called pursuit because this type of eye movement is made when the eyes follow an object. Therefore, to make a pursuit movement, look at your forefinger, at arms length and then move your arm left and right while fixating your finger tip.
Visual scientists have a number of tools at their disposal to record eye moves. Although I do not get into the specifics of these techniques here is one illustration of eye movement data.
At various times reference has been made to fixating ones gaze. The reader will probably find it interesting that even when they fixate a target and attempt to hold their eyes as steady as possible their eyes still undergo very small tremor like movements. These are, in fact, very useful. It has been shown that when there is absolutely no relative motion of an image on the retina that image soon fades and disappears.
The vestibular system has a profound influence on eye movements. You can prove this to yourself by asking a friend to sit in a chair that can be rotated. Before you spin the chair, ask your friend to look a some object opposite them, perhaps a picture on the wall. Note that your friend's eye will be relatively steady. Now spin your friend for 30 seconds or so and when you stop the chair tell the to try and fixate the same target again. You will note that your friends eyes will not be steady, rather they will be moving back and forth. This movement is called nystagmus.
Obviously this is an extreme condition. Yet, there is a constant interaction between the vestibular system and the visual system. It is frequently referred to as the vestibulo-ocular reflex (VOR). This reflex is used to stabilize an image on the surface of the retina during head movement.
As you can well image, the topic of eye movements is much more extensive and complex than the information presented above. For those who would like an authoritative review I recommend Dr. Peter Hallett's chapter entitled Eye Movements which is Chapter 10 in Handbook of Perception and Human Performance Volume 1 Sensory Processes and Perception. Wiley-Interscience, Toronto, 1986. Another useful source are Chapters 5 & 6 by Dr. Ian Howard in his book entitled Human Visual Orientation, Wiley, Toronto, 1982.
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