Imagine you’re watching a tennis match. Or better, imagine you’re the chair umpire. You must focus on the yellow ball, decide whether or not it is in and continuously follow its trajectory. At the end of the match, your eyes will “feel tired”. That’s because you made them work a lot while following the small yellow moving target. Which kind of movements do our eyes perform while tracking a moving object?
So, the situation is that the tennis ball moves quickly from one side of the court to the other and you want to continuously look at it. You have two possibilities: either you follow the ball by rigidly turning your whole body (or just your head) without changing the direction of your gaze, or you move just your eyes by performing really quick changes of direction. In the first case, you’d look quite weird and at the end of the match you’ll be completely sweat; in the second case, which is the solution we normally adopt in all similar daily life activities, you’d properly employ your extraocular muscles in order to optimize the control of your eye movements.
When our eyes quickly jump from a position to another, they are performing the so-called saccades. To have an idea, pick the triangle and the star on the left (in the middle, the extraocular muscles are shown!) and jump with your eyes repeatedly from one to the other, as quick as possible: you’re just performing saccadic eye movements (video at the end of this post). Their maximum angular speed is proportional to their length (the distance they have to cover) and can attain up to 1000 deg/sec. A saccade takes 200 milliseconds to initiate and then lasts from 20 to 200 ms. This makes it the fastest movement produced by the human body, even faster than blinking (300-400 ms). Thanks to specific neuronal mechanisms connected to our eye muscles, time-consuming circuits are bypassed and everything works quite automatically (you don’t actually have to think about how to follow a moving object with your eyes, you just do that). All such amazing neuronal mechanisms are so natural and, in a sense, involuntary that saccades appear even in the opposite case, that is for fixational eye movements.
You, the chair umpire, are not sure whether the ball was in or out. Thus, you take the slow motion of the match and you stop the video on the exact instant when the ball touched the ground. Now you start staring at the still yellow target, which is still. After some seconds of prolonged visual fixation, some small, jerk-like, involuntary eye movements, similar to miniature versions of saccades, will occurr. They are called microsaccades and participate to the maintenance of visibility, even if their precise role in visual perception is is still largely unresolved.
Now imagine you’re watching the dvd of your favourite tennis match and you accidentally push the button that slows the video down . Instead of the real velocity, you’re watching the match at 0.2x speed. The ball has a really slow motion and you try to follow it with your eyes. You can perform smooth movements of your pupils, something that is completely different from the saccadic jumps you were obliged to during the match! This voluntary gaze shift to closely follow a moving object is allowed by smooth pursuit eye movements, which are asymmetric: for example, most humans and primates tend to be better at horizontal than vertical smooth pursuit.
Saccades and pursuits are just two of the main types of eye movements (you can find a complete list here).