“Keep your eye on the ball,” is good advice for a 5-year-old.
It is a rough, approximate instruction. When analyzed with a 5-year-old’s level of understanding, it accurately describes the role the eye is playing during a tennis stroke.
In reality, though, it’s impossible to physically “keep your eye on the ball” while playing tennis. Simply put, the ball’s motion is too fast. While you certainly should be mentally focusing on the ball throughout its entire flight, physically, your eyes will be performing a task that’s a little more complex.
The fovea is the small part of your eye that sees the best. It’s only 1.5mm across, and it possesses the highest density of cones of any part of your retina. Even though the fovea is so small, half of your brain’s visual cortex is dedicated to processing information from the fovea, because it’s the part of your eye that’s capable of observing fine details. Actually, it’s the only part of your eye capable of seeing in 20/20 vision.
Only 1-2 degrees of your visual field is captured by the fovea. This is the vision you perceive as your area of focus. As you read this sentence, for example, your foveal vision is jumping from one group of words to the next. When the word is on your fovea, it’s crystal clear, and you can read it, and if it weren’t on your fovea, you wouldn’t be able to.
Try it for yourself. While focusing on this sentence, try to read the words on the top of the page. You can’t, unless, of course, you jump your focus up there.
Tracking a Moving Object
In an ideal world, your eye would track a moving object by keeping that object’s reflection on the fovea at all times. That way, the object would always appear in crystal clear, foveal focus, and any fine details about it, or its motion, would be readily available to the tracker, throughout the object’s flight.
Sometimes, this is possible, and sometimes it isn’t. It all depends on how fast the object is moving in relation to the eye.
Smooth Pursuit Tracking
If an object is moving slowly enough, the brain is capable of keeping that object on the fovea for an extended period of time, and the eye is moved along with the object in order to do so. Interestingly, most humans are not actually capable of moving the eye smoothly unless they are tracking a moving object, but once a moving object is presented, they are able to smoothly follow it.
The ability to smoothly track an object varies from person to person, and it does appear to be higher in certain kinds of athletes, like tennis players (hey, that’s us!). Every person’s smooth pursuit tracking system has a limit. Once an object’s motion surpasses somewhere between 30 and 100 degrees/second (depends on the person), smooth tracking becomes impossible, and the eye is forced to track in a different manner.
The eye’s second tracking method utilizes quick jumps from one fixation to another, called saccades. As you’ve been reading this article, your eyes have performed many saccades, from one group of words, to the next.
You are blind for a few milliseconds during your saccade. During tracking, when performing a saccade to a new point of interest, your eyes must arrive at the new point before the moving object, so that your saccade is completed by the time the object arrives. If, on the other hand, your saccade is still in progress as the object enters your foveal vision, you won’t see it, and your brain will backfill your memory with a later image to trick you into thinking you did.
Saccadic jumps in athletics are often predictive. The brain predicts a future point of interest, and then jumps the eyes there before the object arrives, giving the image time to stabilize before the object gets there. This way, when the object actually arrives, and reveals its valuable motion information, the eyes are ready and focused, saccade completed, lying in wait.
Just like with smooth pursuit tracking, certain athletes, like fencers and tennis players, have been shown to demonstrate better saccadic accuracy than other athletes of less visually demanding sports, and than control groups.
Smooth Pursuit Tracking in Tennis
When you think of “watching the ball,” you’re thinking of smooth pursuit tracking. “Keep your eye on the ball,” can be specified as “keep the ball on your fovea, such that it’s always in focus.”
And this works… sort of. It works when the speed of the ball is low.
The problem many players run into, as they move up in level (or when returning serve), is that the speed of the ball dramatically increases, and with only smooth pursuit habits built into their vision, their ability to see breaks down.
Predictive Saccades in Tennis
Once the ball is moving sufficiently fast, it’s impossible to keep it on the fovea for its entire flight. As such, we must decide when to see it, and when to be okay with not seeing it. Yes, that’s right. Since you’re blind during your saccades, there will be times during the ball’s flight when you will not see it, and that’s okay.
Actually, it’s better.
Research has shown that elite athletes in tracking sports perform fewer fixations to more important points of interest, and they hold their final fixation for longer, than less skilled athletes. This, ultimately, is how you’re going to transform your tennis vision on faster and heavier balls.
You’re going to focus on gathering a few, critical points of visual information, using predictive saccades. You will perform those predictive saccades accurately, and early enough that they finish before the ball arrives.
When the ball is fast, this will work much better than merely trying to “keep your eye on the ball.”