Answer transcribed from Brightway's Interview with Dr. Bryce Appelbaum:  

With depth perception, I always ask kids if they’ve ever thought about why we have two eyes but we don't see two images. Our eyes being separate from each other in the brain allows for different viewpoints or angles to take in the world. Depth perception is not something that we're born with, it's something that is developed and acquired through our meaningful life experiences where we're learning how to match our motor system with our visual world. It starts really early on in infancy and then in early childhood development.

In order to see in-depth, you need both eyes pointing at the same place for long enough. The sweet spots of both eyes would point at the same place for long enough that the brain can actually put together that information and develop cells in our visual cortex that are only there from two-eyed learning. In research, where rats or certain animals only have one eye or if you don't have the ability to engage in two-eyed learning, you're not going to have depth perception. Hence, if our eyes are pointing or acting like they're in different positions, our brain has three options: we can see a single clear image, two separate images, or a doubled image. 

What we can do, however, is figure out how to suppress or ignore input from one of the eyes to get rid of the confusion. Suppression is where we ignore input from one of the eyes at different pockets of space. It's not always just lights on or off here, it can be when there's movement we suppress or literally different chunks of space where our brain can't take in both eyes input simultaneously. It's almost like a conscious decision at an unconscious level. When we're suppressing input from one of our eyes, whether it's globally or in a finite position, or if we're seeing double because our visual axes are pointing at different places, there is no depth. Therefore, if the root cause of reduced depth perception is an eye turn, ignoring input from the eyes or aligning the eyes in different positions than where the target is, our brains can be taught almost anything if they're forced to because there's tremendous neuroplasticity in the brain.

The old school thought was that there's a critical period for visual development where after age eight, you blow out the candles, and all of a sudden you can't develop depth perception. As silly as that sounds, there are many doctors who practice what they learned in medical school years or decades ago and still stand behind that. I would say at any age the brain can be taught new tricks but there's different malleability at every age. I have a patient right now in his 80s developing depth perception for the first time. There's definitely more neuroplasticity the younger you are and the malleability is much greater than when you're in your 80s. However, if there's the appropriate arrangement of conditions for learning to take place, you can rewire the brain to change how you're using the visual system to develop depth perception. This is assuming the obstacles or reasons for the reduced depth perception are eliminated. I look at any reduction in-depth perception as a kind of a warning sign that the binocularity or eye teaming is not what it could or should be. I think from there, we now see that as we get older and as the visual demands increase, habits become more embedded. If it's identified in most cases, depth perception can be improved if we're able to get to the root cause.

The one area where it would not be is if there's a constant eye turn all the way in or all the way out. With the right motivation compliance, very often the brain can learn how to straighten the eyes and then from there, get the perceptual skills and see the depth that comes with that. However, you need to have the system acting like they're working together before the brain can actually process they're working together. 

So long answer to that, depth perception can be improved with vision therapy. Regarding tennis, if our depth perception is limiting us or if we have trouble knowing where something's located in space, we need a lot of careful understanding of that central and peripheral processing. Regardless, depth perception can be dramatically improved for many people. Also, we can learn how to enhance periphery so that as we're more peripherally engaged, it lets us localize centrally much more efficiently.