How do our eyes move in perfect synchrony?

Humans, like most animals, have two eyes.

And that's for two very good reasons.

"You have a spare one in case you have an accident, and the second reason is depth perception, which we evolved to help us hunt," said Dr. David Guyton, professor of ophthalmology at The Johns Hopkins University. But having two eyes would lead to double vision if they didn't move together in perfect synchrony. So how does the body ensure our eyes always work together?

To prevent double vision, the brain exploits a feedback system, which it uses to finely tune the lengths of the muscles controlling the eyes. This produces phenomenally precise eye movements, Guyton said. 

Related: Meet your muscles: 6 remarkable human muscles

Each eye has six muscles regulating its movement in different directions, and each one of those muscles must be triggered simultaneously in both eyes for them to move in unison, according to a 2005 review in the Canadian Medical Association Journal. "It's actually quite amazing when you think about it," Guyton told Live Science. "The brain has a neurological system that is fantastically organized because the brain learns over time how much stimulation to send to each of the 12 muscles for every desired direction of gaze." 

This isn't innate, it's an acquired ability. "Babies master it usually within three to four months of life," said Guyton. "Most people keep it well into their 80s, but age makes us slowly lose the ability over time."

The feedback learning loop is kick-started when the muscles become slightly out of sync with each other, which happens every now and then throughout life, resulting in a small amount of double vision. Sometimes this is because one muscle is slightly longer than its partner. "Maybe that's just because of a growth spurt," in the eye muscle, Guyton said. These ocular oddities are very often so modest that people don't even consciously notice them, but the brain takes note of these seemingly trivial discrepancies and attempts to straighten things out. "Within less than a second, the brain moves the eyes in opposition to realign them," Guyton said. "If they just carried on moving in the same direction then they wouldn't change their position relative to each other. They'd stay out of sync."

During that recalibration process, the brain uses the data it gathers to help fine-tune the resting length of the muscles that control eye movement. For example, it may realize that one of the muscles has grown faster than the others. "The brain automatically draws a map and then uses it to change the muscle length to relieve the misalignment and that occurs over weeks to months," Guyton said, by adding or subtracting individual building blocks to the muscles. But exactly how the body translates this brain map into muscle changes is still unclear, he added.

But we do know that the brain is perpetually on alert to perfect eye movement through this feedback process. In fact, the muscles' lengths are readjusted so often that most of the proteins in them are less than a month old. "The half-life of the proteins in eye muscles is only about 10 to 15 days so the muscles are turning over all the time," Guyton explained.

Originally published on Live Science.