how do our eyes move ?

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).

sources: i, ii, iii, iv

just wipe that Cataract out of your eye

We live in the Instagram era, when even a simple cup of coffee suddenly becomes one of the most artistic things to be photographed. After sharing your shot with all your friends on the web (so that you’ll have shown that you DO have a social life, yop!), you put back your Reflex (the best camera ever for shooting cups of coffee, we all know that) and notice that its lens is a bit misted up. Inside. So you’ll never be able to get rid of it and all your shots will look blurry. You’ll be able to post only photos of blurry coffee served in blurry cups on blurry tables. You’ll be a blurry person, forever.

Now, transpose this “blurriness” to one of your eyes. Imagine you have one eye which “works” perfectly, like HD vision, and the other one with some misted up lens inside, so that everything you see looks blurry. This is clearly a much worse problem than not being able to shoot cool photos. This problem is called Cataract and nowadays is the most common cause of vision loss in people aged over 40.

Human eye is composed of different layers. Without giving too many details, a very important component of the eye is a transparent tissue known as crystalline lens. This lens is able to change its thickness so that the eye is able to focus on objects at various distances. The changes of thickness are controlled by a suspensory ligament, called Zonule of Zinn, that connects the crystalline to the ciliary body of the eye. Their combined movements are really important for correctly focusing light onto the back of the eye (the retina) so that images appear clear and without distortion.

The crystalline lens is made mainly of water and proteins and it is nourished by the aqueous fluid that is present between the cornea and the crystalline lens. Metabolic changes of the crystalline lens fibers over time lead to the development of opacifications, mainly due to the proteins that clump together. As a consequence, the lens gets “clouded” and this clouding is actually known as Cataract. This process normally worsens the way light enters the eye and, as a result, our sight is blurry.

Nowadays, modern cataract surgery is one of the safest and most effective surgical procedures. Over 90% of operations are successful in restoring useful vision, with a low complication rate. The operation itself is typically performed using only local anesthesia and in one-day hospitals. Very often, the postoperative recovery is really quick and can greatly reduce the patient’s dependence on eye glasses.

The most common surgical procedure for removing cataract is the so called phacoemulsification.

By using a microscope, the surgeon focuses on the lateral side of the eye and makes a small incision on the cornea.

Then, a tiny ultrasound probe is inserted into the eye in order to break the lens. The high vibration frequency (40 kHz) of the probe tip causes the emulsification of the lens material, which gets broken in very small fragments.

By accessing the eye always through the same incision, the crystalline pieces are removed through an aspiration probe.

Next, the same tool is employed again, this time to insert an artificial intraocular lens that actually replaces the crystalline. The artificial lens is usually made of plastic, silicone or acrylic compounds and is supposed to remain in place for the rest of the patient’s life. No stitch is generally necessary, just a protective shield is usually placed over the eye to keep it safe in the early recovery stages.

Recently, lasers have been approved for use in cataract surgery: they improve the accuracy of the surgery, since they reduce the need for surgical blades and other hand-held tools. Moreover, they result to be more efficient than ultrasonic probes for crystalline fragmentation. Laser-assisted cataract surgery is fairly new and significantly increases cataract surgery cost. However, medical consultation is always the best way to evaluate all the possible surgical risks.

sources: one, two and three