Why Can't You See Your Eyes Move in the Mirror?

A fairly recurring complaint in homemade videos is that the person in charge moves the camera too much, causing nausea. The way the eye-brain system works is more complex than we imagine, and any extreme stimulus can affect its performance (giving rise to incredible optical illusions). However, our eyes move all the time during the day, and we don't feel dizzy. The explanation takes us to the strange territory of saccadic movements, the tricks the brain applies, and some exercises to reproduce at home. For example: Why can't you see your eyes move while you're in front of the mirror?

Actually, you can. It's enough to look at a fixed point in the mirror and move your head. I mean the impressive number of movements your eyes make each day. Even while reading this text, your eye activity is startling. These “microjumps” are known as saccadic movements, or simply saccades. Depending on the amplitude of movement, a saccade can last between 20 and 200 milliseconds.

Saccades help us build the equivalent of a three-dimensional map of a scene. The fovea, which is the central part of the retina covered with photosensitive cells responsible for providing “high resolution” in our vision, is too small. Saccades compensate for that limitation and allow us to resolve scenes and objects more efficiently.

Optical Illusions: Eyes That “Don't Move” and the Stopped Clock

One of the most interesting aspects is that the brain doesn't leave a gap during saccades; it fills it using what it has available chronologically. But there's something odd: that information is based on what we see after the jump. At this point, anyone would think it's a mistake, because our eyes can't travel through time. In computer terms, it's a matter of latency.

Processing isn't instantaneous, and the brain tends to attenuate or filter what our senses register. Between the starting point and the endpoint of a saccade, there's a “blur” that the brain can't interpret. What's the solution? Erase that distortion in the middle and fill the symbolic gap with data from the second point (saccadic masking). The result is an optical illusion we know as the “Stopped Clock,” or in more formal terms, chronostasis.

This combination of functions, effects, and tricks means you can't see your eyes move in the mirror. Try it: stand about 45 centimeters away from the mirror. Look at your left eye, then your right. Repeat. And repeat. And repeat. The movement isn't there, but there's no interruption in visual information either. Everything looks continuous and uniform. Now, the question is: how much information is “lost” because of this? On average... 40 minutes a day.

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