The Biology of Saccades: High-Speed Eye Movements

As you read this sentence, your eyes are not gliding smoothly across the text. They are violently jumping from word to word, pausing for a fraction of a second, and jumping again.

These rapid, jerky eye movements are called Saccades. They are the fastest and most frequent movements the human body can make. We perform roughly 3 to 4 saccades every single second of our waking lives.

But this constant, violent movement presents a massive problem for the brain: Motion Blur.

The Blurry Camera Problem

If you hold a video camera and violently jerk it from left to right three times a second, the resulting video will be a nauseating, blurry, unwatchable mess.

Your eyes are essentially cameras. As they whip from one word to the next during a saccade, the image of the world is rushing across your retina at incredible speed. The physical reality of your vision should be a constant, dizzying blur of motion.

Why does the world look perfectly stable and still?

Saccadic Masking: The Neurological Edit

The brain solves the blur problem by simply turning off your vision while the eye is moving.

This phenomenon is called Saccadic Masking (or Saccadic Suppression).

The Pre-Signal: Right before the brain sends the signal to the six extraocular muscles to jerk the eye, it sends a simultaneous, inhibitory signal to the Visual Cortex at the back of the brain.
The Blackout: This signal temporarily suppresses the processing of visual information. For the 20 to 100 milliseconds that the eye is in motion, you are functionally blind.
The Re-activation: The moment the eye stops moving and focuses on the new target, the visual cortex turns back on.

Because the brain seamlessly stitches the "Before" picture to the "After" picture, you never notice the darkness. You never notice that you are completely blind for roughly 40 minutes every day.

The Mirror Experiment: Catching the Mask

You can prove that Saccadic Masking exists right now in a bathroom mirror.

The Action: Look closely at your left eye in the mirror. Now, quickly shift your focus to look at your right eye.
The Result: You will never see your own eyes move. Even though your eyes had to physically cross the space to change targets, the brain completely deleted the footage of the movement. You only see the static start and end points.

Why Do We Saccade? The Fovea Constraint

Why must the eye jump around so frantically? Why can't we just hold still and take in the whole room?

The answer lies in the Fovea (which we discussed previously).

Because high-resolution, sharp vision only exists in a microscopic, 1-degree pinhole in the center of the retina, we cannot "See" a whole face at once.
To recognize a person, the brain must rapidly "Shoot" the fovea at their left eye, their right eye, their nose, and their mouth in a rapid sequence of saccades. The brain then takes these individual, high-resolution "Snapshots" and stitches them together in working memory to give you the illusion that you are seeing their entire face clearly.

Microsaccades: Preventing Blindness

Even when you try to stare perfectly, absolutely still at a single dot on the wall, your eyes are betraying you.

The Drift: The muscles holding the eye are imperfect. They slowly drift.
The Microsaccade: To correct this, the eye performs tiny, imperceptible, involuntary jerks (microsaccades) to pull the target back onto the exact center of the fovea.
The Necessity: If scientists use special equipment to completely paralyze a person's eye muscles, preventing microsaccades, a terrifying biological glitch occurs: The image fades to grey, and the person goes blind within seconds. The photoreceptors in the retina require constant, changing stimulation to fire. If the image is held perfectly still on the retina, the sensors stop firing. Microsaccades are mandatory to keep the visual system "Awake."

Conclusion

Saccades reveal that our visual reality is an aggressive, highly edited reconstruction. To overcome the physical limitations of the tiny Fovea, the eyes act like frantic searchlights, scanning the room in violent jumps. By meticulously editing out the motion blur between the jumps, the brain weaves together a flawless, steady illusion of a continuous world.

Scientific References:

Bridgeman, B., et al. (1975). "Failure to detect displacement of the visual world during saccadic eye movements." Vision Research. (The classic masking paper).
Martinez-Conde, S., et al. (2004). "The role of fixational eye movements in visual perception." Nature Reviews Neuroscience. (The microsaccade blindness study).
Castet, E., & Masson, G. S. (2000). "Motion perception during saccadic eye movements." Nature Neuroscience.