“Listen,” the man on the TV says, “your life could depend on this: don’t blink. Don’t even blink!” He gestures, thumb and middle finger in a circle. “Blink and you’re dead. They’re fast — faster than you can believe. Don’t turn your back, don’t look away, and don’t blink.”

The Doctor is warning Sally Sparrow about the Weeping Angels, aliens on the TV show Doctor Who. The Angels have become one of the series’ most popular monsters because of how scary they are. When you’re staring at them, they’re “quantum locked” and are frozen in stone. They look like any other statue. But when you’re not observing them they move quickly, so quickly that they can rush toward you when you blink. And if they touch you, they’ll send you back decades in time.

It’s a simple but effective concept. The Weeping Angels take advantage of something we do every few seconds without realizing it. You can stop yourself from blinking…for a while. It’s like holding your breath. The longer you go without blinking, the stronger the urge to do so becomes, and all the while a deadly creature is right in front of you, waiting for your moment of weakness.

As you’d imagine, this has led to a lot of online theorizing of how to deal with the Angels, in much the same way that people like imagining what they’ll do when the zombies rise. Most schemes involve blinking first one eye and then the other so that you never stop observing an Angel. As long as you’re watching and Angel, you’re safe.

That won’t work, though, because you’re often blind even though your eyes are open.

Saccades

Go stand in front of a mirror with your nose a few inches from its surface. Look at your reflection’s left eye, then switch to looking at the right eye as quickly as you can. Then look at the left eye again. Then the right. And then ask yourself this: why don’t you see your eyes moving?

Congratulations. You’ve just experienced saccadic masking.

You may think of your eyes like cameras, taking high-definition pictures of everything around you, but they’re not. Your eyes only see in high resolution across a small part of your vision, one that’s roughly the size of your thumbnail when you hold your thumb out at arm’s length in front of you. Away from that central part, your vision gets fuzzier and fuzzier and becomes black-and-white. To compensate, you move your eyes to sweep the narrow spotlight of your high-res vision all around. When you meet someone, your eyes dart to their eyes, their nose, their mouth, their hair. Your brain builds up a composite image of what the person looks like from these snapshots.

These rapid eye jerks, called saccades, aren’t fully under your conscious control. Once one starts you can’t change its direction or how fast your eyes move, and your eyes move fast. Saccades are the fastest movements your body is capable of. They’re so fast that your vision blurs during the movement. To get rid of that blur, your brain performs saccadic masking. Nearly a tenth of a second before your eyes move, your brain shuts down a lot of visual processing so that you’re not aware of your eyes moving and don’t consciously see any blurred images. As soon as the image on your eye is stable, your brain goes back to processing all of the visual data coming from your eyes. Your also lies to you, hiding saccades from you by fiddling with your perception of time during saccadic masking so that it feels like it takes less time than it does. The end result is that you’re effectively blind during a saccade.

It gets worse! If something moves during a saccade, you generally don’t see the motion. If an Angel crept up on you during a saccade, you might not see moving it at all until it was too late.

Microsaccades

Fine, you say, I won’t move my eyes around. I’ll stare fixedly at that Angel. Unfortunately, not even that may save you, thanks to microsaccades. Even when you think your eyes are staying still, they’re not. In part it’s to keep you from going temporarily blind.

As you’re reading this, are you sitting down? Can you feel the texture of your skirt or pants? Chances are, before I asked that question, you couldn’t. The sensory neurons in your legs adapt to the constant stimulus of cloth against them and stop sending signals. The same thing happens to the neurons in your eyes. If you were able to stare at something so that its image was perfectly still on your eye’s retina, then after a while the image would fade away due to neural adaptation. To keep this from happening, your eyes jiggle around, performing a smaller version of a regular saccade. It’s as if the world is filled with ghostly objects that fade if they’re perfectly still, so your eyes jitter to make the objects look like they’re moving. Microsaccades refresh the image on your eye.

You can’t see microsaccades directly. Your brain acts like the image stabilizer in a video camera, smoothing out the shaky image from your darting eyes. But you can see their indirect effect by staring at Akiyoshi Kitaoka’s “Rotating Snakes” illusion. Our brain normally can tell the difference between apparent motion caused by our eyes’ microsaccades and actual motion caused by the thing we’re looking at moving. But with the circular snakes, our brain gets mixed up and mistakes apparent motion for real motion.

It’s Not as Bad as I’m Making It Sound

It’s possible you’ll still be safe from the Angels, because saccades don’t fully blind you. Saccadic masking doesn’t stop your brain from processing all visual information. When you look at something, the visual data moves through successive portions of your brain’s visual cortex. The different parts of the visual cortex look for things like differences in brightness or straight lines. During a saccade, smoother parts of an image are thrown away early in the visual cortex, while parts with a more complex pattern, like text on a page, are still partially processed. And saccadic masking is stronger when your eyes move a lot, but weaker during small motions and microsaccades.

It all gets back to what it means to observe something. Does it count if your brain receives visual information about an Angel and processes it? Or do you have to consciously be aware of what you’re seeing for an Angel to be quantum locked? If it’s the former, then saccades effectively blinding you doesn’t matter. If it’s the latter, though, send me a postcard from the past and let me know.

Even More Information

• Liked the rotating snakes? Kitaoka has a page full of them.
• You don’t see something moving if it happens during a saccade: Failure to Detect Displacement of the Visual World During Saccadic Eye Movements (Bridgeman, Hendry, and Stark)
• Saccades mess with your perception of time: Saccadic Modulation of Neural Responses: Possible Roles in Saccadic Suppression, Enhancement, and Time Compression (Ibbotson, Crowder, Cloherty, Price, and Mustari)
• An overview of microsaccades: Microsaccades: a Neurophysiological Analysis (Martinez-Conde, Macknik, Troncoso, and Hubel)
• Saccadic masking only hides some visual information: Selective Suppression of the Magnocellular Visual Pathway During Saccadic Eye Movements (Burr, Morrone, Ross)