by Gooly (Li Yang Ku)
I was always intrigued by visual illusions and am often surprised by how often we are fooled by our eyes. Some visual illusion is just as good as a good joke. One of my favorite illusion is the spinning dancer, which I can’t easily change the direction I interpret despite knowing it could be both. Understanding visual illusions is also crucial in Computer Vision because they are just side effects produced by the underlying algorithm that helps us see. A great vision algorithm should probably have the same visual illusion as humans do.
Chronostasis is a kind of visual illusion that occurs to you every moment without you noticing. To test it out, you need to find a clock that has a seconds hand; first focus your gaze on some where close so that you can still see the hand ticking from the side view but not too close, then shift your gaze to the seconds hand when it just moved. You’ll notice that the first tick seems to be longer than the other ticks after it.
This illusion is caused by Saccadic Masking, a mechanism that our brain uses to help us see the world without getting dizzy. Our eyes are constantly moving and our head also turns a lot. Saccadic masking shuts down the input when the scenes that shown to your eyes are blurry. So when you move your eyes, the brain has two choices, it can either keep the last image or show you the next stable image in the future. So now you might be yelling “HOW COULD THE BRAIN POSSIBLY SHOW YOU THE IMAGE IT HAVEN’T SEEN!” Yeah, that’s not possible. But remember that there is no clock ticking in your brain and time is just what you feel; so your brain can just freeze your internal clock and wait for the next image then fast forward your internal clock so it syncs back with the real world. And that’s what happened to you when you did that first gaze shift to the seconds hand.
To test out Saccadic Masking you can also find a mirror and stare at your pretty (or nerdy) eyes. First focus on your left eyes, then shift your gaze to your right eye. You won’t be able to see your own eyes saccade because of Saccadic Masking, but if you record yourself doing the same experiment with a smartphone’s forward facing camera, you would be able to see your eyes saccade clearly. (note that smart phone cameras have time delays, so don’t use them as a mirror for testing. It is highly recommended to be used as a mirror outside of the experiment though; it always shows a slightly younger you.)