Scott A. Carr's website

Scott A. Carr

Computer Scientist

A fun experiment with latency


This experiment has two parts. All you need for the first part is a mirror (and working vision). A bathroom mirror works well. Stand in front of the mirror and lean forward a little until your face is about 6 inches from the mirror’s surface. Now look all the way to your left (or right) and then look back to center as quickly as possible. Your goal is to see your eye in an intermediate state, somewhere between all the way left (or right) and center. Spoiler alert, it’s impossible because of saccadic masking.

Your eye really is moving in a nice smooth motion, and the second half of the experiment will prove it to you. This part requires an iPhone (or similar phone with a front facing camera). With your phone held up to your face, turn on your camera app. Switch to the front camera, so you see a nice big picture of your face filling most of the screen. Now do the same eye movement experiment as before. If you’re quick, you’ll see your eye moving. The phone is recording, processing, and displaying the image and each step imposes some delay. You’re seeing what the camera recorded a fraction of a second ago – a glimpse into the past.

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