I love optical illusions, and how a well-designed illusion can provide real insights into how sight works. Not just in terms of light and eyes and lenses but also mentally, as our brain assembles a representation of the world from imperfect information delivered to it by the eyes and other senses. (Here's a great 60-second video that demonstrates how the brain adapts to missing information from the blind spot.) Our personal sense of time can be fooled, too, and these "temporal illusions" give similar insights into how the brain assembles events into our perception of time. David M Eagleman, director of Baylor College of Medicine's Laboratory for Perception and Action writes in the essay Brain Time:
The days of thinking of time as a river—evenly flowing, always advancing—are over. Time perception, just like vision, is a construction of the brain and is shockingly easy to manipulate experimentally. We all know about optical illusions, in which things appear different from how they really are; less well known is the world of temporal illusions. When you begin to look for temporal illusions, they appear everywhere. In the movie theater, you perceive a series of static images as a smoothly flowing scene. Or perhaps you've noticed when glancing at a clock that the second hand sometimes appears to take longer than normal to move to its next position—as though the clock were momentarily frozen.
Try this exercise: Put this book down and go look in a mirror. Now move your eyes back and forth, so that you're looking at your left eye, then at your right eye, then at your left eye again. When your eyes shift from one position to the other, they take time to move and land on the other location. But here's the kicker: you never see your eyes move. What is happening to the time gaps during which your eyes are moving? Why do you feel as though there is no break in time while you're changing your eye position? (Remember that it's easy to detect someone else's eyes moving, so the answer cannot be that eye movements are too fast to see.)
To accomplish this, we engineered a device (the perceptual chronometer) that alternated randomized digital numbers and their negative images at adjustable rates. Using this, we measured participants' threshold frequencies under normal, relaxed circumstances. Next, we harnessed participants to a platform that was then winched fifteen stories above the ground. The perceptual chronometer, strapped to the participant's forearm like a wristwatch, displayed random numbers and their negative images alternating just a bit faster than the participant's determined threshold. Participants were released and experienced free fall for three seconds before landing (safely!) in a net. During the fall, they attempted to read the digits. If higher temporal resolution were experienced during the free fall, the alternation rate should appear slowed, allowing for the accurate reporting of numbers that would otherwise be unreadable.