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Both ELI5 explanations below are still too complicated. Please simplify? Alfenhose
An interesting phenomena, it is because the brain doesn't store what you saw during the time your eye spent moving, instead the brain fills in this time with what you saw when you stopped moving your eye.
When you move your eye or blink the images from your eyes are just blurry or dark and therefore quite useless for your brain to interpret. So the brain use the information from the view before and after the eye movement to fill in the blanks. So if you move your eye to the clock as the second hand is moving your brain does not see the second hand moving and interprets it as if it have been standing still during the entire time you moved your eye. So the first second looks longer because your brain makes the wrong assumption.
The human visual system does not operate in a real-time, continuous way, but rather takes constant ‘snapshots’ of images the eye captures. The perception of dynamic world is based on the brain’s interpretation of these snapshots. Think about it as a ‘frame rate’ of eyes, in which there exist a maximum frame rate that eyes are able to capture. The real mechanism is much more complicated than that, of course, as human observers are able to learn to capture things with much higher ‘frame rate’. Another important thing is, the rate at which eyes capture things changes all the time. For example, when you rotate your eyes, or looking from one point to another (‘saccade’), the sensitivity of the visual system is greatly reduced, and all you see is a blurry image.
Because of the frame rate, the brain would assume the experience is continuous, and automatically fill ‘gaps’ between each frame. It’s like how cartoon works: you see a series of image of a running horse, although these are only still images, you can still kinda perceive continuous motion, and see the horse is running.
The clock illusion originated from the way brain establishes the frame-to-frame relationship from the blurry image captured by the rapid saccade movement. Your visual focus, as well as your eyes, would travel relatively grate distance to look up the clock, therefore there would be a rather large ‘blurry gap’ that needs to be filled in. The brain would trick you into believing what you see after fixing your eyes was kept the same during your whole eye movements. For example, the clock is hitting at 8s mark, and you took 2s to move your eyes from the floor to the clock hanging on the ceiling, and by the time you fixate your eyes at the clock, the clock is already ticking at 10s mark. Your brain would assume the clock was hitting 10s mark already, BEFORE you rotated your eye, and would also add up the 2s you’ve spent on rotating your eye. Therefore the actual 10s appeared to be much longer than it should be.