Why LED signboards shake when I eat an apple?
The signboard LEDs are multiplexed, meaning that each is on for only a very brief time in sequence. Your eye-brain system fuses the flashes together if they are closely enough spaced but if your head is moving relative to the board the flashes can break up into individual dashes or dots. 

The multiplex rate is a design decision of the signboard manufacturers. They could make it so fast that this would not (practically) happen. 

A variation on this is to take an old-school analog oscilloscope with a trace time of maybe tens of milliseconds. The scope has a dot of light that moves from left to right faster than you can perceive so it looks like a horizontal line. Whack yourself on the top of your head and you'll see the impulse response of your eyes.