Move your mouse over the green detectors above (you must have QuickTime 3.0 or better installed). You'll see that you get an "action potential" that stimulates the bell. 1 action potential isn't enough to sound the bell (below threshold), you need 2 arriving at about the same time to make the bell ring. To do that you must move your mouse at the right velocity.
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This is an example of what is called a Reichardt detector (Reichardt, 1969). It is a simple circuit for detecting motion modeled after neural circuits found in the eyes of flies. As you can see, a Reichardt detector is tuned to a certain speed and direction of movement. You have motion detectors in your brain too, but they are more complicated than this simple Reichardt detector.
Reichardt, W. (1969). Movement perception in insects. In Reichardt,
W. (ed), Processing of
optical data by organisms and by machines. New York: Academic Press
Schematic of a Reichardt detector
Implementing Reichardt detectors in computer chips