"A frog hunts on land by vision... The frog does not seem to see or, at any rate, is not concerned with the detail of stationary parts of the world around him. He will starve to death surrounded by food if it is not moving. His choice of food is determined only by size and movement. He will leap to capture any object the size of an insect or worm, provided it moves like one. He can be fooled easily not only by a bit of dangled meat but by any moving small object."
Later in the article, Lettvin explains that they found four different types of neurons that responded selectively for different kinds of stimuli. Here's how Lettvin described the second type:
"To our minds, this group contains the most remarkable elements in the optic nerve... We have been tempted to call them bug perceivers... A delightful exhibit uses a large color photograph of the natural habitat of a frog from a frog's eye view [of course, with] flowers and grass. We can move this photograph... waving it around... and there is no response. If we perch with a magnet a fly-sized object on the... picture... and move only the object we get an excellent response. If the object is fixed to the picture... and the whole moved about, then there is none. Could one better describe a system for detecting an accessible bug?
Why is it important that the neurons not respond when the whole photgraph is moved? What if, for example, the frog is sitting lazily on a rocking lily pad waiting for a bug to fly past?