In direction selectivity, inhibition only occurs in one direction. Naturally, when selectivity detects left-to-right motion, inhibition occurs from right-to-left. This makes sense because when a pitcher throws a ball and the lights in the room go out, you expect the ball to keep moving forward, which is the selective direction. You don't expect the ball to go backwards or jerk back, so if that seems to happen, then it is inhibition. Direction selectivity models this problems through matrices in which a positive "1" represents excitatory inputs, negative "1"s represent inhibitory inputs, and "0"s represent no connection at all between the inputs.
Anastasio, Thomas J. Tutorial on Neural Systems Modeling. Sunderland: Sinauer Associates, 2010. Print.
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