Simon effect

In psychology, the Simon effect states that reaction times are usually faster when stimulus and response occur at the same location than when they do not, even if the stimulus location is irrelevant to the task. It is named for J. R. Simon. Simon originally named the effect reaction toward the source.

A typical case of the Simon effect involves placing a volunteer in front of a computer monitor and a box with two buttons on it, which he may press. The volunteer is told to press the right button when he sees something red appear on the screen, and the left button when he sees something green.

Volunteers typically react much faster to red lights that appear on the right hand side of the screen than when they appear on the left hand side; and vice versa for green lights. This happens despite the fact that the position on the screen is (one would have thought) irrelevant to the task, which requires the subject to note only the colour of the object (ie red or green) and not its position.

Explanations for the Simon effect generally split into two types: those that focus on the spatial nature of the Simon task itself; and those that view the Simon effect as an instance of a more general compatibility phenomenon.

A knowledge of the Simon effect is useful in ergonomic design of aircraft cockpits, or indeed any situation in which a human being has to react quickly to a situation.