Simon effect

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In psychology, the Simon effect is the finding that reaction times are usually faster, and reactions are usually more accurate, when the stimulus occurs in the same relative location as the response, even if the stimulus location is irrelevant to the task. It is named for J. R. Simon who first published the effect in the late 1960s. Simon's original explanation for the effect was that there is an innate tendency to respond toward the source of stimulation.

According to the simple models of information processing that existed at the time, there are three stages of processing: stimulus identification, response selection, and response execution or the motor stage. The Simon Effect is generally thought to involve interference which occurs in the response-selection stage. This is similar to, yet distinct from, the interference that produces the better-known Stroop effect.

Original experiment[edit]

In Simon's original study, two lights (the stimulus) were placed on a rotating circular panel. This device would be rotated at varying degrees (away from the horizontal plane). Simon wished to see if an alteration of the spatial relationship, relative to the response keys, affected performance. Age was also a probable factor in reaction time. As predicted the reaction time of the groups increased based on the relative position of the light stimulus (age was not a factor). The reaction time increased by as much as 30%. (Simon & Wolf, 1963).

However, what is usually seen as the first genuine demonstration of the effect that became known as the Simon effect is by Simon & Rudell (1967). Here, they had participants respond to the words "left" and "right" that were randomly presented to the left or right ear. Although the auditory location was completely irrelevant to the task, participants showed marked increases in reaction latency if the location of the stimulus was not the same as the required response (if, for example, they were to react left to a word that was presented in the right ear).


A typical demonstration of the Simon effect involves placing a participant in front of a computer monitor and a panel with two buttons on it, which he or she may press. The participant is told that they should press the button on the right when they see something red appear on the screen, and the button on the left when they see something green. Participants are usually told to ignore the location of the stimulus and base their response on the task-relevant color.

Participants typically react faster to red lights that appear on the right hand side of the screen by pressing the button on the right of their panel (congruent trials). Reaction times are typically slower when the red stimulus appears on the left hand side of the screen and the participant must push the button on the right of their panel (incongruent trials). The same, but vice versa, is true for the green stimuli.

This happens despite the fact that the position of the stimulus on the screen relative to the physical position of the buttons on the panel is irrelevant to the task and not correlated with which response is correct. The task, after all, requires the subject to note only the colour of the object (i.e., red or green) by pushing the corresponding button, and not its position on the screen.


According to Simon himself (1969), the location of the stimulus, although irrelevant to the task, directly influences response-selection due to an automatic tendency to 'react towards the source of the stimulation'. Although other accounts have been suggested (cf. Hommel, 1993), explanations for the Simon effect generally refer back to the interference that occurs in the response-selection stage of decision making. Neurologically there could be involvement of the dorsolateral prefrontal cortex, as well as the Anterior cingulate cortex, which is thought to be responsible for conflict monitoring. The Simon Effect shows that location information cannot be ignored and will affect decision making, even if the participant knows that the information is irrelevant.

Logical argument for response selection:

The challenge in the Simon effect is said to occur during the response selection stage of judgment. This is due to two factors which eliminate the stimulus identification stage and the execution state. In the stimulus identification stage the participant only needs to be cognitively aware that a stimulus is present. An error would not occur at this stage unless he or she were visually impaired or had some sort of stimulus deficit. As well, an error or delay cannot occur during the execution state because an action has already been decided upon in the previous stage (the response selection stage) and no further decision making takes place (i.e. you cannot make a change to your response without going back to the second stage).

Practical implications[edit]

A knowledge of the Simon effect is useful in the design of man-machine interfaces. Aircraft cockpits, for example, require a person to react quickly to a situation. If a pilot is flying a plane and there is a problem with the left engine, an aircraft with a good man-machine interface design (which most have) would position the indicator light for the left engine to the left of the indicator light for the right engine. This interface would display information in a way that matches the types of responses that people should make. If it were the other way around, the pilot might be more likely to respond incorrectly and adjust the wrong engine.


  • Simon, J. R., and Wolf, J. D. (1963). Choice reaction times as a function of angular stimulus-response correspondence and age. Ergonomics, 6, 99–105.
  • Simon, J. R. & Rudell, A. P. (1967). Auditory S-R compatibility: the effect of an irrelevant cue on information processing. Journal of Applied Psychology, 51, 300–304.
  • Simon, J. R. (1969). Reactions towards the source of stimulation. Journal of experimental psychology, 81, 174–176.
  • Bernard Hommel (1993). "Inverting the Simon effect by intention: Determinants of direction and extent of effects of irrelevant spatial information" (PDF). Psychological Research. 55: 270–279. doi:10.1007/bf00419687.

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