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Classic experiments explained

Simon Task

In short

The Simon Effect (Simon and Rudell, 1967), also known as the stimulus-response compatibility effect, demonstrates that a delay in response time occurs when the stimulus and response position do not correspond. This is tested in tasks where stimulus position is not relevant for the selection of the correct response. Put simply, it is the finding that reaction times are usually faster when the stimulus and response (word and location for example) match, even if that location is irrelevant to the task.

The Simon task

Participants use a computer that has two keys, one on the left and one on the right of the keyboard. The stimulus, often a colour or a word such as ‘left’ or ‘right’, is presented randomly on either the left or right side of the screen.

The participant is asked to respond to the stimulus by pressing the corresponding key that was attributed to that stimulus, so for example the F key for the word left and J for right. The location of the stimulus is unrelated to the task, so if for example they see the word left, regardless of the position of the stimulus ‘left’ appears, they should always press F.

A gif of an example Simon task from the participant's view. A fixation cross is in the middle of the screen, and the 'Left' and 'Right' stimuli appear on either side of the cross.

The accuracy of the responses and reaction times are recorded and compared across congruent and incongruent conditions.

What is the Simon effect?

Even though the location of the stimulus is unrelated to the task, it directly affects the participants’ response selection, since we automatically react towards the source of stimulation. Overall, explanations for the Simon effect refer to the interference which occurs in the response-selection stage of our decision-making.

This effect underscores the powerful automatic influence of spatial information on cognitive processing, illustrating how the arrangement of stimuli can impact motor responses, even where spatial details in situations are supposedly irrelevant to the task’s objectives.

What are some theories behind the Simon effect?

The Spatial Coding Theory proposes that although location is irrelevant, the spatial aspect of the stimulus is automatically processed and encoded, therefore leading to faster responses when stimulus and response locations are congruent.

The Referential Coding theory suggests that the the processing of stimulus location and the selection of the correct response are influenced by a common reference frame. The Simon Effect occurs as stimuli and responses that share the same spatial code (based on the common reference system) are processed more efficiently.

Can I use the Simon task in online research?

Yes, absolutely! In fact, other researchers have studied the Simon effect in an experiment which investigates how face masks affect our social attention over space (Villani et al., 2022).

How does it work in Gorilla?

You can try out and clone our Simon task example. You can also tweak this sample to use your own stimuli.

Have a look: Try the Simon Task in Gorilla

Are there any papers Gorilla users have written about the Simon effect?

Yes, there are! Have a look at the following article:

Wearing the face mask affects our social attention over space


Hommel, B. (2011). The Simon effect as tool and heuristic. Acta Psychologica, 136(2), 189-202.

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(3), 300.

Tagliabue, M., Umiltà, C., & Spera, P. (2009). Interference between nonspatial stimulus features in the Simon effect. The American Journal of Psychology, 122(4), 431-453.

Villani, C., D’Ascenzo, S., Scerrati, E., Ricciardelli, P., Nicoletti, R., & Lugli, L. (2022). Wearing the face mask affects our social attention over space. Frontiers in Psychology, 13, 923558.

This page was written in collaboration with Lizzie Drury