The Simon task and the Simon effect are named after J. R. Simon. Together with his colleague, he first described this effect in 1963. In essence, it shows that people respond faster and more accurately if there is a match between stimulus and response features (e.g., location, when for example stimulus and response are both located on the left side of one’s body).

The effect is also known as a stimulus-response compatibility effect. There are many variants of the stimulus-response compatibility available. Like the Stroop effect, it is easy to notice consciously how difficult a mismatch between a stimulus and response can be.

About this implementation

In this example, you need to respond to the words left and right with the a key (which is on the left side of the keyboard) and with the l key (which is on the right side of the keyboard). The Simon effect here simply means that you will respond more slowly to the word LEFT when it appears on the right side of the screen (i.e., incompatible condition) than when it appears on the left side (compatible condition) of the screen.

  • The Simon effect is here reported as the average response time in incompatible trials minus the average response time in compatible trials. Typically, this is around 35 ms.

  • Note, you can show your response times and copy and paste them to a local file for your own data analysis.

Run the demo

Respond to the word left witht the a key and to the word RIGHT with the l key.

The demo takes less than 2 minutes to complete.

Data output file

In PsyToolkit, the data output file is simply a textfile. The save line of the PsyToolkit experiment script determines what is being saved in the data output file. Typically, for each experimental trial, you would have exactly one line in your text file, and each number/word on that line gives you the information you need for your data analysis, such as the condition, response speed, and whether an error was made.

Meaning of the columns in the output datafile. You need this information for your data analysis.

Colum Meaning


Tablerow number


position of the stimulus (left or right)


required response position (left or right button)


compatibility (compatible or incompatible)


Status (1=correct, 2=wrong, 3=timeout)


Response Time (ms)


If you have a PsyToolkit account, you can upload the zipfile directly to your PsyToolkit account. Watch a video on how to do that. If you want to upload the zipfile into your PsyToolkit account, make sure the file is not automatically uncompressed (some browsers, especially Mac Safari, by default uncompress zip files). Read here how to easily deal with this.

Further reading

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.

Hommel, B. (1993). Inverting the Simon effect by intention: Determinants of direction and extent of effects of irrelevant spatial information. Psychological Research, 55, 270-279.

Prinz, W. & Hommel, B. (2002). Common mechanisms in perception and action: Attention and Performance, Vol. XIX. Oxford: Oxford University Press.