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Why study compatibility effects?

The term Stimulus Response Compatibility had its origins when Fitts and Deininger did an experiment in which they presented subjects with different sets of stimuli and different sets responses, and used several ways of pairing the individual stimulus elements with individual responses (this is called S-R “mapping”). The fastest and most accurate performance was not with one particular set of stimuli or one particular set of responses, but with a special combination of the two, and the way their elements were paired.

For example, suppose you’re in an experiment in which you have the simple task of of saying the name of of a color (e.g. “blue” or “green”) every time that color appears on the screen. You would expect this task to be much easier than if you had to give a random syllable (e.g. “da” or “koh”) as the response.  That’s because in the first case, not only is the set of stimuli similar to the set of responses, but the individual stimuli themselves (colors) are in a sense identical to the individual responses (color names) with which they are paired.  In contrast, in the case of the “da” “koh’ responses, the set of stimuli and the set of responses are totally unrelated so that any stimulus-reponse pairing is just as good as any other.

If you now had to respond to a specific color not with its own name, but with the name of a different color (e.g. color green – response  “blue”),  that task would be even more difficult than responding with a random syllable. That’s because the similarity between the stimulus and the response sets would naturally push you to give the actual name of color that was presented (“blue” to blue), and this is a push that you would have to suppress before you could produce the color name that you were instructed to give.

So, your fastest and most accurate response would be in case 1, when you respond to a color with its own name, the slowest in case 3, when you respond with the name of another color, and in case 2,  when the set of color stimuli and the set of responses are totally unrelated, your time and accuracy would fall between the two.

When the set of stimuli and the set of responses are similar, as in cases one and three, they are said to have set-level similarity (which, as we will see further on, gives them “Dimensional Overlap”) and are “S-R compatible”.  The mapping instructions for compatible stimulus and response sets may be either “congruent” as in case one, or “incongruent” as in case three

Now suppose that the set of stimuli is still Blue and Green, and the set of responses, instead of being color names, are key presses: left for Blue, right for Green.  Just like in the “dah”, “ko” eample, there is no set-level similarity or S-R compatibility.

We start the experiment by randomly presenting the stimuli, one at a time, in the center of the screen and find that there is not much difference in speed or accuracy between the blue and the green stimuli, or between the left and right key-presses. Now, let us introduce one small change: instead of presenting the stimuli in the center of the screen, let us present them either on the left or the right side of the screen.  The instructions remain just as before: left key-press  for Blue, right-keypress  for Green, so nothing has changed other than where on the screen the stimuli are now presented. This small change has a surprising result: when the color is presented on the same side as the response, the response is much faster and much more accurate than when it is presented on the opposite side.

Even though the side on which the stimulus is presented makes absolutely no difference to what the correct response is, and subjects are told to ignore this (i.e. ignore the side because it totally irrelevant), when the side is consistent with (i.e. matches) the side of the response key, it is as if the irrelevant stimuli had been mapped onto their corresponding responses.



The fact that stimulus-response compatibility effects occur with both relevant and irrelevant stimuli has been of great interest to cognitive psychologists.

The study of compatibility effects has given us important insights into the most basic, low-level mental processes involved in perceiving the world, filtering out irrelevant information, making decisions, and executing actions. Over the years, compatibility experiments have been used to gain a deeper understanding of decision-making, attention, practice, priming, similarity, automaticity, rule-learning, implicit associations, and much more.

Although much of the “stimulus-response compatibility” literature seems very dry, being rooted in experiments that use things like numbers and colors as stimuli and responses, the study of compatibility has been used to investigate such wide-ranging social questions as the way that we read emotions in people’s faces and revealing people’s unconscious levels of racism and sexism by testing their levels of “implicit association” with race and sex terms.

Emotional Stroop Stimulus Faces

Image from: Ovaysikia S., Tahir K. A., Chan J. L., DeSouza J. F. X. (2011). Word wins over face; emotional Stroop effect activates the frontal cortical network. Front. Hum. Neurosci.


The dimensional overlap model is an attempt to provide a unified language and unified theoretical framework for understanding all compatibility effects, how they arise, and their relationships to one another.

For a quick summary of the core papers that describe and test the dimensional overlap model, you can check out our timeline of the major developments of the model, and links to the key papers and experiments: A brief history of the model.

If you are interested in a more detailed description of the theory and the experiments behind dimensional overlap, this series of articles provides a good tutorial:


What is Dimensional Overlap?
Taxonomy of DO Ensembles
The Experiments


Once you understand the core concepts, you may want to learn the details of the cognitive processing model itself: that is, the way the the model explains the compatibility effects in terms of mental processing. This series of article will step you through the development of the model, from its initial basic assumptions to its implementation as a full-fledged simulation model:


The Representational Model
The Computational Model
Online Simulation Model (DO2000)


Once you understand the basic theory behind the dimensional overlap processing model and the computational implementation, you can use the online version to set parameters and test out the simulation of different tasks, and get a feel for how the model actually functions under different conditions.

Feel free to browse around the page and explore whatever topics and educational articles interest you. We will periodically add new articles with details about the dimensional overlap model, and the wide and interesting world of compatibility effects.