The Stimulus-Response Compatibility Task

The Stimulus-Response Compatibility Task, also called the SRC Task  is the type of choice reaction time task where there is dimensional overlap between the relevant stimulus set and the response set.  In the dimensional overlap taxonomy, it is considered a  Type 2 task.

The results of performance with SRC tasks were first reported by Paul Fitts (Fitts & Seeger, 1953; Fitts & Deininger, 1954), who called them “Stimulus Response Compatibility Effects”.   In both papers the tasks  were eight-choice  reaction time tasks in which subjects had to move a stylus from a home position to one of eight target positions.  Since that time, many experiments have been run that have confirmed Fitts’ original results, and have used non-spatial stimulus and response attributes.  Thus was  the field of stimulus response compatibility born.

An example of this type of task is described next.  A subject is sitting at a table, on which  is a screen showing  four small,  horizontally arranged squares.  Inside each sqare is a small circle that functions as a light.   A stimulus consists of one of these lights going on.  In front of the screen are four keys.  The subject’s index and middle fingers of each hand are resting on these keys.  When the mapping instructions assign a response to its corresponding stimulus light (congruent mapping), people are much faster and more accurate than when the instructions assign a response to a non-corresponding stimulus light (incongruent mapping).

This effect is  extremely robust, and happens for a wide variety of stimuli and responses. People are slower to say “blue” in response to a red square than they are when saying “red” in response to a red square, even though there is no perceptual similarity between the stimulus and the response. The effect even shows up when the relationship is very abstract: for example, people can be told to respond to lights of different brightness by pressing buttons with different levels of force, and they are faster when they are told to press a button hard for a bright light than when they are told to press a button hard for a dim light.

The key factor that all mapping tasks have in common is the dimensional overlap between the relevant stimulus and the response.

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