Chen-Hui Lu Providence University, Taiwan
Robert W. Proctor Purdue University
Stimulus-response (S-R) compatibility effects are differences in reaction time (RT) and accuracy of responding as a function of the relation or mapping between stimulus and response sets. Such effects typically are attributed to response-selection processes, that is, to processes that intervene between stimulus identification and execution of a specific motor action. Most well known are spatial compatibility effects, which were first demonstrated by Fitts and colleagues. Fitts and Deininger ( 1954) varied the S-R mapping for circular stimulus and response arrays and showed that responses were fastest and most accurate when each stimulus location was mapped to the corresponding response location. Mapping effects of this type are what are examined most commonly in compatibility studies.
Spatial compatibility is described in most textbooks and handbooks on Human Factors and Ergonomics (e.g., Sanders & McCormick, 1993), and other types of compatibility are often mentioned as well, for example, conceptual compatibility, modality compatibility, and so on. Consequently, virtually all ergonomists are familiar with the principle of using compatible display-control sets and mappings when possible. However, the treatments given S-R compatibility typically are rudimentary and do not reflect the level of empirical and theoretical knowledge on the topic that exists at present.
The purpose of this paper is to provide a brief survey of current theoretical and conceptual views regarding the determinants of spatial compatibility effects. We will also examine implications of these views and of pertinent empirical findings for display-control design.
Central to most accounts of S-R compatibility is the concept of coding. In the case of spatial compatibility effects, the emphasis is on spatial codes that mediate between stimuli and responses. Evidence for spatial codes is most apparent in two-choice reaction tasks, where a left or right keypress response is to be made to a left or right stimulus. The spatially direct mapping is faster than the indirect mapping, even when the arms are crossed so that the left response key is pressed by the right hand and the right response key with the left hand (e.g., Dutta & Proctor, 1992). In most situations, the spatial codes are based on relative location, that is, regardless of whether the display or response keys are directly in front of the performer or to the side, the mapping of left-to-left and right-to-right is superior to the reversed mapping (e.g., Proctor, Van Zandt, Lu, & Weeks, 1993). It is worth pointing out that spatial compatibility effects also occur when the spatial information is conveyed by words (e.g., LEFT), symbols (e.g., a left-pointing arrow), and direction of motion (e.g., leftward movement), as well as for vocal responses of a spatial nature (e.g., the utterances LEFT and RIGHT; Wang & Proctor, 1996).
One interesting fact is that stimulus location affects performance even when it is irrelevant to the task. Thus, spatial compatibility cannot be ignored by a designer even when the relevant stimulus information is nonspatial. For example, if you are to respond to a red stimulus with a left response and a green stimulus with a right response, responses are faster when the stimulus and response locations correspond than when