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Psychonomic Bulletin & Review 2002, 9 (2), 212-238 Stimulus–response compatibility and psychological refractory period effects: Implications for response selection MEI-CHING LIEN and ROBERT W. PROCTOR Purdue University, West Lafayette, Indiana The purpose of this paper was to provide insight into the nature of response selection by reviewing the literature on stimulus–response compatibility (SRC) effects and the psychological refractoryperiod (PRP) effect individually and jointly. The empirical findings and theoreticalexplanations of SRC effects that have been studied within a single-task context suggest that there are two response-selection routes—automatic activation and intentional translation. In contrast, all major PRP models reviewed in this paper have treated response selection as a single processing stage. In particular, the response- selection bottleneck (RSB) model assumes that the processing of Task 1 and Task 2 comprises two sep- arate streams and that the PRP effect is due to a bottleneck located at response selection. Yet, consider- able evidence from studies of SRC in the PRP paradigm shows that the processing of the two tasks is more interactive than is suggested by the RSB model and by most other models of the PRP effect. The major implication drawn from the studies of SRC effects in the PRP context is that response activation is a distinct process from final response selection. Response activationis based on both long-term and short-term task-defined S–R associations and occurs automatically and in parallel for the two tasks. The final response selection is an intentional act required even for highly compatible and practiced tasks and is restricted to processing one task at a time. Investigations of SRC effects and response- selection variables in dual-task contexts should be conducted more systematically because they pro- vide significant insight into the nature of response-selection mechanisms. In studies of human information processing, the pro- response selection for one task is affected by response se- cessing stages of stimulus identificationand response ini- lection for another. tiation are typically distinguished from a central process- Although considerable research has been conducted on ing stage that is often called response selection. Issues the SRC and PRP effects individually, few experiments regarding response selection have been investigated thor- have examined the two effects jointly.A reason for the lack oughly in two areas of research, stimulus–response com- of studies manipulating SRC in the PRP paradigm may lie patibility(SRC) and psychologicalrefractory period (PRP) in the belief that response selection for each task occurs the effects. Fitts and Seeger (1953) were the first to demon- same way as it does in a single-task context. However, as strate SRC effects, showing that performance depends not Duncan (1979) pointed out, “In any PRP situation,the re- only on the individual properties of the stimuli and re- sponse sets of two singletasks have been combined to give sponses but also on their relation. A large empirical base a larger total set. It is perhaps unreasonableto suppose that regarding SRC effects exists, along with relatively sophis- this leaves unaffected the complexity of response choice” ticated theoretical accounts of how response selection in (p. 225). Moreover, Hommel (1998) demonstrated that single-task performance is affected by compatibility rela- considerableinteraction occurs between the two tasks. The tions. Telford (1931) was the first to demonstrate the PRP implication is that one cannot assume that response selec- effect, in which, when two tasks must be performed on tion for each task in dual-task contexts is performed in the successive stimuli, the response to the second stimulus is same manner as when each task is performed alone. Sys- delayed when the interval between stimulus onsets is tematic examination of SRC effects in the PRP paradigm short. As for SRC effects, substantial empirical and theo- is necessary to provide a more complete understandingof retical work on the PRP effect has been conducted on how the response-selection mechanism. The purpose of the present paper, therefore, was to re- view SRC and PRP in the literature as they relate to issues We thank Bernhard Hommel, Hal Pashler, and an anonymous re- of response selection and, most importantly,to examine in viewer for their helpful comments on a previous version of this manu- detail those studies that have looked at SRC effects in the script. Correspondence concerning this article should be addressed to PRP paradigm for any additional insights into the nature M.-C. Lien, Mail Stop 262-4, NASA Ames Research Center, Moffett Field, CA 94035 or to R. W. Proctor, Department of Psychological Sci- of response selection that they afford. We begin by review- ences, Purdue University, West Lafayette, IN 47907 (e-mail: mclien@ ing SRC effects and the models developed to account for mail.arc.nasa.gov or [email protected]). them. Such effects have been obtained for relevant and ir- Copyright 2002 Psychonomic Society, Inc. 212 IMPLICATIONS OF SRC AND PRP EFFECTS FOR RESPONSE SELECTION 213 relevant stimulus dimensions in a variety of tasks for response sets or the members withinthe sets.Fitts and Seeger which there is similarity among stimulus and response di- (1953) combined three stimulus panels (Sa,Sb,Sc) with mensions, and most current models include both auto- three response panels (Ra,Rb,Rc) to generate nine matic and intentionalresponse-selectionroutes to account stimulus–response (S–R) ensembles (see Figure 1). The for these effects. We, therefore, discuss the functional as- subject was to move one or two styluses to the response lo- pects of the PRP models,along with pertinentempirical ev- cation(s) corresponding to the stimulus location(s). RT idence.Most PRP models treat response selectionas a sin- was shorter and error rate lower when the response panel gle intentional process that operates separately for each corresponded physically with the stimulus display than task, in contrast to the models of SRC effects. The evi- when it did not—that is, for the combinations of Sa–Ra, dence suggests that there is a bottleneck in response se- Sb–Rb,andSc–Rc. Fitts and Seeger attributed their results lection, structural or strategic, that allows only one re- to the time it took for S–R translation,stating, “A task in- sponse to be selected at a time. Finally, we examine the volves compatible S–R relations to the extent that the en- influence of SRC in the PRP paradigm, showing that the semble of stimulus and response combinations compris- PRP effect is resistant to elimination even with highly ing the task results in a high rate of information transfer” compatibletasks and that cross-talk occurs when there are (p. 199). Set-level compatibilityeffects of the type demon- similarities between the stimuli and responses for the two strated by Fitts and Seeger, in which some combinationsof tasks. We conclude that the conceptionof response selec- S–R sets yielded faster responses than others, are well tion that best fits most results is one that distinguishesre- documented (Kornblum, Hasbroucq, & Osman, 1990). sponse activation, based on long-term and short-term as- Another type of SRC effect, called element-level com- sociations, from final response selection. patibility, occurs when the S–R sets are held constant but the mapping of individual stimuli to responses is varied. STIMULUS–RESPONSE COMPATIBILITY Fitts and Deininger (1954) used circular S–R sets (Sa and IN SINGLE-TASK PERFORMANCE Ra in Fitts & Seeger’s,1953, experiment;see Figure 1), with the mappingbeing direct, mirrored, or random. Direct map- Effects of Relevant Stimulus Information ping involved moving the stylus to the location corre- SRC effects are differences in reaction time (RT) and ac- sponding to that of the stimulus. Mirrored mapping con- curacy, as a function of the mapping between stimulus and sisted of reversing the left–right relations while retaining Figure 1. The three stimulus panels (Sa,Sb,andSc ) and three response panels (Ra, Rb,andRc ) used in Fitts and Seeger’s (1953) Experiment 1, along with the response times and errors for each combination of stimulus and response panels. From “S–R Compatibility: Spatial Characteristics of Stimulus and Response Codes,” by P.M. Fitts & C. M. Seeger, Journal of Experimental Psychology, 46, pp. 199-210,Figure 1. Copy- right 1953 by the American Psychological Association. Adapted with permission. 214 LIEN AND PROCTOR the relationsin the verticaldimension.For random mapping, not between stimuli and responding hands. In most cases, stimuli were arbitrarily assigned to responses. Responses the important spatial relation is the relativepositionof the were fastest for direct mapping (387 msec), intermediate two stimuli and the two responses (e.g., Umiltà & Nico- for mirrored mapping (541 msec), and slowest for random letti, 1990). These results have been interpreted as imply- mapping (1,111 msec). Several authors have attributedthe ing that response selection in two-choice tasks is based on advantagefor mirrored mapping over random mapping to spatial stimulus and response codes (Hommel, 1997). the fact that a single S–R translation rule (respond at the In most SRC studies, only a single mapping, compatible mirror oppositelocation)can be appliedto all stimulusel- or incompatible, is in effect at any time. There are some ements for mirrored mapping but not for random mapping studies, however, that have examined mixed mapping con- (e.g., Duncan, 1977a; Rosenbloom & Newell, 1987). ditions,in which some stimuli are mapped to responses in- Pure and mixed spatial mappings. Most SRC research compatiblyand others compatibly.These studies are rele- since that of Fitts and colleagues(Fitts & Deininger, 1954; vant to the PRP literature because PRP studies typically Fitts & Seeger, 1953) has been conductedwith two- or four- involvetwo different S–R mappings. Duncan (1977b)used choice tasks in which subjects make keypress responses to either a single mapping rule (pure condition) or multiple the alternative stimuli with different spatial mappings.

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