DECISION AND RESPONSE IN DUAL-TASK INTERFERENCE

Experiments with two stimuli (S-1 and S-2) and two responses suggest t he existence of a stage of processing that cannot be shared between tw o concurrent tasks. Widespread support has been found for the hypothes is that response selection for Task(2) is postponed when the S-1 to S- 2 stimulus onset asynchrony (SOA) is short (Pashler, 1994a). At short SOAs, manipulations which impact Task(2) processing prior to response selection (e.g., degradation of stimulus quality) have little effect o n Task(2) response times (RTs). On the other hand, manipulations which art: thought to impact response selection or execution (e.g., Stroop interference) always impact Task(2) RTs. There is, however, one partic ularly compelling demonstration that appears to be inconsistent with t he response selection bottleneck hypothesis: Karlin and Kestenbaum (19 68) report that the RT difference between detection (i.e., 1-choice) a nd 2-choice discrimination dramatically decreases with decreasing SOA. Given that the primary difference between detection and discriminatio n is believed to be at response selection, their result may indicate a processing bottleneck at response execution (Keele, 1973). We fail to replicate the Karlin and Kestenbaum result in two substantive replica tions of Karlin and Kestenbaum's tasks and procedures. In the single e xperiment in which Karlin and Kestenbaum's result is replicated, a sim ple response execution bottleneck account is ruled out by the stabilit y of the difference between 2-choice and 3-choice discrimination times across SOA. Two additional experiments demonstrate that response prep aration and task strategy do not substantially contribute to the atten uation of response selection-level effects with decreasing SOA. (C) 19 97 Academic Press.