[Analysis of interference effects in simultaneous processing of 2 problems].

The aim of this investigation was to analyze dual-task interference in the so called Psychological Refractory Period (PRP) paradigm. In this paradigm subjects have to carry out two choice reaction tasks that overlap in time. A well known result is that reaction time on task 2 (Rt2) increases with decreasing overlap of the two tasks. Thereby reaction time on task 1 is described to be independent of the size of overlap (Rt 1). Usually, this result is explained by the assumption of a PRP which arises in processing of task 2, when serial processing is ongoing in both tasks. It was asked, 1.) whether the PRP is located before or after response selection in the first task and 2.) how the second task influences first task processing. In the experiment subjects had to carry out two choice reaction tasks together. In different conditions the difficulty of response selection in task 2 was systematically increased by varying the number of response alternatives (0, 1, 2, 3). Difficulty of response selection in task 1 was held constant. Overlap between both tasks was varied. This experimental design allows different hypotheses about the sources of interference in both tasks to be examined by use of Schweickert's Critical Path Technique (Schweickert, 1983). Contrary to the results of Karlin and Kestenbaum (1968) the effects of number of response alternatives and size of overlap on Rt2 indicate a localization of the PRP before response selection. The results support models which assume a serial processing in response selection (Welford, 1952). They are contrary to models of parallel processing in this stage (Keele, 1973). The influence of the number of alternatives in task 2 on Rt 1 can be explained by a mechanism of grouping both motor responses.