The authors manipulated stimulus contrast and response-stimulus interval in the alternating runs paradigm to investigate whether early processing could be carried out during a task switch. Subjects alternated between judging the magnitude and the parity of a digit. The results suggested that early processing was not carried out during the task switch (Experiment 1), even in the absence of potentially confounding auditory or visual warning signals (Experiment 2). This processing was, however, carried out in parallel with a demanding operation in a 2nd task (Experiment 3), using the display parameters of Experiments 1 and 2 in the psychological refractory period paradigm. It is concluded that, functionally, task switching may impose a hard bottleneck even for very early stimulus processing. ((c) 2003 APA, all rights reserved)