ABSTRACT
According to original filter theory, there was just one processing channel and therefore task combination could only be achieved by rapid switching of the filter and multiplexing or time-sharing tasks. If it could be demonstrated that two complex tasks that should require continuous attentional processing could be combined without loss of speed or accuracy, then the argument that there was only a single processing channel would have to be abandoned. Allport et al. (1972) asked competent keyboard players to play the piano, sight reading examination pieces that they had not seen before, at the same time as shadowing prose at a rate of 150 words per minute. With only a little practice, Allport and colleagues’ subjects were able to perform both
tasks in combination as fast and as accurately as they could when they performed them separately. This result was interpreted as evidence against a single channel for attentional processing. Experiments like that of Allport et al. (1972) are not without their critics. Broadbent (1982) points out that it is possible to detect decrements in the performance data when the two tasks are combined. Furthermore, it is extremely difficult to determine whether or not each individual task requires absolutely continuous attentional processing. Broadbent would argue that both shadowing prose and sight reading music are tasks involving stimuli that have a certain amount of redundancy in them. Redundancy is a concept from information theory, which was explained in Chapter 2. What redundancy means is that prose and music contain information that allows the subjects to predict what letter or note is likely to come next. If the subject can predict with some certainty what is likely to come next in either task, then at those moments when predictability of the next word or note is high, attention can be rapidly switched, allowing time sharing between the tasks, rather than simultaneous combination. To be certain that there was no time sharing, both tasks would have to be absolutely continuous and include no redundancy whatsoever. We would also have to be certain that each individual task was being performed at the limit of attentional resources. Only if these conditions were fulfilled for the separate tasks and we could be certain that there were absolutely no decrements in either task when they were performed together could we say that there was no limit to dual task combination.
