Does probe rt measure central processing demand?

Abstract

Various authors have tried to assess the processing demands of a range of tasks by presenting a probe at different times during their performance. The assumption behind the technique is that the more processing capacity the main task requires the less will be left for processing the probe. Hence, it is claimed, the reaction time to the probe can be used to infer the capacity demands of the main task. An experiment is reported in which two groups of women combine manual responses to a visual Same/Different letter-match task with responding to an auditory probe. The only difference between the groups is that one responds manually to the probe and the other responds vocally. The two groups produce an entirely different pattern of interaction between the two tasks.

Were there an absolute central demand of the letter-match task which could be measured by the probe technique this should appear irrespective of the form of the probe task. Since the pattern of probe RTs does depend on the form of the response to the probe it is concluded that it is not possible to use a single form of probe task as a neutral measure of the central capacity demand of the main task.