Temporal cost of switching between kinds of visual stimuli in a memory task.

These experiments measured the extra time required to respond when the type of stimulus was changed from one trial to the next. Heretofore, switching costs have been measured for a change in task, but we wished to isolate the cost of changing the sensorial component per se and its necessary analytical processing, as distinct from changing the question being posed or the type of response to be given. Thus, the task was identical throughout the experiments: continuous recognition in a single, 240-trial session, in which the subject was required to distinguish initial from repeat appearances of a stimulus, the single repetition of each stimulus occurring after 1-31 intervening trials. There were two categories of 200-ms stimuli, linguistic (words and non-words) and images (multiple-colored or gray scale panels, human faces, or butterflies); and two conditions of switching, predictable (alternating on each trial) or unpredictable, in which the switch occurred after three to eight trials of one kind. In the majority of cases, there was a robust switching cost, from 24 to 92 ms. The similarity of costs in the predictable and unpredictable modes suggests that this cost is derived at least as much from terminating the modus operandi for the preceding type of stimulus as from a reconfiguration of processing for the new type of stimulus. In the switch between words and images, the costs were "paradoxical" (asymmetrical), in that the switching from an image to a word was more costly than the reverse, that is, changing from the more to the less difficult required the greater time. This, too, is compatible with the idea that termination of the previous mode of processing is a major component of the cost. Thus, in contemplating the neuronal/cognitive events underlying visual memory, consideration must be given to the inertia of pre-existing linkages.