Switching between the forest and the trees: Brain systems involved in local/global changed-level judgments

Visual targets can be coded, in relative terms, at either the local or the global level of stimuli. Previous studies have indicated that targets are i dentified more slowly when they appear at a new hierarchical level, compare d to when they reappear at the same level as in the previous trial. In the present study, we used measures of reaction time and event-related fMRI to investigate factors affecting this switch cost. In particular, we examined the effects of the number of repeated-level trials preceding a switch and w hether the cue to switch was either externally or internally mediated. At t he behavioral level we found that (1) the time taken to identify a target o n a changed-level trial is longer following four repeated-level trials comp ared to two repeated-level trials, but that runs of six do not produce addi tional costs over four, and (2) targets can be identified faster following externally cued switches compared to internally mediated switches. Ne then show that these behavioral effects are associated with distinct patterns of neural activation. Switches performed after two repeated-level trials pref erentially activated the precuneus, while those performed after bath four a nd six activated bilateral inferior parietal cortex and motor hand area. Re lative to external switches, internal snitches activated the putamen, while both kinds of switch conjointly activated the large-scale network proposed to underlie internal/external switches in nonhierarchical tasks. Our data further clarify the mechanisms mediating hierarchical selection. (C) 2001 A cademic Press.