CORTICAL FMRI ACTIVATION PRODUCED BY ATTENTIVE TRACKING OF MOVING TARGETS

Attention can be used to keep track of moving items, particularly when there are multiple targets of interest that cannot all be followed wi th eye movements. Functional magnetic resonance imaging (fMRI) was use d to investigate cortical regions involved in attentive tracking. Cort ical flattening techniques facilitated within-subject comparisons of a ctivation produced by attentive tracking, visual motion, discrete atte ntion shifts, and eye movements. In the main task, subjects viewed a d isplay of nine green ''bouncing balls'' and used attention to mentally track a subset of them while fixating. At the start of each attentive -tracking condition, several target balls (e.g., 3/9) turned red for 2 s and then reverted to green. Subjects then used attention to keep tr ack of the previously indicated targets, which were otherwise indistin guishable from the nontargets. Attentive-tracking conditions alternate d with passive viewing of the same display when no targets had been in dicated. Subjects were pretested with an eye-movement monitor to ensur e they could perform the task accurately while fixating. For seven sub jects, functional activation was superimposed on each individual's cor tically unfolded surface. Comparisons between attentive tracking and p assive viewing revealed bilateral activation in parietal cortex (intra parietal sulcus, postcentral sulcus, superior parietal lobule, and pre cuneus), frontal cortex (frontal eye fields and precentral sulcus), an d the MT complex (including motion-selective areas MT and MST). Attent ional enhancement was absent in early visual areas and weak in the MT complex. However, in parietal and frontal areas, the signal change pro duced by the moving stimuli was more than doubled when items were trac ked attentively. Comparisons between attentive tracking and attention shifting revealed essentially identical activation patterns that diffe red only in the magnitude of activation. This suggests that parietal c ortex is involved not only in discrete shifts of attention between obj ects at different spatial locations but also in continuous ''attention al pursuit'' of moving objects. Attentive-tracking activation patterns were also similar, though not identical, to those produced by eye mov ements. Taken together, these results suggest that attentive tracking is mediated by a network of areas that includes parietal and frontal r egions responsible for attention shifts and eye movements and the MT c omplex, thought to be responsible for motion perception. These results are consistent with theoretical models of attentive tracking as an at tentional process that assigns spatial tags to targets and registers c hanges in their position, generating a high-level percept of apparent motion.