Functional specificity of superior parietal mediation of spatial shifting

Using event-related functional magnetic resonance imaging (fMRI) we determi ned how brain activity changes when an attended target shifts its location. In the main experiment, a white square could appear at 10 possible eccentr icities along the horizontal meridian. It remained on the screen for a vari able period of time and then changed location. At any time the stimulus cou ld dim briefly. Subjects had to press a button when the stimulus dimmed. In order to perform this task attention had to be locked onto the target and shift with it. Half of the runs were performed overtly and half covertly. T he event of interest consisted of the shift in the location of the attentio nal target. The state of maintained attention occurring in between the shif ts constituted the baseline. The superior parietal gyrus was activated bila terally in response to attentional shifts. No other area showed a significa nt response to shifting. On the left side the amplitude of the superior par ietal response correlated positively with the distance of the shift. On the right side a significant correlation was present only for overt shifts. In a separate experiment we compared the maintaining of attention at a single spatial location to passive fixation: the frontal eye fields, anterior cin gulate, right dorsolateral prefrontal cortex, and inferior parietal lobule were significantly activated, indicating that the absence of a shift-relate d response in these areas in the main experiment was due to the fact that; they were equally activated by maintaining and shifting attention. The resp onse to spatial shifts and the correlation with the distance between the or iginal and the new location points to a specific role of the superior parie tal gyrus in shifting the locus of spatial attention. (C) 2001 Academic Pre ss.