Neural systems that encode categorical versus coordinate spatial relations: PET investigations

Participants received three sets of trials while regional cerebral blood fl ow was assessed using positron emission tomography (PET). In one set, the b aseline, they responded when they detected a horizontal bar with an X eithe r above or below it. In another set, the categorical spatial relations judg ment condition, they decided whether the X was above or below the bar. In t he third set, the coordinate spatial relations judgment condition, they dec ided whether the X was within 0.5 in, of the bar. In Experiment 1, the prec ise locations of the bars and X marks were varied, which required participa nts to focus attention selectively. Consistent with previous behavioral and neuropsychological findings, the left hemisphere was generally more activa ted during the categorical judgment task than during the coordinate judgmen t task, whereas the right hemisphere was generally more activated during th e coordinate judgment task. In addition, these apparently simple tasks drew on large and different networks of areas. Experiment 2 was the same as Exp eriment 1 except that the stimuli were always in the same position and were presented very briefly. Three areas in the right parietal lobe were activa ted more by the coordinate task than by the categorical task in both experi ments. In contrast, in neither experiment were any common areas activated m ore by the categorical task than by the coordinate task.