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

Participants received three sets of trials while regional cerebral blood flow was assessed using positron emission tomography (PET). In one set, the baseline, they responded when they detected a horizontal bar with an × either above or below it. In another set, the categorical spatial relations judgment condition, they decided whether the × was above or below the bar. In the third set, the coordinate spatial relations judgment condition, they decided whether the × was within 0.5 in. of the bar. In Experiment 1, the precise locations of the bars and × marks were varied, which required participants to focus attention selectively. Consistent with previous behavioral and neuropsychological findings, the left hemisphere was generally more activated during the categorical judgment task than during the coordinate judgment task, whereas the right hemisphere was generally more activated during the coordinate judgment task. In addition, these apparently simple tasks drew on large and different networks of areas. Experiment 2 was the same as Experiment 1 except that the stimuli were always in the same position and were presented very briefly. Three areas in the right parietal lobe were activated more by the coordinate task than by the categorical task in both experiments. In contrast, in neither experiment were any common areas activated more by the categorical task than by the coordinate task.