Graded functional activation in the visuospatial system with the amount of task demand.

Two studies examined how the amount and type of computational demand are related to fMRI-measured activation in three bilateral cortical regions involved in the Shepard-Metzler (1971) mental-rotation paradigm. The amount of demand for the computation of visuospatial coordinates was manipulated by presenting mental rotation problems with increasing angular disparity (0, 40, 80, or 120 degrees). Activation in both the left and right intraparietal sulcal regions increased linearly with angular disparity in two separate studies. Activation also occurred in the fusiform gyrus and inferior temporal regions, regions that are primarily associated with the processes of object and object-part identification. By contrast, the demand for object recognition and rotation processes was relatively low, and the demand for executing saccades was high in a control condition that required making a systematic visual scan of two grids. The grid-scanning condition resulted in relatively less activation in the parietal and inferior temporal regions but considerable activation in frontal areas that are associated with planning and executing saccades, including the precentral gyrus and sulcus into the posterior middle frontal region. These data suggest that the amount of activation in the various cortical regions that support visuospatial processing is related to the amount, as well as to the type, of computational demand.