The number of attentional foci and their precision are dissociated in the posterior parietal cortex

Many everyday tasks require us to track moving objects with attention. The demand for attention increases both when more targets are tracked and when the targets move faster. These 2 aspects of attention-assigning multiple attentional foci (or indices) to targets and monitoring each focus with precision-may tap into different cognitive and brain mechanisms. In this study, we used functional magnetic resonance imaging to quantify the response profile of dorsal attentional areas to variations in the number of attentional foci and their spatiotemporal precision. Subjects were asked to track a specific spoke of either 1 or 2 pinwheels that rotated at various speeds. Their tracking performance declined both when more pinwheels were tracked and when the tracked pinwheels rotated faster. However, posterior parietal activity increased only when subjects tracked more pinwheels but remained flat when they tracked faster moving pinwheels. The frontal eye fields and early visual areas increased activity when there were more targets and when the targets rotated faster. These results suggest that the posterior parietal cortex is specifically involved in indexing independently moving targets with attention but not in monitoring each focus with precision.