Contrast sensitivity functions (CSFs) were measured in the foveal region while subjects simultaneously performed an instantaneous judgment task, which was designed to maintain their attended areas at a constant size. Spatial attention was sustained over areas that were either narrow (Narrow condition) or broad (Broad condition). We observed that the sensitivity at higher frequencies (over 3 cpd) was greater under the Narrow condition than that under the Broad condition, supporting the argument that attention enhances spatial resolution.