Covert attention, the selective processing of visual information at a given location in the absence of eye movements, improves performance in several tasks, such as visual search and detection of luminance and vernier targets(1-6). An important unsettled issue is whether this improvement is due to a reduction in noise (internal or external)(6-9), a change in decisional criteria(10,11), or signal enhancement(3,5,12). Here we show that attention can affect performance by signal enhancement. For a texture segregation task in which performance is actually diminished when spatial resolution is too high, we observed that attention improved performance at peripheral locations where spatial resolution was too low, but impaired performance at central locations where spatial resolution was too high(4-12). The counterintuitive impairment of performance that we found at the central retinal locations appears to have only one possible explanation : attention enhances spatial resolution,