We studied the correlation between perception and hemodynamic activity in the visual cortex in a change detection task. Whenever the observer perceived the location of a change, rightly or wrongly, the blood oxygenation level-dependent signal increased in the primary visual cortex and the nearby extrastriate areas above the baseline activity caused by the visual stimulation. This non-sensory-evoked activity was localized and corresponded to the perceived location of the change. When a change was missed, or when observers attended to a different task, the change failed to evoke such a response. The latency of the nonsensory component increased linearly with subjects' reaction time, with a slope of one, and its amplitude was independent of contrast. Control experiments are compatible with the hypothesis that the nonsensory hemodynamic signal is mediated by top-down spatial attention, linked to (but separate from) awareness of the change.