The N2pc component of the event-related potential (ERP) is an index of visual-spatial attention. It is not clear whether the N2pc reflects pure top-down attentional activity or an interaction of top-down activity with bottom-up sensory activity. Here, we manipulated stimulus intensity of the items composing the target display. Although the amplitude of the P1 component increased monotonically with increasing stimulus intensity, the amplitude of the N2pc did not vary with stimulus intensity. Instead, the onset latency of the N2pc was delayed for weaker stimuli, suggesting that the strength of the selection cue (target color) influenced the moment at which attention was deployed. The results reveal one way in which early sensory ERP amplitude differences are converted into later latency differences.