The ability to process relevant stimuli selectively is a fundamental function of the primate visual system. The best-understood correlate of this function is the enhanced response of neurons in the visual cortex to attended stimuli(1,2). However, recent results show that the superior colliculus (SC), a midbrain structure, also has a crucial role in visual attention(3-5). It has been assumed that the SC acts through the same well-known mechanisms in the visual cortex(3,5). Here we tested this hypothesis by transiently inactivating the SC during a motion-change-detection task and measuring responses in two visual cortical areas. We found that despite large deficits in visual attention, the enhanced responses of neurons in the visual cortex to attended stimuli were unchanged. These results show that the SC contributes to visual attention through mechanisms that are independent of the classic effects in the visual cortex, demonstrating that other processes must have key roles in visual attention.