The neural underpinnings of attention are still incompletely understood. Debes and Dragoi present causal evidence that the attentional modulation of individual neurons and cell populations is performed by cortical feedback projections. Suppressing cortical area V4 feedback axons to area V1 without altering local and feedforward intracortical inputs virtually abolished the attention-induced benefits on response gain, the accuracy of network computations, and the attentional modulation of postsynaptic area V4 neuron responses. These findings indicate that feedback signals modulate visual cortical responses only when stimuli are attended. Because feedback has been hypothesized to control other top-down processes, these results could have implications for a broad range of phenomena across many modalities.