Significance Neural oscillations have been shown to support a range of cognitive abilities. Here we demonstrate that delta activity (2–4 Hz) in the prefrontal cortex tracked the current task context and modulated sensory processing in a top-down manner. We show that frontal delta and parietooccipital alpha (8–12 Hz) oscillations are functionally coupled and jointly guide visual perception to integrate sensory evidence with current task demands. We observed strong moment-to-moment behavioral fluctuations, which cycled at the rate of the endogenous prefrontal oscillatory brain activity. Our findings suggest that neuronal oscillations provide the functional basis for context-dependent visual perception.