Cortical activity such as recorded by EEG or MEG is characterized by ongoing rhythms that encompass a wide range of temporal and spatial scales. Recent studies have suggested an oscillatory hierarchy with faster oscillations being locked to preferred phases of underlying slower waves, a functional principle applied up to the level of action potential generation. We here tested the idea that amplitude-phase coupling between frequencies might serve the detection of weak sensory signals. To this end we recorded neuromagnetic responses during a motion discrimination task using near-threshold stimuli. Amplitude modulation of occipital high-frequency oscillations in the gamma range (63+/-5 Hz) was phase locked to a slow-frequency oscillation in the delta band (1-5 Hz). Most importantly, the strength of gamma amplitude modulation reflected the success in visual discrimination. This correlation provides evidence for the hypothesis that coupling between low- and high-frequency brain oscillations subserves signal detection.