Simple perceptual decisions are ideally suited for studying the sensorimotor transformations underlying flexible behavior. During perceptual detection, a noisy sensory signal is converted into a behavioral report of the presence or absence of a perceptual experience. Here, we used magnetoencephalography (MEG) to link the dynamics of neural population activity in human motor cortex to perceptual choices in a "yes/no" visual motion detection task. We found that (1) motor response-selective MEG activity in the "gamma" (64-100 Hz) and "beta" (12-36 Hz) frequency ranges predicted subjects' choices several seconds before their overt manual response; (2) this choice-predictive activity built up gradually during stimulus viewing toward both "yes" and "no" choices; and (3) the choice-predictive activity in motor cortex reflected the temporal integral of gamma-band activity in motion-sensitive area MT during stimulus viewing. Because gamma-band activity in MT reflects visual motion strength, these findings suggest that, during motion detection, motor plans for both "yes" and "no" choices result from continuously accumulating sensory evidence. We conclude that frequency-specific neural population activity at the cortical output stage of sensorimotor pathways provides a window into the mechanisms underlying perceptual decisions.