Perceptual experience results from a complex interplay of bottom-up input and prior knowledge or expectations about the world, yet the neural mechanisms by which prior knowledge biases incoming sensory information, as well as the precise stage of neural processing at which these two sources of information converge, is unclear. We investigated this issue by recording electroencephalography (EEG) while observers compared two sequentially presented ambiguous Mooney images of objects. Prior to the main experiment, participants were trained on the meaning of the objects in half of the images, allowing us to experimentally manipulate prior knowledge for a subset of stimuli. Same/different accuracy and response times benefited from prior knowledge of the target identity. This effect was accompanied by a larger occipital-parietal P1 evoked response to the trained versus untrained target stimulus. Time-frequency analysis of the interval between the two stimuli (just prior to the target stimulus) revealed increases in the power of posterior alpha-band (8-14 Hz) oscillations when the meaning of the stimuli to be compared was trained. The magnitude of the prestimulus alpha difference and the P1 amplitude difference was positively correlated across individuals. These results suggest that prior knowledge about visual information prepares the brain for upcoming perception via the modulation of prestimulus alpha-band oscillations, and that this preparatory state influences early (~ 120 ms) stages of subsequent visual processing.