Psychophysical method of study is good at recording input-output relationships of human perceptual behavior. Functional brain imaging gives clue to which area in the brain is activated or how the activation changes with a certain set of stimulus. Both methods should be used in a complementary way: one modality of study supports a hypothesis in the other. Here, an example study about the investigation of the mechanism of perceptual illusion will be introduced. Both psychophysical experiment and functional brain imaging were used to reveal that the reduction of signal-to-noise ratio in the activity of human visual cortex caused the phenomenon.