Humans are constantly perceiving visual and auditory stimuli. Sometimes our perception of these stimuli occurs consciously: we are aware of a stimulus and know that we are perceiving it. But our perception of a stimulus can also occur without our awareness: an image might appear and disappear before our eyes too quickly for us to notice that we saw it, or a sound might be too faint for us to realize that we heard it. This phenomenon—the perception of a stimulus just below the threshold of conscious awareness—is called subliminal perception. Experiments have shown that subliminally perceived stimuli can influence people's thoughts and attitudes.