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Subliminal stimuli, contrary to supraliminal stimuli or "above threshold", are any sensory stimuli below an individual's threshold for conscious perception. A 2012 review of functional magnetic resonance imaging (fMRI) studies showed that subliminal stimuli activate specific regions of the brain despite participants' unawareness. Visual stimuli may be quickly flashed before an individual can process them, or flashed and then masked to interrupt processing. Audio stimuli may be played below audible volumes or masked by other stimuli.