<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">It doesn't always have to be visible<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">
No, it absolutely has to be INVISIBLE.
The point is that the stimulus is picked up well beyond conscious perception. You never see it, you never know that you've seen it, etc.
The fun experiments involve subliminal projection of a stimulus, then blasting the participant with 120dB noise to elicit the startle response.
