We assume an attacker can use any of these methods to
facilitate extraction of private information. In addition, an
attacker can present malicious stimuli in an overt (conscious)
fashion, as well as in a subliminal (unconscious) way, with
subliminal stimulation defined as the process of affecting
people by visual or audio stimuli of which they are completely
unaware . Ways of achieve unawareness typically include
reducing a stimulus intensity or duration below the required
level of conscious awareness.