subliminal vs voluntary

subliminal

adj
  • Without directed awareness or thought; subconscious (dated), unconscious. 

  • Of an advertisement, a message, etc.: conveyed in a manner that a person is unaware of being influenced by it. 

  • Of a stimulus: below the limen or threshold of conscious perception, especially if still able to produce a response; also (generally), below the threshold where a response can be produced. 

noun
  • An advertisement, a message, etc., which is conveyed in a manner that a person is unaware of being influenced by it. 

  • Chiefly preceded by the: something which is below the limen or threshold of conscious perception. 

  • Chiefly preceded by the: the subconscious (dated) or unconscious self. 

voluntary

adj
  • Done by design or intention; intentional. 

  • Done, given, or acting of one's own free will. 

  • Of or relating to voluntarism. 

  • Endowed with the power of willing. 

  • Working or done without payment. 

noun
  • A supporter of voluntarism; a voluntarist. 

  • A short piece of music, often having improvisation, played on a solo instrument. 

  • A volunteer. 

How often have the words subliminal and voluntary occurred in a corpus of books? (source: Google Ngram Viewer )