mental image vs speculative fiction

mental image

noun
  • Perception created in the mind that is not physically present. 

speculative fiction

noun
  • A class of fiction involving fantastic, supernatural or futuristic elements, including fantasy, horror, and science fiction, dealing with worlds, themes, and characters not part of everyday life. 

  • A particular work belonging to this class. 

How often have the words mental image and speculative fiction occurred in a corpus of books? (source: Google Ngram Viewer )