backflash vs mental image

backflash

noun
  • Synonym of flashback (“dramatic device”) 

  • The hazardous situation where the flame propagates backward down the hose of an oxyfuel welding and cutting system. 

mental image

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

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