heart-searching vs introversion

heart-searching

introversion

noun
  • Arrangement of two similar words, lines, etc. to form the middle part of a structure. 

  • The action of turning one's thoughts upon internal or spiritual matters. 

  • A turning inward 

  • A personality orientation towards the self and mental abstraction; behavior expressing such orientation. 

How often have the words heart-searching and introversion occurred in a corpus of books? (source: Google Ngram Viewer )