He asserted that childhood or infantile amnesia was a precursor to the ‘hysterical amnesia,’ or repression, presented by his adult patients.[13] Freud asked his patients to recall their earliest memories and found that they had difficulty remembering events from before the age of six to eight.[14] Freud coined the term "infantile" or "childhood amnesia" and discussed this phenomenon in his Three Essays on the Theory of Sexuality.[14] In 1972, Campbell and Spear published a seminal review about childhood amnesia in Psychological Sciences recapping the research conducted to understand this topic from neurological and behavioral perspectives in both human and animal models.