Why can't Norman Bates say the word "bathroom" in the movie Psycho?

Working on an ignment for my psychology cl and this question has me stumped. I have already explained his dissociative personality disorder, but I'm not sure how this fits into this disorder. Does it cause Norman's mothers personality to try and show itself because he is attracted to Marion? I don't see the connection for his attraction and the word bathroom. Any help would be great!