How did they explain Viki's DID pre-Slezak?
According to the OLTL 40th anniversary book, the explanation for Niki's first appearances on the show is as follows:
"As a little girl, Viki thought she had witnessed her father killing her mother. While Viki had seen her mother's death, the circumstances surrounding it had been misintererpered. Yes, her mother and father had been arguing at the stop of the stairs at Llanfair, but her father had not pushed his wife down the stairs. She had slipped during the argument and fallen. Viki's coming to that revealation in the late 1960s seemed to cure her of her need for the Niki personality."
From there, it eventually morphed into Viki walking in on her father and Irene doing the nasty, to her discovery as a teen that her father had built an underground city, where her first love Roger Gordon was killed (although he would later turn out to be alive), and that her father had had all of that (as well as being pregnant with Megan) wiped from her memory, to the ultimate revealation that her father had sexually abused her as a child in 1995.
