These personalities often go by different names and the stronger new/stronger personality is often aware of the orignal/others whereas the original personality often just recalls 'blacking out'.
Psychologists Thigpen and Cleckley conducted a study on a woman claiming to have 'blinding headaches', which concluded she infact was one of the vary rare cases of reported MPD victims. (For further reading, this study can be found here: http://www.holah.co.uk/, and is explained well in the film based on this study 'The Three Faces Of Eve').
During the interview, Eve White who was normally very self-controlled became distressed and asked whether hearing an occasional imaginary voice made her insane.
She reported that she had on several occasions over the last few months briefly heard a voice addressing her. During this conversation Eve White, as if in pain suddenly put both hands to her head. After a tense moment of silence her hands dropped, and the therapist observed a ‘quick, reckless smile’ and in a bright voice she said: ‘Hi there, Doc’