Wondering about that, too.
But, I think that even when our brain is affected by our illness ~ we still react in a way that isn't a 100% stranger to us, it's rather derived from our personality bank.
The bank includes unconscious parts of our personality which we might not exhibit on a regular day.

Everybody has a life and death wish inside them.
When I'm depressed my death wish is controlling the party (wat a sad party..).
When I'm high the exuberance is blasting out.

The content may be very bizarre and strange and "not us" and not typical of us, but it's still comes from our 'bank' and not some 'bipolar bank'.
The imbalances in the brain causing the expressions of our life&death wishes go out of proportion ..