I have come across a peculiar use of the negative form in northern and central Vermont, USA
For example
First person - "I went to the Mall today"
Second person's response - "So didn't I"

While logically this seems to be saying that the second person did not go to the Mall (and that the first person didn't either), somehow it is understand to mean that the second person did in fact go to the Mall.

How can this be?