Bill Walsh is a mind pioneer. This brief video on one depression biotype - undermethylation - [1 of 5] will likely whet your appetite on the fact that depression is more than just an appearance of sadness.
#5: The Whatever Drop: Less frequent, even more subtle, and often seen as apathetic in association - "Who cares? - not me, whatever, I'm gone." Most characteristic of this drop is the emotional tone, the feeling, behind the expression.