To expand a bit on the implications for depression treatment (NB: not an expert, just someone who's read some things): There are different patterns of how depr

To expand a bit on the implications for depression treatment (NB: not an expert, just someone who's read some things): There are different patterns of how depression symptoms respond to nominally rewarding experiences. Some people with depressive symptoms have a clear but blunted response to rewards: mood elevates, but in a more subdued and fleeting way than would otherwise be expected. This is associated with so-called "atypical" depression, which is named not for its prevalence (it's common), but for the idea that it doesn't fit the profile of "standard" depression. Other people in depressive episodes seem to simply not process rewards at all, often even experiencing guilt over their "failure" to be happy about something. This is associated with "melancholic" depression. At least superficially, it seems to match what might be expected if only the "what could have been" side of the equation is functioning (i.e. everything feels like a failure/disappointment).