Agreed, gotta wonder about this entire study. You can find partial definitions in Introduction (p2,3) [see the PsyArXiv link]. Just skimming it raises all sorts of serious questions about the participants and the authors' subjective definitions of "task" and "rest". Forcing participants to stare at the wall is not "rest", more like "enforced boredom" or "isolation". A gambling "task" or a facial recognition "task" are seriously different. And for the gambling task, presumably the outcome of the gambling and the reward/loss had an effect on affect, even if there was no money being gambled in this particular case.
> Introduction (p2,3):
The participants were "19 large and varied cohorts, totaling 116 healthy and depressed adolescents recruited in person, 1,913 adults recruited online from across the United States, and 26,896 participants performing a gambling task in a mobile app."
> An important but implicit notion amongst behavioural and affective scientists is that each participant has a baseline mood or affective state that will remain constant during an experiment or only vary with emotionally salient events. Mood is modelled as a discounted sum of rewards and punishments, but many models hold that the time scale over which these events unfold is irrelevant and the passage of time itself has no effect on mood.
>
This assumption of a constant affective background has profound methodological implications for psychological experiments. First, consider a “resting state” functional brain scan in which a participant is asked to stare at a fixation cross. Based on the constant affective background assumption, comparisons of resting-state neuroimaging data between (for example) depressed and non-depressed participants are thought to reveal differences in their task-general traits, rather than their response to experimentally imposed rest periods.
> Second, consider an event-related design, such as a gambling or face recognition task, during which participants experience stimuli (wins or losses) that elicit emotional reactions. When analysing these data, responses to task stimuli are thought to occur on top of (and are often contrasted to) the affective baseline, which is presumed to be time-invariant.
> ...
> We find that participants’ mood worsened considerably during rest periods and simple tasks, an effect we call “Mood Drift Over Time” (“mood drift” for short). This downward mood drift was replicated in 19 large and varied cohorts, totaling 116 healthy and depressed adolescents recruited in person, 1,913 adults recruited online from across the United States, and 26,896 participants performing a gambling task in a mobile app. It was not observed when participants freely chose their own activities. We show that mood drift is related to, but not a trivial extension of, the existing constructs of boredom and mind-wandering. We show that mood drift slopes are positively correlated with reward sensitivity and that this relationship is moderated by overall life happiness. These findings may have profound implications for experimental design and interpretation in affective science ..."