That is a good question. Each subject will have experienced many trials of the task, likely more than a hundred. No doubt each subject skipped making a decision on a few trials – either through indecision, or through not paying attention.

As all the hypotheses tested were about gains and losses, so I would assume those skipped trials were left out of the analyses, because they do not provide evidence either for or against the hypotheses (with no decision comes no gain and no loss). One could equal well frame an hypothesis about what brain region(s) would show changes in activity during indecision, and use those skipped trials to test it. But the rate of skipped trials is normally very low – I’d wager less than 5 percent, possibly much less than that.