http://goo.gl/l0sbZR
According to the new theory, as people learn from experiences that are colored by their mood, their expectations come to reflect not only the reward associated with each particular state (such as each stock), but also recent changes in the overall availability of reward in their environment. In this way, the existence of mood allows learning to account for the impact of general environmental factors.
"This effect of mood should be useful whenever different sources of reward are interconnected or possess an underlying momentum," says one of the study's lead authors, Eran Eldar of University College London. "That may often be the case in the natural as well as in the modern world, as successes in acquiring skills, material resources, social status, and even mating partners may all affect one another."