One way mood might affect memory and learning is through attentional bias to mood-congruent stimuli. In a 2011 study by Becker and Leinenger, participants were more likely to notice and remember stimuli that matched their mood state. For example, while working on a primary tracking task, participants in an experimentally manipulated sad mood were more likely to notice the introduction of an unexpected frowning face than those who were in the positive and neutral mood manipulated groups. Findings from this research suggest that how we feel can influence what we pay attention to, what we learn from the experience and how we experience the world. This in turn would also influence what we remember about past events and experiences.
Another way mood might affect learning and memory is through the inability of people suffering from depression to update the contents or working memory. In a study by Joorman and Gotlib (2008), subjects who were identified as clinically depressed; not just in a sad mood, had more difficulty ignoring irrelevant emotional material when the valence was negative. They also found that there was a positive correlation between increased difficulty removing negative information from working memory and rumination.
Since working memory is a limited capacity system that can only hold a select set of representations for a short time, an inability to expel irrelevant or unnecessary information would interfere with and negatively bias new learning. It would also contribute to an ongoing negative mood state.
References
Becker, M. W., & Leinenger, M. (2011). Attentional selection is biased toward mood-
congruent stimuli. Emotion, 11(5), 1248“1254.
Joormann, J., & Gotlib, I. H. (2008). Updating the contents of working memory in
depression: Interference from irrelevant negative material. Journal of Abnormal
Psychology, 117(1), 182“192.