This study investigated how emotion could enhance the effectiveness of a digital training game in improving the cognitive skill of shifting. Participants were randomly assigned to either an emotionally-positive visual design of the game, featuring warm colors, round shapes, and high-intensity character expressions, or an emotionally-neutral design, with grayscale, square shapes, and low-intensity expressions. Emotional valence and arousal were measured throughout the study using the Self-Assessment Manikin (SAM) scale. Results indicate that the positive design led to a significant increase in valence and a significant decrease in arousal, thereby creating a positive-deactivating effect. Higher arousal during gameplay predicted greater gains in training outcomes, with this relationship being fully mediated by gameplay performance. Further analyses revealed a nonlinear effect of emotional design on mood, amplifying the impact of mood on training outcomes. Positive-activating and negative-activating moods were found to be beneficial, while positive-deactivating and negative-activating moods were detrimental. This secondary relationship was not explained by game performance, motivation, or cognitive load, and had a larger impact on the age range of 13 to 15. These findings indicate that in order for emotional design to successfully enhance training outcomes for shifting skills, the design must make significant increases to learner arousal. The mechanism for this enhancement is primarily through increasing the number of successful shifting actions performed during training. Possible causes for the secondary, direct effect of mood on training outcomes are discussed. These findings highlight the complex dynamics of emotional state and training outcomes, as well as the methodological considerations required to successfully capture these relationships. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]