Natural language is highly complex and can be challenging for some learners, yet the contribution of complexity to individual differences in language learning remains poorly understood. This poor understanding appears due to both a lack of consensus among researchers regarding what complexity is, and to on-line language research often employing low complexity, artificial grammar stimuli. This dissertation addresses the first aspect of this problem with an integrative review of complexity theories, and the second aspect with original research. A novel micro language paradigm made it possible to track learning online as participants were trained in a subset of a mini-language based on Japanese. Thirty-two adult native English speakers progressed through four phases of training to learn eight phrases, four at each of two complexity levels, while the acquisition was tracked online with dense-array EEG and frequent behavioral measures of learning. Participants first listened to a soundstream of micro language phrases to familiarize them with the phonology of the language. Next, they completed semantic training and practice. Finally, they listened to the soundstream again, now (presumably) comprehending the phrases. Participants were then divided into high and low learners based upon noun segmentation ability. Overall, findings suggested systematic differences between high and low learner responses to the simple and complex phrases. Both high and low learners were quick to develop a differential response to the noun initial and noun medial syllables, with a higher N1 response to the noun initial syllables emerging around the second minute and a later medial frontal negativity appearing to track engagement in learning. However, high learner electrophysiological response suggested a more strategic response to the noun syllables. It was speculated that the high learner response might constitute a customization of their attention to align with the information content of each syllable. In conclusion, observing learning online and using stimuli of varied complexity provided new insights into the nature of individual differences in learning. The micro language paradigm with the recall behavioral tracking method provides a new way to explore learning of sequential systems such as language. [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.]