For many people, learning a second language as an adult is a challenging endeavor. Much interest in the study of adult second language learning has concerned the type of input that learners receive in their second language, with findings suggesting that second language learners are often exposed to a register of speech called 'foreigner talk' that is slower, more clearly articulated, and grammatically simpler than speech to fluent speakers (Ferguson, 1975; Henzl, 1974, 1979). Notably, a neurocognitive perspective of language called the declarative/procedural model (Ullman, 2020; Ullman et al., 2020) makes predictions for learning from fast vs. slow speech. In this dissertation, I tested these predictions by manipulating rate of speech in an artificial language learning task, thus arguably manipulating speech resembling foreigner talk vs. speech resembling nativelike conversation, and measuring the role of declarative and procedural memory in learning the artificial language. To motivate these predictions, I focus on specific aspects of declarative and procedural memory: Namely, declarative memory appears to be reliant on attention and able to consciously recall facts and experiences, whereas procedural memory is less reliant on attention and appears to learn cognitive and motor skills by seeking quick confirmation of internally generated predictions. It may follow from these characteristics of the memory systems that because learners are more easily able to attend to slow speech (Fernandez et al., 2020; Sajin & Connine, 2017), then slow speech should be supported by declarative memory. In contrast, perhaps because fast speech is more difficult to attend to and/or because procedural memory learns better with items that occur in quick succession (Foerde & Shohamy, 2011; Maddox et al., 2003; Valentin et al., 2014), fast speech should be supported by procedural memory. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]