Influence of Second Language Experience on Auditory Processing in Adolescents: Consequences for Real-World Listening
說明
202 p
附註
Source: Dissertation Abstracts International, Volume: 77-10(E), Section: B
Adviser: Nina Kraus
Thesis (Ph.D.)--Northwestern University, 2016
The sounds of our lives, including the sounds of our language, serve as a training ground that engenders neural plasticity. Experience with more than one language facilitates plasticity beyond that seen in monolinguals, shaping cognitive processes that support juggling two languages in one mind, with the most notable enhancement seen for focusing on relevant, and ignoring irrelevant, information, a process known as inhibitory control. During communication, cognitive and sensory systems interact, making it unlikely that this additional bilingual plasticity is merely within the purview of cognitive systems, but rather is evident in sensory systems as well. This dissertation is built on the hypothesis, tested across five studies, that bilingualism is a form of enrichment that induces plasticity in those auditory processes that interact with cognitive function during real-world communication. The first two studies, which tested subcortical evoked responses in adolescents, asked if bilinguals and monolinguals differed in their sensory encoding. Bilinguals had larger responses to the pitch of speech syllables that tracked with their greater inhibitory control abilities. The third study asked whether bilingual experience drives these sensory enhancements. Subcortical pitch encoding was found to be stronger in children who had more bilingual experience, suggesting this plasticity resulted from experience rather than pre-existing differences. Though these three studies used speech syllables that lacked the complexity of real-world communication, the studies confirm bilingual auditory-processing enhancements and establish their tethering to cognitive processes important for communication. Interestingly, while these findings highlight the potential for bilingual plasticity to facilitate real-world listening, previous literature has found bilinguals to struggle in this realm relative to monolinguals. To understand the interplay between cognitive-sensory advantages and real-world listening disadvantages, the next study compared bilingual and monolingual adolescents on listening-in-noise tests that differed in linguistic complexity. Bilinguals showed an advantage when the targets were tones; monolinguals outperformed bilinguals when stimuli were sentences. The observed performance shift highlights how bilingual experience can engender plasticity that leads to both advantages and disadvantages, which presumably has consequences for real-world communication. Identification of these consequences was the goal of the final study. Auditory selective attention, a fundamental part of everyday communication, involves the very processes found to be enhanced in bilinguals. Bilinguals and monolinguals were compared on this task to yield insights into the consequences of bilingual plasticity for real-world listening. Bilinguals and monolinguals did not differ in performance. Additionally, both bilinguals and monolinguals showed an attention effect on neural processing, in which cortical responses during active listening increased while subcortical responses decreased. However, there were differences in the neural and cognitive processes engaged to complete the task. Consistent with the first study, bilinguals showed greater subcortical encoding of the pitch of the stimuli. They also used inhibitory control to complete this task, as evidenced by a correlation between their inhibitory control abilities and task performance. From these results, we suggest that bilingualism enhances both cognitive and sensory processes important for real-world listening. Furthermore, this series of studies demonstrates that the bilingual cannot be thought of as the sum of two monolinguals, but rather as a mental juggler of two languages that results in unique obstacles and distinct neuroplasticity
