DOI

10.17077/etd.2z2f6emk

Document Type

Dissertation

Date of Degree

Fall 2016

Access Restrictions

Access restricted until 02/23/2019

Degree Name

PhD (Doctor of Philosophy)

Degree In

Psychology

First Advisor

Daniel Tranel

First Committee Member

Michelle W Voss

Second Committee Member

Bob McMurray

Third Committee Member

Molly Nikolas

Fourth Committee Member

Natalie L Denburg

Abstract

Older bilingual adults typically perform better than monolinguals in tasks of executive control, and are diagnosed later with dementia. Studies have also shown structural and functional brain differences between bilinguals and monolinguals. However, it remains poorly understood how language history influences the functional organization of the aging brain. The current study investigated; 1) differences in resting-state functional connectivity between monolinguals and bilinguals in the Default Mode Network (DMN), Frontoparietal Network (FPN), Executive Control Network (ECN), Language Network (LANG), and a network consisting of structures associated with tasks of executive control coined the Bilingual Control Network (BCN); 2) the relationship of cognitive performance with functional connectivity of the BCN; and 3) whether proficiency, age of second language acquisition, degree of second language exposure, and frequency of language use predicts the network’s functional connectivity. Healthy older bilinguals (N=10) were matched pairwise for age, sex and education to healthy older monolinguals (N=10). All participants completed a battery of cognitive tests, a language history questionnaire, and a 6-minute functional scan during rest. Results showed that groups did not differ in cognitive performance, or in the functional connectivity of the FPN, ECN, LANG, or BCN. However, monolinguals had significantly stronger functional connectivity in the DMN compared to bilinguals. Later age of second language acquisition and lower proficiency were also associated with greater DMN functional connectivity. None of these variables predicted BCN’s functional connectivity. However, bilinguals showed stronger functional connectivity with other structures outside of the canonical networks compared to monolinguals. Finally, vocabulary scores, local switch cost accuracy and reaction time were negatively correlated with BCN’s functional connectivity. Overall, these findings illustrate differences in functional brain organization associated with language experience in the DMN, while challenging the “bilingual advantage” hypothesis. The results also suggest a possible neural mechanism by which bilingualism might mediate cognitive reserve.

Public Abstract

As cross-cultural boundaries fade away, business, relationships, employment, education, and the desire for new adventures, have lead to an increasing necessity to communicate in languages other than our mother tongue. Studies suggest that the benefits of being bilingual, or of speaking more than one language, transcend those associated with facilitating communication. Specifically, it has been suggested that bilinguals of all ages are better than monolinguals at executive control tasks (i.e. attention, switching, and inhibition). Scientists have proposed that it is the constant switching of languages what leads to better performance in executive control tasks. This is supported by neuroimaging studies showing that bilinguals engage many of the same brain structures on tasks of language switching and executive control. More recently, studies have suggested that bilingualism protects against dementia onset, as reflected by lifelong bilinguals being diagnosed with dementia (e.g. Alzheimer’s disease) years after monolinguals. These findings have raised great interest in both the scientific and the non-scientific community given the increasing incidence of individuals with Alzheimer’s disease in the United States and the lack of treatments or preventive measures. Although being bilingual does not cure or prevent the disease, it may allow individuals to enjoy a few more years of independency. However, the neural mechanisms underlying this phenomenon remain poorly understood.

The current study aimed to better understand how speaking more than one language and related variables (e.g. proficiency) influence the functional organization of the aging brain. Specifically, the study examined: 1) whether older bilinguals and matched monolinguals differed in the functional connectivity of multiple brain well-established networks observed during rest; 2) whether resting-state functional connectivity in regions proposed to be associated with executive control was significantly associated with performance in tests of cognition; and 3) whether proficiency, age of second language acquisition, degree of exposure to a second language, and frequency of language predict resting-state functional connectivity of structures associated with executive control. Resting-state functional connectivity is acquired through functional magnetic resonance imaging (fMRI) and provides a tool to measure the functional integrity of component brain systems that are likely to have developed as a result of previous experiences. Stronger functional connectivity has been consistently associated with better cognitive functioning and with healthier aging.

My sample consisted of 10 monolinguals matched for sex, age, and education with 10 bilinguals of varying proficiency and language history. There were no significant differences between groups in cognitive functioning. Both groups also showed similar functional connectivity between structures associated with language, and in networks associated with executive control. However, monolinguals had stronger functional connectivity in the Default Mode Network, which has been associated with episodic memory, self-generated thought, and has been shown to be particularly susceptible to neurodegeneration. Further, those who were more proficient in their second language and had acquired it earlier in life had weaker functional connectivity in this network. Interestingly, exploratory analyses showed more widespread functional connectivity in bilinguals. Taken together, these findings raise the possibility that bilingualism might confer compensatory mechanisms to maintain cognitive functioning. That is, the experience of managing two or more languages may reorganize the brain functionally in such a way where if the brain suffers damage in an area usually involved in a function (in monolinguals), the function is not compromised in bilinguals because there are already connections to others areas that help maintain functioning. Findings also suggest that language variables such as proficiency influence the functional organization of the brain, emphasizing the importance of considering the heterogeneity within the bilingual group when studying cognition and brain function in this group. Similarly, given that there were no differences between groups in cognitive performance, this study also raises questions about whether bilingualism is actually advantageous or if it only means that it makes our brains look different.

Keywords

cognitive reserve, fMRI, language, neuropsychology, resting-state functional connectivity

Pages

xiii, 99 pages

Bibliography

Includes bibliographical references (pages 81-99).

Copyright

Copyright © 2016 Edmarie Guzmán-Veléz

Available for download on Saturday, February 23, 2019

Included in

Psychology Commons

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