Does bilingualism increase brain or cognitive reserve in patients with temporal lobe epilepsy?

Abstract

Objective: Bilingual healthy adults have been shown to exhibit an advantage in executive functioning (EF) that is associated with microstructural changes in white matter (WM) networks. Patients with temporal lobe epilepsy (TLE) often show EF deficits that are associated with WM compromise. In this study, we investigate whether bilingualism can increase cognitive reserve and/or brain reserve in bilingual patients with TLE, mitigating EF impairment and WM compromise.

Methods: Diffusion tensor imaging was obtained in 19 bilingual and 26 monolingual patients with TLE, 12 bilingual healthy controls (HC), and 21 monolingual HC. Fractional anisotropy (FA) and mean diffusivity (MD) were calculated for the uncinate fasciculus (Unc) and cingulum (Cing), superior frontostriatal tract (SFS), and inferior frontostriatal tract (IFS). Measures of EF included Trail Making Test-B (TMT-B) and Delis-Kaplan Executive Function System Color-Word Inhibition/Switching. Analyses of covariance were conducted to compare FA and MD of the Unc, Cing, SFS, and IFS and EF performance across groups.

Results: In bilingual patients, FA was lower in the ipsilateral Cing and Unc compared to all other groups. For both patient groups, MD of the ipsilateral Unc was higher relative to HC. Despite more pronounced reductions in WM integrity, bilingual patients performed similarly to monolingual TLE and both HC groups on EF measures. By contrast, monolingual patients performed worse than HC on TMT-B. In addition, differences in group means between bilingual and monolingual patients on TMT-B approached significance when controlling for the extent of WM damage (P = .071; d = 0.62), suggesting a tendency toward higher performance for bilingual patients.

Significance: Despite poorer integrity of regional frontal lobe WM, bilingual patients performed similarly to monolingual patients and HC on EF measures. These findings align with studies suggesting that bilingualism may provide a protective factor for individuals with neurological disease, potentially through reorganization of EF networks that promote greater cognitive reserve.

Keywords: bilingualism; brain function; diffusion tensor; executive function; white matter integrity.

Figure 1.

A) Coronal and B) Sagittal rendering of the cingulum bundle (Cing), uncinate fasciculus (Unc), superior frontostriatal tract (SFS), and inferior frontostriatal tract (IFS) derived from probabilistic diffusion tensor atlas (i.e., AtlasTrack) projected onto a T1-weighted image. The corpus callosum is portrayed in light gray in order to provide additional spatial information. The four white matter tracts of interest are shown for a single individual.

Figure 2.

Y-axis represents estimated marginal means ± standard error for FA, controlling for education, age, sex, WTAR scores, and scanner differences. A) Bilingual TLE had lower FA in ipsilateral Cing relative to monolingual patients, monolingual HC, and bilingual HC. B) Bilingual TLE had lower FA in ipsilateral Unc relative to monolingual HC and bilingual HC. C-D) No differences in IFS or SFS FA were found. FA: fractional anisotropy; Ipsi: ipsilateral; Contra: contralateral; Cing: cingulum’ IFS: inferior frontostriatal’ SFS: superior frontostriatial. Single asterisk represents Bonferroni-corrected significance at 0.05’ double asterisk represents Bonferroni-corrected significance at 0.01.

Figure 3.

Y-axis represents estimated marginal means controlling for education, age, sex, and WTAR scores. Lower scores indicate longer time to completion on TMT-B. Bilingual patients with TLE did not differ from monolingual patients with TLE or either HC groups. Monolingual patients had lower scores relative to monolingual HC and bilingual HC. Double asterisks represents Bonferroni-corrected significance at 0.01.