Dominant hemisphere functional networks compensate for structural connectivity loss to preserve phonological retrieval with aging

Abstract

Introduction: Loss of hemispheric asymmetry during cognitive tasks has been previously demonstrated in the literature. In the context of language, increased right hemisphere activation is observed with aging. Whether this relates to compensation to preserve cognitive function or dedifferentiation implying loss of hemispheric specificity without functional consequence, remains unclear.

Methods: With a multifaceted approach, integrating structural and functional imaging data during a word retrieval task, in a group of younger and older adults with equivalent cognitive performance, we aimed to establish whether interactions between hemispheres or reorganization of dominant hemisphere networks preserve function. We examined functional and structural connectivity on data from our previously published functional activation study. Functional connectivity was measured using psychophysiological interactions analysis from the left inferior frontal gyrus (LIFG) and the left insula (LINS), based on published literature, and the right inferior frontal gyrus (RIFG) based on our previous study.

Results: Although RIFG showed increased activation, its connectivity decreased with age. Meanwhile, LIFG and LINS connected more bilaterally in the older adults. White matter integrity, measured by fractional anisotropy (FA) from diffusion tensor imaging, decreased significantly in the older group. Importantly, LINS functional connectivity to LIFG correlated inversely with FA.

Conclusions: We demonstrate that left hemispheric language areas show higher functional connectivity in older adults with intact behavioral performance, and thus, may have a role in preserving function. The inverse correlation of functional and structural connectivity with age is in keeping with emerging literature and merits further investigation with tractography studies and in other cognitive domains.

Keywords: Aging; DTI; PPI; functional connectivity; language; phonology; word retrieval.

Figure 1

Second level subtractive analysis for words > baseline contrast for old > young groups (P < 0.005, uncorrected).

Figure 2

Left insula PPI for older > younger adults at P < 0.001 uncorrected voxel threshold with clusters >20 voxels.

Figure 3

Parametric model with Reaction time (RT) applied to PPI analysis from left insula (−30, 20, −14) showing negative correlations (at P < 0.001 uncorrected and >20 voxels) between functional connectivity and RT.

Figure 4

Fractional anisotropy (FA) measured on DTI for younger > older adults at P < 0.05 FWE corrected cluster threshold and clusters with >20 voxels (top panel). Correlation between left insula and left inferior frontal gyrus connectivity and FA (bottom panel).