The architecture of cross-hemispheric communication in the aging brain: linking behavior to functional and structural connectivity.
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Contralateral recruitment remains a controversial phenomenon in both the clinical and normative populations. To investigate the neural correlates of this phenomenon, we explored the tendency for older adults to recruit prefrontal cortex (PFC) regions contralateral to those most active in younger adults. Participants were scanned with diffusion tensor imaging and functional magnetic rresonance imaging during a lateralized word matching task (unilateral vs. bilateral). Cross-hemispheric communication was measured behaviorally as greater accuracy for bilateral than unilateral trials (bilateral processing advantage [BPA]) and at the neural level by functional and structural connectivity between contralateral PFC. Compared with the young, older adults exhibited 1) greater BPAs in the behavioral task, 2) greater compensatory activity in contralateral PFC during the bilateral condition, 3) greater functional connectivity between contralateral PFC during bilateral trials, and 4) a positive correlation between fractional anisotropy in the corpus callosum and both the BPA and the functional connectivity between contralateral PFC, indicating that older adults' ability to distribute processing across hemispheres is constrained by white matter integrity. These results clarify how older adults' ability to recruit extra regions in response to the demands of aging is mediated by existing structural architecture, and how this architecture engenders corresponding functional changes that allow subjects to meet those task demands.
Published Version (Please cite this version)
Davis, Simon W, James E Kragel, David J Madden and Roberto Cabeza (2012). The architecture of cross-hemispheric communication in the aging brain: linking behavior to functional and structural connectivity. Cereb Cortex, 22(1). pp. 232–242. 10.1093/cercor/bhr123 Retrieved from https://hdl.handle.net/10161/13476.
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My research centers around the use of structural and functional imaging measures to study the shifts in network architecture in the aging brain. I am specifically interested in changes in how changes in structural and functional connectivity associated with aging impact the semantic retrieval of word or fact knowledge. Currently this involves asking why older adults have particular difficulty in certain kinds of semantic retrieval, despite the fact that vocabularies and knowledge stores typically improve with age.
A second line of research involves asking questions about how this semantic system is organized in young adults, understanding which helps form a basis for asking questions about older adults. To what degree are these semantic retrieval processes lateralized? What cognitive factors affect this laterality? How are brain structures like the corpus callosum involved in mediating distributed activation patterns associated with semantic retrieval?
My research focuses primarily on the cognitive neuroscience of aging: the investigation of age-related changes in perception, attention, and memory, using both behavioral measures and neuroimaging techniques, including positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and diffusion tensor imaging (DTI).
The behavioral measures have focused on reaction time, with the goal of distinguishing age-related changes in specific cognitive abilities from more general effects arising from a slowing in elementary perceptual processes. The cognitive abilities of interest include selective attention as measured in visual search tasks, semantic and episodic memory retrieval, and executive control processes.
The behavioral measures are necessary to define the cognitive abilities of interest, and the neuroimaging techniques help define the functional neuroanatomy of those abilities. The PET and fMRI measures provide information regarding neural activity during cognitive performance. DTI is a recently developed technique that images the structural integrity of white matter. The white matter tracts of the brain provide critical pathways linking the gray matter regions, and thus this work will complement the studies using PET and fMRI that focus on gray matter activation.
A current focus of the research program is the functional connectivity among regions, not only during cognitive task performance but also during rest. These latter measures, referred to as intrinsic functional connectivity, are beginning to show promise as an index of overall brain functional efficiency, which can be assessed without the implementation of a specific cognitive task. From DTI, information can be obtained regarding how anatomical connectivity constrains intrinsic functional connectivity. It will be important to determine the relative influence of white matter pathway integrity, intrinsic functional connectivity, and task-related functional connectivity, as mediators of age-related differences in behavioral measures of cognitive performance.
Ultimately, the research program can help link age-related changes in cognitive performance to changes in the structure and function of specific neural systems. The results also have implications for clinical translation, in terms of the identification of neural biomarkers for the diagnosis of neural pathology and targeting rehabilitation procedures.
My laboratory investigates the neural correlates of memory and cognition in young and older adults using fMRI. We have three main lines of research: First, we distinguish the neural correlates of various episodic memory processes. For example, we have compared encoding vs. retrieval, item vs. source memory, recall vs. recognition, true vs. false memory, and emotional vs. nonemotional memory. We are particularly interested in the contribution of prefrontal cortex (PFC) and medial temporal lobe (MTL) subregions and their interactions. Second, we investigate similarities and differences between the neural correlates of episodic memory and other memory and cognitive functions (working, semantic, implicit, and procedural memory; attention; perception, etc.). The main goal of this cross-functional approach is to understand the contributions of brain regions shared by different cognitive functions. Finally, in both episodic memory and cross-function studies, we also examine the effects of healthy and pathological aging. Regarding episodic memory, we have linked processes differentially affected by aging (e.g., item vs. source memory, recall vs. recognition) to the effects of aging on specific PFC and MTL subregions. Regarding cross-function comparisons, we identify age-related changes in activity that are common to various functions. For example, we have found an age-related increase in bilaterality that occurs for many functions (memory, attention, language, perception, and motor) and is associated with functional compensation.
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