Biology subjects

Dvorak, E.

Publications and source records attributed to Dvorak, E..

3 recordsLinked to original sources

bioRxiv2021

The organization of individually mapped structural and functional semantic networks in aging adults

Language function in the brain, once thought to be highly localized, is now appreciated as relying on a connected but distributed network. The semantic system is of particular interest in the language domain because of its hypothesized integration of information across multiple cortical regions. Previous work in healthy individuals has focused on group-level functional connectivity (FC) analyses of the semantic system, which may obscure interindividual differences driving variance in performance. These studies also overlook the contributions of white matter networks to semantic function. Here, we identified semantic network nodes with a semantic decision fMRI task in 53 typically-aging adults, characterized network organization using structural connectivity (SC), and quantified the segregation and integration of the network using FC. Hub regions were identified in left inferior frontal gyrus. The individualized semantic network was composed of three interacting modules: 1) default-mode module characterized by bilateral medial prefrontal and posterior cingulate regions and also including right-hemisphere homotopes of language regions; 2) left frontal module extending dorsally from inferior frontal gyrus to pre-motor area; and 3) left temporoparietal module extending from temporal pole to inferior parietal lobule. FC within Module3 and integration of the entire network related to a semantic verbal fluency task, but not a matched phonological task. These results support and extend the tri-network semantic model (Xu et al., 2017) and the controlled semantic cognition model (Chiou et al., 2018) of semantic function.

neuroscience↗

bioRxiv2021

Testing the perilesional neuroplastic recruitment hypothesis in aphasia

ObjectiveA prominent theory proposes that neuroplastic recruitment of perilesional tissue supports aphasia recovery, especially when language-capable cortex is spared by smaller lesions. This theory has rarely been tested directly, and findings have been inconclusive. Here, we test the perilesional plasticity hypothesis using two fMRI tasks in two groups of stroke survivors. MethodsTwo cohorts totaling 84 chronic left-hemisphere stroke survivors with prior aphasia diagnosis, and 80 control participants underwent fMRI using either a naming task or a reliable semantic decision task. Individualized perilesional tissue was defined by dilating anatomical lesions, and language regions were defined using meta-analyses. Mixed modeling examined differences in activity between groups. Relationships with lesion size and aphasia severity were examined. ResultsStroke survivors exhibited reduced activity in perilesional language tissue relative to controls in both tasks. Although a few cortical regions exhibited greater activity irrespective of distance from the lesion, or only when distant from the lesion, no regions exhibited increased activity only when near the lesion. Larger lesions were associated with reduced language activity irrespective of distance from the lesion. Using the reliable fMRI task, reduced language activity related to aphasia severity independent of lesion size. InterpretationWe find no evidence for neuroplastic recruitment of perilesional tissue in aphasia beyond its typical role in language. Rather, our findings are consistent with alternative hypotheses that left-hemisphere activation changes during recovery relate to normalization of language network dysfunction and possibly recruitment of alternate cortical processors. These findings clarify left-hemisphere neuroplastic mechanisms supporting language recovery after stroke. Summary for Social Media If AcceptedO_ST_ABSTwitter handleC_ST_ABS@crlgeorgetown What is the current knowledge on the topic?After left-hemisphere stroke, many individuals experience long-term language impairment (aphasia) while others recover their communication abilities. Although there are several hypotheses concerning the kind of brain neuroplasticity that allows some individuals to recover, these mechanisms are not understood in aphasia. What question did this study address?This study tested the perilesional plasticity hypothesis as it relates to aphasia recovery. This predominant theory posits that tissue around the stroke lesion boundary becomes recruited to support recovered language function in post-stroke aphasia. What does this study add to our knowledge?This study clarifies the mechanisms of neuroplasticity in stroke aphasia recovery. The results are not consistent with the conventional perilesional plasticity hypothesis, but rather favor an interpretation that recovery is supported by normalization of language network dysfunction and possibly recruitment of alternate brain regions How might this potentially impact on the practice of neurology?These conclusions will give practicing neurologists a better understanding of how the brain recovers from aphasia after stroke.

neuroscience↗

bioRxiv2020

Global distribution patterns of marine nitrogen-fixers by imaging and molecular methods

Biological nitrogen fixation plays a critical role in marine primary production, yet, our understanding of marine N2-fixers (diazotrophs) is hindered by limited observations. Here, we developed a quantitative image analysis pipeline in concert with mapping of molecular markers for mining >2,000,000 images and >1,300 metagenomes in surface, deep chlorophyll maximum and mesopelagic samples across 6 size fractions (<0.2-2000 m). Imaging and PCR-free molecular data were remarkably congruent. Sequences from diazotrophs were detected from the ultrasmall bacterioplankton (<0.2 m) to mesoplankton (180-2000 m), while images predicted symbiotic and colonial-forming diazotrophs (>20 {micro}m). Imaging and molecular data estimated that polyploidy can significantly impact gene abundances of symbiotic vs colonial-forming diazotrophs. In general our results support the canonical view that larger sized diazotrophs (>10 m) dominate the tropical belts, while sequences from unicellular cyanobacterial and non-cyanobacterial diazotrophs were globally distributed in surface and the mesopelagic. Co-occurring diazotrophic lineages of different lifestyles were frequently encountered, and several new high density regions of diazotrophs were identified in the global ocean. Overall, this work provides an update of marine diazotroph biogeographical diversity and contributes a new bio-imaging-informatic workflow.

microbiology↗