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Long ‐ term acclimatization to high ‐ altitude hypoxia modifies interhemispheric functional and structural connectivity in the adult brain

View Article: PubMed Central - PubMed

ABSTRACT

Background: Structural and functional networks can be reorganized to adjust to environmental pressures and physiologic changes in the adult brain, but such processes remain unclear in prolonged adaptation to high‐altitude (HA) hypoxia. This study aimed to characterize the interhemispheric functionally and structurally coupled modifications in the brains of adult HA immigrants.

Methods: We performed resting‐state functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) in 16 adults who had immigrated to the Qinghai‐Tibet Plateau (2300–4400 m) for 2 years and in 16 age‐matched sea‐level (SL) controls. A recently validated approach of voxel‐mirrored homotopic connectivity (VMHC) was employed to examine the interhemispheric resting‐state functional connectivity. Areas showing changed VMHC in HA immigrants were selected as regions of interest for follow‐up DTI tractography analysis. The fiber parameters of fractional anisotropy and fiber length were obtained. Cognitive and physiological assessments were made and correlated with the resulting image metrics.

Results: Compared with SL controls, VMHC in the bilateral visual cortex was significantly increased in HA immigrants. The mean VMHC value extracted within the visual cortex was positively correlated with hemoglobin concentration. Moreover, the path length of the commissural fibers connecting homotopic visual areas was increased in HA immigrants, covarying positively with VMHC.

Conclusions: These observations are the first to demonstrate interhemispheric functional and structural connectivity resilience in the adult brain after prolonged HA acclimatization independent of inherited and developmental effects, and the coupled modifications in the bilateral visual cortex indicate important neural compensatory mechanisms underlying visual dysfunction in physiologically well‐acclimatized HA immigrants. The study of human central adaptation to extreme environments promotes the understanding of our brain's capacity for survival.

No MeSH data available.


Related in: MedlinePlus

Diffusion tractographic images from a single control subject show between‐group comparison for commissural fiber parameters. (A) Diffusion tractographic images from a single control subject. Fibers connecting the bilateral visual cortex are located in the splenium of the corpus callosum. The inset shows the probabilistic maps of the commissural tract constructed with data from 16 subjects. (B) Scatterplots show the between‐group comparison for the commissural fiber parameters of fractional anisotropy (FA) and fiber length. *P < 0.05, Bonferroni corrected. (C) A significant correlation between mean z‐VMHC index within homotopic visual areas and the length of their commissural fibers. Spearman rho = 0.565, P = 0.023. A longer path length of the fibers connecting the bilateral visual cortex corresponds to stronger interhemispheric functional synchronization. SL, sea level; HA, high‐altitude.
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brb3512-fig-0002: Diffusion tractographic images from a single control subject show between‐group comparison for commissural fiber parameters. (A) Diffusion tractographic images from a single control subject. Fibers connecting the bilateral visual cortex are located in the splenium of the corpus callosum. The inset shows the probabilistic maps of the commissural tract constructed with data from 16 subjects. (B) Scatterplots show the between‐group comparison for the commissural fiber parameters of fractional anisotropy (FA) and fiber length. *P < 0.05, Bonferroni corrected. (C) A significant correlation between mean z‐VMHC index within homotopic visual areas and the length of their commissural fibers. Spearman rho = 0.565, P = 0.023. A longer path length of the fibers connecting the bilateral visual cortex corresponds to stronger interhemispheric functional synchronization. SL, sea level; HA, high‐altitude.

Mentions: The commissural tracts connecting the bilateral visual cortex was not detected for eight of the 15 subjects in the SL group and eight of the 16 subjects in the HA group. Figure 2A shows a diffusion tractographic image from a single control subject. The results found here were in line with previous studies that interindividual differences exist in splenial connectivity, and homotopic connections between the primary visual cortices are variable (Putnam et al. 2010). The distribution of the individuals with detected tracts were comparable between the two groups (Chi‐square = 0.034, P = 1). Therefore, the following structural‐functional correlations were calculated only for the subjects with reconstructed fiber tracts. The between‐group analysis indicated that the reconstructed commissural fibers connecting the bilateral visual cortex were significantly longer (Mann–Whitney U = 9.00, P = 0.015) and the FA was smaller and approached significance (t = −2.101, P = 0.054) in HA immigrants (Fig. 2B). To clearly show through which part of the corpus callosum these fibers went, we produced a population‐based (n = 16) probabilistic map. We found that the fibers connecting the bilateral visual cortex were located in the splenium of the corpus callosum. Using Pearson correlations, fiber parameters (fiber length, FA) were not significantly correlated with neuropsychological measurements, physiological indices, or altitude (all P‐values >0.05).


Long ‐ term acclimatization to high ‐ altitude hypoxia modifies interhemispheric functional and structural connectivity in the adult brain
Diffusion tractographic images from a single control subject show between‐group comparison for commissural fiber parameters. (A) Diffusion tractographic images from a single control subject. Fibers connecting the bilateral visual cortex are located in the splenium of the corpus callosum. The inset shows the probabilistic maps of the commissural tract constructed with data from 16 subjects. (B) Scatterplots show the between‐group comparison for the commissural fiber parameters of fractional anisotropy (FA) and fiber length. *P < 0.05, Bonferroni corrected. (C) A significant correlation between mean z‐VMHC index within homotopic visual areas and the length of their commissural fibers. Spearman rho = 0.565, P = 0.023. A longer path length of the fibers connecting the bilateral visual cortex corresponds to stronger interhemispheric functional synchronization. SL, sea level; HA, high‐altitude.
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Related In: Results  -  Collection

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brb3512-fig-0002: Diffusion tractographic images from a single control subject show between‐group comparison for commissural fiber parameters. (A) Diffusion tractographic images from a single control subject. Fibers connecting the bilateral visual cortex are located in the splenium of the corpus callosum. The inset shows the probabilistic maps of the commissural tract constructed with data from 16 subjects. (B) Scatterplots show the between‐group comparison for the commissural fiber parameters of fractional anisotropy (FA) and fiber length. *P < 0.05, Bonferroni corrected. (C) A significant correlation between mean z‐VMHC index within homotopic visual areas and the length of their commissural fibers. Spearman rho = 0.565, P = 0.023. A longer path length of the fibers connecting the bilateral visual cortex corresponds to stronger interhemispheric functional synchronization. SL, sea level; HA, high‐altitude.
Mentions: The commissural tracts connecting the bilateral visual cortex was not detected for eight of the 15 subjects in the SL group and eight of the 16 subjects in the HA group. Figure 2A shows a diffusion tractographic image from a single control subject. The results found here were in line with previous studies that interindividual differences exist in splenial connectivity, and homotopic connections between the primary visual cortices are variable (Putnam et al. 2010). The distribution of the individuals with detected tracts were comparable between the two groups (Chi‐square = 0.034, P = 1). Therefore, the following structural‐functional correlations were calculated only for the subjects with reconstructed fiber tracts. The between‐group analysis indicated that the reconstructed commissural fibers connecting the bilateral visual cortex were significantly longer (Mann–Whitney U = 9.00, P = 0.015) and the FA was smaller and approached significance (t = −2.101, P = 0.054) in HA immigrants (Fig. 2B). To clearly show through which part of the corpus callosum these fibers went, we produced a population‐based (n = 16) probabilistic map. We found that the fibers connecting the bilateral visual cortex were located in the splenium of the corpus callosum. Using Pearson correlations, fiber parameters (fiber length, FA) were not significantly correlated with neuropsychological measurements, physiological indices, or altitude (all P‐values >0.05).

View Article: PubMed Central - PubMed

ABSTRACT

Background: Structural and functional networks can be reorganized to adjust to environmental pressures and physiologic changes in the adult brain, but such processes remain unclear in prolonged adaptation to high&#8208;altitude (HA) hypoxia. This study aimed to characterize the interhemispheric functionally and structurally coupled modifications in the brains of adult HA immigrants.

Methods: We performed resting&#8208;state functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) in 16 adults who had immigrated to the Qinghai&#8208;Tibet Plateau (2300&ndash;4400&nbsp;m) for 2&nbsp;years and in 16 age&#8208;matched sea&#8208;level (SL) controls. A recently validated approach of voxel&#8208;mirrored homotopic connectivity (VMHC) was employed to examine the interhemispheric resting&#8208;state functional connectivity. Areas showing changed VMHC in HA immigrants were selected as regions of interest for follow&#8208;up DTI tractography analysis. The fiber parameters of fractional anisotropy and fiber length were obtained. Cognitive and physiological assessments were made and correlated with the resulting image metrics.

Results: Compared with SL controls, VMHC in the bilateral visual cortex was significantly increased in HA immigrants. The mean VMHC value extracted within the visual cortex was positively correlated with hemoglobin concentration. Moreover, the path length of the commissural fibers connecting homotopic visual areas was increased in HA immigrants, covarying positively with VMHC.

Conclusions: These observations are the first to demonstrate interhemispheric functional and structural connectivity resilience in the adult brain after prolonged HA acclimatization independent of inherited and developmental effects, and the coupled modifications in the bilateral visual cortex indicate important neural compensatory mechanisms underlying visual dysfunction in physiologically well&#8208;acclimatized HA immigrants. The study of human central adaptation to extreme environments promotes the understanding of our brain's capacity for survival.

No MeSH data available.


Related in: MedlinePlus