18 research outputs found
Nitrogen-limited mangrove ecosystems conserve N through dissimilatory nitrate reduction to ammonium
Earlier observations in mangrove sediments of Goa, India have shown denitrification to be a major pathway for N loss1. However, percentage of total nitrate transformed through complete denitrification accounted for <0–72% of the pore water nitrate reduced. Here, we show that up to 99% of nitrate removal in mangrove sediments is routed through dissimilatory nitrate reduction to ammonium (DNRA). The DNRA process was 2x higher at the relatively pristine site Tuvem compared to the anthropogenically-influenced Divar mangrove ecosystem. In systems receiving low extraneous nutrient inputs, this mechanism effectively conserves and re-circulates N minimizing nutrient loss that would otherwise occur through denitrification. In a global context, the occurrence of DNRA in mangroves has important implications for maintaining N levels and sustaining ecosystem productivity. For the first time, this study also highlights the significance of DNRA in buffering the climate by modulating the production of the greenhouse gas nitrous oxide
Decoding Unattended Fearful Faces with Whole-Brain Correlations: An Approach to Identify Condition-Dependent Large-Scale Functional Connectivity
Processing of unattended threat-related stimuli, such as fearful faces, has been previously examined using group functional magnetic resonance (fMRI) approaches. However, the identification of features of brain activity containing sufficient information to decode, or “brain-read”, unattended (implicit) fear perception remains an active research goal. Here we test the hypothesis that patterns of large-scale functional connectivity (FC) decode the emotional expression of implicitly perceived faces within single individuals using training data from separate subjects. fMRI and a blocked design were used to acquire BOLD signals during implicit (task-unrelated) presentation of fearful and neutral faces. A pattern classifier (linear kernel Support Vector Machine, or SVM) with linear filter feature selection used pair-wise FC as features to predict the emotional expression of implicitly presented faces. We plotted classification accuracy vs. number of top N selected features and observed that significantly higher than chance accuracies (between 90–100%) were achieved with 15–40 features. During fearful face presentation, the most informative and positively modulated FC was between angular gyrus and hippocampus, while the greatest overall contributing region was the thalamus, with positively modulated connections to bilateral middle temporal gyrus and insula. Other FCs that predicted fear included superior-occipital and parietal regions, cerebellum and prefrontal cortex. By comparison, patterns of spatial activity (as opposed to interactivity) were relatively uninformative in decoding implicit fear. These findings indicate that whole-brain patterns of interactivity are a sensitive and informative signature of unattended fearful emotion processing. At the same time, we demonstrate and propose a sensitive and exploratory approach for the identification of large-scale, condition-dependent FC. In contrast to model-based, group approaches, the current approach does not discount the multivariate, joint responses of multiple functional connections and is not hampered by signal loss and the need for multiple comparisons correction
Altered Small-World Brain Networks in Schizophrenia Patients during Working Memory Performance
Impairment of working memory (WM) performance in schizophrenia patients (SZ) is well-established. Compared to healthy controls (HC), SZ patients show aberrant blood oxygen level dependent (BOLD) activations and disrupted functional connectivity during WM performance. In this study, we examined the small-world network metrics computed from functional magnetic resonance imaging (fMRI) data collected as 35 HC and 35 SZ performed a Sternberg Item Recognition Paradigm (SIRP) at three WM load levels. Functional connectivity networks were built by calculating the partial correlation on preprocessed time courses of BOLD signal between task-related brain regions of interest (ROIs) defined by group independent component analysis (ICA). The networks were then thresholded within the small-world regime, resulting in undirected binarized small-world networks at different working memory loads. Our results showed: 1) at the medium WM load level, the networks in SZ showed a lower clustering coefficient and less local efficiency compared with HC; 2) in SZ, most network measures altered significantly as the WM load level increased from low to medium and from medium to high, while the network metrics were relatively stable in HC at different WM loads; and 3) the altered structure at medium WM load in SZ was related to their performance during the task, with longer reaction time related to lower clustering coefficient and lower local efficiency. These findings suggest brain connectivity in patients with SZ was more diffuse and less strongly linked locally in functional network at intermediate level of WM when compared to HC. SZ show distinctly inefficient and variable network structures in response to WM load increase, comparing to stable highly clustered network topologies in HC
Accounting for the complex hierarchical topology of EEG phase-based functional connectivity in network binarisation
Research into binary network analysis of brain function faces a
methodological challenge in selecting an appropriate threshold to binarise edge
weights. For EEG phase-based functional connectivity, we test the hypothesis
that such binarisation should take into account the complex hierarchical
structure found in functional connectivity. We explore the density range
suitable for such structure and provide a comparison of state-of-the-art
binarisation techniques, the recently proposed Cluster-Span Threshold (CST),
minimum spanning trees, efficiency-cost optimisation and union of shortest path
graphs, with arbitrary proportional thresholds and weighted networks. We test
these techniques on weighted complex hierarchy models by contrasting model
realisations with small parametric differences. We also test the robustness of
these techniques to random and targeted topological attacks.We find that the
CST performs consistenty well in state-of-the-art modelling of EEG network
topology, robustness to topological network attacks, and in three real
datasets, agreeing with our hypothesis of hierarchical complexity. This
provides interesting new evidence into the relevance of considering a large
number of edges in EEG functional connectivity research to provide
informational density in the topology.Comment: Accepted for publication in PLOS One, 27th September 201
Association Between the Probability of Autism Spectrum Disorder and Normative Sex-Related Phenotypic Diversity in Brain Structure
IMPORTANCE Autism spectrum disorder (ASD) is 2 to 5 times more common in male
individuals than in female individuals. While the male preponderant prevalence of ASD might
partially be explained by sex differences in clinical symptoms, etiological models suggest that
the biological male phenotype carries a higher intrinsic risk for ASD than the female
phenotype. To our knowledge, this hypothesis has never been tested directly, and the
neurobiological mechanisms that modulate ASD risk in male individuals and female
individuals remain elusive.
OBJECTIVES To examine the probability of ASD as a function of normative sex-related
phenotypic diversity in brain structure and to identify the patterns of sex-related
neuroanatomical variability associated with low or high probability of ASD.
DESIGN, SETTING, AND PARTICIPANTS This study examined a cross-sectional sample of 98
right-handed, high-functioning adults with ASD and 98 matched neurotypical control
individuals aged 18 to 42 years. A multivariate probabilistic classification approach was used
to develop a predictive model of biological sex based on cortical thickness measures assessed
via magnetic resonance imaging in neurotypical controls. This normative model was
subsequently applied to individuals with ASD. The study dates were June 2005 to October
2009, and this analysis was conducted between June 2015 and July 2016.
MAIN OUTCOMES AND MEASURES Sample and population ASD probability estimates as a
function of normative sex-related diversity in brain structure, as well as neuroanatomical
patterns associated with ASD probability in male individuals and female individuals.
RESULTS Among the 98 individuals with ASD, 49 were male and 49 female, with a mean (SD)
age of 26.88 (7.18) years. Among the 98 controls, 51 were male and 47 female, with a mean
(SD) age of 27.39 (6.44) years. The sample probability of ASD did not increase significantly
with predictive probabilities for the male neuroanatomical brain phenotype. For example,
biological female individuals with a more male-typic pattern of brain anatomy were equally
likely to have ASD than biological female individuals with a characteristically female brain
phenotype (P = .40 vs .55, respectively; χ 2
1 = 1.11; P > .05; difference in P values, −.15; 95% CI,
−.10 to .40). This finding translates to an estimated variability in population prevalence from
0.3% to 0.6%, respectively. Moreover, the patterns of sex-related neuroanatomical variability
associated with ASD probability were sex specific (eg, in inferior temporal regions, where ASD
has different neurobiological underpinnings in male individuals and female individuals).
CONCLUSIONS AND RELEVANCE These findings imply that the male neuroanatomical
phenotype does not carry a higher intrinsic risk for ASD than the female neurophenotype and
provide important novel insights into the neurobiological mechanisms mediating sex
differences in ASD prevalence