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| 1 | +Methodology |
| 2 | +================================ |
| 3 | + |
| 4 | +Brain Network Map Selection |
| 5 | +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| 6 | + |
| 7 | +NLA requires the user to specify the network map that will be used to depict the known architecture of the |
| 8 | +human connectome, which is crucial given that the network map selection affects both statistical |
| 9 | +significance testing and interpretation :cite:p:`BellecP`. The current pipeline uses network maps that are generated with |
| 10 | +Infomap, due to its greater congruence with networks derived from task-activation and seed-based |
| 11 | +connectivity studies than alternative modularity algorithms :cite:p:`PowerJ,RosvallM`. Network maps can be generated using |
| 12 | +one's preferred algorithm or one of several published ROI and corresponding network map options that |
| 13 | +will be included in the NLA toolbox :cite:p:`GordonE,PowerJ,ThomasY,GlasserM,ShenX,CraddockR`. The use of standardized ROI and network maps creates a |
| 14 | +common, reproducible framework for testing brain-behavior associations across connectome research |
| 15 | + |
| 16 | +General Linear Model / Edge-wise Statistical Model Selection |
| 17 | +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| 18 | + |
| 19 | +NLA also requires the user to specify the desired statistical model for testing associations between |
| 20 | +behavioral data and edge-wise�or ROI-pair connectivity�connectome data. The analysis pipeline within |
| 21 | +the NLA toolbox offers both parametric and non-parametric correlation. |
| 22 | + |
| 23 | +Connectivity Matrices |
| 24 | +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| 25 | + |
| 26 | +Other software packages are used to create the connectivity matrices that are provided as input into the |
| 27 | +NLA toolbox. One useful option for mapping functional connectivity matrices is CONN - MATLAB-based |
| 28 | +software with the ability to compute, display, and analyze functional connectivity in fMRI. |
| 29 | + |
| 30 | +The NLA Method |
| 31 | +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| 32 | + |
| 33 | +First, connectome-wide associations are calculated between ROI-pair connectivity and behavioral data, |
| 34 | +resulting in a set of standardized regression coefficients that specify the brain-behavior association at |
| 35 | +each ROI-pair of the connectome matrix. Next, network level analysis-consisting of transformation of the |
| 36 | +edge-wise test statistics and enrichment statistic calculation :cite:p:`AckermanM` - is done to determine which networks are |
| 37 | +strongly associated with the behavior of interest. |
| 38 | + |
| 39 | +Both p-value and test-statistic binarization are offered in the current NLA pipeline :cite:p:`EggebrechtA,WheelockM:2018`. Prior research has |
| 40 | +supported the incorporation of a proportional edge density threshold, given that uneven edge density |
| 41 | +thresholds have been shown to unfairly bias results :cite:p:`vandenHeuvelM`. |
| 42 | +For enrichment statistic calculation, NLA offers a number of statistical tests. Prior research has relied on |
| 43 | +chi-square and Fisher's Exact test, as well as a Kolmogorov-Smirnov (KS) test and non-parametric tests |
| 44 | +based on ranks, which compare the distribution of test values within a region to other regions :cite:p:`WheelockM:2018,RudolphM,MoothaV,ZahnJ`. In |
| 45 | +addition, KS alternatives such as averaging or minmax have also shown promise in connectome |
| 46 | +applications :cite:p:`ChenJ,NewtonM,YaariG,EfronB`. |
| 47 | + |
| 48 | +NLA then conducts data-driven permutation testing to establish significance. In the NLA toolbox, network |
| 49 | +level significance is determined by comparing each measured enrichment statistic to permuted |
| 50 | +enrichment p-values which are calculated by randomly shuffling behavior vector labels and computing |
| 51 | +the enrichment statistic many times to produce a null distribution for each network. The FPR is controlled |
| 52 | +at the network level using Bonferroni correction. Therefore, NLA is able to retain edge-wise correlations |
| 53 | +within each network module, but network communities are used to reduce the number of comparisons |
| 54 | +and control the FPR at the network level. After significance is determined, the pipeline allows users to |
| 55 | +create publication quality images to visualize network level findings both in connectome format and on |
| 56 | +the surface of the brain. |
| 57 | + |
| 58 | +**Note**: While the behavior vector labels are shuffled to conduct permutations in the enrichment pipeline, |
| 59 | +functional connectivity data are not shuffled in order to preserve the inherent covariant structure of the |
| 60 | +data across permutations |
| 61 | + |
| 62 | +How Should the Test Statistic Threshold Be Chosen? |
| 63 | +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| 64 | + |
| 65 | +A nominal threshold is used for the thresholding and binarization step of the edge-level tests. The |
| 66 | +nominal threshold is uncorrected and is typically set at 0.05 or 0.01 in the edge-level prob_max field. In |
| 67 | +contrast, a network-level corrected threshold using the Bonferroni method is used in the net-level |
| 68 | +statistics, where the nominal threshold is divided by the number of tests being done to correct for |
| 69 | +multiple comparisons. |
| 70 | + |
| 71 | +How Should the Networks Be Chosen? |
| 72 | +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| 73 | + |
| 74 | +There are many canonical ROI sets and there are many network definitions. Some of these network |
| 75 | +definitions include ROI that are not consistently assigned to any network. These ROI are typically removed |
| 76 | +prior to network level analysis, as is the case in the ``Seitzman_15nets_288ROI_on_TT`` and the |
| 77 | +``Gordon_12nets_286parcels_on_MNI`` network atlases included in this version of the toolbox. Network |
| 78 | +atlases that are not included in this package may also be used, but they must first be formatted into the |
| 79 | +correct structure |
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