MATLAB_connectome_intro.m

%%%% Introduction to using MATLAB for connectome data

% Written by Stuart Oldham

%% Example data

% Run the following command to load in some example data

load('example_connectome.mat')

% There are 4 variables; adj, connectome, COG and Labels. adj is an 82x82 
% group connectome generated from Human Connectome Project Data. connectome
% is a cell and each cell contains an individual subjects connectome. COG 
% are the MNI coordinates of each node and Labels is the name of each node 
% (i.e. what region each node is).

% Connectomes are a map of neural connections (axons, white matter tracts 
% etc)between regions (i.e. brain regions, cells etc). This can be
% expressed as an adjacency matrix (in this example the variable 'adj' is
% such a matrix). Each row i corresponds to a region as does every column
% j. The element (i,j) tells you if region i and j are connected. Regions
% are also called nodes/verticies and connections are also called edges

%% Weighted and unweighted connectomes

% A connectome can either be unweighted or weighted. In an unweighted 
% connectome, the element i,j can either be 0 or 1. 0 indicates there is no
% connection between these regions while 1 indicates there is a connection.
% For weighted networks, each connection can be assigned a value > 0 
% indicating the strength (or weight) of a connection between two regions.

% The connectome here is currently weighted. Typically for dMRI data the
% weight of an edge corresponds to the number of streamlines connecting two
% regions. Here the number of steamlines connecting two regions has been
% corrected to account for variations in region size (i.e. larger regions 
% will have a disproportionate number of streamlines seeded from them)

% To make the connectome unweighted (or binarised) run the following:

bin = double(adj > 0);

% This finds all the elements that have a value > 0 (adj > 0) and creates a
% logical. A logical isa type of data which says if a condition is true or
% not. At every element > 0 the logical will assign a value of 1 and if not
% it will have a value of 0. The 'double' command converts the logical to a
% matrix

%% Visualising connectomes

% Often it can be helpful to visualise the connectome in an image. This can
% help you get an idea of the network topology as well as see if anything
% has gone wrong. The command 'imagesc' is used for this purpose.

% It can also be helpful to change the colour map (i.e. what colours are
% used to represent which colours) as the MATLAB default isn't particularly
% great.

imagesc(log(adj))
colormap('jet')

% We convert the values in the connectome to log values 'log(adj)' in order
% to make for easier visualisation. The colourmap scales according to the
% relative values in the matrix. As a handful of edges have significantly 
% higher weights than the rest of the network, most nodes will be coloured
% blue and be hard to distiguish from the background (i.e. no connections).
% What the log does is the distance between edge weights from absolute to
% relative values, essentially bringing all the values closer together.
% This allows for more of the colourmap to be sampled.

%% Brain Connectivity Toolbox (BCT)
%
% The Brain Connectivity Toolbox (https://sites.google.com/site/bctnet/)
% contains many useful functions for analysing networks. Download from the
% website and unzip. Next you need to create a path pointing to this
% unziped folder using the command 'addpath'. For instance:

addpath E:\BCT

% Simply replace E:\Desktop\Honours_Data\BCT with the path for the BCT on
% your computer

%% Node degree and strength
%
% Degree refers to the total number of connections a node has to other
% nodes. Strength refers to the sum of the edge weights attached to a node.
% To find this values, it is easiest to use the Brain Connectivity Toolbox
% as they have inbuilt functions while will calculate these values. To
% calculate a nodes degree run

deg = degrees_und(adj);

% This will work no matter if the network is weighted or unweighted. The
% output 'deg' will be an array where deg(1) is the degree of node 1,
% deg(2) is the degree of node 2 etc.
%
% To calculate node strength run

str = strengths_und(adj);

% The network must be weighted for this to work. Again 'str' will be an
% array containing the strength of each node

%% Degree distributions
%
% The distribution of node degree and strength are important
% characteristics of a network. This distribution heavily influences many
% other parameters of a network
%
% To find the degree/strength distributions, you simply need to create a
% histogram of the variables deg/str. For instance for the degree
% distribution do

hist(deg)

% A histogram should appear on your screen in a pop out box. Currently the
% histogram has 10 bins (i.e. columns). This can be changed by the
% following 'hist(data,x)' where x is the desired number of bins 

% Also is helpful to label the axis and give a title

title('Degree distribution')
xlabel('Node degree')
ylabel('Frequency')

%% Strength distribution

% The same can be done to produce a strength distribution 

hist(str)
title('Strength distribution')
xlabel('Node strength')
ylabel('Frequency')

%% Degree and strength distributions - subplots

% Subplots can also be useful to display multiple figures in the one
% window. The commands 'subplot(x,y,z)' is used for this purpose. x is the
% number of rows in the subplot, y is the number of columns and z is in
% which position you want to place a figure. So to have degree and strength
% distribution in the same image:

close all
subplot(1,2,1)
hist(deg)
title('Degree distribution')
xlabel('Node degree')
ylabel('Frequency')
subplot(1,2,2)
hist(str)
title('Strength distribution')
xlabel('Node strength')
ylabel('Frequency')

%% BrainNetViewer - interface

% Download BrainNetViewer (https://www.nitrc.org/projects/bnv/) and unpack.
% Add this directory to the current path (again change the directory for
% where you have it

addpath E:\BrainNetViewer

% Next you have to run a little function I wrote to get your data into a
% format BrainNetViewer recognises. It is in the format 
% BrainNetData(COG,Node_colour,Node_size,output_name,labels) where COG is
% the MNI coordinates of each node, Node_colour is how you want each node
% to be coloured (so each node with a Node_colour of 1 will be one colour,
% node with a value of 2 will be another colour etc), Node_size is how
% large you want each node to be, output_name is a string of what you want
% to name the output file and labels is the name of each node (optional).

% One issue with the current variable Labels is there is a space in the
% string (i.e. Left Accumbens, Right Superior Frontal etc). BrainNetViewer
% will freak out if it there are spaces so the spaces need to be replaced.

new_labels = cell(length(Labels),1);
for i = 1:length(Labels)
    new_labels{i,:} = strrep(Labels{i},' ','\_');
end

% Now the file can be generated

BrainNetData(COG,deg,str,'Degree',new_labels)

% This will output a file called 'Degree.node'. We can also create a .edge
% file containing all of the connections

dlmwrite('connections.edge', adj, '\t')

% BrainNetViewer can either be used in one fo two ways. You can either
% interact with it using an interface or do everything through the command
% line.

% To use the GUI (Graphic User Interface) simply run

BrainNet

% You will have to selected a surface, .node file and a .edge file. To
% select the surface file navigate to the BrainNetViewer folder and then go
% to Data/SurfTemplate and select a surface (BrainMesh_ICBM152.nv is what I
% use). I suggest copy what ever surface template you want to use into the
% folder that BrainNetViewer immediately opens up when you clikc 'Browse'
% to speed things up. Next navigate to your .node file and .edge file.
% Select 'Ok' and an options menu should appear. Most of the options are
% self explanatory (under the 'Edge' option in 'Draw Edge' I recommend not
% telling it to draw all edges, instead use the 'Above Threshold' slider.
% will have to experiment to see what give the best result, or just set it
% to the max and have no edges drawn).

%% BrainNetViewer - command line

% To run from the command line simply run the following fuction: 
% BrainNet_generate_views(NodeFileName,EdgeFileName,SurfaceTemplatePath,OutputBaseFilename)
% This will automatically generate axial, coronal and sagiettal views of
% the brain. NodeFileName is your .node file, EdgeFileName is the .edge
% file (although this is optional, enter [] in this spot if you don't want
% edges), SurfaceTemplatePath is the location of the surface template you 
% want to use and OutputBaseFilename is what you want to name the output 
% (don't include the extension).

BrainNet_generate_views('Degree.node',[],...
    'E:\Desktop\Honours_Data\BrainNetViewer\Data\SurfTemplate\BrainMesh_ICBM152.nv','degree');

%% Ploting degree/strength mean and variance across subjects - errorbars

% I believe you need this kind of information so I am going to propose a 
% way to do it but don't know if it is ideal or exactly what you want but 
% anyways.

% So I recommend you store each subjects connectome in a cell, in this
% case called connectome. All that is needed is a simple loop to get the
% degree and strength of each connectome

sub_deg = zeros(length(connectome),length(connectome{1})); 
sub_str = zeros(length(connectome),length(connectome{1}));
for i = 1:length(connectome)
    sub_deg(i,:) = degrees_und(connectome{i}); 
    sub_str(i,:) = strengths_und(connectome{i});
end

% I used the standard deviation to show variance but you can always replace
% this with the actual variance using the 'var' command (just replace any
% instance of std with var. I think standard deviation allows for slightly
% more intuitive understanding

mean_sub_deg = mean(sub_deg,2);
mean_sub_str = mean(sub_str,2);
std_sub_deg = std(sub_deg,0,2);
std_sub_str = std(sub_str,0,2);

% Before I make the figure, I am also going to
% label each subject by their "actual" ID (for illustrative purposes these
% subjects are just going to be named from 1:50). If you chose to do it
% this way just subsitite Sub_id with however you have labelled each
% subject (careful of underscores if you use them). Sub_id doesn't need to
% be a cell it can also be an array

Sub_id = cell(length(connectome));
for i = 1:length(connectome)
    Sub_id{i} = sprintf('Subject %d',i);
end

% Now to make the figure

close all

% Note this command 'figure('Position', [1 1 1400 600])' isn't required, it 
% just makes the figure a nice size automatically. Useful if you want to 
% save the figure automatically. You can change the numbers [1 1 1400 600]
% to change the size of the figure

figure('Position', [1 1 1400 600]);

subplot(2,1,1)
errorbar(mean_sub_deg,std_sub_deg,'rx');
xlim([0 length(connectome)+1]);
ax = gca;
ax.XTick = 1:length(connectome);
ax.XTickLabel = Sub_id;
ax.XTickLabelRotation = 90;
ylabel('Degree')
xlabel('Subject')
title('Degree variance across subjects');
subplot(2,1,2)
errorbar(mean_sub_str,std_sub_str,'bx');
xlim([0 length(connectome)+1]);
ax = gca;
ax.XTick = 1:length(connectome);
ax.XTickLabel = Sub_id;
ax.XTickLabelRotation = 90;
ylabel('Strength')
xlabel('Subject')
title('Strength variance across subjects');


%% Ploting degree/strength mean and variance across subjects - boxplot
% Another possible (and just ever so slightly easier) way is to use 
% boxplots. Note the sub_deg and sub_str needs to be rotated (done by using
% ') so that each column is an individual subject. Unlike the chart done
% above plot the boxplot for degree and strength seperately (i.e. don't use
% subplots, figure will be too cramped)

close all

figure('Position', [1 1 1400 600]);
boxplot(sub_deg')
ax = gca;
ax.XTick = 1:length(connectome);
ax.XTickLabel = Sub_id;
ax.XTickLabelRotation = 90;
ylabel('Degree')
xlabel('Subject')
title('Degree variance across subjects');

figure('Position', [1 1 1400 600]);
boxplot(sub_str')
ax = gca;
ax.XTick = 1:length(connectome);
ax.XTickLabel = Sub_id;
ax.XTickLabelRotation = 90;
ylabel('Strength')
xlabel('Subject')
title('Strength variance across subjects');

%% Ploting node degree/strength mean and variance across subjects - errorbars

sub_deg = zeros(length(connectome),length(connectome{1})); 
sub_str = zeros(length(connectome),length(connectome{1}));
for i = 1:length(connectome)
    sub_deg(i,:) = degrees_und(connectome{i}); 
    sub_str(i,:) = strengths_und(connectome{i});
end

mean_sub_deg = mean(sub_deg,2);
mean_sub_str = mean(sub_str,2);
std_sub_deg = std(sub_deg,0,2);
std_sub_str = std(sub_str,0,2);

close all

figure('Position', [1 1 1400 600]);
errorbar(mean_sub_deg,std_sub_deg,'rx');
xlim([0 length(connectome)+1]);
ax = gca;
ax.XTick = 1:length(connectome{1});
ax.XTickLabel = Labels;
ax.XTickLabelRotation = 90;
ylabel('Degree')
xlabel('Node/Region')
title('Node degree variance across subjects');

figure('Position', [1 1 1400 600]);
errorbar(mean_sub_str,std_sub_str,'bx');
xlim([0 length(connectome)+1]);
ax = gca;
ax.XTick = 1:length(connectome);
ax.XTickLabel = Labels;
ax.XTickLabelRotation = 90;
ylabel('Strength')
xlabel('Node/Region')
title('Node strength variance across subjects');

%% Ploting node degree/strength mean and variance across subjects - boxplot

close all

figure('Position', [1 1 1400 600]);
boxplot(sub_deg)
ax = gca;
ax.XTick = 1:length(connectome{1});
ax.XTickLabel = Labels;
ax.XTickLabelRotation = 90;
ylabel('Degree')
xlabel('Subject')
title('Node degree variance across subjects');

figure('Position', [1 1 1400 600]);
boxplot(sub_str)
ax = gca;
ax.XTick = 1:length(connectome{1});
ax.XTickLabel = Labels;
ax.XTickLabelRotation = 90;
ylabel('Strength')
xlabel('Subject')
title('Node strength variance across subjects');