run correlation measureΒΆ

Matlab output: run_correlation_measure

%% Exercise with cosmo_correlation_measure
% Split-half correlation analysis using cosmo_correlation_measure
%
% #   For CoSMoMVPA's copyright information and license terms,   #
% #   see the COPYING file distributed with CoSMoMVPA.           #


%% Single subject analysis over all categories in VT

% Set the data path
config=cosmo_config();
study_path=fullfile(config.tutorial_data_path,'ak6');

subject_id='s01';
mask_label='vt_mask';
data_path=fullfile(study_path,subject_id); % data from subject s01

% Define data locations and load data from even and odd runs
mask_fn=fullfile(data_path, [mask_label '.nii']); % whole brain

% Load data for odd runs
data_odd_fn=fullfile(data_path,'glm_T_stats_odd.nii');
ds_odd=cosmo_fmri_dataset(data_odd_fn,'mask',mask_fn,...
                            'targets',1:6,'chunks',1);

% Load data for even runs
data_even_fn=fullfile(data_path,'glm_T_stats_even.nii');
ds_even=cosmo_fmri_dataset(data_even_fn,'mask',mask_fn,...
                            'targets',1:6,'chunks',2);

% Combine even and odd runs into single dataset, store in ds_odd_even
% Hint: use cosmo_stack
ds_odd_even=cosmo_stack({ds_odd, ds_even});

% remove constant features (due to liberal masking)
ds_odd_even=cosmo_remove_useless_data(ds_odd_even);

% display dataset
fprintf('Dataset input:\n');
cosmo_disp(ds_odd_even);

% Part 1: compute correlation difference
% Hint: - apply cosmo_correlation_measure to ds_odd_even
%       - because correlation difference and split-half correlations
%         are the defaults, no second argument (options) is required
%         for cosmo_correlation_measure.
ds_corr=cosmo_correlation_measure(ds_odd_even);
fprintf('\nDataset output (correlation difference):\n');
cosmo_disp(ds_corr);

fprintf(['Average correlation difference between matching and '...
            'non-matching categories in %s for %s is %.3f\n'],...
            mask_label, subject_id, ds_corr.samples);


% Part 2: compute the raw Fisher-transformed correlation matrix,
% and store the results in 'c_raw'
%
% (Hint: use a struct 'args' with args.output='correlation' as second
% argument for cosmo_correlation_measure)
args=struct();
args.output='correlation';
c_raw=cosmo_correlation_measure(ds_odd_even,args);
fprintf('\nDataset output (Fisher-transformed correlations):\n');
cosmo_disp(c_raw)

% Because a measure returns .samples as a column vector, the
% confusion matrix is returned in a flattened form.
% The data can be put back in matrix form using cosmo_unflatten.
matrices=cosmo_unflatten(c_raw,1);
imagesc(matrices);
colorbar();

%% Group analysis over all categories in VT using correlation measure

% Compute correlation measure for each subject individually.
subject_ids={'s01','s02','s03','s04','s05','s06','s07','s08'};
nsubjects=numel(subject_ids);

mask_label='vt_mask';

% allocate a cell for the output of the measure for each subject
ds_corr_cell=cell(nsubjects,1);

% Apply correlation measure for each subject using
% cosmo_correlation_measure.
% - compute the correlation measure using cosmo_correlation_measure;
%   this should give a dataset structure with ds.samples of size 1x1
% - because statistics are computed later, add to the output:
%   * .sa.chunks : the subject number
%   * .sa.targets: 1 (for each subject)
% - store the result of the k-th subject in ds_corr_cell{k}

for subject_num=1:nsubjects
    subject_id=subject_ids{subject_num};

    % set path for this subject
    data_path=fullfile(study_path,subject_id);

    % Define data locations and load data from even and odd runs
    mask_fn=fullfile(data_path, [mask_label '.nii']); % whole brain

    data_odd_fn=fullfile(data_path,'glm_T_stats_odd.nii');
    ds_odd=cosmo_fmri_dataset(data_odd_fn,'mask',mask_fn,...
                                'targets',1:6,'chunks',1);


    data_even_fn=fullfile(data_path,'glm_T_stats_even.nii');
    ds_even=cosmo_fmri_dataset(data_even_fn,'mask',mask_fn,...
                                'targets',1:6,'chunks',2);

    % Combine even and odd runs
    ds_odd_even=cosmo_stack({ds_odd, ds_even});

    % remove constant features (due to liberal masking)
    ds_odd_even=cosmo_remove_useless_data(ds_odd_even);

    % apply correlation measure for this subject, and store
    % the result in 'ds_corr'
    ds_corr=cosmo_correlation_measure(ds_odd_even);
    % set targets and chunks for the output, so that cosmo_stat can be used
    % below
    ds_corr.sa.targets=1;
    ds_corr.sa.chunks=subject_num;

    % store result in the cell array 'ds_corr_cell' at the 'subject_num'-th
    % position
    ds_corr_cell{subject_num}=ds_corr;
end

% combine the data from all subjects (in 'ds_corr_cell') into one dataset
% using cosmo_stack, and assign the result to a variable 'ds_all'
ds_all=cosmo_stack(ds_corr_cell);

%% Compute group-level statistics

% run one-sample t-test again zero to compute the t-value and p-values
% against the null hypothesis of a mean of zero.
%
% hint: use matlab stats toolbox's ttest (if present), or
%       use cosmo_stat:
%       * for t-statistics, use 't' as second argument
%       * to convert a 't' to 'p' value, use 'p' as third argument
ds_t=cosmo_stat(ds_all,'t');     % t-test against zero
ds_p=cosmo_stat(ds_all,'t','p'); % convert to p-value

samples=ds_all.samples;

fprintf(['correlation difference in %s at group level: '...
           '%.3f +/- %.3f, %s=%.3f, p=%.5f (using cosmo_stat)\n'],...
            mask_label,mean(samples),std(samples),...
            ds_t.sa.stats{1},ds_t.samples,ds_p.samples);

% Using matlab's stat toolbox (if present)
if cosmo_check_external('@stats',false)
    [h,p,ci,stats]=ttest(samples);
    fprintf(['correlation difference in %s at group level: '...
            '%.3f +/- %.3f, t_%d=%.3f, p=%.5f (using matlab stats '...
            'toolbox)\n'],...
            mask_label,mean(samples),std(samples),stats.df,stats.tstat,p);
else
    fprintf('Matlab stats toolbox not found\n');
end