Hi Pierre,

As I'm not an expert, I would like to ask for a help to understand better about NIAK firm preprocessing.

This is the main part that conducts pca of motion parameters in niak_brick_regress_confounds.m:

========================================================
%% Motion parameters
if opt.flag_verbose
fprintf('Reading (and reducing) the motion parameters ...\n')
end
transf = load(files_in.motion_param);
[rot,tsl] = niak_transf2param(transf.transf);
rot = rot(:,~mask_scrubbing);
tsl = tsl(:,~mask_scrubbing);
rot = niak_normalize_tseries(rot');
tsl = niak_normalize_tseries(tsl');
motion_param = [rot,tsl,rot.^2,tsl.^2];
if opt.flag_pca_motion
[eig_val,motion_param] = niak_pca(motion_param',opt.pct_var_explained);
end
========================================================
I want to better understand about those parameters, 
Are these the Volterra expansion involving parameters at time t-1 as well and their square? 
Could you please briefly explain me what they are?

Thank you in advance.

Kangjoo