I do not know about getting the first time point of each file, but you are using spm_read_vols incorrectly. It should be something like:
fid = fopen('10niftis.txt', 'rt');
K = 0;
while true
this_nifti = fgetl(fid);
if ~ischar(this_nifti); break; end %end of file detected
K = K + 1;
V{K} = spm_vol(this_nifti);
data{K} = spm_read_vols(V{K});
end
fclose(fid);
Maybe you can use
data{K} = spm_read_vols(V{K}(1));
to read only the first time-point -- I have not researched the representation of spm volumes.
