SPM/Percentage signal change

=Percentage of Signal Change=
 * http://blogs.warwick.ac.uk/nichols/entry/spm_plot_units/
 * http://marsbar.sourceforge.net/faq.html#how-is-the-percent-signal-change-calculated
 * http://mindhive.mit.edu/node/86
 * http://link.springer.com/article/10.1007%2Fs12021-008-9042-x
 * http://www.sbirc.ed.ac.uk/cyril/bold_percentage/BOLD_percentage.html
 * http://www.frontiersin.org/Journal/10.3389/fnins.2014.00001/abstract