# Percent change of a biophysical signal

To calculate percent change the formula is: $$\frac{Amount \, ofchanges} {Original \, amount}100=\frac{New−Old}{Old}100$$

because we want to see how it changes compared to where it was before, now if there is no changes the percent changes must be zero. I am a student in neuroscience field, we always measure biophysical signals change (in our field we call it normalized signals, for example the signal that define the amount of blood in cerebral area), but these signals are always start from 1.

Is normalized value and the percent change are the same, and why start from 1 and not 0?