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?
Normalized values usually refer to a ratio, which is not the same as percent change.

Percent change is usually bad because increases and decreases aren't the same:

40 to 60 is a 50% increase. 60 to 40 is a 33% decrease. A 50% decrease from 60 would be 30. It gets especially complicated and unwieldy if you have multiple contributing factors. 

Instead, I recommend taking logarithms and using differences between logarithms: then "0" means no change. Alternatively you can use ratios, which are the same as subtracting logarithms, but then you interpret no change as "1" because it's a ratio; x/x = 1, not zero. Logarithms are great because different additive factors act on the outcome as a product, but you can use linear statistics.