Percent change of a biophysical signal
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.