Intensity Normalization: Where and When?

Dear all,

After analyzing and visualizing the functional preprocessed data (montage; attached) of my subjects, I realized most subjects show dark-inside fMRI scans (hollow-resembling) while others do not. Checking their raw fMRI data showed that the "Prescan Normalization" was not active in their acquisition. I wonder whether there is any need for intensity normalization for the fMRI acquisitions, and whether it is done by "denoising" or not. I tried to do that on one of the subjects. Interestingly, before denoising there was a right-leaning skewness in the probability distribution which converts to a normal distribution after that.

Any help is appreciated.

Bests,
Amir