How to perform this zero center

I am using the 3D CNN to classify the objects. In input layer matlab itself implements the training data to zero-center by default.

I am wondering how it is performing this zero centering? my data dimension consist from row, column, frame, channel (4D).

inputlayer = image3dInputLayer(inputSize,Name,Value)

'Normalization' — Data transformation 'zerocenter' (default) | 'none'

Data transformation to apply every time data is forward-propagated through the input layer,

specified as the comma-separated pair consisting of 'Normalization' and one of the following.

'zerocenter' — The software subtracts its mean from the training set.

'none' — No transformation.

Example: 'Normalization','none'

Data Types: char