Try this
[rows, columns, numberOfColorChannels] = size(yourImage);
% Find location of the center pixel.
middleRow = round(rows/2);
middleColumn = round(columns/2);
% Get its gray level.
centerPixelValue = yourImage(middleRow, middleColumn, :)
To get the max pixel value difference over the whole image do
maxGrayLevelRange = range(yourImage, 'all')
