By default, imhist on a greyscale image outputs 256 bins, just as you want. You then jut have to split this output in half, using simple indexing.
if size(I, 3) == 1 %in that case I(:,:, 1) == I
h = imhist(I); %histogram over 256 bins by default
hlow = h(1:128); %lower part of the histogram
hhigh = h(129:256); %higher part of the histogram
end
To convert to pdf over the total number of pixels, use matrix operations rather than for loops:
pdflow = hlow/numel(I);
pdfhigh = hhigh/numel(I);
or if you want pdfs over the population of each half-histogram:
pdflow = hlow/sum(hlow);
pdfhigh = hhigh/sum(hhigh);
