The zero result indicates the image was uniform (non-changing) within this small patch. Negative or positive sums mean the image was varying (changing) within this small patch of nine photoreceptors. The above matrix is only an approximation to what really happens inside the retina. The differences are:

The above example is called "balanced". The term balanced means that the sum of the negative weights is equal to the sum of the positive weights so that they cancel out perfectly. Retinal ganglion cells are almost never perfectly balanced. The table is square while the centre–surround structures in the retina are circular. Neurons operate on spike trains traveling down nerve cell axons. Computers operate on a single floating-point number that is essentially constant from each input pixel. (The computer pixel is basically the equivalent of a biological photoreceptor.) The retina performs all these calculations in parallel while the computer operates on each pixel one at a time. The retina performs no repeated summations and shifting as would a computer. Finally, the horizontal and amacrine cells play a significant role in this process, but that is not represented here.Here is an example of an input image and how edge detection would modify it. Once the image is spatially encoded by the centre–surround structures, the signal is sent out along the optic nerve (via the axons of the ganglion cells) through the optic chiasm to the LGN (lateral geniculate nucleus). The exact function of the LGN is unknown at this time.
Genetics
Human factor XIII consist of A and B subunits. A subunit gene is F13A1. It is on the chromosome 6 at the position 6p24–25. It spans over 160 kbp, has 14 introns and 15 exons. Its mRNA is 3.9 kbp. It has a 5 UTR of 84 bp and a 3 UTR of 1.6 kbp. F13A1 exon(s)
1 code 5 UTR
2 code activation peptide
2–4 code β-sandwich
4–12 code catalytic domain
12–13 code β-barrel 1
13–15 code β-barrel 2B subunit gene is F13B. It is on the chromosome 1 at the position 1q31–32.1. It spans 28 kpb, has 11 introns and 12 exons. Its mRNA is 2.2 kbp. Exon 1 codes 5 UTR. Exons 2–12 code the 10 different sushi domains. Structure
Factor XIII of human blood is a heterotetramer of two A and two B linear polypeptides or "units". A units are potentially catalytic; B units are not. A units form a dimeric center. Non-covalently bound B units form a ring-like structure around the center. B units are removed when XIII is activated to XIIIa. Dimers containing only A units also occur within cells such as platelets. Large quantities of singular B units (monomers) also occur within blood.