Yes in my opinion the retina is "brain." I don't have a good definition of what is brain, but if I did it would be alot more complex than just being inside the Cranium.
My definitions of the body's organs tend to have much more to do with their unique functions than where they are located. For example, deep within the brain, mainly on the walls of the third and fourth ventricles (also called the "lateral ventricles") there is some soft pink tissue, called the choroid plexus. It has nothing to do with neural activity. It is the source of the cerebrospinal fluid, CSF which the brain "floats in" (for physical shock protection).
The choroid plexus makes about 1cc of CSF every three minutes which eventually flows down the spinal cord with the Dura sheath containing it until near the end, where the nerves are no longer joined into a "cord" but individually separated. (I forget the Latin name for this end of the spinal nerves, but translated it is "horse's tail" as that is exactly what it looks like.) There are little "pop valves" that release the SCF into the abdominal cavity where eventually it is absorbed into the blood.
I know about all this as APL/JHU where I worked made an implanted insulin pump which ran open loop (Best in the world, IMO - now sold by Medtronics.)- I.e. the user had external device to command how much insulin was released and we wanted to close the loop with a sensor of blood glucose level. I designed a way to do that using the clear SCF and the optical rotation that sugar makes. For adequate signal to noise & accuracy, about a 10 cm optical path was required and that required fusing at least three vertebras so we decided not to suggest that. As far as I know, no-one has made an implantable glucose level sensor that will work for more than a couple of months (Body encapsulates foreign objects it cannot dissolve.) But I digress too much.
The choroid plexus is at the very center of the skull, but no one would call it "brain." The brain processes neural signals, as does the retina. (There is almost a 10 to 1 data compression taking place in the retina. I.e. the number of photo sensitive cells in the retina is about 10 times larger than the number of data channels in the optical nerve. Crudely speaking it is mainly edge detection that is done in the retina but much more complex than just that.)
The retina was once an integral part of the tissue that later becomes "brain" and does the same "complex processing of data" function (plus is a transducer of optical radiation). For these reasons I tend to call it brain as whatever would be my functional definition of "brain" the "complex processing of data" function would be the heart of it and its location would not be. For example, the octopus has eight brains, one at the head of each leg and none (I think) inside its skull. On second thought, there probably is some central processing of the signals from the eyes that then gets sent to the eight leg brains.
BTW, while working closely with several doctors at JHU hospital, including some in the Wilmer eye clinic, I suggested that it might be possible to detect, non-invasively, the early stages of Alzheimer’s disease by careful measurements of the wavelength spectrum reflected by the retina as it was “brain tissue.” They all agree that might be feasible but we had no idea what to look for, no sponsor, etc. and nothing was done. As far as I know there is still no way still to diagnose Alzheimer’s except tissue biopsy (post mortem, usually).
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