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A creature has 3 faces, 1 faces left, 1 faces forwards, and 1 faces right. It also has 4 eyes. 2 of these eyes are on the border between the front and side faces, and can be used for stereoscopic vision by either face. Could there be an optic chiasma that could handle this, and if so, how would it be structured?

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  • $\begingroup$ Is this based on Earth biology? Is the creature the product of evolution, or engineering? Organs more unlikely than those you describe exist in the real world, so evolution doesn't rule it out. Engineering can definitely make it possible. Earth biology has some stumbling blocks (at least for vertebrates), alien biology might have converged on paradigms that make it all but inevitable. $\endgroup$
    – John O
    Aug 26, 2021 at 16:04
  • $\begingroup$ The optic chiasm is a purely passive structure. It does not "handle" anything. (More in depth: some sensory nerve fibers come from the left eye, some come from the right eye. All those nerve fibers converge in the optic chiasm. From there some of them go to the left hemisphere of the brain, some go to the right hemisphere of the brain. No optical nerve fibers stop at the chiasm, no optical nerve fibers originate in the chiasm. No processing happens there. Moreover, the structure is specific to vertebrates. Cephalopods and insects, for example, do not have a similar structure.) $\endgroup$
    – AlexP
    Aug 26, 2021 at 16:43
  • $\begingroup$ @AlexP The optic chiasma does make sure that each hemisphere gets input from both eyes, which is important for stereoscopic vision $\endgroup$ Aug 26, 2021 at 16:47
  • $\begingroup$ (1) As I said, it is a structure specific to vertebrates. Insects are flying creatures, and do not have a similar structure. (2) The same effect could be obtained in other ways, such as routing the signals from within the brain. (3) It's more complicated than "each hemisphere gets input from both eyes". Each hemisphere gets about half the visual field, not an arbitrary mixture of fibers from both eyes. This can be easily arranged for the four eyes in the question. (4) But then again, the optic chiasm is a purely cable routing box. You can route the cables in many other ways. $\endgroup$
    – AlexP
    Aug 26, 2021 at 16:55
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    $\begingroup$ Support in the brain for our vision system is quite voluminous. Lots of regions are involved/connected in either hemisphere. I don't think a creature with 4 eyes and 2 function groups would have two hemispheres, the brain could evolve into 3 or 4 separate parts instead of 2 parts. $\endgroup$
    – Goodies
    Aug 26, 2021 at 22:48

2 Answers 2

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If you look to the right of your screen, your screen will be on your left - the image will be projected to the right side of your two retinas, and then get to the right side of your brain - if you now look towards the left of your screen, the same thing happens vice versa.

In both cases, the information on the screen was transported from two retinas to one communal brain area - the Chiasma opticum being no more than a street crossing , where different bus routes take different exits. Please also note that, while looking to the right and then left of your screen, you only ever saw one screen, and never had the experience of having two screens that somehow phase in and out of existence.

Now, there are some medical conditions (albinism being one), where the midline of the eye may be shifted, or the visual projections to the brain are not separated by hemisphere as usual. In such cases, stereo vision can be limited

Taking these two datapoints together gives us this: It seemingly is beneficial for stereovision to have the retinal signals from the same object get together at the earliest, but, it is not really necessary for the visual experience of seamlessly mergin the input from two eyes.

Now, for your quad-eyed creature: however the fields of vision would work out, whether the middle two eyes had two irises to enable staring straight ahead and straight to the sides at the same time, or whether the middle two eyes would turn to enable staring straight to the side, it would always be possible to route the info from corresponding parts of the image on the retina to corresponding brain areas, especially since nerve connections need not be 1:1 but can be n:n. That would enable stereo vision for all three pairs of eyes. Not all of that routing would be called a chiasma (as that is reserved for crossings) but what's in a name, really? Visual experience is hard to imagine: Would it be a near 360° experience? Would the input be translated to a 'view' that is only part of the combined visual field of the four eyes? No problems either way.

Now the evolution of this might be a little tricky - in humans the crossing probably happens because in a two hemisphere brain it is efficient to have the hemisphere that gets the picture of an arm also control that arm - i am guessing your quad-eyed creature only posesses two arms, so you might also want to take that into consideration while tracing connections.

Just to reiterate: It is simply historical reasons why the human visual system is as it is - a person might route all information from both retinae to one hemisphere, and still retain stereo vision (as in accurate depth perception et al) or route the lower half of both retinae to the right and the upper half to the left, no matter. The important part is that the information from corresponding images of points in space will land near each other, for fast and efficient networking. Visual experience, is, as we see in ourselves, not dependent on that at all (and thus slower as some of the more spatial functions) and can be upheld even with the two brain hemispheres klunkily communicating over the corpus callosum.

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There is no need for shared optical chiasmas there, as long as the three faces share a single brain.

See the case of the Hogan sisters, who are cojoined twins of the rarest type (craniopagus). They share neural tissue. Due to the high level of interconnectivity between their brains, they can see through each other's eyes.

Your creature's faces could be able to see what each other is seeing and, just like the Hogan sisters, share sensorial input of all kinds and motor control.

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