Yes, reptiles have separate hemisheres. Most animals have brains with clear left/right separation from worms to us. The sides aren’t always called hemispheres but still close enough.
Reptiles do not have a visual cortex, though; they have an optic tectum (homologous to our superior colliculus) but the projection of left side of the world to the right side of the brain is the same.
I just opened a textbook in front of me to check reptile visual wiring and back in the 70s scientists transplanted 3rd eyes onto tadpoles and looked at the frog brains. Inputs from the third eye formed just like inputs in our brain with alternating columns of inputs from the 2 eyes that view same part of the world. So thats most likely the case with this
In the case of humans, at least, each half of each retina sends signals to the opposite hemisphere. This is the main reason why the optic nerves cross.
Birds do the cross-hemisphere thing, but though they have binocular vision, they also favor one eye or the other for certain tasks (plus they see in additional colors and can also sleep while flying, tho of course none of this is trinocularity):
I’m not completely sure about this. I would also like to see the anatomy, including the neural anatomy, of that one specimen properly analysed after it’s death. There is a very slight chance something went wrong with it’s parietal eye. However, that would be very strange, since as far as I remember from my systematics lectures, not even the light reception works the same way as in the other two eyes…
I hadn’t thought that they already have a “third eye” there. The parietal eye doesn’t have a lens, lid, or retina, though. The third eye of this snake is a clearly a third regular eye which means that there must have been a duplication of one of the other two during development.
Exactly. That’s why I’m rooting for a thorough analysis. The parietal eye itself is a remnant of a paired structure, so getting a single (in this case: third) eye from a pair isn’t unprecedented.
Also, it’s development is separate from the ‘regular’, phylogenetically more recent eyes. However, both are extrusions of the brain, which makes me wonder what happend here. Could be some developmental genes got expressed in cells where they are usually dormant, or the whole structure got duplicated by a mitosis gone wrong. Either way, growing to 40cm is is quite an accomplishment for that particular danger noodle, and it might contribute to our understanding of developmental processes, so: double win!
The reptilian brain visual system is a lot simpler. Snakes do not see in stereo as the eyes are on opposite sides of the head. Even this guy has them at about 90 degrees, so there is probably little overlap. This means there is no need to coordinate both eyes together the way we do. Chameleons do use stereo vision, but they do it differently - they target an insect with one eye, then bring in the second one to confirm depth just before firing the tongue. So, maybe a third eye would work fine on a snake, but not on a mammal because of the extra wiring we have.
The reptile brain does have a sort of LGN, which is where a lot of the fusion of our two eye images happens in our heads. Maybe something with three eyes needs three of these, or maybe six for full trinocular vision. But the third eye is looking up, so it is hard to guess what information it could fuse with the other views.
Nice try, evolution, but I don’t think it will catch on.
That’s interesting, as I tend to think of snakes as primarily predators, rather than prey, but I’ve read in a few places that stereo eye placement is for predators, and side-eye placement is for prey to watch for predators. I suppose like all things in science-for-pedestrians that is a very broad generalization.
Snakes are often predators, but not visual predators. They use smell and vibrations for finding prey and vision more for seeing things (often birds) that are coming from above to eat them.
