Not only that, but the right half of the visual field is processed by the left hemisphere, and vice versa. Note it’s not the sight from the right eye that’s crossed over, but the right half of the field of view.
That’s a very good mystery - what evolutionary factor promoted the tendency for nerves to crossover like this?
I mulled over this in college with my professor and his reasononing is that there are a lot of things about the body, and especially the nervous system, that don’t “make sense” to our intuitive Newtonian, Darwinian, macroscopic perspectives. But we have to remember that most of these things evolved early on, by natural selection at the cellular level. So the factors that selected for many of the things we think are bizarre may have been quite obvious, but they’re lost in the murk of evolutionary history.
I suggested that there might be some advantage during early embryonic development for nerves to grow across the axis of the spinal column instead of away from the axis. Prof said that was probably as good a guess as any.
Right. Natural selection is one of the most powerful explanatory scientific theories ever devised.So it must explain why vertebrates developed contralateral sensorimotor nervous system control over the body. Neuroanatomy shows the decussations of the nerve pathways clearly enough, but the existence of the decussations does not explain why they exist. That answer must come from a consideration of the ways bodies interact with their environments and how those bodies evolved.
The decussation of the visual pathways in vertebrates suggests an explanation for contralateral control, but the argument needs to be developed. Actually, it is at the pupil of the chambered eye that the visual stimuli from the environment become inverted and reversed before being projected upon the retina. The purpose of the optic chiasm is to combine the reversed retinal images of the world from the two eyes into one “cyclopian” image projected onto the visual cortical areas.
Thus, organisms that rely heavily on precise visual sense from pupillary eyes have a selective advantage in developing crossed motor circuitry to keep the inputs from the right visual hemispace in close connection with the central motor areas of the brain, and this leads to the fact that vertebrates (and invertebrates like cephalopods with chambered eyes) have crossed nervous systems.
This assumes that the optical chiasm must of necessity cross the images, and redirect to opposite hemispheres in order to combine them? Do we know that?
What we know is that nerve fibers originating in the nasal half of the retina cross here. That corresponds to left visual field of the left eye and the right visual field of the right eye. Once the fibers pass the chiasm, what you have represented in the the right tract are fibers originating from the temporal (outside)half of the right retina, traveling with fibers originating in the nasal half of the left retina. Together, these fibers represent the left visual world and are processed by the right cerebral hemisphere. The left optic tract is composed of fibers originating in the left temporal retina and the right nasal retina. This tract transmits information about the right visual world.
Check out figure 2 on this webpage for a good diagram of optical pathways.
[aside][sub]One of my favorite medical conditon names is homomynous hemianopsia. Ran into that little gem for the first time in first year med school. Just try to pronounce it correctly on the first try. This condition develops when a mass (classically a pituitary adenoma) presses on the optic chiasm, causing the crossing fibers to malfunction. The patient experiences a loss of the left visual field as perceived by the left eye and a loss of the right visual field as perceived by the right eye[/aside][/sub]
[GD mode]Neurodoc, I’ve heard the reversed visual fields argument for sensorimotor decussation before. It makes sense, but is difficult to prove.
However, it’s probably know approximately when, phylogenetically speaking, the modern visual and sensorimotor systems developed. If a crossed sensorimotor system developed much earlier than the visual system, this would argue that sensorimotor crossing has nothing to do with our reversed visual fields.[/GD mode]
The crossed visual system leads to crossed sensorimotor hypothesis makes sense only for organisms where there is overlap of the visual fields covered by the two eyes. Many animals (herbivores like cattle and sheep come to mind) sacrifice overlapping fields to obtain an overall wider field of vision by placement of the eyes on the sides of the head. Presumably, decussation of nasal retinal fibers occurs in these animals. In such animals, the left cerebral hemisphere processes information from the left anterior and right posterior visual fields and the right hemisphere processes information from the right anterior and left posterior visual fields. An advantage for a crossed sensorimotor system is less obvious in these animals.
To argue, therefore, that the sensorimotor systems are crossed to simplify rectification of visual inputs with sensorimotor inputs/outputs, you’d need to know whether the ancestral organism possessed overlapping visual fields.
Homonymous hemianopsia is a concordant visual field loss from both eyes, e.g. loss of the left hemifield from both eyes or loss of the right hemifield from both eyes. The condition described for the chiasmatic lesion described in the earlier post is “bitemporal hemianopsia.”
The argument for the optical transformation of sensory input leading to crossed neural pathways does not depend upon the optic chiasm, and it does not depend upon the organism being binocular. A cyclopian organism with a single pupillary eye would be subject to the argument.
I presented the argument in greater detail in an article that was published in Medical Hypotheses 45, 471-495.
Damn! I was posting from a dim recollection. You got me.
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I think you think I’m not understanding you. I agree that whether an organism had one pupillary eye, or sixty, the image projected onto the retina would be inverted (up is down, left is right). Therefore, the visual system is crossed secondary to the optical properties of the pupil/lens.
You ask why the sensorimotor pathways are crossed as well. Fine and dandy. There doesn’t seem to be an obvious physical constraint on these systems as there is in the case of vision.
Your hypothesis is that since the physics of vision dictates a crossed system, and since seeing is very important, the peripheral sensorimotor systems adopted a crossed configuration to jive with the visual system. This is what you are saying, right?
I raised two hypothetical objections to this hypothesis:
If the crossed sensorimotor systems were in place prior to the development of a crossed visual system (ie. the modern eye did not develop that early), this undermines your hypothesis. If vision came first, your hypothesis is bolstered. I have no knowledge regarding the timings of the origins of these pathways; nonetheless, it sounds like the sort of thing that someone would know.
Your argument is based on the assumption that since (let’s say) left-sided visual field information is processed by the right hemisphere, there exists an advantage to having left-sided sensorimotor information processed there as well. However, in animals that have laterally positioned eyes (cows, sheep, horses) the left-sided visual field is only perceived by the left eye and is then sent to both the right and left hemispheres. The anterior portion of the left visual field falls on the posterior portion of the left retina (analogous to the temporal portion in humans) and these fibers pass (without crossing) to the left hemisphere. The posterior portion of the left visual field is perceived by the anterior left retina (analogous to the nasal half of the human retina) and these fibers cross at the chiasm to be processed by the right hemisphere. With such an arrangement, having a crossed sensorimotor systems is of no clear benefit; the right hemisphere is receiving visual inputs from right anterior visual field and the left posterior visual field, whereas the sensorimotor fibers originate from the left side only of the organism. My contention is that if the mammalian visual and sensorimotor pathways originated in an organism that had laterally placed eyes, then the inference you make regarding the benefit of a crossed sensorimotor configuration is invalidated.
You can’t demonstrate that the mutually crossed configuration is more efficient; you infer this based on a notion (albeit inaccurate-but that’s for another day) of Nature’s untiring drive for ever improved design. I’m not saying this isn’t an attractive hypothesis, just pointing out that it requires this infererence.
[sup]Anyone want to give me odds that this thread is in GD by the end of the weekend?[/sup]
Well, your comments are thoughtful and indicate that you do appreciate the details of the argument.
Comparative anatomy and phylogeny show that crossed nervous systems are associated with organisms that depend on image-forming eyes (i.e. pupillary eyes). Vertebrate nervous systems show a high degree of crossing-over. So do cephalopods. There is much less in more primitive invertebrates.
Carnivores and herbivores (like cows) with laterally placed eyes still have overlapping visual fields. This includes non-owl birds. They are all capable of stereopsis.
So I think that my hypothesis is quite plausible, and in fact is the most plausible of all the various hypotheses that have been suggested to explain this “mystery” of crossing-over in the nervous system. Can you think of a better explanation?
I don’t even know of the alternative hypotheses. Is there a convenient reference you could point me to?
Another thought: Do you know the details of the experiments in which right/left, up/down, or both are reversed by a subject’s wearing of special glasses. I have never read these studies in detail, but my recollection from Psych 101 (oh so many years ago:() is that initially, subjects had a great deal of difficulty coordinating movement, but that gradually they adapted and coordination improved. Since your argument depends on the mutually crossed visual and sensorimotor systems conferring an advantage, it is vital to know the degree of adaptation the subjects of the above experiment could attain. Perhaps they regained functionality (could perform ADLs, etc.) but never quite made it back to baseline performance?
If these subjects demonstrated no deficits on testing after adaptation, then your hypothesis is less plausible. If they never returned to baseline performance, that doesn’t prove your hypothesis (the remaining deficit could be secondary to limited neural plasticity), but it helps a little, I think.