left-right reversal of CNS: Why?

kenneth paul collins KPCollins at postoffice.worldnet.att.net
Sun Sep 15 02:25:30 EST 1996


Jerry Larson wrote:
> 
> > dantso at cris.com wrote:
> > >       You are confusing image inversion with visual field representation.
> > >         The optics of the eye have nothing to do with crossed CNS
> > > representations.
> > >         Humans are among the few species that have an appreciable binocular
> > > overlap, perhaps to provide stereopsis, or whatever.
> > >         Crossed CNS representations are ancient, many species having very
> > > poor or no vision, or no single retinal image. Many species in which other
> > > senses far dominate vision. The visual *field* (not image) representation
> > > crossing followed the previous cross of the rest of the CNS. It did not
> > > drive it.
> >
> Sounds right to me.  I think there's some confusion of levels in this
> whole thread.
> 
> This whole business about the topological equivalences and deformations of
> the various homunculi (and generic animal-unculi), I just don't get it.
> The brain doesn't know its own topological layout. 

The nervous system =is= so self-aware... such selfe-awareness is built right 
into, and derives from, the TD E/I nerual Geometry that I described in earlier 
posts... it follows from the way that "neighborhood" relationships are 
preserved, globally, within the neural topology... it's this one global 
ordering principle that allows "self" to be distinguished from not-self within 
neural-activation states... (I'm discussing "consciousness", not conventional 
notions of "immune-system function")...

> If you could take and
> twist it around through space, it would still work; in fact just that has
> happened.

...not if neighborhood relationships are not preserved within the global 
Geometry... frogs whose optical tecta are surgically relocated, for instance, 
strike with their tongues, at insects, in the wrong direction... missing the 
insects... (technical point... beyond normal joint movement, the nervous 
system doesn't respond well to being subjected to "twisting" manipulation)...

> I'm inclined to think that there was some good reason for
> crossed control circuits, but it's likely to have predated the elaborate
> encephalization of mammals, especially human beings.

...we've been over this ground already...

> btw, I remarked earlier that many cranial nerves have ipsilateral or
> bilateral representation; one person agreed, another disagreed, saying
> that they map onto the same, contralateral, cortical homunculus.

...in my prior response I over-generalized... I was actually addressing 
=only= the spinal trigeminal tract... and I stand on what I posted.

 
>   I'll admit that I was thinking in large part of hearing (bilateral),
> smell (bilateral or nonlateralized), and vision, which works by crossed
> fields rather than crossed eyes, as I said.  But for many of the other
> cranial nerves, yes, it's true that the _cortical representation is in the
> opposite hemisphere.  However, the first (most rostral) nucleus concerned
> with that nerve is generally on the ipsilateral side of the brainstem,
> whereas the somatic nerves, having already decussated at the level of the
> medulla, find their first nucleus on the contralateral side of brainstem
> or thalamus. So there are crossed fibers, but they're postsynaptic;
> they're crossing after they get to the brain, whereas the spinal nerves
> cross just before, or just as, they get to the brain.  And then, of course
> there are vision, hearing and smell, but maybe they don't count, since
> they're not directly concerned with the CNS representation of the body.
> 
> This does all seem to make some kind of functional sense, with the somatic
> sense and motor control for one side of the body, plus the the
> corresponding visual field, all in one hemisphere (and not necessarily so
> for smell, hearing, and the face).  Why it needs to be the contralateral
> hemisphere, that I still don't see.  I don't find the business about
> avoidance movements convincing, since the crossing phenomenon, and the
> avoidance movements, probably predated advanced cerebral and cerebellar
> development,

...the topology that was described =permeates= the nervous system... it's 
functionality has only become augmented as evolutionary development 
occurred...

> but obviously, if you have legs or fins, or even paired
> muscles as in snakes, and you want them to work in a coordinated way,
> there has to be cross-talk of some kind.  Many quadrupeds, e.g. horses,
> can stand or even "walk" by spinal reflex when decerebrated, showing there
> must be crossed connections at the spinal level.
> 
> Obviously the issue arises with bilateral symmetry.  As soon as you have a
> left and right, and a front and a back, you can have direction; you can
> put eating, fighting and sensing equipment in the front, and you'll want a
> lot of nervous system at that end to control all that stuff; and you'll
> want more nervous system running from that end clear to down to the other,
> and you'll want locomotor equipment (legs, fins, wings, whatever) on both
> sides so you don't just go in circles, and you'll need some kind of way to
> coordinate the two sides.
> 
> Basically, if you bump into something on the left and you want to avoid
> it, the right limbs have to do something.  Maybe what they have to do is
> relax, so the left ones can push you away.  Of course, you also need a way
> to do the opposite, so you can go after things you want, and that sort of
> decision, whether to go for it or avoid it, that is just the sort of thing
> a brain would come in handy for.  But even just to go in a straight line,
> you need some sort of feedback mechanism.  If you haven't upgraded your
> nervous system to a brain-based one yet, some reflexes that take
> information from the left and do something on the right would be just the
> ticket.  In other words, crossed circuits,i.e.decussated fibers!
> 
> If you want to build a robot with a complex brain, you can probably make
> it work by ipsilateral control.  I don't know if it can compete with
> contralaterally-controlled robots or not, but it will work fine.
> But if you have really simple control circuits, some of them are going to
> have to be crossed, so that you can, for example, have the left rear and
> right front leg work as a functional unit.

...and the more elegant the connectivity strategy that's implemented, the 
easier it will be to "grow complexity"... and the easier it is to "grow 
complexity", the faster the elegantly-wired organism will out-pace it's 
competition... especially with respect to stuff like individual combat... the 
decussed nervous system will "just" compute faster than any 
ipsilaterally-wired jointed-thing behavioral behavioral controller... and 
it'll be easier to build in the first place (relative to an 
ipsilaterally-wired jointed thing of equal complexity, all other things being 
held constant)... 

> 
> See, when we ask the question why there is "left-right reversal of CNS",
> we (at least those of us who aren't experimental neuroscientists) are
> thinking about "the left side of the brain controls the right" as Paul
> Simon says, i.e. contralateral _cortical representation.  But cerebral and
> cerebellar cortex came long after bilateral symmetry.  The conclusion I'm
> coming to here is that the crossed representation was there _before the
> encephalization, in fact it was necessary exactly because there wasn't
> much central control yet.
>
> Does anybody know offhand, what is the lowest thing on the evolutionary
> ladder that demonstrates contralateral representation?  Do insects have
> it?  I would guess they do, because they don't have much of a brain, so
> there have to be "subcortical" control circuits that coordinate the two
> sides of the body.
> 
> Do all vertebrates have it, even critters like snakes without legs or
> fins? (I would guess they do, because they USED to have feet or fins in
> their evolutionary derivation). How about planaria?  (Planaria, i.e.
> flatworms, have a pair of nerve trunks, right and left, running lengthwise
> down the body.  I believe they do have some sort of rudimentary brain, but
> how do the two sides work together-- are they controlled by the brain,if
> any,  or do they talk directly to each other?)
> 
> Jerry

...as has been discussed, the antecedants of mammalian decussation can be 
traced all the way back to organisms having radial symmetry... ken collins



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