@odenskrigare,
Further matters will come up later
(if I can get to them) but in recapping what has been covered so far on brain build, and the motor system, we can see that there are a number of structures besides just the cortical areas. One important hub is the basal ganglia. It works much more so on a
inhibition process, than on an
excitation process. We know that neurotransmitters and neuromodulators are important, and that deficits in them, as well as deficits in volume or structural integrity, equate to deficits in the production of movement
(and this is also true of the cerebellum, as well). Also, the motor system's priming is often at a level of conscious that is not firstly registered in the state of consciousness, and individual structures have their own states and levels of processing (conscious). And finally, we have come to see that it is most clearly
brain that is working which makes movement happen, and that there is no evidence for, yet more than enough evidence against, any idea of the brain's having to recieve 'signals' to produce bodily actions by the CNS.
Here, I will go into the area of what has been learned thus far, from research related to, and studies and tests done with, people (and monkeys) after callosotomy.
When we talk about the brain, we are not talking about just the neo-cortex; we are not looking at just one single pathway and then reaching a conclusion on aggregate operation of the brain. For this reason, I had stressed that there is a necessary difference between using the term
THE brain, as opposed to simply and only, the term
brain. Although it is necessary to look at each local structure or module in the first place--each process, each pathway, map, and system and so on--in developing the overall view,
we have to adjust our understanding as we add overlaps, synaptic assemblies, and body/environment/genetical inputs;
bottom-up averaged by top-down.
The procedure of callosotomy is to keep epileptic seizures on one side from crossing over to the other side. While it is not as dangerous an operation as it had been back when it was first done in the 1940s by Dr.William Van Wagenen--
there is a very thin layer of cells between the corpus callosum (CC) and the lateral ventricals, and if you cut into that, you'll have a bit of a problem--it is yet a '
last resort' choice. The CC, which can be listed in five sections (four principle parts in bold)
rostrum,
genu,
body,
plenium, and isthmus
(not to be confused with brain stem part with the same name, and thus, I guess, not used often), is not exactly the same in each and every normal brain, but is very, very close.
The CC is the major
(but not only) commissure , and is made up of some 200 million fibers--axons--that cross through from cortical layers II and III. These are both ipsilateral and contralateral projections to correspondings areas of each cortical hemisphere. Incoming axons with connecting neurons are generally in vertical-like structuring called cortical columns which constitute functional units. Layer VI and V of the cortex primarily send axons down to the thalamus, and the striatum, brain stem, and spinal chord respectively.
Here we have to keep one thing one thing in mind, as touched on by Aedes before, namely, there is only one brain stem, and the subcortical structures are sitting on top of that, and wired from there, along with the ipsilateral projecting of the cerebellum to the upper structures and the neo-cortex even, to some degree. So even if all the commissure fibers of the corpus callosum were cut (sometimes some are left in split-brain surgery), we are not fully and absolutely cutting the whole brain into two sections. This fact must be kept in mind.
Also, it must be kept in mind that the cognized and acknowledged state of consciousness is always projected as a singularity, at any given moment, regardless. For this reason, split-brain patients did not notice any major lasting differences (in most cases) after surgery, and original examinations of the first patients did not show any feared '
two people in one person' results. It seems to not have given much concern, in the early days, that the studies done with monkeys did present evidence of difference in hemispheric conscious (not consciousness). At present, those who have done perhaps the larger bulk of work in this area, with Sperry, Gazzaniga, LeDoux, Funnell, Kingstone, and a few others leading the way, have provided a rather clear picture of how each hemisphere has its own level of conscious (not used here in the sense of that full state of consciousness). I will next go into that.
