Hemispheric control....

Richard Norman rsnorman at mediaone.net
Tue Jan 23 20:41:55 EST 2001


You are right.  The "homunculus" in the sensory and motor cortex is
twisted every which way and it makes not the slightest difference.  It
makes absolutely no difference that the retinotopic mapping in
the primary visual cortex is twisted, convoluted, and mightily
distorted and misshapen.  And no one ever was concerned with
whether the mapping in the geniculate was upside down!

All that matters is that this particular neuron somehow "knows"
that it is associated with this particular part of the visual field,
or part of the body.


"Andrew T. Austin" <slightlynervous at NOSPAMbtinternet.com> wrote in message
news:94k7tu$kbl3 at eccws12.dearborn.ford.com...
> I have often wondered about the supposed "upside down image on the visual
> cortex" thing.  Just because the lense will invert the image, doesn`t
> necessarily mean that this is how it is represented on the visual cortex,
> surely?
>
> A quick `twist` in the optic nerve would surely solve this problem.
>
> Plus, it doesn`t seem to me that the image is `projected` as though onto a
> viewing screen.
>
> Can anyone provide further elucidation?
>
> Thanks,
>
> Regards,
>
> Andrew "likes to use long words unnecessarily" Austin.
>
> Richard Norman <rsnorman at mediaone.net> wrote in message
> news:Ef2b6.40322$ft6.843643 at typhoon.mw.mediaone.net...
> > The image reversal in the visual system  is certainly not the source
> > of contralateral control.  Consider the contortions that the optic nerve
> > must undergo at the optic chiasm to ensure that retinal ganglion cells
> > originating on the nasal half of each retina cross, but those
originating
> > on the temporal half stay ipsilateral.  And consider that most
vertebrates
> > don't have much bilateral vision -- their eyes are on the sides of their
> > heads and all axons must cross.  Squid, which also have camera eyes,
> > and inverted images, don't have this silliness.  No, the "upside down
> > and backwards" image on the retina is not really related to anything.
> > Neurons don't care about which way is "up".
> >
> > "MS" <marshmallow5 at yahoo.com> wrote in message
> > news:LLXa6.3717$mo2.429194 at news1.news.adelphia.net...
> > > I'm assuming what you mean by contralateral control is that the left
> > > hemisphere governs motor and sensory functions for the right side of
the
> > > body and vice-versa. The only explanation that I have found suggests
> that
> > > it's a result of the visual system.  All of the other sensory and
motor
> > > systems could hypothetically be represented ipsilaterally (i.e. left
> side
> > of
> > > the body on the right side of the brain, etc.)  However, an optical
> > property
> > > of the visual system is that the images reaching the retina are upside
> > down
> > > and backwards, so that the left visual field reaches the right
> hemisphere,
> > > etc.  The rest of the senses (and motor systems) have to be aligned
> > > accordingly, so that things seen in the left visual field are also
felt
> in
> > > the left visual field, and reached for the same visual field. So
> > basically,
> > > the reversed visual system causes everything else to need to be
> reversed.
> > >
> > > Marcello Spinella, Ph.D.
> > >
> > >
> > > Andrew T. Austin <slightlynervous at NOSPAMbtinternet.com> wrote in
message
> > > news:942lop$5j14 at eccws12.dearborn.ford.com...
> > > > This might be a silly question, but can anyone tell me what the
> > advantage
> > > of
> > > > having contra-lateral control is?
> > > >
> > > > What benefit IS there by having the left hemisphere controlling the
> > right
> > > > side and vice versa?
> > > >
> > > > Thanks in advance,
> > > >
> > > > Andrew "clicking his amygdala" Austin.
> > >
> > >
> > >
> >
> >
>
>







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