Hemispheric control....

Richard Norman rsnorman at mediaone.net
Mon Jan 22 17:12:52 EST 2001


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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