Right visual field to left hemisphere. Why?

F. Frank LeFever flefever at ix.netcom.com
Mon Oct 5 21:16:41 EST 1998


In <6vauve$bva$1 at fremont.ohsu.edu> Matt Jones <jonesmat at ohsu.edu>
writes: 
>
>In article <barnes.183-0110981901060001 at pc318.psy.ohio-state.edu>
Cosimo,
>barnes.183 at osu.edu writes:
>>Basic stuff, we all know that the left hemisphere gets sensory info.
from
>>the right side of the body and sends motor commands to the right side
of
>>the body and vice versa for the right hemisphere.  Someone hits me
with a
>>pretty good question the other day.  Why are we like this?  What
advantage
>>does this wiring system have over simply wiring left to left and
right to
>>right?
>
>One adaptive advantage can be seen by considering a really simple,
>freely-moving organism, with just rudimentary vision. A simple fish,
or
>planarium or something. Chances are that this organism mainly uses
vision
>to avoid being eaten by bigger organisms. Because of the mechanism
that
>the animal uses for locomotion (i.e., it wiggles its body back and
>forth), to avoid an object in its right visual field, it needs to
>contract the muscles on the left side of its body. A direct connection
>between sensory apparatus on the right side, and locomotor apparatus
on
>the left side would make this escape behaviour much more efficient.
>Perhaps this is why vertebrates are organized like this, and we
inherited
>it from such simple ancestors.
>
>Just a theory.
>
>Matt Jones


 Maybe I've misread, or maybe, your being so heavily invested at the
molecular level (I checked out your two abstracts in upcoming Society
for Neuroscience program), you really have slipped up on a molar (gross
anatomy) level.  Both sensory and motor fibers are crossed, so
something appearing in the right visual field would go to left
hemisphere, yes, but most direct motor response from that hemisphere
would be to the muscles on the RIGHT side of the body, not the left.

IF we assume adaptive advantage was based on avoidance or escape, too
bad!  However, if you alter your example to specify approach, i.e.
turning TO the right, by flexing right-side muscles, to approach
stimulus source (e.g. soemthing small and edible), it's plausible...

Just to complicate things: T.C. Scheirla (under whom I studied, long
ago; and under his students/colleagues, Jay Rosenblatt, Daniel Lehrman,
Ethel Tobach) suggested ways in which weak stimuli would produce
approach, and strong ones, withdrawal--at many different phylogenetic
levels. 

Further food for thought: perhaps it's just a matter of eyes moving
from side of head to pointing ALMOST the same direction (slight
binoocular disparity providing an important depth cue), but there
appears to be some evolutionary progression such that some fibers from
each EYE cross and (in humans) some do not.  However, the net result is
tht each eye's right visual field goes to the left hemisphere, and vice
versa for left field.

Interesting consequence of this evolution: a postdoc in Roger Sperry's
lab (classmate of Gazzaniga) (yes, yes, name-dropping again; but I do
want to convey a sense that there is a history to neuroscience and that
it depends on a tradition of many people working towards its
goals)--ANYWAY, the postdoc I studied under told me of another postdoc
who wanted to do split-brain work with pigeons (Sperry et al. worked
with humans and cats, etc.); being intent on being thorough in
restricting visual cues to the intended hemisphere, he followed the
example of other researchers and cut the optic chiasm also.  With cats,
monkeys, etc., this means that one can present stimuli to one
hemisphere at a time simply by covering one eye at a time.

In pigeons, however, ALL fibers cross, so after surgery he found he was
unable to use visual stiumli at all--the pigeons were blind!

F. Frank LeFever, Ph.D.
New York Neuropsychology Group





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