Crossed nervous systems

Paul Grobstein pgrobste at brynmawr.edu
Fri Jul 22 16:57:21 EST 1994

> Brian Scott (scott at PSYCH.TORONTO.EDU) wrote:
> > I think this would be the place to ask such a question, so here it is.
> >
> > What are the current theories as to why the nervous system is mostly crossed?
> > What are the benefits which are thought to be gained from such a setup?

Sorry to come in late in this discussion.  Somehow missed original posting
from Brian, would be interested in whether I missed any responses.  The
earliest I've seen is that from Stephen Jou, to which I'm responding.  He
gives a nice summary of the classical hypotheses for a problem that has
interested me for years.  For what it is worth, I don't think any of them
hold.  Explanations based on vertebrate ontogeny, as well as those based on
differences between vertebrate and invertebrate ontogeny run into
difficulties because a not bad generalization that neural systems cross
holds for many invertebrate situations as well as vertebrate ones. 
Explanations based on idiosyncracies of visual system organization run into
troubles because crossing patterns are strikingly similar in a variety of
both sensory and motor systems.  The biggest problem with classical
hypotheses, however, is that crossing (in both vertebrates and
invertebrates) is a "not bad" generalization, which is to say it doesn't
ALWAYS hold: there are noteworthy exceptions, of which the best known is
probably the somatosensory projections to the cerebellum.  My own research
is concerned with a descending tectofugal pathway in the frog (probably in
all vertebrates) which also violates the "one side of the body relates to
the opposite side of the world" principle, and there is evidence for a
similar organization in invertebrates (cf. chapter in Visual Structures and
Integrated Functions (Arbib and Ewert, editors, Springer-Verlag, 1991; yes,
of course, happy to respond with reprints to e-mailed addresses).  

Yes, a VERY interesting problem, currently unsolved so far as I know.  And
one most likely to be solved by looking to see which systems do and do not
fit the generalization in a variety of organisms.  I would love to know of
other cases of uncrossed systems, and would be happy to post a summary of
responses if enough relevant information turns up.     

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