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?
Good question! Anyone who has encountered the extensive amount of crossed
arrangement in nervous systems must ask why it occurs, what evolutionary
selection pressure there is favouring such a predominance of contralateral
Bryan Kolb and Ian Q. Whishaw's _Fundamentals of Neuropsychology_ (3rd
edition), W. H. Freeman, 1990 discuss this briefly on pp27-30, although the
answer remains far from complete.
Briefly, they summarise three hypotheses by three people:
(1) The great Santiago Ramon y Cajal suggested that "crossing in the visual
system is necessary so that a continuous representation of an object is
retained in the visual cortex." Figure 1-16 in the book gives an example
of this. In words, an object crossing the visual field from right to
left would, in an ipsilaterally-wired brain, appear discontinuously,
first in the right hemisphere moving to the right, then in the left
hemisphere moving to the left. However, in a contralateral brain,
the image's projection "moves" in one smooth motion from left to right
across the hemispheres.
Given recent research on vision, it seems doubtful that this can account
for contralateral wiring in general, especially for the nonvisual systems.
(2) The coil-reflex theory, credited to Coghill, as exemplified by the
marine vertebrate, _Amblystoma_. Essentially, the contralateral
arrangement of the _Amblystoma_'s nervous system allows a coil reflex, so
that the animal flexes AWAY from sensory stimulus which may indicate a
predator or noxious stimuli.
This is a nice hypothesis because it clearly assigns survival value
to a crossed system. Kolb and Whishaw further point out that William
Webster suggested that this might also explain why the olfactory system
is uncrossed: in primitive animals, the olfactory bulb may be to bring
the animal TO food, so it is the IPSILATERAL muscles which must be
contracted, not the contralateral muscles as in the other systems which
demand coiling away.
One wonders if other neurologists working with other animals can lend
their knowledge here? Is there a species where it would be evolutionarily
advantageous to coil TOWARDS a sensory input, and that system has turned
out to be uncrossed?
(3) An interesting theory by Kinsbourne, who proposed that in evolving from
invertibrates to vertibrates, the body has rotated 180 degrees with
respect to the head. The argument for this is that invertebrates have
an uncrossed and ventral nervous system, dorsal heart and posterior
flow of blood, while vertebrates have a crossed and dorsal nervous
system, ventral heart, and anterior flow of blood.
Kolb and Whishaw point out some deficiencies in this hypothesis, the
strongest one to me being that no adaptive value of a 180 degree rotation
This is quite out of my field, but an engaging question. Perhaps one of the
expert neuroanatomists or developmental neurologists can shed some light on
> A novice wants to know. :-)
So does this one!
>scott at psych.toronto.edu | "They are in you and in me; they created us,
> Brian Scott | body & mind; and their preservation is the
> Department of Psychology | ultimate rationale for our existence."
> University of Toronto, Canada | - Richard Dawkins (The Selfish Gene)
Stephan F. Jou | Institute of Biomedical Engineering
stephan at spine.med.utoronto.ca | Rosebrugh Building, University of Toronto
MSc student and computer geek | Toronto, Ontario, M5S 1A4, CANADA