Stephan Jou (stephan at spine.med.utoronto.ca) wrote:
: 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?
: 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.
Good point -- vision is the only system I can think of where the crossing is
of the receptive field rather than of the actual somatic division of the
: (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.
This hypothesis doesn't really answer why both the sensory AND motor pathways
are crossed in higher vertebrates, does it? If you wanted flexion of muscles
contralateral to the sensory input, wouldn't you only cross one or the other?
Relatedly, what does it mean that some of the sensory paths are crossed
immediately upon entering the spinal cord, whereas motor tracts are crossed
in the brainstem?
: (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
: is given.
Indeed! Again, why would some tracts cross at different points in the
: > A novice wants to know. :-)
: So does this one!
Count me in, too.
Steven W. Barger, Ph.D.
Sanders-Brown Center on Aging