Crossover of information - (Dec/19/2005 )

Why do brains have two halves, and why is information crossed over?

good question, most of it I won't be able to say, but in terms of sensory and motor cortex it appears that the left hemisphere receives/control the right part of the body, and vice-versa.

Not so sure about lower structures such as the hippocampus or the cerebellum. It's funny that structures that are present in only 1 specific localization like the thalamus appear to be on the brains main axis.

Hi,

I had the same question, and after I spent some hours digging the internet (Holy Google ), there seem to be 3 possible explanations:

1) The primitive coiling reflex in response to noxious stimuli.
(Imagine a worm.... Impulses from the sensory nerve fibres had to be conducted to the motor neurones on the immediate opposite side of the spinal cord to allow the primitive rod-like animal to bend away from the noxious stimulus. )

2) The visual system.
(Obviously, stereotopic sight, which requires both eyes is an evolutionary advantage. But as lenses invert the image, the optic fibers have to decussate to ensure a proper representation of both eyes in the cortex.
Also, it is easier if the info from the right eye is processed in the same half of the brain, which is responsible for the motor output to the right arm.)

3) Complex motor actions like walking require the coordination of both sides and therefore the information has to cross the midline. (I think that's a bit fishy, but ...ok, that's what I've found...)

Btw, the split brain is of advantage, because it makes parallel processing possible.

Just a question concerning the walking...
I know that its the corpus callosum that enables the brain hemispheres to communicate. But I thought it connects mainly the higher cortex areals. So how do the motor centers communicate?
I guess there is also another way in addition to the CC...because otherwise split-brain-patients would have extreme problems to walk, since the Central Pattern Generators for the left and the right leg wouldn't be able to communicate. But actually the behave quite normal.

it's not really a strict left-controls-right and right-controls-left relationship. some processes are mostly handled by a single hemisphere.