Thank you Dr Gupta.

It really is an exciting question and as far as I know with no definitive answer today. Maybe it could be a process similiar to embryological forming of a primitive tube from the plate (the cells form a tube with a central hole which is circumferentially surrounded by cells). Regarding this process, a secondary 'circumferenting', i.e. cellular migration continuing with a 'second looping' ends up with crossed neural pathways. Just my thought, hope you get the idea I ment. 

Looking forward to other answers, theories etc.

Dear Luiz,

An interesting question, and I agree with the other responses above.  I don't know if anyone looks at "why?" in the way you are asking the question.  Most axonal pathfinding studies look at "how?".

I would like to point out that not all axonal pathways cross the midline of the organism.  In the case of the visual pathways, there are some retinal ganglion cell axons that stay on the same side (ipsilateral) as they project to the visual centers.  It will vary between species, but it is how binocular vision is achieved.  In the case of the sense of smell, as far as I know the pathway remains ipsilateral.  (Someone may correct me here--I work on vision, not smell).

In addition, as more sensitive tract tracing techniques became available, the ideas about the extent to which some tracts remain ipsilateral has changed.  For example, see

http://www.ncbi.nlm.nih.gov/pubmed/19125408

Still, you have asked an interesting question. I also look forward to other theories.

Best,

Jill

An interesting explanation by Daya Gupta, but I fail to see the extension to hand / eye / ear preferences as well as lateralization of language and praxis. Giving the uncrossed cerebellar fibers and their relation to motor control this can't be all.

I concur that most likely the "how" question might have to be answered before the "why" might be addressed. What is the difference between crossed and uncrossed systems in terms of development? There seems to be no consistent differences between midline structures and lateral structures, subcortical and cortical, brainstem, etc... In the brain, only cerebellum and olfaction comes to my mind.

Interestingly, the lateralization of language and praxis seems to be far from constant, as language lateralization or crossed apraxia indicate.

When do the first signs of fibre crossing occur in development?

I expect, in case of visual pathways, cross over of neurons of the central nervous system between left and right brains fecilitates depth of the vision, which is required for realistic image of the outside world. Similar explanation should apply to other aspects of mental facalties.  

Why is only the visual pathway taken as an example? It has contralateral, but IPSILATERAL prijections as well. This is also valid for auditory cortex. 

In addition, what is the explanation with prople born blind? Also the claim that both eyes are needed for septh isn't true, since there are even professional sportsman who are missing one eye due to an accident, but still have perfect depth perception (as judged by their continuing sporting activity and top performance).

What is the explanation for pyramidal tract crossing?

I agree with the statement that the answer can be found in tracking evolutionary pathways. This truely asks for someone with more experience in neurodevelopmental embryology to get engaged in this discussion.

To contribute to this interesting debate, please see this paper:

Vulliemoz, S., Raineteau, O., Jabaudon, D. (2005) Reaching beyond the midline: why are human brains cross wired? The Lancet Neurology, 4(2):87-99. DOI: http://dx.doi.org/10.1016/S1474-4422(05)00990-7

Best regards,

Francisco.