Nervous Crossroads

Ever wonder why the left side of your brain controls the right side of your body? So have I. And so have a few neuroscientists whom I’ve asked. None have an answer, much less a good one. The internet has proven to be equally uninformative on the topic: many sources are quick to tell you about the crisscrossing of sensorimotor information flow, but none address its evolutionary advantage. I mean, really, why does it make any sense (har) for sensorimotor tracts to be crossed?

Perhaps this decussation stems from the lateralization of visual processing, which DOES make sense. Light entering the eye passes through the lens and then hits the opposite side of the retina. Spatially, it’s logical for such visual stimuli to get relayed to the brain hemisphere closest to the stimulus receipt site, which occurs contralateral to the stimulus. Evolutionarily, it would be advantageous for motor and sensory information related to a particular field of vision to coincide anatomically with the processing of that field.

This could be bollocks, of course. Take a species without any visual organs (not including species with ancestral visual organs, like cave fish who have vestigial eyes) and demonstrate contralateral connectivity of sensorimotor information and brain hemisphere, and you’ve got yourself a negation on this theory. Until then, folks.


2 responses to “Nervous Crossroads

  • Colleen

    It’s funny that you wrote about this because I just had a conversation about this topic when I was working over break. A lady at the office was asking why this is and I told her that scientists didn’t really have an answer.

    This sort of question borders of the kind of thing that science can’t really explain, anyway. You can talk about a particular design being more efficient, causing that mutation to become the predominant one, but evolution is not deterministic, so you can’t say why the mutation appeared in the first place. There are plenty of examples of inefficiencies in organisms. Maybe cross-wiring is just a bizarre quirk. Maybe cross-wiring is more efficient on certain body plans, but not on others and the only reason it appears on us is because an ancestor had a body plan that made it more efficient. I’m beginning to throw out random speculation now, but sometimes there isn’t a good answer because the answer is simply “that’s just the way it is.”

  • sasha

    I have a professor this semester who is particularly expertly in evolutionary neuroscience. She was a perfect person to ask about this, of course. As in the past, she said there was no obvious answer, but that an evolutionary fluke is probably the very best explanation. It’s disheartening on the one hand, but then again, it’s interesting how flukes can become standard (all vertebrates possess this crossed property, it’s most amusing). But in any case it does sort of toss out this hodge-podged idea. Oh well, it was an attempt.

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