Wednesday, April 9, 2008

Are there parts of the brain that are genuinely not used?

Trauma and Cortical Map Plasticity

Generally, the answer to this question is no. However, certain types of trauma can result in brain areas that are no longer being used – at least for a little while.

For example, imagine that you've just lost your middle finger in a wood workshop accident. Oops! Now the area of your brain that received sensory input from this finger is receiving silence – it's no longer being used.

But the competition for cortical space is fierce and the price of not using is often losing. Thus, during the weeks of subsequent disuse following your accident the area of your brain that was once devoted to your middle finger undergoes a major rewiring. In time, no sensory representation of your middle finger remains; instead, the space it once occupied has been overtaken by neurons that represent your two adjacent (healthy and attached) fingers.

Another outcome is possible and it is responsible for the “phantom limb” sensations reported by a majority of amputees. In the case of phantom limbs neurons become cross wired as the deprived cortex seeks input and active, adjacent cortex pushes into it's area.

The image to the right depicts the location of sensory representation for different parts of the body across the primary sensory cortex. Notice that the area of your brain that receives sensory input from your hand is adjacent to the area that receives sensory input from your face. Where you to lose your entire hand in the workshop accident you might wind up like many amputees who have also lost a hand -- that is, feeling sensations from your “phantom” hand when someone touches cross wired regions of your face! Some amputees have complete maps of their missing hand on their faces.

Note, you may have heard that violinists have unusually large sensory areas devoted to the fingers of their left hand (this is the hand that fingers the strings). This is true! Just as disuse leads to atrophy and replacement, increased stimulation leads to expansion. Cortical maps are constantly changing and adjust proportionately to the amount of stimulation they experience. This type of plasticity occurs in the visual, auditory and motor systems as well.


Chris said...

Wow! A most complete answer to my earlier question. For me, this TOTALLY makes the claim that we use only part of our brains a misguided one. Imagine saying we only use 10 percent just because only 10 percent is active AT ONE TIME. That's like saying I only use one 26th of the letters in the alphabet since I only type one at once. Silly!

Thanks for teaching me something new.

Rachael said...

No problem, and on that note -- while it sometimes happens that we are only using 10% of our brains at a given moment, we're usually using more than that. :)

A good intuitive example of this would be to recall some of the fMRI images that we've gotten used to seeing on TV and in magazines. You should remember seeing areas all around the brain lighting up in those images, far more than 10%.

Unfortunately, I couldn't find any information on the "average % of active use" -- I'm guessing this is because there's too much variation so the statistic wouldn't be very meaningful.

I do like your alphabet analogy. I think I'll use it in the future, with the addition that the brain often uses "many letters at once" -- but as noted before, a full 26 is a seizure! :D