NOTICE: Posting schedule is irregular. I hope to get back to a regular schedule as the day-job allows.

Monday, February 21, 2011

How do we know that?

Just a quick post today to clarify something from yesterday...

Just how do we *know* what all of these brain areas do?  It's not like they can tell us, right?

Well, actually, they do.  Until the past 50 years, mostly we found out what role a brain area fulfilled by losing it.  Losing the brain area and the function, that is.  The most famous case in Neuroscience I have mentioned before - that of H.M. who had surgery to remove part of his temporal lobe, and lost the ability to form new memories.

Likewise, most of what we have learned about brain function has been and still is learned by studying loss.  In humans we look at the consequences of epilepsy, stroke and brain injury.  In rats we can perform experiments to temporarily or permanently inactivate small regions of the brain.  By doing this, we learn about fucntion, regrowth and adaptation following injury. 

The most profound work was done by Canadian Neurosurgeons Wilder Penfield and Herbert Jasper.  Penfield treated epilepsy by destroying the brain cells that triggered seizures.  To do this, he first had to determine which brain areas - and cells - were damaged.  In the course of surgery, he would apply a probe delivering a small electrical stimulus to various brain parts of the brain surface (while the patient was awake!) and asking what the patient experienced.  Penfield and Jasper published "Epilepsy and the Functional Anatomy of the Human Brain" in 1951 which mapped the connections between touch sensations and muscle control for the whole body onto the motor and sensory cortex of the brain.  We'll speak more of the "motor homunculus" and "sensory homunculus" in later blogs.

In the past 60 years medical science has developed astonishing new techniques for imaging the functional activity of the human brain.  Functional magnetic resonance (fMRI) detects areas of the brain with increased blood oxygen flow and consumption.  Positron emission tomography (PET) tracks the flow of radioisotope-labeled glucose into neurons. Magnetoencephalography (MEG) detects minute electrical currents to depths of 50-70 mm into the brain (Note the human brain stem is about 100-150 mm from the surface) providing thousands of times the resolution of EEG.  On the noninvasive stimulation side, we have transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS) which can stimulate many neurons without surgery - albeit these techniques are not precise (i.e. minimum volumes of 5-10 cubic *centimeters* and typically only to depths of 10-20 mm.  While still not perfectly precise - the imaging resolution is measured in cubic millimeters, which can *still* contain over one million neurons - it is now possible to place a subject in an MRI scanner, ask them to perform a task such as reading a book or imagining playing a musical instrument, and watching which parts of the brain "light up" in real time. 

So, yeah, we do have a pretty good understanding of the functions of various brain areas.  The interesting part is not just what we know, but how we learned it. 

Until next time...

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