Curious things we learned from epilepsy surgery...

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In the previous series of blogs from The LabRats Guide to the Brain,I mentioned that many surgical treatments have been used to remove the neural tissue that acts as a site of origin for epileptic seizures.  One of the pioneers of this work was Dr. Wilder Penfield, who has been mentioned in this blog in the past (click on the content tags for previous blogs).  Penfield's medical research involved identifying the point of origin for epileptic activity, then surgically removing that piece of the brain.  Before the surgery, however, Penfield and colleagues need to find out what function is served by that area of brain.  Very low voltage/amperage electrical stimulation was delivered via hair-thin stimulating electrodes.  Clearly some stimulations caused physical body movement, and unfortunately some triggered the very epileptic activity being treated.  But some of the other results were astounding:  patients described sensations, feelings or complete memories.  These studies gave rise to much of the mapping that we know today - including that of the motor and sensory homunculi are courtesy of such recordings as preparation for epilepsy surgery.

Another striking result of epilepsy surgery was the case of patient H.M., very well known to neurosurgery and neuroscience students alike.  H.M.s epilepsy originated in the hippocampus, an area near and dear to my... memory.  Yes, the hippocampus is the structure most associated with processing of new memories.  On September 1, 1953, neurosurgeon William Beecher Scoville removed part of the medial Temporal Lobe on both sides of H.M.'s brain.  From that point onward, his ability to form new memories was impaired, although his ability to learns skills and remember past events appeared unaffected.  The surgery removed most of the hippocampus, amygdala and entorhinal cortex, thus removing a critical link in the brain circuitry necessary for processing memory.  H.M. died in 2008, and for 55 years he became the most studied patient in Neuroscience!

Another strange but true tale involves the split-brain.  Epileptic seizures can originate in a small area of brain and eventually spread to involve major portions of both hemispheres.  As a last-chance attempt to halt the progress to the opposite hemisphere and limit the duration and extent of seizure, a procedure called corpus callosotomy is performed to cut the connections between hemispheres of the brain.  The resulting separation resulted in much of what we now understand to be the lateralization of functions to two different halves of the brain.  However, it is the behavior of the "split-brain" patients that is most curious.  As discussed in the sections on vision, each retina projects the left half of the visual field to the right brain, and the right half of the field to the left brain.  Without an intact corpus callosum connecting the two hemispheres, the patient cannot put both halves of the visual field together.  As a result, they constantly scan their eyes back and forth and move their head to allow both halves of the brain to "see" the entire field.  Split-brain patients have been observed to be in conflict between intended actions controlled by the separate halves of the brain - expressed as one handing restraining or slapping the other, or two hands attempting to do the same task. 

Fortunately these drastic surgeries not performed with much frequency, rather medication and more precise surgical techniques suffice for most epilepsy patients.  Still, drastic cases exist, and parts of the brain may need to be removed.  The resulting responses, deficits, and conflicts are very important to our understanding of the brain!

Until next time - take care of your brain - you don't want to lose it!