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Tuesday, September 22, 2009

The Man With Half A Brain

A lovely new paper reports in fascinating detail on a man who lost a uniquely large portion of his brain: Bilateral limbic system destruction in man.

The authors, Feinstein et al from Iowa City, have studied the patient, "Roger", for 14 years. Roger was born in 1952, and lived a fairly uneventful life until he contracted herpes simplex encephalitis (HSE) at the age of 28.

HSE is an extremely rare condition in which the herpes virus infects the central nervous system. Untreated, it is fatal in 70% of people. Survivors suffer varying degrees of neurological damage. Roger suffered more than most - his is the worst case of herpes encephalitis damage among patients currently alive, and there are only three recorded cases of similarly extensive lesions. Roger lost almost his entire "limbic system":
The amount of destroyed neural tissue is extensive and includes bilateral damage to core limbic and paralimbic regions, including the hippocampus, amygdala, parahippocampal gyrus, temporal poles, orbitofrontal cortex, basal forebrain, anterior cingulate cortex, and insular cortex. The right hemisphere is more extensively affected than the left, although the lesions are largely bilateral.
"Limbic system" is an old, vague, but still popular term for a collection of brain structures located deep in the centre of the brain (but not to be confused with the basal ganglia). It's often thought of as the "primitive", "emotional" part of the brain, and there is some truth to this. Roger's limbic system was profoundly damaged on both sides; on the right side, the lesion included the whole temporal lobe and most of the ventral prefrontal cortex as well.

What happened to Roger's mind when his brain suffered such injury? In many ways, remarkably little. His only major impairment is profound anterograde amnesia: he is unable to remember anything that has happened since the infection, which was 28 years ago.
For Roger, not much has changed over the past 28 years. He has virtually no episodic memories for any events that have transpired over the past three decades. For example, he has no recollection of 9/11, and when shown pictures of the planes crashing into the World Trade Center he often responds with bewilderment, speculating that Russia must be attacking America.
This is, obviously, a disabling deficit: Roger cannot lead a normal life. But in other areas of mental functioning, he is quite normal. His IQ is above average; his speech and language abilities are excellent; his vision and hearing are normal, although he has no sense of taste or smell. His short term (working) memory, attention, and reasoning abilities are unimpaired. His motor abilities are fine - he is reportedly an excellent bowler - and he is able to improve motor skills through practice. And his recall of things which happened before the infection is largely preserved, although the few years just before the infection are partially lost.

Fascinatingly, Roger's personality and emotional life seems to have been changed by the infection as well, but in a rather fortunate way -
Roger appears remarkably unconcerned by his condition. He hardly ever complains and, in general, shows little worry for anything in life. Both of his parents and his sister fervently claim that “Roger is always happy,” an observation that is consistent with our own impression. Moreover, based on his family’s report, Roger is paradoxically happier now than he was before his brain damage. ... His premorbid disposition of being somewhat reserved and introverted has shifted to being outgoing and extroverted...

Most conversations with Roger involve animated speech that is replete with prosody, gesture, and, often times, laughing. He readily displays signs of positive emotion including happiness, amusement, interest, and excitement. As previously noted, Roger’s positive mood has remained essentially unchanged over nearly three decades.
His only other reported quirks are an insatiable appetite, and a habit of collecting and holding onto everyday items.

What does all this mean? Neuroscientists will find little about the case surprising. No textbooks are going to have to be rewritten. Roger's inability to form new memories, combined with preserved memory of events up to the few years before the damage, is similar to that seen in other cases of bilateral hippocampus damage. The most famous being the sadly recently deceased patient "H. M.", but there have been plenty of others. The hippocampus seems to be necessarily for forming new long term memories, but the memories themselves are stored elsewhere.

Roger's happy-go-lucky disposition is also not too unexpected, given that he suffered bilateral damage to the ventromedial prefrontal cortex (vmPFC). Last year I wrote about a study from the same Iowa team finding that damage to this area seems to protect against depression. And this is the same region which was targeted by the infamous prefrontal lobotomies of the 40s and 50s - which, for all their ethical shortcomings, sometimes did seem to relieve people of mental anguish.

For me, Roger provides two main lessons, both rather satisfying ones. Firstly, even after losing large parts of the brain, life goes on. The brain is modular, and we can live without many of the modules. And secondly, if our emotional circuitry is damaged, we generally feel better, rather than worse. To put it another way, perhaps, happiness is our default state, and emotions just have a habit of getting in the way.

ResearchBlogging.orgFeinstein, J., Rudrauf, D., Khalsa, S., Cassell, M., Bruss, J., Grabowski, T., & Tranel, D. (2009). Bilateral limbic system destruction in man Journal of Clinical and Experimental Neuropsychology, 1-19 DOI: 10.1080/13803390903066873

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