For the first time, neuroscientists have described in exhaustive detail the underlying neurobiology of an amnesiac who suffered from profound memory loss after damage to key portions of his brain.
Principal investigator Larry R. Squire, PhD, professor in the departments of Neurosciences, Psychiatry and Psychology at the University of California, San Diego School of Medicine and Veteran Affairs San Diego Healthcare System (VASDHS) - with colleagues at UC Davis and the University of Castilla-La Mancha in Spain - recount the case of EP, a man who suffered radical memory loss and dysfunction following a bout of viral encephalitis.
EP's story is strikingly similar to the more famous case of HM, who also suffered permanent, dramatic memory loss after small portions of his medial temporal lobes were removed by doctors in 1953 to relieve severe epileptic seizures. The surgery was successful, but left HM unable to form new memories or recall people, places or events post-operation.
His brain was removed and digitally preserved at The Brain Observatory, a UC San Diego-based lab headed by Jacopo Annese, PhD, an assistant adjunct professor in the Department of Radiology and a co-author of the latest study
Like Molaison, EP was also something of a scientific celebrity, albeit purposefully anonymous. In 1992, at the age of 70, he was diagnosed with viral encephalitis. He recovered, but the illness resulted in devastating neurological loss, both physiologically and psychologically.
Not only did he also lose the ability to form new memories, EP suffered a modest impairment in his semantic knowledge - the knowledge of things like words and the names of objects. Between 1994, when he moved to San Diego County, and his death 14 years later, EP was a subject of continued study, which included hundreds of different assessments of cognitive function.
After his death, EP's brain was also processed at The Brain Observatory. The last five years have been spent parsing the data and painting a full picture of what happened to EP and why.
Squire, a Career Research Scientist at the VASDHS, said EP's viral encephalitis infection wreaked havoc upon his brain: Large, bilateral, symmetrical lesions were found in the medial temporal lobe, portions of the brain responsible for formation of long-term memory; and whole, crucial structures were eliminated - the amygdala and hippocampus among them.
Additionally, other brain regions had atrophied and white matter - the support fibers that transmit signals between brain structures - had become gliotic or scarred.
Squire said EP provides new and surprising twists in understanding how memory functions and fails.
The researchers' finding appeared in this week's Online Early Edition of PNAS.