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A new examination of the brain of Patient H.M. - the man who became an
iconic case in neuroscience when he developed a peculiar form of amnesia
after parts of his brain were removed during surgery in 1953 - shows that
his surgeon removed less of his brain than thought.

At age 27, H.M., whose real name was Henry Molaison, underwent an
experimental surgical treatment for his debilitating epilepsy. His surgeon
removed the medial temporal lobe, including a structure called the
hippocampus.

Thereafter, H.M. was unable to form new memories. His case brought about the
idea that the hippocampus may have a crucial role in retaining learned
facts, replacing the notion that memories are scattered throughout the
brain. H.M. became the focus of more than 50 years of memory research,
working closely with the researchers who had to introduce themselves every
time they met.

"Much of what we know about human memory, it has one way or another to do
with H.M.," said study researcher Jacopo Annese, director of The Brain
Observatory in San Diego.

After H.M.'s death in 2008, Annese and his colleagues cut the patient's
frozen brain into 2,401 slices, each 0.07-millimeters thick. They took a
picture of every slice, and created a high-resolution, 3D model of his
brain. [See Images of the Brain Slices of Patient H.M.]

In the new study detailed online today (Jan. 28) in the journal Nature
Communications, they report that a significant portion of the hippocampus,
which was thought to have been removed in surgery, was actually intact.

What happened to H.M.?

Research on H.M. showed that there are in fact different kinds of memory. He
was unable to learn new facts, remember the events happening around him or
learn people's names, but he was able to recall events from his childhood.
He also could learn skills, for example, he could get better at a new motor
task with practice.

"Over 50 years of studies, the picture [of memory] was a little bit
complicated," because H.M. had some types of memory but not others, Annese
said.

The only way to start teasing out H.M.'s memory impairment in light of the
anatomy of the brain was to know what exactly had happened during the
surgery, Annese told LiveScience.

Until the 1990s, the researchers had only sketches drawn by the surgeon, Dr.
William Scoville, to refer to. But after the advent of neuroimaging,
researchers scanned H.M.'s brain in 1992 and found that a portion of the
hippocampus had been spared.

In the new study, Annese and his colleagues measured the exact length of
H.M.'s hippocampus, and found the spared portion was even larger than what
brain scans had shown.

The posterior part of the hippocampus deals with memory, and the brain
slices show this part wasn't removed, and in fact, was undamaged at the
cellular level, the researchers said.

"The most beautiful finding I think was the fact that we realized . that
Scoville missed the posterior hippocampus," Annese said.

The memory impairment

The new findings shed light on what happened to H.M., but likely won't
revolutionize what researchers know about memory, and are in fact in line
with modern views of hippocampal function, Annese said. [Inside the Brain: A
Photo Journey Through Time]

Almost all connections from the hippocampus to the cortex go through a part
of the temporal lobe called the entorhinal cortex, which Annese found had
been removed from H.M.'s brain. As this region connects the hippocampus to
other brain regions, the surgery may have nearly isolated the hippocampus
from the rest of the brain.

This may mean that H.M.'s amnesia had more to do with the entorhinal cortex
being removed, than with the parts of the hippocampus being removed, Annese
said, although more study is needed to know for sure.

The new study presents "an extremely detailed post-mortem investigation of
the remaining anatomy of [H.M.'s] brain," said Neil Burgess, a memory
researcher at University College London, who wasn't involved in the new
analysis. "These extra details will no doubt continue to fuel the debate as
to which bits of the medial temporal lobe are responsible for which aspects
of memory."

Editor's Note:This story has been updated on Feb. 3 to correct the thickness
of brain slices, which were 0.07 millimeters, not 0.7 millimeters.

Email Bahar Gholipour or follow her @alterwired. Follow us @LiveScience,
Facebook & Google+. Original article on LiveScience.

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