In death as in life, HM's brain reveals insights into memory

Date: Jan-30-2014In 1953, Henry Gustav Molaison of Hartford, CT, was 27. After undergoing a brain operation to correct seizures - which involved the removal of his hippocampus and surrounding tissue - he was left unable to form new memories, although his personality, language and intellectual ability remained intact.

"Patient H.M.," as he became known worldwide, lived for another 55 years, during which his

willingness to undergo test after test contributed enormously to our understanding of human

memory and the role of the hippocampus, in particular.

H.M.'s inability to form new "declarative memories" became the catalyst for over 50 years of

scientific discoveries, resulting in thousands of published papers that have shaped basic

understanding of how memory works.

At the time, around the 1950s, scientists believed that memory was distributed throughout the

brain.

H.M. could learn new skills but not remember doing so

H.M.'s contribution changed all that and helped scientists discover that memory comprises at

least two systems - declarative memory and motor learning.

Declarative memory helps us record names, faces and new experiences and stores them until we

consciously retrieve them. This system uses the medial temporal areas of the brain and the

hippocampus.

Motor learning uses other brain systems and happens more subconsciously. This is why people

who have not ridden a bike for years can just get on one and ride off as if they had never stopped.

So for H.M., while he could learn new skills, he could not remember doing so. He was

constantly surprised that he knew how to do something or that it was easier to learn than he

was expecting. What he did not realize is that he had learned it already.

As technology improved over the years, and MRI scans were introduced, it was possible to look

in more detail at the impact of the 1953 surgery on H.M.'s brain. However, the full

extent could not be properly assessed while he was alive.

Postmortem study of H.M.'s brain released, including 3D model

Now, thanks to a detailed postmortem study of his brain, scientists around the world will

finally see a detailed neurological picture of a case that has come to define modern studies of

human memory.

Study leader and neuroanatomist Dr. Jacopo Annese, of the University of California San Diego

and Brain Observatory, also in San Diego, and colleagues have published their work in the

journal Nature Communications.

In the open paper, they describe how they used histological sectioning, which

involves making stained slides of thin slices of brain tissue, and digital 3D

construction, to reveal a microscopic anatomical model of H.M.'s brain.

The level of sampling and image quality is a significant advance over the MRI scans that were

taken while H.M. was alive.

The work started in 2009, when Dr. Annese and his team dissected H.M.'s brain into 2,401 thin

tissue slices and preserved them cryogenically in serial order.

While they prepared each slice, the team also recorded a series of digital images of the

surface and archived them. This was used to create the microscopic 3D model of the whole

brain.

The 1953 operation on H.M.'s brain did not fully remove hippocampus

Of particular interest are the detailed 3D measurements of the medial temporal lobe region,

where H.M.'s hippocampus and some of the lobe structures were removed in the 1953 operation.

The reconstruction shows that the operation had not fully removed the hippocampus but had

left, as the authors note, "a significant amount of residual hippocampal tissue with

distinctive cytoarchitecture."

Dr. Annese says:

"Our goal was to create this 3D model so we could revisit, by virtual dissection, the

original surgical procedure and support retrospective studies by providing clear anatomical

verification of the original brain lesion and the pathological state of the surround areas of

HM's brain."

The study also reveals something that had not been seen before: a small, circumscribed lesion

in the left orbitofrontal cortex. Dr. Annese suggests this lesion was most likely created in the

1953 operation.

In 2012, a study of young recruits at the Swedish Armed Forces Interpreter Academy in

Uppsala, found that learning

languages helps parts of the brain to grow, including the hippocampus and three areas of the

cerebral cortex. The researchers said their findings suggest learning languages could be a good

way to keep the brain shape.

Written by Catharine Paddock PhD




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