New research reveals that a significant portion of his hippocampus - thought to have been completely removed in surgery in 1953 - was actually intact. (Agencies)
How did the new research reach this conclusion?
After the death of Patient H.M. - whose real name was Henry Molaison - in 2008, a team led by Jacopo Annese, director of The Brain Observatory in San Diego, California, and his team cut the patient's frozen brain into 2,401 slices - each 0.7-millimetres thick.
They took a picture of every slice, and created a high-resolution, 3D model of his brain.
A painstaking analysis showed that a significant portion of the hippocampus was actually there, said a study that appeared in the journal Nature Communications.
Patient H.M. became an iconic case in neuroscience when he developed a peculiar form of amnesia.
At age 27, H.M. underwent an experimental surgical treatment for his debilitating epilepsy.
His surgeon removed the medial temporal lobe, including hippocampus.
Thereafter, H.M. was unable to form new memories.
"Much of what we know about human memory, it has one way or another to do with H.M.,” Annese was quoted as saying.
After the surgery, H.M. was unable to learn new facts or remember past events. But he could learn skills.
Annese 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,” claimed the researchers.
The new findings shed light on what happened to H.M. but would not drastically change what researchers know about memory, Annese said.
New research reveals that a significant portion of his hippocampus - thought to have been completely removed in surgery in 1953 - was actually intact.