Why the Hippocampus May Be the Most Important Region in Your Brain

This real-life story shows how memory forms our identity

By Richard Restak, MD, The George Washington University School of Medicine and Health Sciences
Edited by Kate Findley and proofread by Angela Shoemaker, Wondrium Daily

Memory is the natural extension of attention and learning. The act of memory facilitates the formation, activation, and retention of circuits that contribute to the brain’s optimal functioning. Dr. Restak explains how we are the sum total of the memory we retain. Without memory, we wouldn’t know who we are. 

Brain memory digital concept
A key region of the brain called the hippocampus regulates incoming information prior to it being distributed to the rest of the brain where it gets stored as long-term memory. Photo by Alexander Supertramp / Shutterstock

The Hippocampus and Memory

The hippocampus, a portion of the brain located in the temporal lobe of each cerebral cortex, is the entry portal for information to be remembered. If the hippocampus is damaged, we have difficulty forming new memories. 

This was demonstrated by Patient H. M., whose real name was Henry Molaison. He started having seizures when he was 10 years old. By age 20, he was completely incapacitated. 

Since he could be felled with one of the sudden seizures at any time, he couldn’t work or form relationships, and lived at home with his mother. At age 27, in 1953, he underwent a new type of operation. 

The operation, however, was not a corpus callosum operation, where the hemispheres are split by cutting the corpus callosum—a band of nerve fibers linking the two hemispheres—resulting in a split brain perforation. 

It was a different type of surgery, altogether. A finger-sized portion of the anterior temporal lobe—the portion of the brain responsible for semantic, or factual, memory—was removed, containing both hippocampi.

Memory Loss Impacts Learning

After the operation, H. M. seemed to be normal for the first hour or so, but then very unusual things were noticed. He could no longer learn or remember new information for more than a few seconds. 

For a time, people thought maybe he was playing a joke on them. They would come in and introduce themselves, and then they’d leave. When they came back, he wouldn’t remember who they were. 

This is how it remained for the rest of his life. After meeting someone new, several minutes later, he wouldn’t be able to recall having met them, and had to be reintroduced.

It was so bad that after his mother died, people were instructed to stop telling him about her death because each time someone mentioned it to him, he would grieve again because he didn’t remember that it had occurred. 

He remembered things clearly that had happened before the operation, but he couldn’t form new memories of things after the operation. This included names, events, and places—all were forgotten almost immediately. 

How Memory Forms Identity

Over the years, his face in a mirror surprised him because he only remembered what he looked like as a young man. Every question was new, even if asked moments before.

“In 1993, his psychologist played a tape for me—I was doing a book at the time on memory,” Dr. Restak said. “This was a conversation she had with H. M.”

H. M. had no memory of what he did early in the day or anybody he had met. He wasn’t certain of her identity, even though she had worked with him for 25 years. When she told him he should know who she was, he became reticent and said, “Well, I think you and I went to high school together.” 

“Now, throughout this discussion of the tape I was hearing, he sounds very tentative,” Dr. Restak said. “He wasn’t convinced of her assertions or what she was asking him. What he said he didn’t sound like he was very clear about. Finally, with fatigued resignation he admits that his memory was severely deficient.”

H. M. died on December 2, 2008, at the age of 82. Since then, his brain has been dissected and converted into high-resolution digital images. Jacopo Annese, founder of the Brain Observatory, who undertook this project, describes the images as “a neurological biography that survives in glass and pixels.” 

H. M.’s tragic experience led to the new insight that memory is controlled by a key region, the hippocampus, which regulates the inward flow of information prior to its distribution to the rest of the brain where it becomes long-term memory.  

Memory forms the basis for personal identity, as illustrated by H. M.’s story. Memory is also key to thinking.

This article was edited by Kate Findley, Writer for Wondrium Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for Wondrium Daily.
Dr. Richard Restak is Clinical Professor of Neurology at The George Washington University School of Medicine and Health Sciences. He earned his MD from Georgetown University School of Medicine. Professor Restak also maintains an active private practice in neurology and neuropsychiatry in Washington, D.C.