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Henry Gustav Molaison

The Curious Case of Patient H.M.

By Erin Heaning, published Jan 13, 2022

Key Points

  • Henry Gustav Molaison (often referred to as H.M.) is a famous case of anterograde and retrograde amnesia in psychology.
  • In an attempt to control his seizures, H. M. underwent brain surgery to remove his hippocampus and amygdala. As a result of his surgery, H.M’s seizures decreased, but he could no longer form new memories or remember the prior 11 years of his life.
  • He lost his ability to form many types of new memories (anterograde amnesia) such as new facts or faces, and the surgery also caused retrograde amnesia as he was able to recall childhood events, but lost the ability to recall experiences a few years before his surgery.
  • The case of H.M and his life-long participation in studies gave researchers valuable insight into how memory functions and is organized in the brain. He is considered one of the most studied cases in medical and psychological history

Who is H.M?

Henry Gustav Molaison, or “H.M” as he is commonly referred to by psychology and neuroscience textbooks, lost his memory on an operating table in 1953.

For years prior to his neurosurgery, H.M suffered from epileptic seizures believed to be caused by a bicycle accident that occurred in his childhood. The seizures started out as minor at age ten, but they developed in severity when H.M was a teenager.

Continuing to worsen in severity throughout his young adulthood, H.M was eventually too disabled to work. Throughout this period of time, treatments continued to turn out unsuccessful, and the epilepsy proved a major handicap and strain on H.M’s quality of life.

And so, at age 27, H.M agreed to undergo a radical surgery that would involve removing a part of his brain called the hippocampus — the region believed to be the source of his epileptic seizures (Squire, 2009).

For epilepsy patients, brain resection surgery refers to removing small portions of brain tissue responsible for causing seizures. Although resection is still a surgical procedure used today to treat epilepsy, use of lasers and detailed brain scans help ensure valuable brain regions are not impacted.

In 1953, H.M’s neurosurgeon did not have these tools, nor was he or the rest of the scientific or medical community fully aware of the true function of the hippocampus and its specific role in memory. In one regard, the surgery was successful, as H.M. did in fact experience fewer seizures.

However, family and doctors soon noticed he also suffered from severe amnesia which persisted well past when he should have recovered. In addition to struggling to remember the years leading up to his surgery, H.M also had gaps in his memory of the 11 years prior.

Furthermore, he lacked the ability to form new memories — causing him to perpetually live an existence of moment-to-moment forgetfulness for decades to come.

In one famous quote, he famously and somberly described his state as “like waking from a dream…. every day is alone in itself” (Squire et al., 2009).

H.M soon became a major case study of interest for psychologists and neuroscientists who studied his memory deficits and cognitive abilities to better understand the hippocampus and its function.

When H.M died December 2, 2008, at the age of 82, he left behind a life-long legacy of scientific contribution.

Surgical Procedure

Neurosurgeon William Beecher Scoville performed H.M’s surgery in Hartford, Connecticut in August, 1953 when H.M was 27 years old.

During the procedure, Scoville removed parts of H.M’s temporal lobe which refers to the portion of the brain that sits behind both ears and is associated with auditory and memory processing.

More specifically, the surgery involved what was called a “partial medial temporal lobe resection” (Scoville & Milner, 1957). In this resection, Scoville removed 8 cm of brain tissue from the hippocampus — a seahorse shaped structure located deep in the temporal lobe.

Bilateral resection of the anterior temporal lobe in patient HM.

Bilateral resection of the anterior temporal lobe in patient HM.

Further research conducted after this removal showed Scoville also probably destroyed the brain structures known as the “uncus” (theorized to play a role in sense of smell and forming new memories) and the “amygdala” (theorized to play a crucial role in controlling our emotional responses such as fear and sadness).

As previously mentioned, the removal surgery was partially successful in reducing H.M’s seizures; however, he also lost the ability to form new memories.

At the time, Scoville’s experimental procedure had previously only been performed on patients with psychosis, so H.M was the first epileptic patient, and he showed no sign of mental illness. In the original case study of H.M which is discussed in further detail below, nine of Scoville’s patients from this experimental surgery were described.

However, because these patients had disorders such as schizophrenia, their symptoms were not removed after surgery. In this regard, H.M was the only patient with “clean” amnesia along with no other apparent mental problems.

H.M’s Amnesia

H.M’s apparent amnesia after waking from surgery presented in multiple forms. For starters, H.M suffered from retrograde amnesia for the 11-year period prior to his surgery.

Retrograde describes amnesia where you can't recall memories that were formed before the event that caused the amnesia. Important to note, current research theorizes that H.M’s retrograde amnesia was not actually caused by the loss of his hippocampus, but rather from a combination of antiepileptic drugs and frequent seizures prior to his surgery (Shrader 2012).

In contrast, H.M’s inability to form new memories after his operation, known as anterograde amnesia, was the result of his loss of hippocampus.

This meant that H.M could not learn new words, facts, or faces after his surgery, and he would even forget who he was talking to the moment he walked away.

However, H.M could perform tasks, and he could even perform those tasks easier after practice. This important finding represented a major scientific discovery when it comes to memory and the hippocampus. The memory that H.M was missing in his life included the recall of facts, life-events, and other experiences.

This type of long-term memory is referred to as “explicit” or “declarative” memories and they require conscious thinking.

In contrast, H.M’s ability to improve in tasks after practice (even if he didn’t recall that practice) showed his “implicit” or “procedural” memory remained intact (Scoville & Milner, 1957). This type of long-term memory is unconscious, and examples include riding a bike, brushing teeth, or typing on a keyboard.

Most importantly, after the removal of his hippocampus, H.M lost his explicit memory but not his implicit memory — establishing that implicit memory must be controlled by some other area of the brain and not the hippocampus.

After the severity of the side-effects of H.M’s operation became clear, H.M was referred to neurosurgeon Dr. Wilder Penfield and neuropsychologist Dr. Brenda Milner of Montreal Neurological Institute (MNI) for further testing.

As discussed, H.M was not the only patient that underwent this experimental surgery, but he was the only non-psychotic patient who exhibited such a degree of memory impairment. As a result, he became a major study and interest for Milner and the rest of the scientific community.

Since Penfield and Milner had already been conducting memory experiments on other patients at the time, they quickly realized H.M’s “dense amnesia, intact intelligence, and precise neurosurgical lesions made him a perfect experimental subject” (Shrader 2012).

Milner continued to conduct cognitive testing on H.M for the next fifty years, primarily at the Massachusetts Institute of Technology (MIT). Her longitudinal case study of H.M’s amnesia quickly became a sensation and is still one of the most widely-cited psychology studies.

In publishing her work, protected Henry’s identity by first referring to him as the patient H.M (Shrader 2012). In the famous “star tracing task”, Milner tested if H.M’s procedural memory was affected by the removal of the hippocampus during surgery.

In this task, H.M had to trace an outline of a star, but he could only trace the star based off the mirrored reflection. H.M then repeated this task once a day over a period of multiple days.

Over the course of these multiple days, Milner observed that H.M performed the test faster and with fewer errors after continued practice. Although each time he performed the task he had no memory of having participated in the task before, his performance improved immensely (Shrader 2012).

As this task showed, H.M had lost his declarative/explicit memory, but his unconscious procedural/implicit memory remained intact. Given the damage to his hippocampus in surgery, researchers concluded from tasks such as these that the hippocampus must play a role in declarative but not procedural memory. Therefore, procedural memory must be localized somewhere else in the brain and not in the hippocampus.

H.M’s Legacy

Milner’s and hundreds of other researcher’s work with H.M established fundamental principles about how memory functions and is organized in the brain.

Without the contribution of H.M in volunteering the study of his mind to science, our knowledge today regarding the separation of memory function in the brain would certainly not be as strong.

Up until H.M’s watershed surgery, it was not known that the hippocampus was essential for making memories, and that if we lost this valuable part of our brain we would be forced to live only in the moment to moment constraints of our short term memory.

Once this was realized, the findings regarding H.M were widely publicized so that this operation to remove the hippocampus would never be done again (Shrader 2012).

H.M’s case study represents a historical time period for neuroscience in which most brain research and findings were the result of brain dissections, lesioning certain sections, and seeing how different experimental procedures impacted different patients.

Therefore, it is paramount we recognize the contribution of patients like H.M who underwent these dangerous operations in the mid-twentieth century and then went on to allow researchers to study them for the rest of their lives.

Even after his death, H.M donated his brain to science. Researchers then took his unique brain, froze it, and then in a 53-hour procedure, sliced it into 2,401 slices which were then individual photographed and digitized as a three-dimensional map.

Through this map, H.M’s brain could be preserved for posterity (Wb et al., 2014). As neuroscience researcher Suzanne Corkin once said it best, “H.M was a pleasant, engaging, docile man with a keen sense of humor, who knew he had a poor memory but accepted his fate.

There was a man behind the data. Henry often told me that he hoped that research into his condition would help others live better lives. He would have been proud to know how much his tragedy has benefitted science and medicine” (Corkin, 2014).

About the Author

Erin Heaning is a member of the Class of 2023 at Princeton University. She studies psychology with a minor in neuroscience and works as a research assistant at the Princeton Baby Lab. Outside of school, she works for BMS Pharmaceuticals and as a writer for After graduation, she plans to study clinical psychology with a focus on women’s mental health.

How to reference this article:

Heaning, E. (2022, Jan 12). The Curious Case of Patient H.M.. Simply Psychology.


Corkin, S. (2014). Permanent present tense: The man with no memory, and what he taught the world. Penguin Books.

Hardt, O., Einarsson, E. Ö., & Nader, K. (2010). A bridge over troubled water: Reconsolidation as a link between cognitive and neuroscientific memory research traditions. Annual Review of Psychology, 61, 141–167.

Scoville, W. B., & Milner, B. (1957). Loss of recent memory after bilateral hippocampal lesions. Journal of neurology, neurosurgery, and psychiatry, 20(1), 11.

Shrader, J. (2012, January). HM, the man with no memory | Psychology Today. Retrieved from,

Squire, L. R. (2009). The legacy of patient H. M. for neuroscience. Neuron, 61, 6–9.

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