Henry Molaison (more commonly known as “HM”) is one of the most intriguing gentlemen in the history of neuroscience.
Following a surgical procedure to treat epilepsy, he developed severe memory defects. He was widely studied from 1957 until his death in 2008, playing a crucial role in studies that led to understanding the biological basis of learning and memory. For this week, I would like to tell you more about him and why he is such a legend.
HM was born on Feb. 26, 1926 in Hartford, Connecticut. At the age of seven, he suffered from a bicycle accident that resulted in a head injury. He was fine for several years until he began to have seizures, which is not unexpected because traumatic brain injuries often lead to seizures. The episodes got worse and worse until he was referred to William Scoville, a neurosurgeon at the Hartford hospital.
Scoville determined that the seizures were arising from regions of the brain known as medial temporal lobes. It was known that if the seizure focus (where seizures are coming from) could be removed, then the epilepsy patient has a considerable chance of becoming seizure-free. As a result, HM underwent a surgical procedure to take out the medial temporal lobes on both sides of the brain. (It was actually later determined that only one side was epileptic, so removing the medial temporal lobe on both sides was completely unnecessary.)
What happened next is fascinating. Indeed, HM no longer had seizures. However, he could not recall events that occurred before the surgery. Neither could he form new memories. For example, he was regularly studied by Brenda Milner. Although Milner often came to his house, he was never able to recognize her. As they talked throughout the session, HM was able to learn her name and participate in conversations. However, when Milner came back the next day, HM had completely forgotten who she was, and he regarded her as a stranger.
With the advent of sophisticated neuroimaging techniques, HM’s brain was imaged many years following the surgery to determine the exact places that were removed. Turns out, most of his hippocampus had been removed. The name “hippocampus” was inspired by its striking resemblance to a seahorse (the Greek hippos for horse and kampos for sea monster). HM’s case was direct human evidence for the critical role of the hippocampus in learning and memory. Clearly, the hippocampus must be doing something important in memory formation and recall.
Although many of his memory functions were disrupted, HM did not lose everything. In one of Milner’s studies, HM was asked to trace a star while looking at a mirror. HM performed this task over the course of three days, with several trials each day. Surprisingly, his ability to draw the star improved each day, even if he did not remember having done the task the previous day. This means that HM could still learn motor skills and retain some forms of long-term memory. This study led to the belief that although the hippocampus is important for memory formation, there are certain types of memory that can be generated independent of the hippocampus.
The case of HM is one of the most important studies in the field of neuroscience, leading to further investigations into the hippocampus as the brain center for learning and memory. That is why HM is such a legend.
