How A Man’s Surgical Procedure Led To Neuroscience

How Henry Molaison’s permanent loss of memory expanded our insights into the brain

Photograph of Henry Molaison. Image provided by Popular Science

In 1953, Henry Gustav Molaison, who suffered from an intense case of epilepsy, underwent brain surgery in hopes of improving it. He was twenty-seven at the time, and had been experiencing epileptic seizures for over a decade; it had resulted from a biking accident during his childhood. The experimental procedure involved removing a small portion of Molaison’s brain. After the surgery, he no longer experienced epilepsy…or the ability to form new memories. Henry Molaison was left an amnesiac for life.

Prior to Molaison’s surgery, it had been hypothesized that memory and learning were maintained and processed by the brain as a whole. The results of Molaison’s surgery suggested that this was not the case; instead, it revealed that distinct parts of the brain play roles in these areas. His surgery helped in understanding the functions of the hippocampus and amygdala, which are both parts of the medial temporal lobe. Overall, the temporal lobe is one of five lobes in a person’s brain used to process signals received from your senses. The hippocampus and the amygdala have two different but related functions within the limbic system.

• Humans have two hippocampi, on the left and right sides of the brain. The hippocampus stores and encodes declarative or explicit memories, which require the conscious recollection of information, such as facts or experiences.

• The amygdala processes strong emotions associated with memories, such as fear. There is one amygdala located above each human ear.

After his surgery, Molaison was no longer able to form declarative memories. However, he could still form implicit memories, which are retrieved non-consciously. This allowed Molaison to learn certain motor skills despite his inability to remember how, where, or when he learned them. As this suggests, motor skills are stored as implicit memories.

Being unable to form long-term memories, Molaison experienced what is known as anterograde amnesia. This involves the brain failing to encode and retrieve new experiences as memories. Molaison also experienced temporally graded retrograde amnesia, Retrograde amnesia prevents someone from recalling memories from the past. Molaison’s case was special; while he was incapable of recalling memories from the decade before having his surgery, he was adept at retelling memories from his childhood.

Molaison’s loss of memory allowed for discoveries of the brain, essentially creating the field of neuroscience. He was referred to as “H.M.” in studies to protect his identity and was observed under many, such as his neurosurgeon, William Beecher Scoville, neuropsychologist Brenda Milner, and Suzanne Corkin, a mentee of Milner’s. Corkin went on to study H.M. for over 46 years, up until his death in 2008. She formed a unique bond with him, which was detailed in her book, Permanent Present Tense. After his death, Molaison’s  brain was cut into 2,401 pieces; and later on, his brain was transported to The Brain Observatory, a research institute located in San Diego, CA, for the study titled “Project H.M.”