Epilepsy surgery with severe consequences
At the age of 27, he was offered epilepsy surgery by neurosurgeon William Beecher Scoville in Hartford, Connecticut. The surgeon emphasised that the procedure was experimental, but also noted that he had prior experience with psychosurgery (prefrontal lobotomy). To stop the seizures, the surgeon believed he would need to remove most of the medial parts of the temporal lobes bilaterally, including most of the hippocampus and the amygdala. This would prove to be a serious misjudgement, leaving Henry with severe anterograde amnesia for the rest of his life.
Following the operation, he was largely unable to acquire or retain new information, with the exception of some motor tasks. New experiences were forgotten in less than a minute. His working memory, necessary for tasks such as holding a conversation, and his procedural memory, including skills like swimming and cycling, remained intact. The amnesia effectively confined him to a life lived entirely in the present. He also exhibited retrograde amnesia, which ultimately restricted his memory to events occurring within approximately one year before surgery. Henry described his new existence as: 'like waking from a dream – every day is alone in itself' (5).
Following the operation, he was largely unable to acquire or retain new information, with the exception of some motor tasks. New experiences were forgotten in less than a minute
There was no impairment of his intellectual functions, and he exhibited no perceptual deficits; however, he had developed profound anterograde amnesia for both verbal and non-verbal material in all sensory modalities. Henry's specific memory deficits demonstrated the existence of two distinct types of long-term memory: explicit/declarative memory, responsible for recalling previous experiences and information, and implicit/non-declarative memory, responsible for skills that do not require conscious recall, such as cycling or swimming.
Together with one of the founders of neuropsychology, Brenda Milner, Scoville published an article describing the surgery and its consequences for HM; the abbreviation by which he subsequently became known in the literature (6).
Neuropsychologist Suzanne Corkin studied him for much of his adult life, from 1962 until his death in 2008. She has since published a book on him (4). In 2009, at the Brain Observatory in San Diego, his brain was sliced into 2401 sections, each 0.07 millimetres thick. The entire 53-hour procedure was broadcast live, with each section photographed and assembled into a 3D model. One of the findings from the dissection was that slightly more of the hippocampus had been preserved than previously thought. However, the entire entorhinal cortex was absent, indicating that this region is also critical for normal memory function.
Following completion of a digital reconstruction in 2014, Henry's brain can now be studied digitally via the website of the Brain Observatory at the University of California, San Diego (7, 8). According to Corkin, this digital resource marks the beginning of a new chapter in the study of the most extensively investigated case in the history of neuroscience (9).