HM, a high school graduate who worked as a motor winder, was hit by a bicycle as a child, sustaining a 5-minute loss of consciousness. At age 10, HM began to experience “minor” seizures; after age 16, “major” seizures developed. By 1953, despite escalating doses of anticonvulsants, HM was having up to 10 seizures daily, affecting his interictal mental functioning and his ability to work.
In an effort to ameliorate HM’s epilepsy, William Beecher Scoville, a Hartford Connecticut neurosurgeon with a faculty position at Yale, bilaterally resected HM’s medial temporal lobes using a procedure he had developed, called “fractional undercutting.” This involved suctioning the medial temporal tissue extending 8–9 cm caudally from the temporal tips through bilateral supraorbital burr holes.
Unilateral medial temporal resection for treatment of intractable epilepsy had been pioneered by Wilder Penfield at the Montreal Neurological Institute (MNI) in 1928. However, applying bilateral “fractional undercutting” to the treatment of HM’s seizures in 1953 was, according to Scoville, “frankly experimental.”
Although HM’s epilepsy did improve and his post-operative neurological examination was reportedly normal, Scoville noted “one striking and totally unexpected behavioral result”— HM’s remarkable loss of ability to form new 41 memories Scoville first reported this surgical result in April 1953 in a speech at the Harvey Cushing Society. His talk was published in 1954 as “The Limbic Lobe in Man” and included mention of “a very grave, recent memory loss” in two patients “undergoing bilateral resection of the entire [limbic lobe] complex including the hippocampal gyrus extending posteriorly for a length of 8–9 cm from the tips of 45 the temporal lobes.” One of these patients was HM.
“HM” is arguably the single most intensively studied and best-described patient in the history of neuroscience. Henry Gustav Molaison unexpectedly developed a severe impairment in his ability to form new memories following bilateral medial temporal lobe surgical resection for intractable seizures in 1953, when he was 29 years old. Postoperatively, HM’s “loss was immediately apparent... [T]his young man could no longer recognize the hospital staff nor find his way to the bathroom, and he seemed to recall nothing of the day-to-day events of his hospital life…. His early memories were apparently vivid and intact.
Neuropsychologist Brenda Milner, who began a career-long study of HM beginning two years after HM’s surgery. HM displayed a complete loss of memory for events since his surgery, partial retrograde amnesia for the 3 years before the procedure, and remarkably intact early memories.
What made HM unique was that his mental status findings were “restricted to his inability to remember new episodic, autobiographical events and not confounded by other neurological or psychological disorders. In light of earlier reports that unilateral lesions had no impact on memory, Scoville and Milner concluded that bi lateral lesions produce “persistent impairment of recent memory whenever the removal is carried far enough posteriorly to damage portions of the anterior hippocampus and hippocampal gyrus.” “The degree of memory loss appears to depend on the extent of hippocampal removal.”
Importance of this case:
Prior to studies of HM, memory was believed to be a widely distributed function, associated with, rather than separate from, other cognitive and perceptual abilities. The function of the hippocampal formation was unclear, having been suspected of having roles in motor, olfactory or emotional function.
HM’s case illustrated the importance of the medial temporal lobes for memory. Contrary to popular interpretation, HM’s case alone did not prove that bilateral hippocampectomy resulted in complete anterograde amnesia, since the amygdalae were also removed. However, when HM was considered with their two unilateral hippocampectomy cases, Penfield and Milner were able to conclude that removal of the hippocampi bilaterally does, in fact, result in anterograde amnesia.
Despite having no recall of having learned a motor task, nevertheless his performance of complex motor skills improved with repetition, establishing that procedural memory relied on different brain networks than did declarative memory. In addition, HM could retain information as long as his attention to a task was sustained by continual mental rehearsal, establishing that working memory involved extrahippocampal areas.
HM also agreed to have his brain preserved for further study after his death. An extremely detailed postmortem examination of HM’s brain showed that he had approximately cm3 of retained hippocampal tissue bilaterally, so the surgical resections were not complete.
Although HM was arguably the most extensively studied patient in neuropsychiatric history and his case demonstrated that remote memory and procedural learning involved brain areas other than the hippocampus, there is still debate as to whether his hippocampi were entirely ablated and whether a small iatrogenic frontal lesion may have contributed to his amnesia. Nonetheless, the case of HM had a lasting influence on the field of neuropsychiatry, demonstrating that memory was neither a widely-distributed brain function nor a unitary process.
Excerpted from: “Six Landmark Case Reports Essential for Neuropsychiatric Literacy”
Authors: Sheldon Benjamin, M.D., Lindsey MacGillivray, M.D., Ph.D., Barbara Schildkrout, M.D., Alexis Cohen-Oram, M.D., Margo D. Lauterbach, M.D., Leonard L. Levin, M.S. L.I.S., M.A.