In Memoriam, H.M.

by Sanjay Kairam

Those of you who are students of Psychology or Neuroscience are surely aware of H.M., a patient known only by those initials, who incurred significant memory impairment as the result of an experimental brain surgery performed in 1953.  H.M., now revealed as Henry G. Molaison, passed away this week at the age of 83, leaving behind his enormous, if unplanned, contribution to the field of brain research and neuropsychology.

For those of you who don't know about this extraordinary man, H.M. was born in 1926, and suffered from an early age from epilepsy (thought to be the result of a head injury sustained at the age of 9 when he was knocked down by a bicyclist).  This epilepsy remained unresponsive to current treatments for many years, and in 1953, H.M. finally was introduced to surgeon Robert Scoville, the man who would perform the radical brain surgery that would profoundly change H.M.'s life (excerpt from BrainConnection):

The idea behind the surgery was simple. Seizures, as Scoville correctly reasoned, are caused by uncontrolled electrical impulses that start in a localized area and then spread throughout the rest of the brain. If one could remove the part of the brain where the seizures originated, it should be possible to cure the epilepsy. Henry had the most common form of the disease, called temporal lobe epilepsy, which meant that his seizures began in the tissue located on either side of his brain. Dr. Scoville removed a large chunk of Henry's right and left temporal lobes, which was a crucial decision because the brain is symmetrical and thus most important structures are duplicated. Altogether, Henry lost about a fist-sized portion of his brain, which encompassed (on both sides) the hippocampus, the amygdala, and the entorhinal and perirhinal cortices. As it turns out, the hippocampus is crucial for memory storage. When he lost his hippocampi, Henry became frozen in 1953, remembering very well the events before his operation but unable to create any new memories.

This type of memory loss, known as anterograde amnesia, is similar to that popularized by Leonard, the character played by Guy Pearce in Memento: while some or all memories formed before the accident may remain intact, the patient is unable to commit new events to long-term memory.  This can be differentiated from the type of amnesia more commonly referenced by pop culture, retrograde amnesia, which consists of the loss of long-term memories from before the time that trauma occurred.  H.M. suffered from a combination of the two, though it was the anterograde amnesia that was particularly striking.  The following excerpts are from the landmark paper published by Scoville and neuropsychologist Brenda Milner in 1957 (which can be found here):

There have been no gross changes in personality [referring to a group of patients].  This is particularly clear in the case of the epileptic, non-psychotic patient [H.M.] whose cheerful placidity does not differ appreciably from his pre-operative status and who, in the opinions of his family, has shown no personality change.  Neurological changes in the group have also been minimal.

As far as general intelligence is concerned, the epileptic patient has actually improved slightly since operation, possibly because he is less drowsy than before.  There has been one striking and totally unexpected result: a grave loss of recent memory in those cases in which the medial temporal-lobe resection was so extensive as to involve the major portion of the hippocampal complex bilaterally...In the non-psychotic patient [H.M.] the loss was immediately apparent.  After operation this 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.  There was also a partial retrograde amnesia, inasmuch as he did not remember the death of a favourite uncle three years previously, nor anything of the period in the hospital, yet could recall some trivial events that had occurred just before his admission to the hospital.  His early memories were apparently vivid and intact.

Beyond simply providing an insight in the localization of brainstructures involved in memory formation, H.M.'s condition provided a profund insight into the differences among short-term, long-term, and procedural memories.  Another key finding made by Milner through continued study and interaction wtih H.N. was that he was able to acquire new motor skills, even if he was not explicitly aware that he was doing so (from Wikipedia):

In a study conducted by Milner in the early 1960s, HM acquired the new skill of drawing a figure by looking at its reflection in a mirror (Corkin, 2002). Further evidence for intact motor learning was provided in a study carried out by Corkin (1968). In this study, HM was tested on three different motor learning tasks and HM demonstrated full motor learning abilities in all three tasks. HM's ability to learn certain problem-solving procedures has also been shown with the Tower of Hanoi task (Kolb & Whishaw, 1996). Experiments involving repetition priming underscored HM's ability to acquire implicit (non-conscious) memories, in contrast to his inability to acquire new explicit semantic and episodic memories (Corkin, 2002). These findings provide evidence that memory of skills and repetition priming rely on different neural structures than memories of episodes and facts; whereas procedural memory and repetition priming do not rely on the medial temporal structures removed in HM, semantic and episodic memory do (cf. Corkin, 1984). The dissociation of HM's implicit and explicit learning abilities along their underlying neural structures has served as an important contribution to our understanding of human memory: Long-term memories are not unitary and can be differentiated as being either declarative and non-declarative (Smith & Kosslyn, 2007).

The findings made from working with H.M. continue to inform memory research and will provide a canonical example for students of psychology for many years in the future.  We can imagine that every day he sat down with researchers, it was to him as if it were the very first; and yet had it not been his disposition to agree each time, research on the brain would surely have been impoverished.  It is in tribute and without irony that we can say that in this way, H.M.'s memory will persist forever.