How did Patient H.M. change the course of neuroscience?

There have been of course several case studies in neuroscience- scientists trying to understand the unknown and how to overcome such issues. However, there is one case that stands out the most- the clinical case of Henry Molaison, more commonly referred to as Patient H.M.


Who was Patient H.M?

Patient H.M was born on February 26th , 1926. For more than 50 years, neuroscientists studied H.M, performed various tasks, however, he was unable to recall some of this.  It had started when H.M had been knocked down by a bicycle at the age of 7 (or 9 in some other scientific reports), where he began to have seizures, worsening by the time he got to the age of 16. Despite this problem, he decided to work on an assembly line, but by 27 he was almost controlled by these seizures, so he stopped working. He was prescribed antiepileptic drugs, but this failed to stabilise his epilepsy.  Therefore, he was referred to neurosurgeon William Scoville who, alongside his team of neurosurgeons at Hartford Hospital, discovered many vital facts about the brain after the bilateral removal of the hippocampus (which plays a major role in memory) on both sides of the medial temporal lobe.  The hippocampus, alongside some minor areas of the medial temporal lobe, was removed in the hope to control the seizures.  After this was removed, neurosurgeons were able to comprehend much more about the brain.

Prior to the surgery, H.M didn’t suffer any previous memory impairments however, after the surgery, he suffered anterograde amnesia (he also suffered from retrograde amnesia for a while, but that diminished over a period of time).  Thus, his brain was unable to create any new memories after the surgery.  From this piece of information and carrying out further tests, neuroscientists were able to conclude many facts.  William Scoville and Brenda Milner, a neuropsychologist, reported their findings of Molaison. When Brenda first saw H.M, his seizures were now much more controlled, however, his memory was deteriorating more rapidly than two patients who encountered epileptic seizures- they both had a unilateral removal of the left temporal lobe.  Brenda saw that H.M wasn’t able to retain daily activities as fast as they happened, even “underestimated his own age” and wasn’t even able to remember the people he had been introduced to just a few minutes ago.  Clearly, his long-term memory followed the “Ebbinghaus Forgetting Curve“, however, his retention was very poor.  In Milner’s reported discoveries, H.M was described as “like waking from a dream… every day is alone in itself“. There was no apparent change in his personality or any abrupt behaviour but wasn’t able to execute new activities for his explicit memory.  Thus, he was not able to remember any new semantic memory (recall of general facts) or episodic memory (recall of personal facts) after the removal of the hippocampus. However, his impairment in semantic memory is disputed amongst scientists. Despite this, his brain was able to create long-term procedural memory- so any motor and cognitive skills were learnt, but he wasn’t able to remember that he did it.

Although his long-term memory was heavily affected, particularly his explicit, his short-term memory was not affected.  When presented with a piece of information, such as numbers or a string of letters, he was able to retain that information for up to 15 minutes with continuous rehearsal, but when attention was distracted by another topic, he failed to recall any of that previous information.  It was more difficult with a material, which was forgotten much more quickly- within less than a minute.  Material included were faces, decorations or just furniture.  H.M’s visuospatial sketchpad wasn’t functioning as well as his phonological loop, but his short-term memory was much stronger than long-term memory, so his central executive was functioning fairly well, which enhanced through constant rehearsal, else H.M wouldn’t have been able to retain some information.  He was also asked during a test about two voices and the similarity in their pitch- there was an exponential relationship- as the duration increased between the two voices, the ability to give a reasonable response decreased.  This particular experiment, which also had an interference tone between the two tones, illustrated great results- his ability to retain information was much more than a normal patient, between 0.25s to 2s.  This indicates that H.M is more able to rehearse in the presence of a distraction and remember the necessary information.  This is different from when H.M was unable to remember a string of digits when his attention was diverted elsewhere.

Crossword findings

It was believed that he could form some semantic memory, as in his later life, Dr. Brian Skoto  and some of his colleagues -from Duke University- used crosswords to see H.M’s ability to create new semantic memory.  The following findings were based on this experiment:

  • As he wasn’t able to create any new memories after the operation, the crosswords were pre-1953 based.  
  • However, information post-1953, he could connect his old memories with any new information by seeing to establish any possible connection.  
  • The results were presented in a report which compared how H.M did in three different types of crosswords with healthy volunteers. 
  • One was a mixture of pre-1953 and post-1953; H.M answered 1/5 of them correctly on the first day, but his results improved from the same crossword until he got just under 1/2 right on day 5.  
  • It’s an indication that he formed new semantic memory.  The presentation of the new information was the result of nondeclarative memory- he unconsciously remembered some of the information.  
  • This study was done twice over a three-year .  

Overall, we found that the Model of Memory didn’t follow for Molaison as when he got a sensory input it was transferred to the short-term memory but wasn’t encoded to long-term memory.  Short-term memory would only function for a long period through constant rehearsal, which H.M did during his tests.

Model of Memory- shows what happens with information at each stage

From all of these results, neurosurgeons had been able to understand that as H.M didn’t have a hippocampus, the brain wasn’t able to encode any long-term memory, but was still able to have sensory memory encoded to short term memory.  The medial temporal lobe was the bigger area, where the hippocampus was enclosed in and this was the area for long-term to be encoded.  As both hippocampi were removed, it caused severe anterograde amnesia. This meant that the process of consolidation couldn’t take place as there was no place to transfer the memory to. The working memory wasn’t hampered because the prefrontal cortex is believed to be responsible for that- the prefrontal cortex is near the front of the brain. Brain plasticity (can the brain change according to experience?) was quite limited in Patient H.M as both of his hippocampi were removed.  Nowadays, only one hippocampus is removed if a patient suffers from epilepsy- thus suffers less major memory impairment as H.M.  This is because when one hippocampus is removed, the other one can’t only continue to function as intended, but also cover the loss of the removed hippocampus.  This is allowed as of brain plasticity- the brain can adapt to the change.  However, H.M had both hippocampi removed, thus, brain plasticity wasn’t as extensive therefore suffered more severe memory impairment.  Considering he was an adult when the operation happened, brain plasticity is less efficient- so even less reorganisation of memory function. 

Undoubtedly, all of this had a massive impact on his life- he wasn’t able to make new memories about current events around the world, or even just activities that he did.  Experiments were conducted on him to help with research regarding the brain.  When he wasn’t required for research, he was dependent, but not fully.  This is because he wasn’t able to remember his tasks, however if he had repeated a task numerous times, he would know how to do it, but couldn’t recall actually doing it.  This is implicit memory– it is unconsciously remembering an event or a skill. As with his daily life routine, it was more negative than positive, as he had false memories of previous activities, meaning it was hard to control him at times.  During his later years, he did crosswords for entertainment and was even tested on crosswords, anagrams and some game activities.

What did we find?

Without Henry Molaison, our view of the brain would be vastly different.  It gave us many interesting findings and gave us reasons why we should continue studying the brain? 

  • Firstly, there was declarative and non-declarative memory.  H.M only had non-declarative as he was able to do activities, particularly those he did repeatedly but failed to recall.  Non-declarative memory is stored near the back of the brain- partly in the cerebellum and other parts.  
  • Due to these parts of the brain being unaffected, H.M was still able to unconsciously store some activities for the long term.  
  • His inability to create new long-term memories, but the ability to retrieve long-term memories, indicates that the two processes are done in two different parts of the brain.  
  • If they were in the same place, in the hippocampus, Henry Molaison would not have been able to encode nor retrieve long-term memories.

Due to the study of H.M, we were able to see a distinction between short-term memory and long-term memory. Since they are both stored in different places within the brain, they would either be both; affected and unaffected when the bilateral removal of the hippocampus took place.  We acknowledged that the long-term memories are encoded in the hippocampus, but the short-term memories are encoded and processed primarily in the prefrontal cortex.  It was clearly evident when tests were conducted, where H.M performed well in short-term memory, but not that good in the long-term memory tasks.  Although not mentioned thoroughly in this report, H.M suffered retrograde amnesia for a temporary period of time after the operation.  H.M stated he was able to remember memories from a long time ago, such as childhood memories, but not memories just before the operation. This shows that the older memories are not reliant on the medial temporal lobe, unlike the recent long-term memories.  Now we know that remote long-term memories are dependent on areas of the brain, such as the cortex.  Due to that, consolidation didn’t happen in H.M, so no links through neurons were made.

It is important for neurosurgeons to distinguish different types of memory, as, without it, we will not know where a particular patient is lacking- whether lacking in retrieving facts or if their procedural skills are intact. 

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