H.M. (patient)

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Revision as of 08:51, 28 April 2008

Background

One of the most famous patients in the history of neuropsychology, Henry M., more well-known as H.M. was born in 1926 and underwent an experimental surgery in an attempt to cure his severe seizures. The experiment confirmed the important possesses of particular neural structures, predominantly, the medial temporal lobes and their functions in memory formation and processing. H.M.'s case study helped define the different memory disorders.

History

Born a normal functioning individual, H.M. had no seizure complications growing up, until he was knocked down by a bike at age 9. This left him unconscious for five minutes, and was believed to be the source of his seizures. He began having absent seizures at this age. His condition worsened and by the age of 16, he began having generalized seizures in conjunction with absent seizures. Between the ages of 16 and 27, H.M. usually experienced 10 absent and 1 generalized seizure per week. Doctors gave him a medication called Phenytoin, an anticonvulsant medication, but this was ineffective. With symptoms worsening, H.M. withdrew from school due to embarrassment and frequency of his epileptic fits.

The Surgery

[1] At age 27, H.M. opted for a bilateral medial temporal lobectomy. In this procedure, the doctor removed parts from the medial temporal lobe on each side of the brain, completely destroying, most of the hippocampus, parahippocampal gyrus, ethorhinal cortex, and part of the amygdala. In addition to the surgery, the anti-seizure medication, Phenytonin's toxicity effects caused degeneration of the cerebellum.

The Results of the Surgery

As the doctors suspected, H.M.'s seizures became less frequent and intense, he began having less than 1 generalized seizure and 5 absence seizures per year, and his IQ was generally the same as prior to the surgery. What they did not suspect were the other, negative, side effects associated with this experimental surgery. He stared rehabilitation daily from 1967- 1977. H.M. Developed:

• Global amnesia, manifesting itself in two types, anterograde and retrograde.
  • Anterograde amnesia is the loss of memory after the injury sustained. H.M. exhibits this because he is unable to form new long term memories.
  • Retrograde amnesia is the loss of memory from before the injury was sustained. H.M. shows retrograde amnesia because after the age of 16, he remembers nothing. He is unable to recall famous songs, events, political figures from this period of time, but can recall events that happened in his childhood before age 16.
• Long Term Memory Loss

H.M.'s long term memory was severely impaired. He was unable to learn new stories, block patterns, songs, drawings, new vocabulary. Although, he was able to store and use some new information using repetition and priming methods.

• Inability to Update Memories

H.M. frequently retells stories, can't remember his uncle or mother's deaths, does not recognize himself in a mirror, does not have recognition of anyone since he was 16, can't tell you what he did earlier in the day, and does not know the day, month, year season or his age. H.M. struggles to learn new information, a characteristic of anterograde amnesia. Although, in an experiment where one of H.M.'s nurses was told to be very nice to H.M. and the other to be less friendly. When asked which nurse he would rather be friends with or which was the "good nurse" vs. "bad nurse", he was always right in his selection of the "good nurse", implying that he did get to know people, he just doesn't remember the context in which it happened.

• Explicit Memory Problems

H.M. exhibits implicit learning without explicit knowledge of completing tasks. He has no idea what he did five minutes ago. He is unable to remember consciously and internally previous experiences. An example of this is his aptitude in learning the Mirror Drawing Task, a task requiring visuospatial abilities. Over a period of three days of training, H.M. improves on this task, making less errors and completing the task in a less amount of time, indicating that he has learned new material. Although, if you are to ask H.M. what materials he used for the mirror drawing task, or if he had ever completed a mirror drawing task before, he would replay that he had no recollection of this. Another example of this is seen when H.M. broke his ankle at age 60. He became a proficient user of the walker, but did not understand why or how he knew how to use this apparatus.

• Working Memory Problems

H.M. is capable of maintaining conversations and can remember people's names, process sentences, keep attention, but if there is a disruption or a distraction, H.M. will forget what he is doing and you must start the conversation over or even reintroduce yourself. He often is negligent about self-care. H.M. frequently forgets to change his clothes, eat lunch, and shave his face, because he is unable to remember tasks that he would like to do. He is unable to temporarily store and manipulate new information when he is distracted. An example of this would be, when read a list of words, H.M. would perform better in regards to the recency effect. H.M. would be unable to use primacy recollection due to the fact that he would get distracted by the new words being said, and not able to recall the first words stated.

• Disassociation between Semantic and Episodic Memories

Semantic memories are the context free, general knowledge held by the general population. Whereas, episodic memories are those that are biographical records of personal experiences paired with time and place. H.M. was unable to learn words he didn't already know prior to his surgery, meaning he was incapable of encoding new semantic information. In the mirror drawing task, he remembers the procedure of the task, because of his marked improvement, but he is unable to remember the even taking place, or the actual learning process. He is unable to initiate new topics of conversation. A good example of this is portrayed when a nurse asks H.M. if he remembers why he has all of the birthday cards in his room. He is unable to recall having a birthday party the day before.

• Inability to respond emotionally in particular situations

This problem could be attributed to the possible removal of parts of the amygdala from both hemispheres of the brain and the use of antiepileptic medication for years.

Conclusion

H.M. helped to define the different memory disassociations based on the side effects of his surgery. In his case the first was spared and the second was impaired.

1. immediate memory vs. long term memory
2. remote memory vs. anterograde memory
3. implicit memory vs. explicit memory
4. verbal memory vs. nonverbal memory
5. semantic memory vs. episodic memory

References

• Ogden, Jenni A. (1996). Fractured Minds. Oxford: New York.
• Stirling, John. (2002). Introducing Neuropsychology. Psychology Press: New York.