H.M.'s Memory Deficit

HM's Amnesic Conditions Examined

You can probably recall quite easily where you were five minutes ago, or the person you just chatted with, or even what you did yesterday evening. Many amnesics (i.e., persons with amnesia) would be unable to remember these simple things, and may have no idea of what they have done with their day so far.

A famous case study unfolds below about a young man who underwent surgical operation for his frequent epileptic seizures. Although the operation was successful, but he came out of the operating theatre a different man—his hippocampal system was severely damaged and consequently his long-term memory affected.

H.M.'s Case Study: The man who never got older
In response to a seizure disorder that could not be controlled effectively by anticonvulsant medications, in 1953 a young man sought surgical help for his epileptic seizures. In his desperation, the surgeon removed the hippocampus from both brain hemispheres—that scientists only now know are critical to memory—because this was the seat of his seizures. The operation did reduce his epilepsy but it also had a dramatic effect on his memory. The patient alluded here is known in the scientific literature by his initials, H.M. (his actual name was Henry Molaison). After the surgery, H.M. was unable to remember the events of his life or the people he met after the surgery, such as his physicians and other caregivers, or to learn new facts about the changing world around him. Even more severe was the damage that H.M. was unable to tell his age, the current date, or any aspect of his recent history (such as where he was living and how long he had lived there). In fact, on occasion in his later years, he misidentified a current picture of himself as a picture of his father. His memory was no better for people in the public eye or the public events in which they figured. Such deficits persisted until his death in 2008 at the age of 82. Nevertheless, throughout his life, H.M. still expressed a wide range of memory abilities. He could reason and solve problems, recognize objects, and perform voluntary and reflexive motor acts appropriate to all manner of objects and situations. These abilities, along with his full range of linguistic skills, demonstrated that he could access the considerable store of knowledge that he had acquired early in life before the surgery. His ability to remember the remote past prior to his surgery seemed largely intact, as was his ability to hold information in memory temporarily while working with it, as long as he was not interrupted. Because of the mixture of memory loss and memory retention, H.M.’s life had some surreal qualities. For example, he enjoyed solving crossword puzzles and could happily do the same crossword puzzle over and over again, because he didn’t notice the repetition. Although he also enjoyed watching television shows, they were difficult for him to understand because the commercials interspersed throughout a show caused him to forget the story line.

H.M.'s Case Study: The man who never got older

In response to a seizure disorder that could not be controlled effectively by anticonvulsant medications, in 1953 a young man sought surgical help for his epileptic seizures. In his desperation, the surgeon removed the hippocampus from both brain hemispheres—that scientists only now know are critical to memory—because this was the seat of his seizures. The operation did reduce his epilepsy but it also had a dramatic effect on his memory.

The patient alluded here is known in the scientific literature by his initials, H.M. (his actual name was Henry Molaison). After the surgery, H.M. was unable to remember the events of his life or the people he met after the surgery, such as his physicians and other caregivers, or to learn new facts about the changing world around him.

Even more severe was the damage that H.M. was unable to tell his age, the current date, or any aspect of his recent history (such as where he was living and how long he had lived there). In fact, on occasion in his later years, he misidentified a current picture of himself as a picture of his father. His memory was no better for people in the public eye or the public events in which they figured. Such deficits persisted until his death in 2008 at the age of 82.

Nevertheless, throughout his life, H.M. still expressed a wide range of memory abilities. He could reason and solve problems, recognize objects, and perform voluntary and reflexive motor acts appropriate to all manner of objects and situations.

These abilities, along with his full range of linguistic skills, demonstrated that he could access the considerable store of knowledge that he had acquired early in life before the surgery. His ability to remember the remote past prior to his surgery seemed largely intact, as was his ability to hold information in memory temporarily while working with it, as long as he was not interrupted.

Because of the mixture of memory loss and memory retention, H.M.’s life had some surreal qualities. For example, he enjoyed solving crossword puzzles and could happily do the same crossword puzzle over and over again, because he didn’t notice the repetition. Although he also enjoyed watching television shows, they were difficult for him to understand because the commercials interspersed throughout a show caused him to forget the story line.

Observation of H.M.'s condition continues ...
He could also hold a perfectly reasonable conversation, except that his conversation was devoid of current content; he could not tell you about recent weather conditions or the books that he had most recently read. If you avoided such topics in your conversation with him, you would be hard-pressed to notice any memory deficit at all. However, if you left for a short while, even for only a few minutes, upon returning you would find that he could not remember what you had been conversing about minutes earlier, and, most likely, he could not remember having ever met you! What was nearly as striking as his near-total inability to recollect his experiences was his intact ability to be affected by his experiences. Remarkably, he was able to acquire and express a variety of new skills, such as learning how to read words backward. He did so despite being unable to remember that he had ever been asked to read such words. Moreover, like neurologically normal individuals, his improvement in performance with practice was larger for the items he had previously seen than for new items presented for the first time. But the improved performance with repeated items occurred despite the fact that he was unable to judge which items were the repeated ones and which ones were new.

Observation of H.M.'s condition continues ...

He could also hold a perfectly reasonable conversation, except that his conversation was devoid of current content; he could not tell you about recent weather conditions or the books that he had most recently read. If you avoided such topics in your conversation with him, you would be hard-pressed to notice any memory deficit at all. However, if you left for a short while, even for only a few minutes, upon returning you would find that he could not remember what you had been conversing about minutes earlier, and, most likely, he could not remember having ever met you!

What was nearly as striking as his near-total inability to recollect his experiences was his intact ability to be affected by his experiences. Remarkably, he was able to acquire and express a variety of new skills, such as learning how to read words backward. He did so despite being unable to remember that he had ever been asked to read such words.

Moreover, like neurologically normal individuals, his improvement in performance with practice was larger for the items he had previously seen than for new items presented for the first time. But the improved performance with repeated items occurred despite the fact that he was unable to judge which items were the repeated ones and which ones were new.

Because amnesia can be so selective, affecting only certain memory capabilities while leaving other aspects of memory fully intact, it indicates that memory is a collection of abilities supported by a set of brain and cognitive systems that operate cooperatively, each system making different functional contributions. Notably, even when retrograde amnesia Opens in new window is observed, it typically is limited to memories for particular events or episodes of the past. This type of memory is called episodic memory Opens in new window.

As long as the damage is limited to the hippocampal system Opens in new window and does not extensively involve neocortical brain regions, patients will have intact memory Opens in new window for the basic perceptual Opens in new window, motor Opens in new window, linguistic, and cognitive competences Opens in new window they had before onset of amnesia. They also retain information learned early in life about language, objects, and the world in general (Cipolotti & Bird, 2006).

This dissociation—between the ability to retain information in general and the ability to remember specific life events—is an important attribute of amnesia and plays a central role in theories of the neural underpinnings of memory.

Spared Abilities

Interestingly, while amnesic patients display a deficit of long-term memory, other aspects of memory are spared. The fact that certain abilities are spared after hippocampal damage implies that other somewhat independent memory systems must exist in the brain.

Spared Working Memory

In contrast to H.M’s long-term memory Opens in new window that got damaged, his working memory was unaffected. A classic experiment illustrates the dissociation between long-term Opens in new window and working memory Opens in new window. In this experiment, a digit span task, in which the person has to report back a sequence of items, such as digits, read one at a time by the experimenter. H.M’ performance was within the normal range (7 +/- 2 items), indicating an intact working memory span. However, once his working memory span was exceeded, his performance suffered.

This deficit was demonstrated by an extended digit span task, in which the same digit string is presented on each trial but with an additional digit added to extend the span. For example, participants are repeatedly given a string of digits that surpasses their digit span by one digit (e.g., 2-7-9-1-3-4-8-6) until it can be correctly recalled.

Then they are given multiple trials with the same initial string but with an additional digit at the end (e.g., 2-7-9-1-3-4-8-6-5) until it can be recalled, and so forth.

Neurologically intact subjects can recall strings of at least 20 digits using this procedure, because they use long-term storage in addition to working memory to complete the task. H.M. could not recall even a single string that was one digit larger than his span, despite 25 repetitions of the same string. In other words, he was unable to use long-term memory to extend the list of digits beyond his working memory span (Drachman and Arbit, 1966).

Because patients with amnesia have intact working memory, they perform normally when the delay between the exposure to information and the memory test is short, or when the amount of material to be remembered is small. Thus, they can comprehend episodes and events normally if those events unfold over a relatively short time, and they can engage in reasonable discourse if the conversation remains on topic. However, because their memory impairment emerges with longer delays, they are unable to retain this information for the long-term.

Consequently, these individuals exhibit little cumulative learning across events or episodes. For example, you can comprehend this paragraph and then integrate it not only with the knowledge acquired from the previous paragraphs in this literature, but also with information from the preceding ones. Patients with amnesia, such as H.M., cannot do this.

Indeed, patients with the most severe amnesias often comment on the difficulty that reading presents to them. As a result, they have great difficulty learning new facts and data about themselves or about the world. When amnesics do show evidence of such learning, it is usually strongly tied to information they acquired prior to the amnesia that is highly salient to them and likely relies on regions outside the hippocampus Opens in new window. For example, capitalizing on H.M.’s love of crossword puzzles, scientists were able to show that he could learn some new information.

Spared Skill Learning

In the course of the extensive studies of H.M.’s memory deficit, researchers noticed a funny thing: In spite of his pervasive and profound long-term memory impairment he nonetheless appeared to be able to learn on some tasks! Furthermore, even though H.M. was getting better and better at these tasks, he seemed to be totally unaware that he was learning anything. Rather, he would just comment “Huh, this is easier than I thought it would be.”

One of the first examples of skill learning that scientists observed in H.M. was on a mirror-tracing task, which involved tracing the outline of a figure (such as a star) by looking in a mirror (Milner, 1962; Corkin, 1968).

For this task, word triplets are presented in mirror-image orientation, and the viewer reads them aloud as quickly and accurately as possible. Of critical importance, half of the word triplets are presented multiple times, appearing once in each block of trials, and half are presented only once during the experiment.

With practice, neurologically intact participants show improvements in reading both the mirror-imaged words they have seen before and the new mirror-imaged words. In this way, they acquire the skill of reading any word presented in mirror-imaged text, not just specific instances of mirror-reversed words. Like neurologically intact people, amnesic patients show improvements on both the practiced and new mirror-imaged words.

What is even more remarkable is that this spared learning occurs even when the patients cannot recollect the training events during which the new skills were acquired, cannot recall or recognize the material on which the increasing skill is demonstrated, and have no insight into their improved performance!

Specialties

H.M.'s Memory Deficit

HM's Amnesic Conditions Examined

Spared Abilities

Spared Working Memory

Spared Skill Learning

See also: