The Case of HM: Story on Memory and Brain Functions of the Anonymous Patient HM

In 1953 an anonymous patient by the name of HM underwent one of the most drastic and educational surgeries that neuroscientists and psychologists had ever seen. One doctor Scoville removed a section of HM's brain no larger than a fist, but encompassing "the hippocampus, the amygdala, and the entorhinal and perirhinal cortices." (Schaffhausen).

Due to this loss, especially of the hippocampus as research would suggest, HM lost the ability to form new memories, because he lacks the ability to consolidate the new information he is taking in. HM suffered, and suffers still from retrograde and anterograde amnesia. However, despite HM's lack of declarative memory and long-term memory consolidation, HM is still retains his long-term memory, short-term memory, and procedural memory consolidation. It is his lack of long-term memory consolidation, however, that seems to hinder the development of a new life. Even in 2004, researchers noted that HM thought Eisenhower was still president and that he still looked the way he used to look (a full head of dark brown hair) in the 1950s. (Koenigshofer). So why is it that HM can remember how to tie his shoes, how to play a modern computer game, and even remember that Eisenhower was the president in 1953? This phenomena gave researchers a fantastic look into the workings of the human mind, the areas of the brain associated with memory, and those areas functions. At this point, although the brain and memory remain an area of mystery to researchers and psychologists, there are different forms of memory and those different forms are regulated and run by different parts of the brain.

Today, the most prominent model for memory is the Atkinson-Shiffren model. This model involves a multi level memory storage system containing sensory memory, short-term memory, and long-term memory. Sensory memory deals with the retention of sensory information such as auditory and visual stimulus after the stimulus has ceased. Sensory memory is very short in duration; however, it allows us to recall very vivid and detailed images, tastes, smells, feelings (touch), and sounds. Yet, since the capacity of the sensory memory is so small these memories are very quite fleeting. After several seconds these sensory memories are generally lost. It is interesting to note, however, that we are able to recall some of the sensory information from years ago, this is due to the encoding (transfer of information) of sensory information from the sensory memory to the next form of memory, the short-term memory. If they are worth keeping, such as the sights from your trip to Venice, Italy, some sensory information may also be encoded into a more lasting long-term memory. George Sperling, who did experiments on iconic or sensory memory in the 1960s, noted that iconic memory (sensory memory) was a sort of buffer allowing sensory information more time to be recorded into a more permanent categorical manner (Sperling). However, it is also important to note that our brains do filter out a great deal of sensory information, and for it to even make it into the sensory memory we have to actually give the stimuli "attention." This is the process by which we "decide" what information we will take note of, and what information we will put aside and disregard. It is also interesting to notice that it is the responsibility of the sensory memory to identify stimulus information enabling us to construct meaning (Romero and Kemp 241-255).

The second form of memory, and one with a longer retention duration is short-term memory. This form of memory is also called the working memory due to the fact that the short-term memory is where we pull information from when we use it. However, short-term memory has quite a small capacity and can only hold around 7-9 chunks of information of varying sizes. (Koenishofer). Retention time is around 30 seconds, so if information isn't processed into long-term memory, it is lost. Learned information from our years and years of schooling is actually stored in the long-term memory, but used in the short-term memory state.

Generally though rehearsal methods, short-term information is transferred into the long-term memory. Romero and Kemp note that it is the long-term memory that "makes learning and intelligence possible" (241-255). The long-term memory has a seemingly infinite capacity, and due to this capacity we're able to call upon various pieces of information when we need it, even information from our childhoods. The long-term memory also has a set of subsystems, one of which was previously discussed in the section on HM. More specifically, declarative and procedural memory are part of the long-term memory subsystem. Due to HM's case (losing declarative memory consolidation but retaining procedural memory consolidation) it seems that these two subsets are also a result of different parts of the brain. Other subsets included in the long-term memory system are implicit and explicit memory, semantic and episodic memory. However, it is the declarative and procedural memory subsets that will be discussed in HM's case.

Again, after HM's surgery he was still able to remember events before the surgery. Additionally, he retained normal functioning of his short-term memory. He additionally was able to learn new skills such as playing an instrument or a computer game and he was able to retain that information. What he wasn't able to do, was learn new information. He wasn't able to recall newly learned facts, events, and the like. He wasn't able to consolidate information from the short-term memory into the long-term memory. This distinguishes short-term from long-term memory, and gives support to the fact that the hippocampus isn't involved in short-term memory function.

The hippocampus, one of the areas removed by the Scoville surgery in 1953 is believed to be responsible for memory recognition, and the storage of factual and intentional information into long-term memory (Memory, Learning, and Intelligence). The latter being the declarative memory aforementioned. Additionally the amygdala, another part of the brain that was removed in the HM case, is responsible for emotional memory, memory of fear, and memory consolidation. Conversely, the Basal Ganglia and Cerebellum are responsible for the creation and storage of implicit memory (skills, habbits, and simple classical conditioned responses). The posterior pariental cortex is responsible for the storage capacity of short-term memory. The prefrontal cortex, for the working memory or short-term memory. (Memory, Learning, and Intelligence). These physiological discoveries made in part by HM's surgery help us to explain what happened to HM's memory system as well.

Since both the hippocampus and amygdale were removed, it serves to explain why exactly he no longer has the ability to make new memories or consolidate information into long-term memory. However, this would raise the question as to why HM can recall old memories, after all the hippocampus is gone and is responsible for the functions needed for long-term memory. Some research has hinted at the fact that the role of the hippocampus in the long-term retention of memory is reduced as time passes. While many functions of the memory are still a mystery, this suggestion would explain why HM can recall how he looked in 1953 and who was president, but not be able to create new memories of what he did the night before. It seems that the HM case proves that the hippocampus is responsible for the formation of long-term memories, but not the retrieval of them.

To further explain HM's case of procedural memory we look to the Basal Ganglia and Cerebellum. These two area remained in-tact in HM's brain and thus explain why he could record these procedures in his memory, even if he had no recollection of ever doing it before. Additionally, the prefrontal cortex, another part of the brain left in-tact after surgery, provides the functioning necessary to keep the short-term or working memory in-tact.

Although HM's surgical procedure seemed to freeze him in the 1950s, it is important to note that the seizures (the reason for the surgery) did cease with the procedure. Additionally, the research side effect of the surgery is the increased amount of information researchers and psychologists have been able to draw from both the surgery and further follow-up studies on HM. While it was a tragedy for so many memory processes to be hindered with the removal of parts of his brain, HM's case has changed the way researchers and psychologists see memory and memory research.

Sources:

Koenigshofer, Kenneth. "LECTURE: Introduction to Memory ." University of Maryland University. Web Tycho, Maryland. 2004.

"Memory, Learning, and Intelligence." University of MarylandUniversityCollege Web Tycho. .

Romero, Anna, and Steven Kemp. Psychology Demystified. New York City: McGraw Hill, 2007.

Schaffhausen, Joanna. "The Day His World Stood Still." Brain Connection. 2007. Scientific Learning. 15 May 2007 .

Sperling, G. "A model for visual memory tasks". Human Factors, 5, 19-31. (1963).