Patient HM is probably the most intensely studied patient in the history of behavioral neuroscience. He is the most carefully documented patient having an amnesic disorder characterized technically by the term anterograde long-term memory loss. This is the type of memory loss that is displayed by the main character, Leonard, in the movie Memento. (And also by a second character, Sammy, who also plays a role in the movie. More about that later.)
The types of amnesia that are best known in popular culture, and frequently depicted in films, are forms of retrograde amnesia. In a typical case, reported in a newspaper story or depicted in a film, a person is discovered who does not know who he is and has no recollection of his past. The onset of these cases of retrograde amnesia is typically attributed to some traumatic event, either a physical event such as a blow to the head, or psychological such as being witness to some horrific event. In these cases there is no anterograde loss of memory function going forward from the traumatic event. If the patient is interviewed weeks, months, or even years after the traumatic event, he has normal memory for all of the events that have happened during the interim period. The deficit is in the form of an inability to retrieve memories of what preceded the traumatic event, thus the term retrograde. Sometimes the retrograde loss covers the period from birth up until the traumatic event such that the patient does not know anything about who he is or his past history. Other times the retrograde amnesia covers a shorter period of events around the time of the trauma. Here at the Tacoma Film Club we recently saw an example of this kind of limited retrograde memory loss depicted when we viewed the film The Machinist.
The amnesia documented in patient HM is of interest to scientists for a couple reasons. First, he has a fairly “pure” anterograde long-term memory loss without other accompanying symptoms. For example, his procedural memories (habits) are unaffected, and there is no retrograde loss of his declarative memories. (See my previous post for descriptions of these technical terms). Second, the location of the brain damage that is responsible for his deficit is relatively small, and has been carefully documented. His damage is restricted to an area in and around the hippocampus in the medial temporal lobe of the brain.
There are other documented cases of patients, in addition to HM, who exhibit anterograde long-term memory loss, but it is typically accompanied by additional neurological deficits, presumably because the brain damage is not as localized. Cases of relatively pure forms of this particular neurological deficit are extremely rare. For this reason, the likelihood that two individuals who know each other would both have this condition (as depicted by the characters Leonard and Sammy in the film Memento) is probably something on the order of being struck twice by lightning. This provides us with an important potential clue we can try to use as a tool to try to interpret the events depicted in the film. Lets evaluate some of the possibilities.
Possibility #1 is that the screenwriter simply used coincidence as a plot device. In that case, both Leonard and Sammy actually have this rare neurological disorder. What evidence would we expect to find in the plot if that possibility is the correct interpretation? At a minimum, we should be able to expect that all of the events depicted in the film are consistent with that interpretation.
Possibility #2 is that character Sammy did not really have the condition, but Leonard did. This is the possibility pitched to us by Leonard as we watch the film. Leonard tells us that Sammy was faking this condition in order to carry out an insurance scam. Leonard, in his job as insurance adjuster, performed several tests in which he tried, unsuccessfully, to catch Sammy in this scam. If that possibility is the correct interpretation of the film, then a basic theme of the film is irony. It is ironic that Leonard, having worked so hard to prove that Sammy was faking it and did not really have the condition, later becomes afflicted with the same condition himself. Evidence that would support this interpretation would have to include some details in the plot (perhaps subtle or hidden) revealing that Sammy was faking, and all of the details regarding Leonard would have to be consistent with him really having the condition.
Possibility #3 is that character Sammy really had the condition, but Leonard did not. Perhaps Leonard, while observing Sammy, got the idea that it might be a good scam to fake the condition displayed by Sammy. If this interpretation is correct, we would expect, at a minimum, that there will be clues embedded in the plot that reveal Leonard does not really have memory loss, but is faking it. There should also be clues revealing, or at least suggesting, what that scam was.
Possibility #4 is that the character Sammy does not really exist, but is simply a made-up character, or perhaps an alter-ego, of Leonard. Just to be complete, we should probably consider two variations on this theme. The first is that Leonard really does have memory loss, but in addition, is lying to us (or is hallucinating) when he tells us about Sammy. The second is that Leonard does not have memory loss at all. In that case, we need to diagnose what is going on with Leonard. Is he psychotic, having an imaginary clinical condition as well as one or more imaginary friends? Is it all a big con game of some sort, and if so, what exactly is the con? If either of these variations is the correct interpretation, we would expect the plot to provide enough clues that we can figure out what is going on.
I suppose there might be other possible interpretations as well (if you have come up with others that are internally consistent with all of the facts in evidence in the film, please post them here as comments, and we can discuss them as well). In future postings we will examine some of the evidence revealed to us in the film that can help us decide between these possible interpretations.
The next post in this series is Part 4.