There’s a difference between “unilateral” and “bilateral” strokes. In simple terms, the brain has two hemispheres, both of which have a hippocampus; a stroke that affects both is pretty devastating, but a stroke that affects just one hemisphere is more manageable. Much has been learned about the memory system from subjects who have suffered varying memory deficits from strokes, or even weirdly precise injuries. One subject referenced in scientific studies on memory was an amnesia sufferer whose condition resulted from somehow getting a pool cue lodged right up his nose to the point where it physically damaged his brain.37 There’s really no such thing as a “non-contact” sport.
There have even been cases where the memory-processing parts of the brain have been removed deliberately via surgery. This is how areas of the brain responsible for memory were recognized in the first place. In the days before brain scans and other flashy technology, there was Patient HM. Patient HM suffered severe temporal-lobe epilepsy, meaning the areas of his temporal lobe were causing debilitating fits so often that it was determined that they had to be removed. So they were, successfully, and the fits stopped. Unfortunately, so did his long-term memory. From then on, Patient HM could remember only the months leading up to surgery, and no more. He could remember things that happened to him less than a minute ago, but then he’d forget them. This is how it was established that the temporal lobe is where all the memory-formation workings are in the brain.38
Patients with hippocampal amnesia are still studied today, and the wider-reaching functions of the hippocampus are constantly being established. For example, a recent study from 2013 suggests that hippocampal damage impairs creative thinking ability.39 It makes sense; it must be harder to be creative if you can’t retain and access interesting memories and combinations of stimuli.
Perhaps as interesting were the memory systems HM didn’t lose. He clearly retained his short-term memory, but information in short-term memory no longer had anywhere to go, so it faded away. He could learn new motor skills and abilities such as specific drawing techniques, but every time you tested him on a specific ability, he was convinced it was the first time he’d ever attempted it, despite being quite proficient at it. Clearly, this unconscious memory was processed elsewhere by different mechanisms that had been spared.?
Soap operas would lead you to believe that “retrograde amnesia” is the most common occurrence, meaning an inability to recall memories acquired before a trauma occurs. This is typically demonstrated by a character receiving a blow to the head (he fell and hit it in an unlikely plot device), regaining consciousness and asking, “Where am I? Who are you people?,” before slowly revealing he can’t recall the past twenty years of his life.
This is far more unlikely than TV implies; the whole blow-to-the-head-and-lose-whole-life-story-and-identity thing is very rare. Individual memories are spread throughout the brain, so any injury that actually destroys them is likely to destroy much of the whole brain as well.41 If this happens, remembering your best friend’s name probably isn’t a priority. Similarly, the executive regions in the frontal lobe responsible for recollection are also pretty important for things such as decision-making, reasoning etc., so if they’re disrupted then memory loss will be a relatively minor concern compared with the more pressing problems. People can and do demonstrate retrograde amnesia, but it is usually transient and memories eventually return. This doesn’t make for good dramatic plots, but it’s probably better for the individual.
If and when retrograde amnesia does occur, the nature of the disorder means it’s very hard to study; it is difficult to assess and monitor the extent of someone’s memory loss from their earlier life, because how would you know anything about this time? The patient could say, “I think I remember going to the zoo on a bus when I was eleven,” and it seems as though their memory is returning, but unless the doctor was actually on the bus with them at the time, how can anyone be sure? It could easily be a suggested or created memory. So in order to test and measure someone’s memory loss from their earlier life, you’d need an accurate record of their whole life to measure any gaps or losses accurately, and having such a thing is rare.
The study of one type of retrograde amnesia resulting from a condition known as Wernicke-Korsakoff syndrome, typically the result of thiamine deficiency due to excessive alcoholism,42 benefited from an individual known as “Patient X,” a sufferer who had previously written an autobiography. This enabled doctors to study the extent of his memory loss more precisely as they had a reference to go from.43 We might see this happening more in the future, with more and more people charting their lives online via social media sites. But then, what people do online isn’t always an accurate reflection of their lives. You can imagine clinical psychologists accessing an amnesia patient’s Facebook profile and assuming their memories should consist of mostly laughing at funny videos of cats.
The hippocampus is easily disrupted or damaged—by physical trauma, stroke, various types of dementia. Even Herpes Simplex, the virus responsible for cold sores, can occasionally turn very aggressive and attack the hippocampus.44 And, of course, as the hippocampus is essential for the formation of new memories, the more likely type of amnesia is anterograde: the inability to form new memories following a trauma. This is the sort of amnesia Patient HM suffered from (he died in 2008 at the age of seventy-eight). If you saw the film Memento, it’s just like that. If you saw the film Memento but don’t really remember it, that’s not quite so helpful (but is ironic).
This is just a brief overview of the many things that can go wrong with the brain’s memory processes, via injury, surgery, disease, drink, or anything else. Very specific types of amnesia can occur (for example, forgetting memory for events but not for facts) and some memory deficits have no recogniz-able physical cause (some amnesias are believed to be purely psychological, stemming from denial or reaction to traumatic experiences).
How can such a convoluted, confusing, inconsistent, vulnerable and fragile system be of any use at all? Simply because, most of the time, it does work. It’s still awesome, with a capacity and adaptability that puts even the most modern supercomputers to shame. The inherent flexibility and weird organization is something that’s evolved over millions of years, so who am I to criticize? Human memory isn’t perfect, but it’s good enough.
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