Tuesday, 29 November 2011

DECEMBER 7th [Science Comedy Advent Calendar]

December 7th: Patient HM

For many people, the Christmas period is used as a sort of 'anchor' for memory. It is generally accepted, regardless of the evidence to support this, that Christmas is a happy time, a period of childhood in particular where there is a strong emotional component, where a lot of good things are condensed into one short period and so tend to stick in the memory.

In a way, the Christmas period gives us the opposite of Post Trauamtic Stress Disorder, PTSD. PTSD is a serious issue for many people, and one of the hallmarks of it is a tendency to mentally 'relive' the memory of the incident that led to the trauma. This may seem illogical. Why would we want to keep repeating things we find unpleasant? After a bout of food poisoning, we don't go straight back to the kebab shop we stumbled into at 2.30 am and order another discount 'box of meat', just to repeat the colourful experience. But with PTSD, people do apparently constantly relive the cause of their distress, like a diabetic injecting syrup into their veins for 'fun'.

It's a product of the way our memory systems work. The traumatic experience kicks our autonomic nervous system into high gear, with our fight-or-flight coordinating sympathetic nervous system flooding our bodies with adrenaline and the like. and one of the consequences of this is that it amplifies the memory processing aspects of the limbic system, which is also heavily involved in emotion processing. It makes sense, in an evolutionary context. If you find yourself in a dangerous position, facing a considerable threat, it would be best to remember it as vividly as possible, so that you rapidly learn to avoid it in future. Humankind may not have survived if our memories of sabre-tooth tiger encounters were recalled as just 'brown snarling thing, big teeth, or possibly tusks?'. As such, our brains are wired for intense recall of emotionally intense experiences. These ones stand out from our daily memories like a big shiny star on a Christmas tree, which is just a green mass with shiny bits on it.

Admittedly, this is not how memory is described in many textbooks, but then academic literature has never really targeted the festive market. It's a niche sadly overlooked, one that this blog is hoping to corner.

But if PTSD can cause us to relive unpleasant experiences, perhaps Christmas can have the same effect in the opposite way? For a child, it's an intense period of pleasure, gratification, acquisition and all things fun and magical, perhaps the flood of positive emotions cause the memories formed at this time to be more salient? We always tend to look back at childhood Christmases with a rosy glow. Unless they were shit.

But or most people, childhood Christmases are period that they wish would never end. But look at that realistically for a second. What kind of life would that be? Never moving on, never progressing, it's just always 'now'. Sounds horrible, really.

One person who knew exactly what this was like (or, more realistically, could have known but was unable to do so) was patient H.M. Although it sounds a bit like the name given to someone undergoing some sort of super-soldier research project, the trusth was far more mundane and far more useful to science.

Patient HM suffered from crippling temporal lobe epilepsy. In order to treat this, surgeons basically removed his temporal lobes. Presumably they'd mainly treated people suffering appendicitis before this point.

Although it cures his epilepsy, patient HM didn't know this. He didn't know much about anything after the surgery, as he lost the ability to form new memories. He still had his short term memory, but that has a capacity of about 30 seconds, which isn't much use really. He was like the guy from Memento, but without a burning desire for revenge to motivate him to do stuff.

For the remainder of his life, HM's most recent long term memory was preparing for the surgery. Thanks to his condition, we've learned a great deal about the human memory system, as we're not really allowed to cut out bits of people's brains in order to see what happens, even if they're not bothered about it.

Patient HM contributed a great deal to the study of the human mind, despite the fact that he knew nothing about this. And Christmas is all about generosity, isn't it? And what could be more generous giving a great deal without even the minimal conscious awareness of it? Although that latter also describes victims of theft, so maybe it's not an ideal comparison.

Patient HM passed away almost 3 years ago exactly, with almost 6 decades of his life that he had no memory or awareness of, all of it acknowledged briefly but then discarded like the umpteenth pair of hideous socks given to you for Christmas by a doddery relative.

Patient HM; gone, but not forgotten.

…actually, given the context, that last bit is quite insensitive. Apologies.

Twitter: @garwboy



Dave Steele said...

I've never really bought this evolutionary "just so" story explanation for PTSD flashbacks. PTSD flashbacks are crippling which is no good if its primary use is to stimulate avoidance or amplify the flight response due to "better" encoded memories.
Controversially there's also the phenomenon where women who've been sexually assaulted after being drugged who don't remember the traumatic incident itself but still get PTSD.
Sorry, that was pointless and not very festive. Um...ho, ho, ho.

Dean Burnett, Neuroscientist said...

I could probably have detailed this a bit more clearly. I do see how an evolved mechanism would leads to temporarily heightened memory encoding, but the way I've written it seems like I'm saying 'PTSD is a useful evolved mechanism', which, as you say, is daft as it's essentially crippling. It's sort of like me saying the appendix evolved, so appendicitis is a useful evolved trait. Which it isn't.

That's twice I've mention appendicitis now in a piece about memory formation. Maybe my subconscious mind is trying to tell me there's a link between the two? to the lab!!!

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