photomontage: jetheriot
banana image courtesy: Fir0002/Flagstaffotos
If your plastic doll lost one of her legs in a playground accident, you could pop it back into her hip socket. Even if her leg were badly mangled by a lawnmower or melted off with a blowtorch, you could order a replacement, theoretically, from a toy warehouse in China. Snap the new leg on – she’s as good as she ever was.
Your leg would be, if not as good as new, pretty darn good. Just ask Oscar Pistorius, the South African runner who, despite being born without a complete pair of legs, competed at the London Olympics, holding his own against athletes running on their native legs.
Prosthetic legs, prosthetic teeth, prosthetic hands, prosthetic faces – we’ve become proficient at replacing parts of ourselves. Wouldn’t it be nice to have replacement parts for the brain too? Wouldn’t it be nice to be able to walk into a big warehouse stacked from floor to ceiling with prosthetic memory storage units, pull a cartridge down from the shelf and snap it into your brain?
Is that even possible? Several aspects of human memory make it a challenging candidate to target for replacement with a prosthetic brain part. For starters, which part of the brain would be replaced? It’s not like memory lives in a single region of the brain. That’s not how memory works. There’s nowhere in a brain I could put my finger on and say, “This is where memory lives.”
Memory networks stretch from one side of your brain to the other, down your spinal cord and into the nerves that infiltrate your body from head to toe. Memory isn’t a circumscribed region of your brain. It’s not an organ or an appendage. Memory is the very “backbone” of the entire nervous system that animates a human organism.
And memory isn’t only one thing. There’s a memory for facts, a memory for movements, a memory for recent occurrences, a memory for remote events . . . How can you replace a brain part if you can’t even put your finger on it?
But some brain parts are more necessary than others, and you have to start somewhere. Dr. Berger, a professor of biomedical engineering at the University of Southern California, decided to start with the hippocampus. He and his team set out to create a prosthetic hippocampus, and, at least in a small way, they succeeded.
The first step in replacing a brain part is figuring out exactly what that part does. The hippocampi – there are two of them, one on each side – are brain ridges rolled deep inside each of your temporal lobes. They’re one of the most well-studied areas of the brain, and while we don’t know exactly everything they do and how they do it, we know that you have to have at least one working hippocampus to be able to hold on to new information and remember it as a lasting memory.
Without a functioning hippocampus, I could pop a pink balloon right in front of your face, and five minutes later, you wouldn’t be able to tell me what color the balloon was, or that a balloon had even been popped.
Tucked and rolled delicately into the interior of your brain, your hippocampi are exceedingly fragile. They’re more sensitive to fluctuations in oxygen than most of your other brain parts, so they’re susceptible to damage when the brain is starved of oxygen, for example, during cardiac arrest. I’ve treated patients whose only main cognitive impairment after prolonged cardiac arrest was the inability to convert short-term experiences into long-lasting memories. In other words, the drop in oxygen wasn’t severe enough to damage the entire brain. Only the sensitive hippocampi sustained injury.
Hippocampi are also vulnerable to the effects of chronic alcohol use, and Alzheimer’s disease takes an early toll on them as well. Even a minor blow to the head can have a serious impact on hippocampal function. When they’re damaged, the hippocampi shrivel up to such an extent that the shrinkage can be seen and measured on an MRI. And unfortunately, hippocampi aren’t like lizard tails. If you lose one, you can’t just grow another.
So you can see why a prosthetic hippocampus would come in handy. Giving someone the ability to hold on to new information, the ability to remember again, could be the difference between a person requiring around-the-clock supervision and being able to live independently.
Dr. Berger chose the prosthetic hippocampus as his object of study, not only because of its crucial role in memory, but also because its anatomy has been so well studied. We know a lot about how hippocampi are wired, and to a lesser extent, what they do.
When a pink balloon pops in front of your face, that event triggers a certain response in you and in your nervous system, a response that echoes through your hippocampi. And when your hippocampi are working, the balloon popping can be recalled at a later point. The popping of the balloon, a brief moment in time, has been made sticky somehow, so that instead of being forgotten, it sticks around, and you remember it.
The prosthetic hippocampus Dr. Berger devised wasn’t meant to replace an entire hippocampus. That would have been too enormous an undertaking. The device was meant to replace a small segment of hippocampus only. After recording the electrochemical chatter of a rat’s healthy hippocampus, Dr. Berger selectively damaged a small section of it, rendering the hippocampus non-functional.
In rats, the hippocampus is shaped like a banana, so if you imagine a healthy banana with a rotten stretch in the center, you get the picture. Two healthy yellow banana tips on each end, a brown segment in the center. His task was to re-wire a route around the damaged section of the banana, to bridge the two still viable tips of the banana together through an external computer chip – the artificial hippocampus – which would mimic the native banana. It would do whatever the damaged section of the banana it bypasses used to do.
His “artificial hippocampus” was an array of precisely arranged wires and electrodes connecting the viable tissue at one end of the rat hippocampus, through a computer chip, to the viable tissue at the other end of the hippocampus, a kind of detour around a damaged section of the native hippocampus, around a broken stretch of neural highway.
The function the short stretch of damaged rat brain provided when it was healthy is now mimicked through artificial means, which is quite a remarkable achievement. Despite lacking a functional native hippocampus, rats were able to make new memories using this new computer chip, this replacement part, at least for the part that remembers to press levers.
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A prosthetic limb needs to be in physical contact somehow with the rest of the body it attempts to complete: suction cups, a strap, velcro, stretchable skin, something to keep the two parts together.
Prosthetic legs provide a cosmetic function, often going unnoticed beneath a pair of jeans, but their primary purpose is to provide an adequate structure for balance and locomotion. A prosthetic leg is a continuation of the body’s skeleton – it just happens to be a removable one – so it must be connected with the rest of that skeleton somehow. Attachment systems for prosthetic legs typically involve some kind of stump-and-socket system. On rarer occasions they’re semi-permanently attached via titanium bolts.
Prosthetic limbs provide a structural function, and physical contact with the body is necessary. For example, the crucial balancing service a big toe provides can only be provided by a prosthetic toe that connects with the skeleton it’s balancing. In other words, your prosthetic big toe wouldn’t work if you kept it in your purse. Prosthetic limbs don’t operate at a distance.
But as long as it retained neural contact with the brain, there’s no reason a prosthetic hippocampus couldn’t. Instead of a rat with an artificial hippocampus, pretend it’s you who has one. Let’s say you blew out your hippocampi during a prolonged cardiac arrest. You can’t remember anything anymore, and you can’t remember that you can’t remember, so you’ve been outfitted with a prosthetic hippocampus. Wires, much longer wires than those used in the rat experiment, criss-cross your skull and penetrate your brain and connect to the artificial hippocampus you carry around in your pocket.
The cord stretching from your head to your mid-section is a few feet long, allowing for some slack. The circuitry bypassing your damaged hippocampus, perhaps a bit more circuitous than the course the rat’s wires took from its brain to its prosthetic hippocampus, is safely housed in a fiber-optic cable emerging elegantly through tiny holes carefully drilled into the back of your head, sprouting like a golden ponytail, neither fragile nor prone to tangling. Your golden ponytail connects your brain with an artificial hippocampus humming softly in your pocket.
With your prosthetic hippocampus, you see a balloon pop, and five minutes later you’re able to recall that it was pink, and that a balloon had popped. You’re able to remember again.
We can take it one step further. What if your pocket hippocampus communicated with your brain, not through precisely arranged wires, but through precisely arranged wireless transmissions? It’s not so far-fetched. That’s how prosthetic cochleas work. They’re implanted in the skull and communicate with the brain using wireless transmissions.
A prosthetic hippocampus with wireless capability would be highly desirable, removing, as it does, the need for tiny holes to be drilled through your skull, never a great idea. With wireless, it wouldn’t matter where you kept your artificial hippocampus, as long as it was in neurologic contact with the brain. Closer would be better, but as long as it communicated clearly and accurately over the air waves, it could be in Saskatchewan.