Adventures in Memory. Hilde østby
Читать онлайн книгу.
In a way it’s, in very general terms, a kind of voluntary ‘memory’ reading,” Eleanor Maguire says.
So far, she can see the track in the vinyl record, but she can’t hear the music.
“The next step would be to be able to see what people remember without having decided on a fixed set of memories beforehand. But it’s a long way until we get to that level,” she assures us. We can safely leave mind reading to science fiction films and books.
Maguire isn’t doing this because she thinks memories can be reduced to a checkerboard pattern in an MRI image. To her, memories are vastly complex—they are unique experiences that can only be fully known by the one who keeps them. They are also not static. She has observed that something happens to memory traces over time: two weeks after an initial memory is encoded, its memory trace is visible in the front of the hippocampus, but much older memories from ten years previous are processed further back in the hippocampus.
“Memories contain many pieces of the initial experience that are later brought back and put together again,” she explains. “When the memories are still fresh, they are more easily accessible; we can easily picture the episode and how it happened. In the beginning, it is readily available within the hippocampus. As a memory ages, the pieces are stored in other parts of the brain and it takes more effort to reconstruct it and bring it back. The hippocampus puts all the pieces together in a coherent scene.”
But what is she actually looking at? What gives the memories a unique “signature” in the fMRI images of the hippocampus? Eleanor Maguire believes that there are groups of neurons working together on one memory.
“The fact that we can see unique patterns for each memory must mean that information about the person’s experience is present there; it has to be in some way related to the biological memory trace.”
But because the resolution of an fMRI image is extremely coarse, we can only see large groups of nerve cells activated at the same time, as opposed to individual neurons.
“While it is important to study memory on a cellular level, we should also think of a memory rather like a big cloud of activity. A memory is more than the single synapses—it is much more complex than that,” says Maguire.
To her, episodic memories are first and foremost about scenes. “All the little pieces that together make up a memory don’t mean anything unless they are placed in a scene. The action takes place somewhere.”
But as an episode is tied to a place and forms a scene in your mind’s eye, an important component of this may be a set of grids—the map within the hippocampus and entorhinal cortex. The memory is tied down by all the little synapses being strengthened through long-term potentiation. Synapse by synapse, the memories are clicked into place.
“We are hoping that our discovery can help solve the enigma of Alzheimer’s disease. Long before there are other symptoms, people with Alzheimer’s experience spatial navigation problems,” Edvard Moser says. The newest episodic memories also suffer first when the disease sets in. They go before all the knowledge we have gathered throughout life does, and also long before mature memories from long ago dissolve, like clouds of sparkling particles that swirl out to sea, never to return.
BUT WHAT ABOUT our divers? You haven’t forgotten, have you, that we sent ten men down into the ice-cold water of the Oslo Fjord at the beginning of this chapter?
The rain is dripping from the eaves of the diving center here on dry land, and we’re rubbing our cold and wet hands together in a futile attempt to stay warm, our teeth chattering. The divers are, of course, voluntary participants; nobody is forcing them to do this. Still, with only a few remaining bubbles on the surface reminding us that they are down there, it’s easy to be a bit worried. What if something were to happen? And what if they remember as poorly as, well, a jellyfish? We will return to the divers shortly, but since we brought it up: Do jellyfish remember?
“We don’t know if jellyfish remember,” biologist Dag O. Hessen says. “But jellyfish do have a kind of ‘will,’ since they swim in a certain direction, even if they don’t have a brain, only nerve fibers. However, all animals, even the simplest ones, have a certain capability to learn.”
How did human memory become as advanced as it is? Why do we remember the way we do and not the way jellyfish do? What might the alternatives have been?
“We have not been able to prove that animals have memories that work like human memories. We believe that other animals’ memories associate to a situation and pop up when they see or feel something, as when for example a cat sees a cupboard door and remembers that it hurt its tail there once,” Hessen explains.
So there’s no proof that zebras can stare melancholically into the sunset and remember the great loves of their lives, or that a dog can suddenly bark mournfully because it’s thinking of a sad episode from its youth. No gazelles cringe because they’re thinking about an embarrassing moment two years ago, no leopards experience a flash of happiness when a memory hits them of how they killed their first prey. At least, not that we have been able to prove.
“We believe that only humans do this: look back in time regardless of context. All animals and plants have some form of memory, in the sense that they adapt to the environment. It’s beneficial to learn to avoid dangers and remember how to secure food and partners. It’s obviously an evolutionary advantage for all living organisms, even for short-lived ones, to be able to remember and not only live in the moment. What’s special about humans is our ability both to see the past before us and to create visions of the future. To be able to envision the future is possibly a byproduct of memory,” Hessen believes.
The biologist suggests that there’s another reason for humans to have developed a large brain with advanced memory, something that has to do with our social groups.
“We know that social animals have larger brains and more memory than animals that aren’t social animals.”
An example: all bats are, in a sense, social animals, but vampire bats are particularly social. They live in groups, and they can’t survive for more than three days without fresh blood. According to Hessen, researchers have found that bats—sympathetically enough—help each other by regurgitating blood for others, even bats outside their family, and it seems as if they remember favors that have been done for them earlier. There’s a form of reciprocity between vampire bats that’s very similar to humans, like friendship.
“Many believe that humans have good memory because people are social animals with many hierarchies and exchanges of favors. Sympathies and antipathies depend on remembering. And the longer one lives, the more one has to remember complicated social structures,” says Hessen.
Animals that live longer remember more. An example is the elephant. It does actually remember—like an elephant.
This is just one of many anecdotes about elephant memory. In 1999, as the zookeepers in the elephant sanctuary at Hohenwald, Tennessee, introduced their elephant Jenny to a newcomer named Shirley, Jenny became agitated. Shirley also seemed more than usually preoccupied with Jenny. The two elephants behaved as if they knew each other. Upon investigation it turned out that, for a short while more than twenty years earlier, the two elephants had worked together in the touring Carson & Barnes Circus. According to Hessen, researchers that have followed elephants over long periods of time have found that elephant herds are highly dependent on good memory. The matriarch of the herd must be old enough to have the experience to lead her herd to safety if there is a fire or to find water during dry spells; younger matriarchs risk making fatal mistakes.
The elephants Shirley and Jenny acted as if they really had human-like emotional memories of each other. But memories can also be far less complicated, without being less impressive. Several animals have a kind of instinct—or memory—for time and place. Puffins return to the west coast of Norway on exactly the same date year after year, regardless of weather. American and European eels swim all the way to the Sargasso Sea to spawn. Monarch butterflies have multiple generations each year, of which only one lives long enough to migrate south and back. It’s impossible for the new generation of migrating monarchs to