WE EXPERIENCE something—as we find ourselves in a specific situation at a specific location—and it becomes a memory in the brain, it spreads out across the cortex until we recall it. A memory is composed of thousands of connections between neurons; it is not one connection that makes a memory. A memory is more than Terje Lømo’s long-term potentiation.
But what does a memory look like? Can we see a complex memory the way we can see a simple memory trace? To be able to do this, we must exit the rabbit and mouse brains and enter the human brain. And we must watch the brain while memories are recalled. Fortunately, we don’t need to sedate humans and open their heads to get a glimpse of their memories. As we learned in chapter 1, Eleanor Maguire at University College London has used an MRI scanner and some reminiscing volunteers to observe the traces of their memories as they relive their past experiences.
An MRI machine uses a strong magnetic field to take pictures of the body. Different body tissues react differently to the magnetic field, which results in detailed images. The MRI machine can be programmed in a certain way to read the oxygen level in the blood flowing through the brain. Since neurons use oxygen to function, we can tell from the images where there’s a lot of activity. We then know where in the brain nerve cells are most active while the test subjects remember things. This is called functional magnetic resonance imaging (fMRI)—images of the brain while it is working—as opposed to structural MRI, which shows us only what the brain looks like. The memories light up like tiny flashlights underwater, flashes that light up the sea in little spurts.
But is it really possible to see what memory a person is recalling? In Eleanor Maguire’s laboratory, participants allowed themselves to be scanned by an MRI machine at the same time as they were asked to remember their own experiences. The professor actually managed to figure out what they remembered by studying the fMRI images. Maguire watched the activity in the hippocampus while the test subjects were thinking of episodes from their past, and she could see that each memory had a unique pattern of activity. She had a computer program that learned which of the test subjects’ memories were tied to which patterns of activity. From that, the computer program could pick out which fMRI images hung together with which memories.
Is this simply a mind-reading machine?
“These are memories we had agreed with the volunteers, before the scanning took place, that they would recall, not random memories. 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
