his cortex were strengthened, until they could manage without the help of the hippocampus. This process may take many years. That’s why Henry didn’t remember anything from the last couple of years prior to the surgery. The memories from this period were simply too unstable and dependent on the hippocampus. For a long time, it was believed that this was the full explanation and that the hippocampus wasn’t necessary at all when it came to recalling early memories. But then researchers, like Eleanor Maguire and others, started to notice that things happen in the hippocampus when we retrieve a memory.
They didn’t question whether or not Henry’s memories were real, but they did point out that a memory is not just a memory. A memory may have turned into a story that includes facts about what happened, not unlike an anecdote. On the other hand, a memory can also be something completely different: a re-creation of the experience, filled with sensory experiences, emotions, and details of how the episode unfolded in time and space. Henry’s memories were probably more like the first kind, resembling book knowledge or simple tales, called semantic memory. He seldom gave particularly detailed descriptions of his childhood. Often, the stories began with “I used to … ,” followed by facts about where he’d gone to school, where he’d vacationed, and who his family was. He possessed a rather dry encyclopedia about himself. Presumably, he could not recall lifelike, smelly, noisy, emotional memories. After having known Henry for years, researcher Suzanne Corkin was convinced that his memories lacked the vividness so characteristic of episodic memories. BACK AT GYLTE Diving Center, we’ve split the divers into two groups and numbered them from one to ten. The divers are completing their first memory test, the one we use for comparison, measuring their normal memory. The men are visibly sweating over the twenty-five words we gave them to remember. Not only because the test is hard; they have to look at their list of words for two minutes, then go for a little walk and return to the table to write down what they remember. But with the diving gear already halfway on, they are hot and perspiring more than they might like. The divers manage to remember between six and seventeen correct words, completely normal results.
That day by the fjord, the rain on our skin feels like pins and needles of nervousness as the first group goes down into the water. What if we don’t find out anything at all? What if the men are diving in vain and don’t get to prove anything about memory and context?
Of course, we can’t go through life relying on our surroundings to help us remember everything. Godden and Baddeley also pointed out that this was an unreasonable idea. In An Essay Concerning Human Understanding (originally published in 1689), the philosopher John Locke described a man learning to dance in a room with a large trunk. He could do the most elegant dance steps, but only as long as the trunk was there. If he was in a room without the trunk, he was hopeless on the dance floor. This sounds very strange, and fortunately the story probably isn’t true. It highlights, though, the idea of context-dependent memory. The point Godden and Baddeley made was that our memory may rely on context to a certain degree. Can this be useful in some way? Should we cram for an exam in the location where we’ll be taking it? Or remain in the same apartment until our dying day for fear of losing the memories that have been made there?
Fortunately, we do have access to our memories when we are not in the same environment as where we experienced an event. The divers at Gylte can recount the amazing experiences they’ve had in the water even when they are safely on shore.
Our memory networks—our fishnets of memories—benefit from context beyond just our physical surroundings. We create the strongest memory networks on our own, when we learn something truly meaningful and make an effort to understand it. Someone who is passionate about a particular subject, such as diving, will more easily learn new things about diving than about something she’s never been interested in before. This is because she already has a large memory network devoted to diving where she can store her new knowledge, and because she is motivated. It’s as if we can add another layer of netting just because the self is involved; memory is self-serving. Memories are linked to what concerns you, what you feel, what you want. Too bad, then, that so much of what we actually need to remember is so darned uninteresting!
Lately, others have tried to test context-dependent memory in other ways. Do we remember things we learn while skydiving? The researchers concluded that the stress level of skydivers was so high that it erased all effects of context. This may not be so strange—if we are so high on adrenaline that we barely notice where we are, there are no surroundings to support those memories. More practical were the researchers who wanted to examine if medical students remembered more when they were in the classroom where they had first been taught. The classroom, in this case, was either an ordinary classroom or an operating theater, where the students were dressed for surgery. Fortunately for the future patients of those medical students, it turned out in the experiment that the differences were so minimal that doctors can safely continue practicing medicine far from the context of learning.
In our experiment at Gylte, we split the divers into two groups. The divers in the first group would be tested on what they remembered on land after trying to memorize twenty-five words underwater. The others had to both learn and recall the words underwater.
The five divers in the first group come ashore, splashing as they go. They wriggle out of their masks and flippers, unhook leaden oxygen tanks and sit, legs spread apart, on the bench along the wall of the Diving Center.
Their results are miserable.
One of them remembered only words from the first test—the one for comparison—and got a zero on the underwater words. The best one remembered thirteen words from the list he saw underwater, but this too was worse than he’d done during the first test on land. The average result of the comparison test, inside the Diving Center, was 8.6 correct words. The divers remembered an average of 4.4 words when they emerged from the water.
“I sort of thought I had the words there while I was underwater, but then we got up on land, and it was as if my mind changed completely, and I lost it,” one of the divers says.
The removing of flippers, tottering up from the edge of the pier along the walk to the Diving Center, lifting the tanks from their backs, and grabbing a piece of paper may of course have disturbed their trains of thought and pushed the words out of the way. Duncan Godden and Alan Baddeley had pondered this possibility and tested whether all the trouble of getting back onto dry land could have thrown off the result. They let one group of divers learn the words on land, then dive and come up again, and compared them with a group who’d learned the words on land and waited the same amount of time, but without moving. The group that dived in the middle remembered just as much as those who had remained still in the same place. So all the hassle of changing location could not have explained why the divers who learned in the water remembered less on land.
Deep beneath the surface, the divers in the second group have taken out their flashlights and waterproof notepads that make it possible for them to write underwater. Bubbles from their breathing pop on the water’s surface; they are fifty or fifty-five feet down, and it’s hard to handle the plastic-covered sheet to write the twenty-five new words. They’ve gathered in a circle in the dark, and short flashes from the flashlights tied to their arms shine through the water every time they move their hands and write. Like the words they learned on land, these are mainly one-syllable words: short and concrete and easy to write with gloves on.
This group had remembered an average of 9.2 words when they were tested in the Diving Center. But what happened when they tried to learn twenty-five words underwater and were supposed to remember them underwater? As the bubbles grow bigger and the divers slowly rise to the surface, those of us on the pier are long since soaked through and clinging to empty and wet paper coffee cups. Even the seagulls have stayed at home today.
The divers, on the other hand, are not in a hurry. They rest for a while a few feet under the surface before they get out of the water. Around us, clumps of old snow lie between tufts of rotten grass. Our excitement has been building this whole
