PSYCHOLOGY

Take a Walk on the Blind Side

Brain Requires Time to Make Connections Through Nonconscious Processing

In 1884, the Russian czar Alexander III commissioned the great goldsmith Peter Carl Fabergé to craft the first of a series of enameled eggs, each opening to reveal an unexpected object—a hen, a cherub, and even a replica of the Kremlin. Using their own version of the Fabergé eggs, Jeffrey Ellenbogen, Matthew Walker, and their colleagues have been probing a mental quality that lies at the root of human creativity—the mind’s power to draw connections between different areas of knowledge and experience. They have recently uncovered a few surprises of their own.

Graham Ramsay

“There’s a danger in thinking that what’s in our minds is all that’s going on,” said Matthew Walker (right), shown with Jeffrey Ellenbogen.

Many have assumed that the ability to see patterns in the welter of human experience is the product of conscious attention and thought. What Ellenbogen, HMS clinical fellow in medicine at Brigham and Women’s Hospital, Walker, HMS assistant professor of psychology in the Department of Psychiatry at Beth Israel Deaconess Medical Center, and colleagues have found is that such inferential knowledge may be hatched outside the glare of consciousness, during a period of nonconscious, or offline, processing.

The researchers came to their conclusion after asking 56 students to perform a simple inference task. Subjects who were tested shortly after an initial learning period performed, as a group, no better than chance. Groups of subjects tested after a period of at least 12 hours had a much higher success rate—nearly 80 percent of their inferences were correct. And those tested after a night’s sleep were able to draw more distant connections. The findings appear in the May 1 Proceedings of the National Academy of Sciences.

“Even we were a little taken aback by the size of this benefit,” said Ellenbogen.

Failure at an apparently simple task may not be something any of the subjects—who were all from Harvard—are familiar with. But it may be that the first group can blame biology. “They just don’t have the ability to make these leaps of judgment yet,” said Walker. “The aperture of memory hasn’t opened up.”

Intriguingly, the more successful, later test-takers reported feeling no more confident of their answers to the inference task than the first group. This raises all kinds of questions. What was going on in their minds that enabled them to make correct inferences? How exactly does the aperture of relational memory open? And how did sleep enhance that ability? Finally, could offline processing be impaired in certain psychiatric illnesses such as posttraumatic stress disorder (PTSD) or schizophrenia?

Hard Day’s Night
Walker first became aware of the power of offline processing during his research on the effects of sleep on learning motor tasks, such as a finger-tapping sequence. Subjects would initially learn the motor sequence by breaking it down into chunks, which made for a choppy performance. “But when you brought them back after a night of sleep, it was as though those problem points had been smoothed out,” he said.

In attending lectures and reading journals, he and Ellenbogen found that researchers studying relational memory had devised an experimental paradigm similar to the one they were using for motor-skill learning. In this “transitive inference” paradigm, subjects are presented with chunks of information such as A and B, B and C, C and D, such that A is greater than B, B is greater than C, and so on. Subjects may be trained to learn the information over the course of several days. They are then tested on the relationship between pairs they have not seen before, such as B and D. Most of these subjects answer correctly.

Reasoning that people rarely encounter information in such a repetitive manner—and suspecting from their previous work with motor memory that the correct answers might have more to do with the extended time course of the experiments than the learning periods—Ellenbogen, Walker, and their colleagues took a different approach. They created a series of hierarchically related objects—six elegantly colored ovals. “We call these our Fabergé eggs,” Walker said. They presented the ovals two by two to 56 college students (see figure below). In each case, the students were asked to identify the higher ranked oval—a smiley face appeared each time they guessed correctly. Once the students had learned the relationships between the six individual “premise pairs,” they were divided into four groups.

Members of the first group were tested 20 minutes after the initial learning period—not just on the premise pairs but also on never-before-seen pairs. These students were unable to infer which was the more valuable oval—the percentage of correct answers was the same as if they were guessing. The next group learned the pairs in the morning and were tested 12 hours later. More than 80 percent of their answers were correct. The same success rate applied to the third group, which learned the pairs in the evening, slept, and took the test in the morning; and to the last group, which learned the pairs in the evening, slept, and took the test the next evening.

Though the additional benefit of sleep did not show up in the simple B and C inference task, it was evident in more difficult inferences. Subjects who slept were much more likely to see the relationship between more distantly separated ovals, such as B and E.

Adapted from original courtesy Matthew Walker

“Fabergé egg” hunt. Six hierarchically ranked ovals with unique designs (bottom) were paired consecutively (top). The pairs were presented to student subjects, who were asked to identify the higher ranked of the two. A smiley face appeared after correct identifications (top middle). Subjects—brought back either 20 minutes, 12 hours (with or without sleep), or 24 hours later—were tested not just on the original pairs, but also on the relative rank of never-before-seen pairs such as B and E (top right). The students’ ability to identify the higher ranked ovals on the retest increased markedly with adequate time away from the task.

Disease Links
Walker believes the hippocampus, known as the gateway to memory, almost certainly plays a central role in this offline processing. Bits of information enter the curlicue-shaped structure and are relayed to different regions of the cortex. “Think of them as balloons with strings, all bound by the hippocampus,” he said. The hippocampus replays the memory, out of the light of consciousness, which strengthens the synaptic connections between cortical regions. Eventually, the ties to the hippocampus are cut, opening the door to a universe of potential inferences.
“As soon as you start embedding bits of information within the cortex, you’re not only connecting them to each other, you’re connecting them to this vast store of additional information within the cortex,” Walker said.

On the flip side, difficulties with sleep, which occur in aging and in schizophrenia, could diminish the time spent processing offline, and in this way, cause memory problems. Conversely, people with PTSD consciously replay painful memories over and over, possibly because of defects in offline processing. “It’s interesting that a hallmark of PTSD is repetitive nightmares,” Walker said.

“There are a lot of steps that need to take place for us to understand what this means for patient populations, but we’re excited to explore them,”
said Ellenbogen.

—Misia Landau

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