How the human brain can learn without thinking

May 26 2005

Watch out -- you may learn something and not even know it, says Takeo Watanabe, an associate professor of psychology at Boston University's Center for Brain and Memory. Watanabe and his team recently pinpointed the mechanism that makes subliminal learning work. Watanabe will present the team's findings at the American Psychological Society meeting in Los Angeles, May 27 and 28.

Long considered the realm of science fiction, subliminal learning occurs when individuals are influenced by a stimulus they are unaware of, like words played back below the threshold of hearing or images flashed on screen faster than the eye can perceive. Watanabe's recent findings grew out of his team's previous work in which they established that subliminal learning is real and that the brain is capable of learning without consciously focused attention.

In this latest research, Watanabe and his team uncovered the mechanism that primes the subconscious, enabling individuals to learn a task without actually realizing it. They also showed this type of learning is retained, giving a new interpretation to how long a learned behavior is retained in the visual cortex -- an area of the brain thought to be fixed very early in life.

To establish how the mechanism worked, Watanabe's team devised a series of perception tests. Initially, participants watched a computer screen as a series of letters flashed by and were instructed to signal when they saw a gray letter. As individuals concentrated on watching for gray letters, sets of dots jiggled on the screen in areas that were at the periphery of the visual field. Five to 10 percent of the dots moved together in a coherent direction -- a fraction smaller than that easily detectible by the human eye. Letters flashing on the screen were randomly paired with the moving dots.

From this test, the researchers established the length of time it took each participant to identify the direction and coherent movement of the dots on the screen.

Participants were next exposed to a similar set of computer screens, this time with the fraction of the moving dots faded to just below the level of human perception. As participants watched for gray letters to appear in the center of the screen, the imperceptible dots moved coherently just outside their field of vision. Participants were exposed again and again to the imperceptible, moving dots as they signaled the gray letters and were later tested to see if their recognition time had improved.

In a subsequent round of tests, participants showed marked improvement in the time they took to recognize the coherently moving dots. Watanabe says this improvement demonstrates the participants learned to recognize and better identify the movement during the trials, even though their attention was focused somewhere else and the moving dots were faded to the point of being imperceptible.

The experiment differed from previous studies in having participants focus on something other than the moving dots -- in this case, the letters on the screen -- while being exposed to the movement of the imperceptible dots. Watanabe says that having subjects focus on letters activated an internal "reward" pathway in their brains, priming their subconscious to learn more efficiently.

According to Watanabe, the visual cortex, the area of the brain tested in his experiments, has long been considered unchangeable in humans past 6 months of age. Watanabe found it could be "changed" and that the changes could last for a considerable period; individuals were tested again six months after the initial trials and show little or no deterioration in their ability to recognize moving dots in a visually noisy background.

"It's possible that other parts of the brain could work this way too," Watanabe says. "People might be able to improve their pronunciation of a new language, if it's presented simply, without paying attention. It's possible the brain could be changed without a lot of effort."

The Watanabe group plans to repeat their experiments using functional magnetic resonance imaging, or fMRI, to peer into the brains of participants. Using fMRI, the team will essentially be able to look directly into the portion of the brain involved in subliminal learning.