Early learning induces brain changes used later in life
BY WILLIAM A. WELLS
Lessons learned early in life can, at least in owls, leave a permanent mark on the brain. This allows an adult owl to relearn a task it learned early in life, though the same task can never be learned by an adult who has not had such training as a juvenile.
So concludes Eric Knudsen, Sewall Professor of Neurobiology, in a research report in the March 6 Science.
"Changes in the brain that are induced by early experience result in a persistent effect that can be reused later in life," Knudsen said. "The learning that occurred early can be expressed later on."
Parts of the human brain may work in similar ways, he said. This would explain the critical nature of childhood learning, which may lay down structures in the brain needed to tackle tasks encountered only much later in life.
Knudsen's task for his owls was locating objects emitting sounds. Owls naturally determine the position of a squeaking mouse or a chirping cricket by computing when the sound from the object reaches the owl's ears. If the sound reaches the left ear first, the cricket must be to the left of the owl. The brain then directs a head movement so that the owl is staring straight at the cricket, ready to make it dinner.
For owls under Knudsen's care, however, the task may be complicated by a pair of glasses. The prism glasses shift the owl's world view, so that an owl looking straight ahead sees objects to its right. Responses to chirping crickets are shifted to compensate for the change. A young owl learns that a sound coming from the right demands a straight-ahead glare, whereas a sound from straight ahead necessitates a head movement to the left.
Knudsen had previously established that owls can learn this modified behavior only early in life, by forming new brain connections. The connections link two spatial maps in the owl's brain one based on sounds, the other on vision.
In the new study, Knudsen reexamined owls' ability to adapt to the glasses late in life. As he had observed in earlier studies, two older owls with no training were unable to modify their behavior appropriately. But three other adult owls, who had been trained as juveniles and then readapted to life without glasses, quickly relearned the altered head-turning demanded by the glasses.
Young owls still have an advantage over the older, trained owls, Knudsen said. The older owls can relearn only the exact task they encountered as youngsters. If given glasses that shift their view even farther to the right, they adapt poorly or not at all, and they are completely flummoxed by glasses with a left-shift. Young owls can adapt to all of these challenges and more.
The results suggest that a marker specific to the learned task is laid down for later use, Knudsen said. His best candidate for the marker is the altered nerve cell connections that he has observed in previous studies. "We hypothesize that it is this anatomical change that results in the behavioral change," he said, "but that's a hypothesis that needs to be tested."
Knudsen believes that when the glasses go on or come off, the brain picks up on its own errors and uses the errors to teach itself to adapt. "There has to be a signal what we call an instructive signal that says the system isn't working and then corrects the system," he said. In the juvenile owls the brain can reprogram itself in any way, but in the older owls Knudsen suspects the brain can switch only to pathways previously laid down.
Knudsen knows what part of the brain is involved in the owl's learning task, so he can test the response of single cells as the glasses are changed. Although other animals lack this experimental advantage, they show similar types of learning, he said. Certain songbirds, for example, must learn their songs at a critical period early in life, although they do not start singing until they are sexually mature.
In humans, the use of cochlear ear implants suggests that the interpretation of language must be learned early in life. These implants stimulate nerves directly, bypassing the hair cells in the ear whose absence or poor functioning is at the root of some forms of deafness. Implant recipients who became deaf as adults are quick to interpret the new "sounds," but individuals who have always been deaf find the sounds more confusing, said Knudsen, whose recent owl study was funded by the National Institute on Deafness and Other Communication Disorders.
Only future work will show whether
the owl findings have parallels in adults who are relearning
language or in children learning school lessons, he said.