Mar. 6, 1998 STANFORD -- Lessons learned early in life can, at least in owls, leave a permanent mark in the brain. The mark allows an adult owl to re-learn a task that 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.
These are the conclusions of a study to be published in the March 6 issue of Science by Eric Knudsen, Professor of Neurobiology at Stanford University School of Medicine. The work was funded by the National Institute on Deafness and other Communication Disorders.
'Changes in the brain that are induced by early experience result in a persistent effect that can be re-used later in life,' said Knudsen. '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 only encountered 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 tasty 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 owl's brain then directs a head movement so that the owl is staring straight at the cricket, ready to make it dinner.
If the owl is under Knudsen's care, however, its 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 sounds from straight ahead necessitate 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 and the other on vision.
In the new study, Knudsen re-examined the owl's ability to adapt to the glasses late in life. As he observed in earlier studies, two older owls with no training were unable to modify their behavior appropriately. But three owls, who had been trained as juveniles and then re-adapted to life without glasses, quickly re-learned the altered head-turning demanded by the glasses.
The young owls still have an advantage over the old, trained owls. The older owls can only re-learn the exact task they encountered as youngsters. If given glasses that shift their view even further to the right, the older owls 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, said Knudsen, that a marker specific to the learned task is laid down for later use. His best candidate for the marker is the altered connections between nerve cells that he has observed in previous studies. 'We hypothesize that it is this anatomical change that results in the behavioral change,' said Knudsen, '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 re-program itself in any way, but in the older owls Knudsen suspects the brain can switch only to pathways that have previously been 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. 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.
Only future work will show, said Knudsen, whether the owl findings have parallels in humans who are re-learning language or in children learning school lessons.
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