Birds burst into song as you step into the room. Behind you, the door closes with a small metallic click and the birds’ song fades as the buzzing of electric lights and machinery begin to overpower it. Despite the white noise, you notice that one bird sounds different from the others. This bird is one of the test subjects in research conducted by scientists at
The study, published online in Neuron on January 13, 2010, is the first to identify an auditory feedback pathway in the brain that controls vocal learning. This research lays the foundation for improving human speech in people whose auditory nerves are damaged and who must learn to speak without the benefit of hearing their own voices.
At this stage, using humans in experiments is impossible, but these songbirds are working well as substitutes. Amazingly, the brains of songbirds and humans function similarly during the process of vocal learning. Richard Mooney, PhD, Duke professor of neurobiology and senior author of the study, said, “The problems that juvenile birds solve when they learn a song from a tutor bird are similar to the problems humans solve when we learn to speak, and birds and humans exploit similar neural systems to reach this solution.”
The team sought answers to how the brain programs and channels auditory feedback to shape the vocal performance in juvenile birds that are learning to sing.
Initially, the scientists identified the neurons in the songbird brain that convey the auditory feedback needed to learn songs. Then they passed a brief pulse of electricity through the implanted electrodes to alter the brain’s activity associated with one of the notes that the pupil was learning to sing.
Three interesting dynamics emerged. First, the distortion in the bird's singing was delayed and showed up anywhere from hours to weeks after the bird first heard the electrical noise pulse in its song. Second, the distortion always came in the same place in the bird’s song, at the exact point in the song where the electrical "noise" was introduced. Third, by disrupting neural activity at different stages of the learning process, they determined that the distortion effects were strongly age-dependent. Younger birds sometimes showed changes within an hour while older birds kept singing properly for a while but eventually changed over a period of weeks.
Mapping the brain pathways for this auditory feedback might eventually make it possible to find a way to stimulate those pathways in humans. Speaking intelligibly when you cannot hear is not an easy task and one which can only be accomplished after years of exhausting training. If an easier way is found, it could help to facilitate communication between people who can and cannot hear.