While the birds are awake and singing the neurons fire in a pattern that is unique to the note and syllable components of each bird's individual song. When the awake bird hears its own song, these neurons do not fire in response.
But in the sleeping bird listening to a recording of its own song, the neurons do fire in the pattern identical to song production, though the bird produces no sound. This pattern of firing during listening, like the pattern of firing necessary to produce song, actually anticipates the next song "syllable," or set of notes.
"The learned song is a temporal code that uses the nerve impulse spikes of single cells in precisely matched patterns for hearing and singing. The two patterns can be 'mapped' to each other with spike-by-spike precision," said Margoliash. "The bird is using the preceding sound to predict how to generate the next syllable."
Understanding how patterns of behavior are represented in the brain has been a major problem for neurobiologists.
"Previously we found that during singing song is represented as a temporal code. Now, much to our surprise, we find this correspondence in single cells of matched sensory and motor patterns. Forming this mapping of sound and action is the process of learning," said Margoliash.
During undisturbed sleep, the researchers discovered, the neurons spontaneously fired the same complex song production patterns in bursts. Interestingly, these activity patterns were at slight variance, as if the bird was rehearsing a variety of slightly different songs, sometimes with slower or faster tempos.
How does the bird learn to correct its song when by the time it hears it the neuron is now engaged in the production of the next sound? Practice during sleep may be part of the answer.
"In contrast to the prevailing idea that it learns by making moment-to-moment adjustments, we think the bird stores the song production pattern and reads it out at night, an
Contact: Jeanne Galatzer-Levy
University of Chicago Medical Center