We only remember few things that happened before the age of four, this part of our childhood is critical. During this time, we learn to walk, navigate human society and speak — with some help from mom and dad, of course. Zebra finches are not all that different in this respect. Only the males sing, and they learn it all while they’re still babies by attentively listening to a tutor. Scientists tapped into the brains of the zebra finches and uncovered a neural mechanism that may be key to shaping the brains of the birds, but also that of humans, in this critical stage of their development.
“For young animals, the early sensory experiences are very important and strongly affect brain development,” said Prof. Yoko Yazaki-Sugiyama and Dr. Shin Yanagihara from Okinawa Institute of Science and Technology Graduate University (OIST). “This stage is called the ‘critical period’ where the brain circuits are very flexible and can be easily changed and modified. We wanted to know how the early sensory experiences during the critical period shape brain circuits and lead to appropriate behaviours.”
Only male zebra finches learn how to sing songs. These songs attract mates, and as such are essential to reproduction. If a young can’t master the complex songs, then he has no chance of passing on his genes.
To learn the song, juvenile males attentively list to the father’s song and memorizes it. Animal behaviorists believe young finches store the song in their auditory memory, gradually adapting it as they vocalize until the birds develop their own songs, similar to the ones they’ve been tutored. This is very similar to human speech development which occurs during a critical period when children are all eyes and ears to what parents say.
Yanagihara and colleagues predicted that when the juvenile zebra finches are tutored by their fathers, the experience modifies the young brain circuits to form memories.
The team set out to confirm this prediction by looking what kind of response neurons from the higher auditory cortex had to the sound of a tutor’s song. The brains of juvenile birds were monitored when they listening to different songs — their own, the tutor’s or other zebra finches, as well as songs from other songbird species. Isolated control zebra finches were also monitored. You can listen to how these various songs sound like below.
The investigation suggests that the zebra finches respond to all sounds with non-selective neurons. There are, however, selective neurons that only ‘fire’ almost exclusively to the sound of a tutor’s song.
“In the normal, tutored birds, we encountered a group of neurons that responded very strongly to the tutor song, after they had learned the song, but did not respond to the other songs,” Yanagihara said.
“However, for the birds which had no tutor experiences, we did not see any response to the tutor-song, or in this case the genetic-father-song, and no selective neurons at all.”
Five percent of the neurons in the higher auditory cortex (27 neurons) reacted to the tutor song, indicating where early auditory memory might be located in the zebra finch brain. This is important because, for both finches and humans, these critical brain circuits are still poorly understood.
“This study gives some idea of how the brain acquires memories during the critical period,” Yanagihara said. “This is a step in understanding how the neuronal mechanisms of memory and early sensory experiences form brain circuits in the early developmental stage, not only in birds, but also in humans and other species.”