A new study published in Science shows how a neural circuit in Zebra finches, involved in forming the learning of a courtship song, can be manipulated. Scientists effectively implanted a new song in the mind of the birds, in the absence of an elder tutor bird.
Zebra finches learn mating songs from elder tutors, usually a mother to daughter and father to son. Song lessons are critical for juvenile birds because it can affect their ability to find a mate.
Memories are two-fold, making them and then storing them. This study focused on the first step by attempting to disrupt the brain during tutoring to see if it would inhibit the memory of the lesson. Cells in the neural circuit responsible for song-learning were genetically engineered to be sensitive to light exposure. Then birds were subjected to interval light exposure during lessons.
The researchers set up two experimental groups. In the first group of juvenile birds, brain cells were activated with light while they spent time with their tutor. The second group of juveniles was isolated from tutors all together. For reference, cells in a control group were activated when they were alone, and during typical tutor sessions. Surprisingly, it was the control group that showed an exciting development.
The birds in the control group sang a unique song. It was different from the songs sung by birds who had never met a tutor and different from the birds who had interacted with a tutor with light interference. It was unclear where this alternative tune came from, so researchers investigated further.
A second experiment stimulated juvenile birds with light for 50-300 milliseconds over five days, during the time that they would typically be interacting with a tutor. Except, during these lesson times, no elder bird was present. Researchers found that the length of the notes in the courtship song corresponded to the amount of light exposure the birds received. For example, the birds that received short intervals of light exposure held their notes close to 50 milliseconds. Birds subjected to more prolonged light exposure held their song notes for longer. All of this occurred without the juveniles being taught the song.
Researchers also measured the quantity of information conveyed in the song. They found that birds who received traditional tutoring, and birds who received short light exposure conveyed about the same amount of information in their courtship song. However, birds who received prolonged light exposure conveyed less information.
Finally, they tried exposing birds to the light before meeting a tutor and comparing it to birds who were only subjected to light exposure during or after lessons with a tutor. If stimulated before meeting a tutor, the juvenile would not attempt learning from the tutor, but if met tutor first or during exposure, they had no problem learning song. This test is more evidence that the neural pathway is involved in making a memory of the lesson, but not storing it.
Throughout all of these experiments, other aspects of courtship songs, including pitch, harmony, how frequently juveniles practiced, and timeline of vocal development were not affected. Learning courtship songs is a very complicated process, and each of these elements may be processed by separate neural networks.
The next step for researchers is to find out how memories of song lessons are stored.