Post by Anastasia Sares
What's the science?
Vocal learning is at the core of human linguistic and musical abilities, allowing us to imitate sounds produced by others and use them for communication. Only a few other species are capable of vocal learning, and songbirds are one such species. This makes the songbird an excellent model organism to help scientists characterize vocal learning circuitry in the brain. A number of brain regions have been identified as crucial to this process— a central one is Area X, within the basal ganglia (structures responsible for learning and initiating behavior). Other notable regions in the vocal learning circuit are involved in receiving auditory inputs (AIV), coordinating motor output (the RA), and processing motivation and reward (VTA). However, we still don’t have a full picture of how this ensemble functions and how different brain regions might be involved. This week in Neuron, Ruidong Chen and colleagues showed that another area in the basal ganglia of birds, the ventral pallidum (VP) was an important part of the vocal learning system.
How did they do it?
The authors combined data from anatomical tracing, electrical stimulation, lesions, and response to distorted auditory feedback to demonstrate the importance of the ventral pallidum.
To trace the connections of neurons between different regions, they used two methods. The first was retrograde tracing with viruses: in this technique, special viral proteins are modified to be fluorescent and then injected in the target region. They naturally climb backwards from the end of a neuron and cause the region of origin to light up. The second method is antidromic spiking: it’s a similar concept but with electric signals. Stimulating the target of a neuron causes electrical signals to move backwards up it, and these backwards-moving charges can be recorded at the region of the neuron’s origin. Once they mapped out the system, they tested how the VP reacted to singing and vocal errors. Again, the authors employed a two-pronged approach. First, they performed a surgery to disrupt the function of the VP (lesion) and observed its consequences on the bird’s song development. Second, they implanted recording electrodes in the VP and put birds into an enclosed environment with speakers that would play back a distorted version of the bird’s song at specific points while the bird was singing. Finally, they also played the bird’s own song back when it wasn’t producing any song, which should only activate audition-related areas.
What did they find?
The authors found anatomical and functional evidence supporting the idea of a loop in the songbird vocal learning system incorporating the ventral pallidum (Area X→VP→VTA→Area X). The VP also received inputs from a variety of vocal learning areas. Disrupting the VP in juvenile birds resulted in abnormal song learning, indicating that it was a necessary part of the learning network. Some neurons in the VP were related to auditory information in general, as they fired during song performance and during a song played back to them later. Other neurons seemed to be calculating and responding to singing errors. During singing, these neurons responded to differences between distorted sounds and undistorted sounds, but they did not respond to songs or movement in general. Signals from these error-detecting neurons were the ones that left the VP and traveled to their next stop (the VTA).
What's the impact?
This research adds the VP in the middle of a complex neural network controlling vocal learning, helping to map out its relationship with other already-established areas. Though the VP is usually thought of as a region processing emotion, reward, and motivation, the authors contend that it can act as an internal “critic,” helping the birds to continuously refine their songs. Since the basal ganglia are fairly well preserved across species, studying these circuits will help us understand what is going on in human vocal learning as well. Further research into these systems may help us to understand internally-driven learning processes more generally.
Chen, Ruidong et al. Songbird Ventral Pallidum Sends Diverse Performance Error Signals to Dopaminergic Midbrain. Neuron (2019). Access the original scientific publication here.