How Birds Sing

Humans do it, whales do it, but it’s the beautiful creatures we call songbirds who are perhaps the true masters of the art of singing. The complex neurological functions that allow birds to create their lovely songs gave Arielle Spellun, freshly minted graduate of Penn’s Biological Basis of Behavior program, her senior honors thesis.

Spellun worked with Assistant Professor of Biology Marc F. Schmidt to study how the song system works in the brain of the zebra finch, examining in particular the role of a part of the brain’s neural motor pathway called the robust nucleus of the arcopallium (RA, for short).

In humans, the two hemispheres of the brain communicate and coordinate with each other through an interconnecting structure known as the corpus callosum. But birds lack a corpus callosum, making the workings of complex processes requiring input from both sides of the brain a mystery. Each hemisphere of a songbird’s brain contains a complete song system pathway that controls one half of the bird’s double-sided vocal organ, the syrinx, with each side in perfect sync with the other. But what happens if one side of the bird’s song system isn’t functional?

Spellun’s work built on previous experiments in which ibotenic acid was injected to knock out the left or right RA in adult birds and also in juveniles, who were still developing neurologically and learning their songs from adults. As expected, disabling an RA in either hemisphere of an adult zebra finch’s brain permanently affected its singing ability (though not eliminating it completely). But the young zebra finches affected by an RA lesion before their songs were fully learned and crystallized fared quite differently, maturing into adult birds with a fully functional song system.

“Like humans, there’s a critical period of [neurological] plasticity," Spellun explains. “If you lesion the RA prior to song crystallization and then compare the adult song to a normal adult zebra finch, there aren’t any changes. So we hypothesize that maybe there’s some sort of crosshemispheric plasticity that makes up for the fact of the RA being lesioned in the juveniles, and that something is going on there to correct for losing that single RA.”

Spellun and her colleagues looked for signs of crosshemispheric changes that might explain the recovery of song in finches that were given RA lesions as juveniles. They injected fluorescent tracers into the RAs of normal birds and lesioned birds, using confocal microscopy to examine and compare the RA projections (connections between the cerebral cortex and the rest of the nervous system). The current experiments have provided the first quantitative evidence that these projections are strongly ipsilateral (situated on the same side as their RA), as previous studies had speculated.

Although Spellun’s active involvement with the project has ended with her recent graduation, the zebra finch research will be continued by Schmidt and his colleagues, including senior researcher Judith McLean and student Emma Kaplan, C’14. Aside from providing fresh insights into the beautiful mysteries of birdsong, the work could stimulate new ideas in other areas. “There’s a lot of homologies between song learning and language learning,” Spellun points out. “Although our research doesn’t directly connect to it, there’s a lot going on in trying to see how we can take what we know about the song system and maybe using it to understand why some children aren’t able to learn language, or why some language pathologies and problems develop.”

Issues of communication and learning have always fascinated Spellun. Taking a sign language course inspired her to set up an ASL (American Sign Language) student club at Penn, and also “got me interested in doing something with research with communication, animal communication. It helped me to incorporate that into my life.” She also found time to start a student-run tutoring program in several West Philadelphia elementary schools. “We work with kids who are a couple or more grade levels behind in reading. I knew I always wanted to be a pediatrician, but it’s actually turned my focus toward working in community medicine in urban populations.”