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Can Zebra Finches Tell Us How We Learn to Talk - and Walk
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A baby zebra finch
| Talking the Talk
Stringing It Together
Singing In the Lab
The Learning Curve
Who's Your Daddy?
The Recording Studio
The Cutting Room
A Sensitive Shell
Different Brains, Similar Pathways
Better Understanding, Better Treatment
References
Additional Sources
Glossary
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Talking the Talk
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Baby songbirds learn to sing much the same way that human infants learn to mimic their parents’ speech. Both start out babbling. Eventually, through trial and error, they master the syllables and rhythms of their parents’ sounds, or vocalizations.
Charles Darwin noted this similarity in his 1871 book, The Descent of Man. He called birdsong the nearest analogy to language in the animal kingdom. Both humans and songbirds must learn from an adult how to use their vocalizations to communicate. Almost all other animals make sounds instinctively. A kitten will meow whether raised by cats, cows or dogs. Deaf dogs growl and bark just like hearing dogs.
Only a few other animals must learn their vocalizations: bats, whales, and three types of birds (parrots, hummingbirds, and songbirds). Of those, songbirds, particularly zebra finches, are easiest to study in the lab. Scientists can record their songs to analyze, and compare those recordings to brain activity. Conveniently, the zebra finch sings a short song that is always the same and is simple enough to study in detail.
No wonder scientists flocked to the zebra finch as a model for studying speech perception and production in humans. More recently, though, researchers see song learning as a way to model many other forms of complex human behavior. For example, speech and song are motor behaviors. (They involve controlling the muscles of the vocal tract, such as the throat and mouth.) For neuroscientists, then, bird song is an easy-to-record-and-measure way to study the brain basis for learned movements in general.
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Stringing It Together
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Speech and song also exemplify the trial-and-error learning we must master before performing any task involving a sequence of movements, according to Michael Fee, a neuroscientist in the McGovern Institute for Brain Research at the Massachusetts Institute of Technology who studies zebra finch. For example, infants first flail their arms and legs, like a muscular babbling, before they figure out how to wave bye-bye, drop toys from the highchair and, one day, dribble a basketball. They must perfect each tiny movement and string them together in just the right sequence with just the right timing. In fact, most of the activities that we value are learned through similar practice-makes-perfect ways.
Discovering the neural basis for these learned actions is easier in songbirds than in humans. These discoveries can help scientists understand and better treat conditions as diverse as stuttering and delayed speech to movement disorders and certain neurodegenerative diseases.
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Singing In the Lab
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The first thing scientists observed about song learning is that it follows a predictable pattern. The baby zebra finch listens to its father’s song, remembers it, and practices over and over to imitate it. (Only male zebra finches learn their songs.) At first, the baby makes the erratic vocalizations, or subsong, that Darwin compared to babbling. The baby must compare its own sounds to its memory of Dad’s song, because it experiments with new versions and gradually improves. Soon, it sings a more structured, less variable version, called a plastic song. This song resembles Dad’s, but still has errors, similar to how a toddler may mispronounce words.
Around three months, that practice pays off. The bird has mastered the adult song, which it will sing for the rest of its life without variation. This unchanging version is called a stereotyped song. To hear and see these changes in recordings from Frank Johnson’s lab at Florida State University, visit http://www.psy.fsu.edu/~johnson/johnsonlab/johnson.htm#the%20animal]
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The Learning Curve
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Sarah Bottjer, a neuroscientist at University of Southern California, loves birds, but she studies birdsong because she wants to understand how experience and learning change the brain. The brain is changeable, or plastic, because new experiences can re-wire its circuitry, setting up new connections among neurons. The younger you are, the more plastic your brain and the more easily you can learn new things! That may be why it is hard to teach old dogs new tricks, and why children under the age of 14 can learn a second language more easily than adults.
Of course, adults continue to learn many new things, so brain remodeling happens throughout life. However, certain types of brain remodeling must happen “now or never” – during sensitive periods of development. At these times, animals are primed to learn new skills and the brain is easily molded by experience.
For example, zebra finches have a sensitive period for song learning. They must learn their song between around 4 to 12 weeks of age. Otherwise, they will babble throughout their lives.
Vocal learning in humans has a sensitive period, too. Scientists think that young children raised without hearing adult language could not learn to speak recognizably later. It would be unethical to conduct such an experiment, though! That is where songbirds come in handy. Bottjer and other scientists use this sensitive period to study how plasticity works at the level of brain pathways and individual neurons.
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Who's Your Daddy?
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To see how learning changes the brain, scientists in some labs sometimes have a songbird from a different species “adopt” a baby zebra finch, something called cross fostering.
When first hatched, the hatchling naturally homes in on the sounds of its own species and tunes out other species. But a zebra finch raised by a Bengalese finch will copy the foster dad’s songs. Still, the youngster will sing the song with the phrasing and timing of its native species, as if speaking with an accent. This means that there is some genetic influence. But experience shapes brain wiring (patterns of connectivity) during learning, and that makes song learning a tool for learning about the brain’s plasticity.
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The Recording Studio
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To trace how song learning influences brain wiring, researchers compare audio recordings of the song itself with electrical recordings of neurons firing during different stages of learning and singing. Graphs representing the audio recordings, called spectrograms, show the frequency of sound over time. Scientists analyze them for features that match the syllables, or individual units, of the song.
The neural recordings show neurons tracking along with the syllables, like a bouncing ball following a score. At each point, a set of neurons fires and then quiets down while the next set cues up.
These recordings from many different labs over the years reveal that different brain regions have different responsibilities for birdsong. One set forms a learning pathway that remembers what dad’s song sounded like and tries to match it. Another set forms a motor pathway charged with making the sounds.
These learning pathway’s brain regions go by the acronyms LMAN, DLM, and Area X. The motor pathway contains regions known as the high vocal center (HVC) and RA, which send signals to the muscles that produce the sounds.
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The Cutting Room
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To find out how these two pathways work during different stages, scientists inactivate or eliminate specific brain regions to see the effect on song learning and production.
In this way, Bottjer discovered that LMAN is very important for learning in young birds, but not so much for adults who already know the song. In juveniles, eliminating LMAN makes all learning stop. The song is frozen in time and will never mature. However, pulling the plug on this region causes little problem in adults. Thus, the bird needs LMAN to refine its song during the sensitive period for vocal learning, but not beyond.
Building on this work, Fee, the MIT scientist, showed why LMAN is so important. Just as someone bumping your arm while you play piano could change your tune, bursts of neurons in this region jar the motor pathway that is producing the song. This disturbance generates the variations on a theme that the young bird compares to the tutor’s song. Fee thinks this type of “exploration” is essential to learning and wonders if something similar happens in a similar part of the human brain that helps us be creative.
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A Sensitive Shell
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Meanwhile, Bottjer continued to work on how LMAN manages to refine the song. She found an important clue when her lab discovered that this region contains a specialized shell that doubles in size during the sensitive period for song learning. After the bird has learned the mature song, the shell shrinks back to its original size.
Neurons in the shell make a series of connections with another learning region (DLM). These connections create a feedback loop to LMAN. Bottjer thinks this feedback allows the young bird to compare sounds of its own vocal effort with its memory of its tutor’s song. If the juvenile’s song sounds like Dad’s, he keeps it. Otherwise, he tries, tries, and tries again. The feedback tells the juvenile, “Oh, I sound bad! I need to try something different,” or “Yes! I’ve nailed it!”
Manipulating this feedback by delaying it or adding artificial sounds produces something similar to stuttering in birds. In other words, changing the auditory experience rewires the bird’s neural circuits to change song behavior. Other learned behaviors, such as learning to fly in birds or learning to walk or talk in humans, require similar feedback mechanisms.
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Different Brains, Similar Pathways
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Surprisingly, many scientists are finding that these song learning pathways resemble pathways in the human brain that control learned behaviors. For example, Area X is analogous to the human basal ganglia, which is involved in trial and error learning and habit formation. LMAN is similar to the prefrontal cortex (PFC), which is the seat of reasoning, concentration, temper and personality. Likewise, regions that control song production resemble human motor centers.
This realization has put to rest the historical bias that birds are stupid, according to neuroscientist Erich Jarvis at Duke University Medical Center. The view that a bird’s brain is dominated by primitive structures that drive instinctive behaviors derived from 19th century “father of neuroanatomy,” Ludwig Edinger. He thought that birds have only a thin cortex, the outer folds of the brain that we use for higher functions of perception and complex motor control. Now scientists realize that the cortex also dominates the songbird’s brain. Its learning to produce songs is anything but instinctual and involves complex interactions between the cortex and other brain regions.
Thus, studying the song system provides basic details about how humans learn and produce complex behaviors, too. Bottjer believes that better understanding of the interactions of these pathways in songbirds will also help explain how plasticity works in general. Plasticity is important not just during early development, but also in aging and in the brain’s response to injury and disease.
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Better Understanding, Better Treatment
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One benefit of studying birdsong is an improved understanding the neural basis of human speech, speech disorders, and auditory processing. About 3 million people in the United States stutter and 8-9% of young children have speech disorders. All told, about 8 million Americans have some language impairment.
Also, many human disorders arise from disruptions to the brain regions involved in learning and producing complex sequential behaviors. For example, Parkinson’s disease (PD) arises when specialized neurons in the basal ganglia begin to die. Among other symptoms, patients lose control of learned movements. PD afflicts 500,000 plus Americans but will increase as the population ages.
The basal ganglia is also involved in obsessive compulsive disorder (OCD), which affects 2.2 million American adults age 18 and older in a given year. Sufferers do not receive the signal that a sequential task is completed, so they repeat it over and over. The first symptoms of OCD often begin during childhood or adolescence.
These diverse disorders have no cures and few effective treatments. That is partially because they are still poorly understood. The National Institutes of Health supports many studies of Bottjer’s and other birdsong neuroscientists because better knowledge of how birds learn and produce complex behaviors will provide the basis for better treatments for these human disorders.
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References
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- Bottjer, S.W. (2004). Developmental regulation of basal ganglia circuitry during the sensitive period for vocal learning in songbirds. Annals of the New York Academy of Sciences, 1016, 395-415.
- Bottjer S.W., Brady J.D., & Cribbs B. (2000). Connections of a motor cortical region in zebra finches: relation to pathways for vocal learning. Journal of Comparative Neurology, 420 (2), 244-60.
- Bottjer S.W., & Johnson F. (1997). Circuits, hormones, and learning: vocal behavior in songbirds. Journal of Neurobiology, 33(5), 602-18.
- Jarvis E.D, Güntürkün, O., Bruce, L., Csillag, A., Karten, H., Kuenzel, W., et al. (2005). Avian brains and a new understanding of vertebrate brain evolution. Nature Reviews Neuroscience, 6, 151-159
- Nottebohm F. (2005). The Neural Basis of Birdsong. PLoS Biology, 3(5), e164.
- Ölveczky B.P., Andalman A.S., & Fee M.S. (2005). Vocal experimentation in the juvenile songbird requires a basal ganglia circuit. PLoS Biology , 3(5), e153.
- To a zebra finch: How the brain cultivates birdsong. (2005). PLoS Biology, 3(5), e162.
- Aronov. D, Andalman, A.S., & Fee, M.S. (2008). A specialized forebrain circuit for vocal babbling in the juvenile songbird. Science 2008, 320, 630-634.
- Listen to an interview with Michale Fee about this paper on the Canadian Broadcasting Centre's science show, "Quirks and Quarks" http://www.cbc.ca/quirks/archives/07-08/may03.html (2nd item at this link)
- Listen to the journal Science’s podcast of Michale Fee discussing this study.http://podcasts.aaas.org/science_podcast/SciencePodcast_080502.mp3
- Listen to the difference between a baby zebra finch’s babbling and an adult’s song:http://web.mit.edu/~aronov/www/babbling/
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Additional Sources
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- National Institute on Deafness and Other Communication Disorders (NIDCD) http://www.nidcd.nih.gov
- National Institute of Neurological Disorders and Stroke (NINDS) http://www.ninds.nih.gov/index.htm
- State of the Science Conference: Developmental Stuttering, March 21-23, 2005http://www.nidcd.nih.gov/nidcdinternet/Templates/InternetContentPage.aspx?NRMODE=Published&NRNODEGUID=%7b896947A4-C4B5-45FA-A5F6-2B418877D0F1%7d&NRORIGINALURL=%2ffunding%2fprograms%2fvsl%2fstutteringwrkshop%2ehtm&NRCACHEHINT=NoModifyGuest#5
- Speech and Language Developmental Milestoneshttp://www.nidcd.nih.gov/health/voice/speechandlanguage.asp
- About Parkinson’s Diseasehttp://www.ninds.nih.gov/disorders/parkinsons_disease/parkinsons_disease.htm http://www.ninds.nih.gov/disorders/parkinsons_disease/detail_parkinsons_disease.htm http://www.nlm.nih.gov/medlineplus/parkinsonsdisease.html
- About Obsessive Compulsive Disorderhttp://www.nimh.nih.gov/health/topics/obsessive-compulsive-disorder-ocd/index.shtml
- About Stuttering http://www.nidcd.nih.gov/health/voice/stutter.htm
- About Speech and Language Disorders http://www.nidcd.nih.gov/health/statistics/vsl.asp#2\http://www.nlm.nih.gov/medlineplus/speechandcommunicationdisorders.html
- PBS: Nova, “Bird Brain” http://www.pbs.org/wgbh/nova/sciencenow/3214/03-brain.html
- PBS: Nova, Profile: Erich Jarvis http://www.pbs.org/wgbh/nova/sciencenow/3214/03.html
- Avian Brain http://avianbrain.org/
- Art Arnold Laboratory, University of California at Los Angeles http://www.physci.ucla.edu/html/arnold.ht
- Michale Fee Lab, Massachusetts Institute of Technology http://web.mit.edu/feelab/
- Erich Jarvis Lab, Duke University Medical Center http://www.jarvislab.net/
- Frank Johnson Laboratory, Florida State University http://www.psy.fsu.edu/~johnson/johnsonlab/johnson.htm
- Richard Mooney’s Lab, Duke University http://www.neuro.duke.edu/faculty/mooney/
- Marc Schmidt’s Lab, University of Pennsylvania http://www.bio.upenn.edu/faculty/schmidt/marc/
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