We are not the only species to develop speech impediments
Birds that stutter and mice that make ultrasonic squeaks could help reveal the origins of spoken language
Moheb Costandi
Dozens of small birds called zebra finches have made their home in Erich Jarviss lab at Duke University Medical Center in Durham, North Carolina. The birds are mostly grey and white, but they have striking black-and-white stripes around their beaks and eyes hence the name.
Zebra finches are known for their complex, boisterous songs. But these finches cannot sing properly. They constantly get stuck on one note, which they repeat over and over again. In a word, they stutter.
These remarkable birds can help us understand how songbirds produce their amazing, intricate songs, and how their ability to sing first evolved. Whats more, they could even offer insight into how human language evolved.
The sweet twittering of songbirds may seem far removed from the intricacies of human speech. In one sense, that is literally true: birds are separated from us by over 300 million years of evolution. Nevertheless, our feathered friends can teach us a thing or two about ourselves.
Both birdsong and human language are learned by listening to, and imitating, others
This idea has a long pedigree. Over 2,000 years ago, Aristotle argued that birdsong might serve as a useful model for studying human speech.
In general the birds produce most voice, and with most variety, when they are concerned with mating, he wrote in Historia Animalium. He went on to note that a mother nightingale has been observed to give lessons in singing to a young bird, from which spectacle we might obviously infer that the song was... capable of modification and of improvement.
However, by the end of the 19th Century a combination of evolutionary theory and religious doctrine had led scientists to assume that other animals had only the most limited brainpower.
And yet, research published over the past two decades shows that Aristotle was probably right.
For starters, both birdsong and human language are learned by listening to, and imitating, others.
Japanese tits use syntax to combine the notes in their vocal repertoire in different ways
Each human language contains a finite set of words. These can be combined to express infinitely many meanings.
Each language has a set of rules by which words can be combined into meaningful phrases and sentences. These rules known as compositional syntax were thought to be unique to our species.
However, a study published in March 2016 showed that Japanese tits use syntax to combine the notes in their vocal repertoire in different ways, creating different meanings. Birdsong really is remarkably similar to human language.
In humans, this delicate system sometimes goes wrong and the result is impediments such as stutters.
They not only stutter but also lose their ability to learn new songs
People with a stutter involuntarily repeat syllables, prolong certain speech sounds, and often hesitate and pause. Stutters are most common between 3 and 6 years of age, when children are learning to speak.
Most children who have a stutter will eventually recover, and around 80% will be fluent as adults, says neuroscientist Sophie Scott, who studies the neural basis of vocal communication at University College London, UK. But the problem persists in the rest, and they will have a problem for life.
In the past two years, two research groups have independently created songbirds with stutters. They are the first of their kind.
In a study published in 2014, Jarvis and his colleagues used electrical currents and neurotoxins to selectively destroy neurons in one region of their zebra finches brains. This region, known as Area X, is considered to be the bird equivalent of a part of the human brain called the striatum, which becomes active when people imitate the speech of others.
What we hear as stammering is actually the person trying to cope with their disfluency
The brain damage affected the tempo of the birds songs and the sequencing of syllables within songs. The birds also repeated syllables much more often, and about a month after the damage was done they started making stuttering-like sounds.
When you do this in young animals, you prevent their ability to imitate during their critical period of vocal learning, says Jarvis. But when you do it in adults after imitation has been completed, they not only stutter but also lose their ability to learn new songs.
Not only do the birds stutter similarly to humans, Jarvis has found evidence that the underlying mechanism may be similar.
In humans, stuttering may be an unfortunate by-product of the person trying to correct an underlying speech deficit.
Quite often, what we hear as stammering is actually the person trying to cope with their disfluency, says Scott. Repetition can occur because people are trying different things to get past the problem and keep their speech going.
Humans with damage to the basal ganglia get a stutter
Something similar may be happening in the brain-damaged zebra finches. A chance discovery by Jarvis and his colleagues suggests that the birds stuttering may actually be a result of their brains trying to repair themselves.
We think its due to the brain forming new neurons, he says. When the new cells enter Area X they try to form new connections, but they dont do it properly at first, and this causes stuttering.
Jarvis thinks the same could be true of people with stutters.
Several areas of the human brain, including the striatum, produce small numbers of new brain cells throughout life. Humans with damage to the basal ganglia get a stutter, says Jarvis. I predict that this could be due to the production of new neurons in the striatum.
When these results are combined with those of the second research group, they reveal clues to how the vocal learning pathway works.
The second group, led by Wan-Chun Liu of the Rockefeller University in New York, made genetically-engineered zebra finches. The birds carried human mutations associated with Huntingtons Disease: a neurodegenerative disorder marked by involuntary and repetitive dance-like movements, and by problems with speech and communication.
They found that the entire song-producing system had been thrown out of whack
The zebra finches carrying the Huntingtons mutations grew up with profound vocal deficits. They stuttered, and struggled to imitate other birds songs. Whats more, over time both their syllables and their syntax deteriorated.
The team published their results in October 2015.
Lius team was hoping to unravel why Huntingtons affects peoples speech in the way it does. We know that neurons die in a part of the brain called the striatum, but the question is how this affects the rest of the brain.
Lius zebra finches had damage in their song-producing brain circuitry, in particular in Area X. This pattern is similar to that seen in the brains of people with Huntingtons Disease. BBC
