Stammering speech, or stuttering, has traditionally been thought to be an indication of anxiety or stress. A large amount of evidence, however, has long supported the idea that it must have some genetic component. For instance, identical twins often both stutter, as do other family members. In 2008, scientists at the National Institute of Deafness and Other Communication Disorders identified that those who stuttered often had a mutation in the gene Gnptab. The discovery was particularly interesting, because, previously, the gene had only been believed to be related to “general housekeeping” of the body, such as digestion. How could a gene that affects every cell of the body cause something so mechanical as stuttering?
To further investigate, the researchers decided to duplicate the same Gnptab mutation in mice. While human speech is complex, much of it involves the control of breath and the timing of muscles in the mouth and tongue. As Tim Holy, one of the senior researchers on the study said, “Those kinds of things may be shared all the way from mice to people.” Recording the affected young mice for 3-5 minute intervals, the team noticed that these mice produced a greater frequency of single-syllable squeaks with longer pauses between them when compared with other mice. While not technically the same as stuttering, it is similar to how the hesitations that break up the speech of people who do stutter. Needless to say, the results were encouraging, and carried the promise of potentially finding assistance for millions of people who suffer from the affliction.
Mice, of course, make noises for a reason. When they are in pain, when they meet other mice, or when they want to attract a mate are all reasons why mice use sound to communicate. As the group studied the mice sounds, they discovered that the mice emitted ultrasonic sounds, particularly when the mice detected sex pheromones from a potential mate. These sounds were similar to those heard from whales, dolphins, and birds. But how exactly were mice producing these ultrasonic noises? The general consensus was that was that it must be a mechanism similar to either the whistle of a tea kettle or the resonance from the vibration of vocal cords. Neither would prove to be correct.
A team of scientists from across the world, including those from Cambridge University, Washington State University, and the University of Southern Denmark, used high-speed video cameras (up to 100,000 frames per second!) to make images of the mice larynx and saw something very interesting. The vocal folds remained completely still while the mice produced the ultrasonic sounds. Instead, what was happening was that the mice were pointing a small air jet coming from the windpipe against the inner wall of the larynx. Such a production of ultrasonic sound is completely different than what has been observed in any other animal.
“This mechanism is known only to produce sound in supersonic flow applications, such as vertical takeoff and landing with jet engines, or high-speed subsonic flows, such as jets for rapid cooling of electrical components and turbines,” Dr. Anurag Agarwal, study co-author and head of the Aero-acoustics laboratories at Cambridge’s Department of Engineering, said in a press release about the research. “Mice seem to be doing something very complicated and clever to make ultrasound.”
Future work will look at how the mice use their brains to control the muscles producing these sounds. The study will also help researchers refine the way they look at “mouse speak” as an analogy for human speech. Scientists will be better able to identify which mutations cause changes in vocalization and what the relevant changes in brain structure are for these.
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