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The act of hearing is a group effort for the human body's organs, involving the ears, the eyes and also, according to the results of a new study, the skin.
In 1976 scientists discovered the importance of the eyes to our sense of hearing by demonstrating that the eyes could fool the ears in a peculiar phenomenon named the McGurk effect. When participants watched a video in which a person was saying "ga" but the audio was playing "ba," people thought they heard a completely different sound—"da." Now, by mixing audio with the tactile sense of airflow, researchers have found that our perception of certain sounds relies, in part, on being able to feel these sounds. The study was published November 26 in Nature.
Normally when we say words with the letters "p," "t" and "k," we produce a puff of air. This puff helps the listener distinguish words with these letters from those with the similar sounding "b," "d" and "g," respectively, even though the puff is so subtle that most of us do not even notice feeling it. "Unless you're a microphone manufacturer or a radio jockey or a phonetician, this isn't something that you're aware of," says Bryan Gick, an associate professor of linguistics at the University of British Columbia in Vancouver, and lead author of the study. Donald Derrick, a graduate student in the University's Department of Linguistics, is the other author on the study.
Gick and Derrick set out to determine if these puffs of air help us to perceive "p" and "t" sounds. The pair had 66 participants listen to sessions of recorded sounds through headphones. In one session, the participants heard a combination of "pa" and "ba," and, in the other, "ta" and "da."
The researchers also sent light bursts of air from thin tubes placed over participants' skin, over either their hand, neck or in their ear. The participants were blindfolded so they did not know where the tube was placed. In some cases, puffs were released with the appropriate sounds, "pa" and "ta," and in other cases, they coincided inappropriately with "ba" and "da." As Gick notes, the puffs were about half as forceful as what we would feel in a normal conversation, and most participants were not even aware of them over the course of the experiment.
The researchers found that if there was no air puff, participants misheard "pa" for "ba" and "ta" for "da" 30 to 40 percent of the time. The accuracy improved 10 to 20 percent when an air puff over the hand or neck accompanied "pa" and "ta." No improvement occurred, however, if an air puff was sent through the tube in the ear, suggesting that the participants were not simply hearing the airflow.
The opposite effect was observed when the participants received an air puff with the inappropriate sounds— "ba" and "da." While subjects correctly identified these sounds in about 80 percent of cases when played without the release of air, the accuracy decreased by about 10 percent if the sounds were accompanied by puffs of air.
"Largely, in English, the difference between 'pa' and 'ba' is this puff of air," Gick says.
The ability of the skin to contribute to hearing could be due to the fact that the largest organ in the body is covered in mechanoreceptors. Gick says that he has even found that air puffs sent to the ankle can help the listener comprehend those "p," "t" and "k" sounds. These receptors in skin cells, which are similar to the ones in the ears, respond to the pressure created by airflow.
In the real world, the cues available to a listener vary. Standing a foot or closer to someone speaking normally should produce tactile puffs, Gick says. However, if the conversation were taking place on a windy street, this sensory input would be destroyed. Although people can hear sounds in the absence of airflow, these sensory cues could make it easier to distinguish between two words, such as "tall" and "doll," especially if there is a lot of ambient noise.
The feel of sounds could be exploited in devices for groups such as the hearing impaired. Gick is in the early stages of exploring how to incorporate into hearing aids airflow-detecting sensors that would produce a synthetic puff to the side of the neck. Because the skin mechanoreceptors among the hearing impaired typically function normally, Gick says, this additional tactile stimulus could help the person wearing the device perceive sounds. A similar concept could aid pilots in their noisy work environments.