BETHESDA, Md. (May 26, 2005) -- How someone tells you to "keep quiet" affects whether or how you might comply. But what happens when you're asked to "talk louder," or you're talking and the background noise level suddenly goes up?
Purdue University researchers found that how you get louder is a function of how you're told to speak louder and environmental cues.
Far more surprising, they discovered that trying to talk louder in response to verbal or other cues involves different sets of muscles and setting internal performance goals -- all accomplished subconsciously, involving neural control of the respiratory system.
"It's entirely shifted my thinking about how the respiratory system works in speech," lead researcher Jessica E. Huber said. "We never viewed respiration as a flexible system, just whether it was efficient or not. But we found that respiratory control is very context-dependent, and changes in the linguistic or cognitive load of the speech task alters the neural control of the respiratory system," Huber added.
The study, entitled "Changes to respiratory mechanisms during speech as a result of different cues to increase loudness," appears in the June issue of the Journal of Applied Physiology, published by the American Physiological Society. The research was conducted by Jessica E. Huber, Bharath Chandrasekaran and John J. Wolstencroft, Department of Speech, Language and Hearing Sciences, Purdue University.
Rethinking childhood speech therapy, helping adults with injuries or Parkinson's
The results have the potential to greatly influence speech therapy in cases ranging from spinal injury to Parkinson's disease, cerebral palsy, multiple sclerosis, stroke or other brain injury. The study said it is "important to consider the efficiency of the patterns elicited by these cues, from a work perspective, when planning a treatment."
"Since the respiratory system provides the pressure that allows us to speak, it's critical that we understand how it's controlled," Huber noted. "From a neural control perspective it's an interesting study, but in modeling speech, understanding how kids learn to speak, and how diseases change the system, these results give us a whole new integrated system to consider," she added. "Based on our results, we think that the differential physiological mechanisms are all related to the internal model or target of what people think they need to do, at least partly based on what we told them, the cues we gave them through those instructions."
Surprising and differing results from three simple tasks
The researchers asked 15 men and 15 women in their early 20s to read short and long sentences at four "sound pressure levels" (SPL): comfortable, twice as loud as the comfortable level, 10 decibels (dB) higher than comfortable (with a visual clue for judging the SPL), and finally with artificial noise piped in but no instructions on how loud to speak.
A surprising thing happened. The average comfortable speech level was around 80dB, about the sound level of manual tools or a doorbell ringing. The 80dB was for all subjects, regardless of sex or interaction effects. Even more amazing was that in all three of the test conditions all subjects hovered right around the 90dB level, despite the different instructions, or lack of instruction. (90 dB is about what a tractor or a shouted conversation would sound like. The traditional jackhammer chimes in at about 130 dB.)
What was different was how subjects produced the requested sound pressure level (SPL):
-- 10 dB higher: subjects took a deep breath, thus increasing lung volume to take advantage of higher "recoil pressures," the natural "push" lungs provide, partly because they "snap" back into position. The researchers believe "subjects may have perceived maintaining an SPL at nearly 90dB as difficult and planned in advance to achieve this goal."
-- Twice as loud: subjects increased their expiratory muscle tension and pressure by squeezing their abdominal or stomach muscles. This was the "only condition where there were significant changes from 'comfortable' in the abdominal measurements." This large increase "may suggest that subjects underestimated the amount of pressure that would be required--.They may have realized the need for more driving pressure as they moved through the utterance," the paper reported.
-- Noise: In this more normal situation and without specific instructions on how to cope with the noise, subjects perhaps felt the most natural. The subjects used a combination of increased recoil pressure and increasing expiratory muscle tension, but less strongly than in the other situations. The paper speculates that this respiratory combination spreads "the work across a larger set of muscles--appeared to be the most efficient, and required the least muscular effort from the speaker." Indeed, participants said the noise scenario was the "easiest" in terms of producing SPL.
'Noise' prompts higher SPL, but also effort for clarity by slowing down
Most interesting of all the findings was that in the "noise scenario," all speakers slowed their speech markedly. Why? "Speakers may have perceived the need to use a slower speech rate in the noise condition to improve the intelligibility of the speech signal in the noise," the paper states. "These data suggest that goals for speech production were more complex in the noise condition than in the other two loud conditions; ie., to improve intelligibility in addition to increasing loudness."
Differences in how people perceive the task of speaking affect how they use their respiratory systems to support speech. This means researchers "need to look at specific cognitive or linguistic loads to study exactly what is changing relative to the control of the respiratory system during speech production," Huber said.
"Previously we used to debate whether the respiratory aspect of speech was context-dependent, impacted by what kind of instructions are given, but it's never been demonstrated before. Now we need to determine how changes in the linguistic or cognitive load alter the neural control of the respiratory system. We thought that the 'higher order speech components' like the articulatory system were more finely tuned to cognitive and linguistic load compared with the respiratory system, but this may not be the case. The respiratory system deserves just as much attention in the future," Huber concluded.
Source and funding
The study, entitled "Changes to respiratory mechanisms during speech as a result of different cues to increase loudness," appears in the June issue of the Journal of Applied Physiology, published by the American Physiological Society. The research was conducted by Jessica E. Huber, Bharath Chandrasekaran and John J. Wolstencroft, Dept. of Speech, Language and Hearing Sciences, Purdue University, West Lafayette, Indiana.
Research funded by National Institute on Deafness and Other Communication Disorders.
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