Consider a common scenario in a conversation: your friend is in the middle of asking you a question, and you already know the answer. To be polite, you wait to respond until your friend finishes the question. But what are you doing while you are waiting?
You might think that you are passively waiting for your turn to speak, but the results of this study suggest that you may be more impatient than you think. In analogous circumstances recreated experimentally, speakers move their vocal organs—i.e. their tongues, lips, and jaw—to positions that are appropriate for the sounds that they intend to produce in the near future. Instead of waiting passively for their turn to speak, they are actively preparing to respond.
To examine how speakers control their vocal organs prior to speaking, this study used real-time magnetic resonance imaging of the vocal tract. This recently developed technology takes a picture of tissue in middle of the vocal tract, much like an x-ray, and it takes the picture about 200 times every second. This allows for measurement of rapid changes in the positions of vocal organs before, during, and after people are speaking.
A video is available online (http://youtu.be/h2_NFsprEF0).
To understand how changes in the positions of vocal organs are related to different speech sounds, it is helpful to think of your mouth and throat as a single tube, with your lips at one end and the vocal folds at the other. When your vocal folds vibrate, they create sound waves that resonate in this tube. By using your lips and tongue to make closures or constrictions in the tube, you can change the frequencies of the resonating sound waves. You can also use an organ called the velum to control whether sound resonates in your nasal cavity. These relations between vocal tract postures and sounds provide a basis for extracting articulatory features from images of the vocal tract. For example, to make a “p” sound you close your lips, to make an “m” sound you close your lips and lower your velum, and to make “t” sound you press the tip of the tongue against the roof of your mouth.
Participants in this study produced simple syllables with a consonant and vowel (such as “pa” and “na”) in several different conditions. In one condition, speakers knew ahead of time what syllable to produce, so that they could prepare their vocal tract specifically for the response. In another condition, they produced the syllable immediately without any time for response-specific preparation. The experiment also manipulated whether speakers were free to position their vocal organs however they wanted before responding, or whether they were constrained by the requirement to produce the vowel “ee” before their response.
All of the participants in the study adopted a generic “speech-ready” posture prior to making a response, but only some of them adjusted this posture specifically for the upcoming response. This response-specific anticipation only occurred when speakers knew ahead of time exactly what response to produce. Some examples of anticipatory posturing are shown in the figures below.
Figure 2. Examples of anticipatory postures for “p” and “t” sounds. The lips are closer together in anticipation of “p” and the tongue tip is raised in anticipation of “t”.
Figure 3. Examples of anticipatory postures for “p” and “m” sounds. The velum is raised in anticipation of “p” and lowered in anticipation of “m”.
The surprising finding of this study was that only some speakers anticipatorily postured their vocal tracts in a response-specific way, and that speakers differed greatly in which vocal organs they used for this purpose. Furthermore, some of the anticipatory posturing that was observed facilitates production of an upcoming consonant, while other anticipatory posturing facilitates production of an upcoming vowel. The figure below summarizes these results.
Figure 4. Summary of anticipatory posturing effects, after controlling for generic speech-ready postures.
Why do some people anticipate vocal responses while others do not? Unfortunately, we don’t know: the finding that different speakers use different vocal organs to anticipate different sounds in an upcoming utterance is challenging to explain with current models of speech production. Future research will need to investigate the mechanisms that give rise to anticipatory posturing and the sources of variation across speakers.
Sam Tilsen – email@example.com
Peter Doerschuk – firstname.lastname@example.org
Wenming Luh – email@example.com
Robin Karlin – firstname.lastname@example.org
Hao Yi – email@example.com
Ithaca, NY 14850
Pascal Spincemaille – firstname.lastname@example.org
Bo Xu – email@example.com
Yi Wang – firstname.lastname@example.org
Weill Medical College
New York, NY 10065
Popular version of paper 2aSC8
Presented Tuesday morning, October 28, 2014
168th ASA Meeting, Indianapolis