Brittany Bernal - Sensorimotor Adaptation of Vowel Production in Stop Consonant Contexts

The purpose of this research is to measure the compensatory and adaptive articulatory response to shifted formants in auditory feedback to compare the resulting amount of sensorimotor learning that takes place in speakers upon saying the words /pep/ and /tet/. These words were chosen in order to analyze the coarticulatory effects of voiceless consonants /p/ and /t/ on sensorimotor adaptation of the vowel /e/. The formant perturbations were done using the Audapt software, which takes an input speech sample and plays it back to the speaker in real-time via headphones. Formants are high-energy acoustic resonance patterns measured in hertz that reflect positions of articulators during the production of speech sounds. The two lowest frequency formants (F1 and F2) can uniquely distinguish among the vowels of American English. For this experiment, Audapt shifted F1 down and F2 up, and those who adapt were expected to shift in the opposite direction of the perturbation. The formant patterns and vowel boundaries were analyzed using TF32 and S+ software, which led to conclusions about the adaptive responses. Manipulating auditory feedback by shifting formant values is hypothesized to elicit sensorimotor adaptation, a form of short-term motor learning. The amount of adaptation is expected to be greater for the word /pep/ rather than /tet/ because there is less competition for articulatory placement of the tongue during production of bilabial consonants. This methodology could be further developed to help those with motor speech disorders remedy their speech errors with much less conscious effort than traditional therapy techniques. Sensorimotor Adaptation in Stop Consonant Contexts 3 The goal of this study is to gain an understanding of how typically functioning speakers respond to acoustic perturbations to their speech, and to evaluate the amount of vowel sensorimotor adaptation that takes place upon exposure to these experimental conditions in different consonant contexts. Articulatory changes were assessed by measuring formant values, which are high-energy acoustic resonance patterns that reflect the articulatory working space during the production of speech. The effects of coarticulation were analyzed by requiring participants to say the words /pep/ and /tet/. The resulting articulatory changes that took place in participants based on the varying consonant contexts were compared. The methods used in this line of research can be further developed as a potential form of neurorehabilitation for disordered speakers. Review of the Literature I: Articulatory Properties of Vowels and Consonants Vowels are typically produced with voicing by the larynx, due to the vibration of the vocal folds, but different vowels require distinct articulatory positions. Vowels are produced with an open vocal tract. On a spectrogram, a graphical display of acoustics characterizing time, frequency, and intensity, vowels have very well defined resonances or formant patterns (Shriberg & Kent, 2003). The center frequencies of the two lowest frequency formants (F1 and F2, measured in Hertz) uniquely distinguish among the various vowel sounds. Vowels are primarily lingual, meaning the tongue is the main articulator during vowel production (Hixon, Weismer, and Hoit, 2008). Thus, changes in the frequency of F1 and F2 reflect changes in lingual position. Roughly speaking, the frequency of F1 decreases as lingual height increases, and the frequency of F2 increases as lingual position advances forward in the oral cavity (Fant, 1970). The vowel Sensorimotor Adaptation in Stop Consonant Contexts 4 analyzed in this study was /e/, which is produced with the tongue positioned in the middle and front of the vocal tract. Stops are the types of consonants that were the focus of this study. Stops are classified as obstruents, which are sounds that have either a complete or narrow constriction within the vocal tract (Shriberg & Kent, 2003). Stops begin with occlusion of the airway at the place of articulation. Then, there is a buildup of oral air pressure behind the occlusion in the tracheal space. After the pressure builds up, the airway is abruptly opened and a burst of air is released. Typically accompanying this release is a burst of noise, which is prominent for voiceless stops because of the significant amount of intraoral air pressure. Voiceless stops usually are produced with aspiration that promptly follows the burst noise (Shriberg & Kent, 2003). Aspiration is noise that is generated as air rushes through the open vocal folds. Stop sounds that were analyzed in this study included /p/ and /t/. /p/ is a voiceless, bilabial stop. Bilabial signifies that the two lips are involved in the production of the sound. /t/ is a voiceless, alveolar stop. Alveolar sounds are those in which the tongue comes to contact the upper gum ridge inside the teeth (Hixon, Weismer, and Hoit, 2008). Taking into consideration the differences in the place of articulation for the consonants /p/ and /t/, there is less competition for articulatory placement of the tongue when saying the word /pep/ compared to /tet/. This is because vowels are lingual productions, whereas /p/ is bilabial, meaning that the lips are the primary articulator. Thus, the tongue can move more fluidly from one place in the vocal tract to the next rather than maneuvering through production of two adjacent sounds using similar places of articulation. Conversely, for the word /tet/, both the vowel /e/ and the consonant /t/ are lingual, because the articulatory production of /t/ places the tongue at the alveolar ridge. There is less competition for placement of the tongue in the Sensorimotor Adaptation in Stop Consonant Contexts 5 word /pep/ than the word /tet/, and the tongue has more freedom to move in /pep/. Based on the differing coarticulatory demands of the two target words in this study, it is reasonable to hypothesize that the amount of sensorimotor adaptation will be greater in the word /pep/.