THE TROUGH EFFECT – AN AERODYNAMIC PHENOMENON ?

The trough effect is a phenomenon occuring in a VCV-sequence, where both vowels are identical and the consonant is produced with an articulator which is thought to be unspecified for vowel production. It can be described as a momentary deactivation of tongue movement when the consonant is a bilabial. Several interpretations have been given to the causes and implications of the trough with a speech material consisting of /b/ and /p/. In order to verify a possible aerodynamic effect we carried out three different experiments and included additionally /m/ in our corpus. Our results provide evidence that aerodynamic requirements play a role. Nevertheless, aerodynamics alone do not cause the troughs. HORS-D’ŒUVRE: THEORETICAL BACKGROUND The trough appears during the oral closure of the bilabial and has been described as a momentary deactivation of the tongue musculature responsible for the surrounding vowel production (Bell-Berti & Harris 1974, Gay 1975). Troughs have also been found for lip movements in /uCu/-sequences (Bell-Berti & Harris 1974, Perkell 1986), i.e. lip rounding for the vowel is deactivated during the consonant. In a recent paper by Lindblom et al. (2003) characteristics of the trough have been summarised: “(1) the trough effect clearly exists for both tongue and labial articulations; (2) the trough effect can most simply be described either as a ‘turning off’ or diminution of underlying articulator activity for the vowel; (3) the trough cannot be attributed to (aspiration induced) acoustic or aerodynamic constraints, and (4) the trough effect appears to exhibit language-specific effects, e.g. its absence in Turkish and French. ” (p.261). Lindblom et al. conclude that the trough effect would provide evidence for a segment-bysegment activation in opposition to Öhman’s model of an underlying independent vowel cycle with a superimposed consonant gesture. Although we will further concentrate on the troughs found in tongue movements, we would like to mention a possible explanation for the troughs found in /utu/ and /usu/ for lip movements for which Perkell (1986) proposed a specific target production of the intervocalic consonants. Our explanation is related to the cross-sectional area at the two lips. The cross-sectional area is very small in /u/, resulting in a high frequency damping. This damping effect is counterproductive for the production and perception of the voiceless stop or fricative. For voiceless stop production Maeda (1996) has pointed out that the cross-sectional area of the supralaryngeal articulators has to be larger than the one at the glottal level in order to guarantee the appropriate noise magnitude. Hence, we propose that lip deactivation during /utu/ or /usu/-sequences could be a strategy to increase the cross-sectional area at the lips. In opposition to what Lindblom et al. (2003) pointed out, there is evidence in the literature for possible aerodynamic effects on the tongue surface in a comparable dataset, even though it comes from a totally different perspective. Svirsky et al. (1997) tried to answer the question of Fuchs et al. The Trough Effect From Sound to Sense: June 11 – June 13, 2004 at MIT C-26 whether oral cavity enlargement in the production of voiced stops is an active or passive mechanism. By means of EMA (4 subjects) and combined EMA/intraoral pressure recordings (1 subject) they observed tongue displacements and estimated tongue compliance during the production of /ama/, /aba/ and /apa/. Svirsky et al.’s results provide evidence for a significantly larger tongue displacement, i.e. tongue lowering in /b/. There was less lowering for /p/ and no significant differences from zero for /m/. In terms of the trough effect this could question its existence in /ama/. The intraoral pressure differences in Svirsky et al.’s result were in the p>b>m direction. The authors suggest “that tongue deformation may be passive and pressure driven for /apa/ and that active vocal-tract expansion is superimposed on the passive, pressure driven deformation for /aba/” (p.570). Another intriguing aspect has been brought up by Vilain et al. (1999, 2000). They modeled /aba/ sequences with 2 linear anthropomorphic articulatory models which were based on 2 cineradiographic corpora of 2 French speakers. Using the first model, Vilain et al. found that “strikingly, [b] appears here to necessitate a complete recombination of the articulators.the new position of the tongue compensates for the high position of the jaw implied by the lip occlusion. And this new combination cannot be considered as an anticipation towards a new [a] configuration, since the second [a] is produced exactly as the first one was. The recombination is therefore specific for the consonant. ” (Vilain et al. 1999, p.2). With the second model (differing only in vocal tract morphology) tongue and jaw are not recombined, both articulators contribute to the vowel articulation and also to the bilabial with a less open jaw position. These findings support the hypothesis that the trough would have its origin at a motor control level and that tongue and jaw movements involved in the vowel production are not unspecified for the intervocalic consonant. The theoretical implications of the trough effect seem to be remarkable and have often been discussed in the literature with respect to coarticulation, anticipation and speech motor control. We were interested in investigating the following questions: (1) Do troughs occur in VCVsequences where C is a nasal? If so, an aerodynamic effect due to intraoral pressure differences could be excluded, since in nasals intraoral pressure does not rise as in voiced or voiceless stops. (2) Can troughs be explained by a recombination of tongue and jaw movements (as in Vilain et al. 1999, 2000)? If so, what happens when tongue-jaw coordination is perturbed due to a bite block? PLAT PRINCIPAL: PRELIMINARY RESULTS FROM AN EMG EXPERIMENT By means of hooked wire Electromyography (EMG) and acoustics we recorded 3 native speakers of German and 1 bilingual speaker of Japanese and German. Activity of the following tongue muscles was registered for all subjects: Genioglossus posterior (GGP), Genioglossus anterior (GGA), and Styloglossus (SG). The speech material consisted of VCV-sequences where V was either /i/, /a/ or /u/ and C /p/, /b/ or /m/. Each target word was embedded in the carrier sentence: Habe X besucht (Visited X) and repeated up to 17 times. Root mean square amplitudes were calculated and smoothed with a cut-off frequency of 25Hz. In a next step ensemble averages of the GGP signals were calculated by time-warping each consonant to the average length of the consonant in each vowel context, and then extending the averaging window by 100ms before the onset and after the offset of each consonant. Ensemble averages for GGP activity were lined up with the consonant onset, defined as the F2 offset in the acoustic data. Preliminary results from one subject for GGP activity are shown in figure1. The left graph corresponds to /u/-context and the right to /i/-context. Fuchs et al. The Trough Effect From Sound to Sense: June 11 – June 13, 2004 at MIT C-27 -100 0 100 200 50 100 150 200 250 GGP lined-up at consonant onset