Vocal effort modulates the motor planning of short speech structures

Speech requires programming the sequence of vocal gestures that produce the sounds of words. Here we explored the timing of this program by asking our participants to pronounce, as quickly as possible, a sequence of consonant-consonant-vowel (CCV) structures appearing on screen. We measured the delay between visual presentation and voice onset. In the case of plosive consonants, produced by sharp and well defined movements of the vocal tract, we found that delays are positively correlated with the duration of the transition between consonants. We then used a battery of statistical tests and mathematical vocal models to show that delays reflect the motor planning of CCVs and transitions are proxy indicators of the vocal effort needed to produce them. These results support that the effort required to produce the sequence of movements of a vocal gesture modulates the onset of the motor plan.

Figure 1

Timing variables for the different consonantal transitions. Left panels show onset delays Δ and consonantal transitions τ measured from sound and spectrogram, for representative cases of the fricative-fricative (a), fricative-plosive (b), plosive-fricative (c), and plosive-plosive (d) types. In the right panels we show that vocal onset delays Δ are not significantly correlated with the normalized transitions ${\tau }^{\prime }=\tau /\mathrm{T}$ for the six fricative-fricative (a), 11 fricative-plosive (b), and nine plosive-fricative (c) combinations. On the contrary, timing variables are positively correlated for the 11 plosive-plosive combinations (d).

Figure 2

Frequency of occurrence of consonants in CCVs. Rows and columns correspond to the first and the second CCV consonants, respectively. Low [blue (dark gray)], medium [light blue (medium gray)], and high [yellow(light gray)] frequency of occurrence values of the these combinations in Spanish. Occurrences of fricative-fricative and fricative-plosive combinations present high variability, while only one plosive-plosive combination presents a high occurrence level.

Figure 3

Elements of the articulatory model used to calculate vocal effort. (a) Elements of the vocal folds model of Eq. (1)). Abduction effort is needed to separate the folds, increasing $z_0$ in Eq. (1)) to deactivate oscillations during unvoiced plosives. This is modeled as a constant $E_0$ for the group (bca, bta, dca, gta, dta) shown in green. (b) Vocal tract movements, described by its cross section $A\left(x,t\right)$, require an effort proportional to its deformations with respect to a relaxed configuration (shadowed area). (c) The transitions τ measured on the recorded CCVs are positively correlated with vocal effort $E$ computed with Eq. (3)). The distributions of CCVs with $E$ and τ are shown along the horizontal and vertical axis, respectively, for $E_0=9\%$ of the mean tract effort.