3.1. Boundary effects on kinematic measures

Boundary effects on each of the four measure types (DUR, T-to-PKVEL, DISP, and PKVEL) are plotted in two sets of graphs in Figure 3. First, the bar graphs show the mean values obtained from each boundary level of each closing and opening gesture, separately for the Info-Structure levels (new or given). Second, line graphs are provided for simplified presentation of the magnitude of the boundary effects, visualizing Δ(final-medial) values (i.e., the mean difference between the boundary levels) in each level of Info-Structure, so that a larger Δ value indicates a larger boundary effect. The results of LME models for the main effects of Boundary and Info-Structure are summarized in Table 2. See Appendix 1 for the results of other control predictors included in the model (i.e., Consonant sequence, Vowel sequence, their interactions with Boundary, and interaction between Boundary and Info-Structure). In Figure 3, significant Boundary effects are indicated by 3 levels (*<.05, **<.01, ***<.001) for the sake of information, but with no a priori implication that p-values below 0.05 can be used for assessing the robustness or the size of the significance. The figure (and Table 2a–b) also contains %-increase, indicating the mean relative increase in each measure from IP-medial to IP-final condition. In the presentation of the results, gestures labelled ‘C1-closing’ and ‘C1-opening’ are associated with C1 in C₁VC₂V(C₃), and those labelled ‘C2-closing’ and ‘C2-opening’ with C2 in C₁VC₂V(C₃). For C3 in the closed syllable context, only ‘C3-closing’ is included as explained in the method section.

Duration (DUR, Figure 3a) shows Δ(final-medial) significantly above ‘0’ in all gestures of CV.CV and CV.CVC words. The distribution of PBL over the bisyllabic words can be characterized as being ‘progressive’ in that the magnitude of PBL (estimated by the %-increase and β in Table 2) is progressively decreasing from the right edge. Notably, Δ(final-medial) in DUR decreases with a substantial drop from C2-opening to C2-closing as the gesture becomes distal from the boundary. As can be seen by comparing the left and the right panel in Figure 3a, such a drop was much steeper in the open syllable condition where C2-opening was the right most gesture, than in the closed syllable condition where C2-opening was not the rightmost gesture. It was also noticeable that even the most distal C1-closing gesture and the following C1-opening gesture in both C₁VC₂V and C₁VC₂VC₃ show a significant PBL effect on the movement duration (DUR), though with a more attenuated magnitude. This suggests that PBL in Korean can extend to the leftmost closing gesture in bisyllabic words.

Another durational measure Time-to-PKVEL (acceleration duration), however, indicates more restricted distribution of PBL. As can be seen in Figure 3b, it shows a significant boundary effect only on those gestures that belong to the final syllable whether open or closed, and there is no distal effect on the gestures (C1-closing, C1-opening) of the first (non-final) syllable. (This will be discussed in relation to the scope of the temporal modulation by the π-gesture in the discussion section.) Within the final syllable, a progressive decrease of the magnitude in duration from the right edge is also evident in Time-to-PKVEL, with a substantial drop from C2-opening (into the following vowel) to C2-closing (from the preceding vowel), though in a lesser degree.

Displacement (DISP, Figure 3c) and Peak velocity (PKVEL, Figure 3d) also show a progressively decreasing trend from the right edge, though not clearly to the leftmost distal gestures. As can be seen in Figure 3c, the Boundary effect on DISP is not significant in initial gestures (i.e., there was no effect on C1-closing and C1-opening in C₁V.C₂V, and C1-closing in C₁V.C₂VC₃). But Δ(final-medial) in DISP decreases (again with a substantial drop from C2-opening to C2-closing) as the gesture becomes distal from the boundary. These displacement results indicate that PBL is accompanied by substantial spatial extension. As for PKVEL, the significance of Δ(final-medial) is limited to gestures that are proximal to the boundary (i.e., only at the final gesture (C2-opening) in C₁V.C₂V and at the three gestures that belong to the final syllable (C2-closing, C2-opening, C3-closing) in C₁V.C₂VC₃) with relatively smaller %-increase values compared to the effect on DISP. Thus, the PKVEL results show faster articulatory movements at the IP-final position compared to the IP-medial position for proximal gestures near the prosodic juncture, although there is not a clear correspondence between the variation in PKVEL and the durational measure (DUR) of PBL.

When examining the bar plots, one can notice a difference in the nature of the progressively decreasing trend from the right edge between the spatial measure (DISP) and the temporal measure (DUR). As can be seen in the lower panels (bar plots) of Figure 3a, DUR values in IP-medial conditions consistently stay low throughout the entire target word. In contrast, in IP-final conditions, DUR values gradually increase toward the right edge, illustrating the waxing boundary effect. DISP exhibits somewhat different patterns. Notably, in the first gesture (C1-closing gesture), the overall DISP values are relatively larger in both IP-medial and IP-final conditions compared to the following gestures within the target word, possibly indicating a word-initial strengthening effect. Interestingly, in the IP-medial condition, the degree of DISP continues to decrease toward the end of the target word, indicating a continuous phrase-medial reduction. Conversely, in the IP-final condition, the degree of DIPS begins to increase from the second gesture (C1-opening gesture) onwards, reaching its maximum displacement at the phrase-final gesture, showing articulatory strengthening on the right edge. However, when considering the mean differences (Δ) presented in the upper panels of Figure 3c (and the β values provided in Table 2), the progressively increasing trend in DISP towards the right edge still appears to originate from the initial C1-closing gesture and extend to the right edge. Thus, it appears that this increasing pattern, contributing to significant boundary effects in the spatial dimension, results from both the gradual increase in articulatory strengthening represented by DIPS in the IP-final condition and the gradual reduction observed in the IP-medial condition.

These results, taken together, indicate some similarities and differences between CV.CV and CV.CVC. As shown in Figure 3a, PBL spreads leftward to a similar extent irrespective of the presence or absence of an additional gesture of C3-closing in C₁V.C₂V vs. C₁V.C₂VC₃. In particular, the magnitude of PBL on the C2-closing gesture is comparable (i.e., 44%-increase and 47%-increase in C₁V.C₂V and C₁V.C₂VC₃, respectively). On the other hand, the magnitude of the PBL (208%-increase) for C2-opening when it is rightmost in C₁V.C₂V (i.e., in the absence of C3-closing) is clearly greater than that of either C2-opening (105%-increase) or C3-closing (123%-increase) in C₁V.C₂VC₃.⁶ It appears that the effect of PBL on the final syllable is comparable regardless of whether the final syllable is open or closed. Interestingly, however, Time-to-PKVEL, as can be seen in Figure 3b, shows a somewhat different pattern—i.e., the magnitude of PBL reflected in Time-to-PKVEL is comparable on the C2-opening gesture (33%-increase vs. 38%-increase in C₁V.C₂V and C₁V.C₂VC₃, respectively), while it was much larger on the rightmost, C3-closing gesture (109%-increase) in C₁V.C₂VC₃ than on the rightmost, C2-opening gesture (33%-increase) in C₁V.C₂V. Thus, there appears to be some differences in temporal distribution as a function of the syllable structure or whether the rightmost gesture is consonantal opening (C2-opening) versus closing (C3-closing). Nevertheless, what remains invariant is a general pattern of progressive effect (i.e., the more proximal the gesture is to the boundary, the larger temporal expansion the gesture shows in a gradient fashion regardless of whether the final syllable is open or closed).

Another noticeable difference as a function of syllable structure (or whether the final gesture is an opening or a closing one) can be observed with other non-temporal measures (i.e., DISP and PKVEL), which indicate much more pronounced boundary effects on the lip closing movement into the coda consonant in the closed syllable. As can be seen in Figure 3c–d, the magnitude of the boundary effect on DISP and PKVEL for the C2-opening gesture is far greater in the closed syllable (C₁V.C₂VC₃) context (118%-increase in DISP and 54%-increase in PKVEL) compared to that in the open syllable (C₁V.C₂V) context (58%-increase in DISP and 18%-increase in PKVEL). Moreover, the magnitude for each measure for the rightmost gesture (C3-closing) in C₁V.C₂VC₃ (closed syllable condition) is also far greater than for the rightmost gesture (C2-opening) in C₁V.C₂V (open syllable condition). The rightmost C3-closing gesture in C₁V.C₂VC₃ shows much more boundary-induced spatial expansion (160%-increase in DISP) along with much faster movement (87%-increase in PKVEL) compared to the rightmost C2-opening gesture in C₁V.C₂V (58%-increase in DISP and 18%-increase in PKVEL). These results indicate that the boundary effect on the gestures that constitute a final syllable is larger in magnitude when the syllable is closed (or when the final gesture involves a closing movement) than it is open.

Additionally, as summarized in Table 2b and also seen in Figure 3a–b, the effects of Info-Structure on the temporal expansion appear to be much weaker and less systematic as compared to the Boundary effects. The duration measure (DUR) showed a small but significant increase only in two cases (i.e., on C2-closing in the open syllable (C₁V.C₂V) condition and on C3-closing in the closed syllable (C₁V.C₂VC₃) condition). Similarly, Time-to-PKVEL showed a significant increase only in two cases that are different from the effects on DUR (i.e., on C2-closing in the open syllable (C₁V.C₂V) condition and on C1-closing in the closed syllable (C₁V.C₂VC₃) condition). Moreover, as can be inferred from the β coefficients and %-increase provided in Table 2, even for these sparsely observed significant effects, the magnitude in the focus-induced increase (e.g., about 3% increase in DUR for C2-closing in C₁V.C₂V) was far smaller compared to the boundary-related increase in duration (e.g., about 44% increase in DUR for C2-closing in C₁V.C₂V).

On the other hand, DISP and PKVEL reveal robust Info-Structure effects across all gestures except for one case (i.e., DISP of C2-opening in C₁V.C₂VC₃ (which showed a marginal effect) as can be seen in Table 2b). As can be inferred from %-increase values in the table, the magnitude of change in spatial displacement and movement peak velocity due to Info-Structure appears to be largely similar across all gestures. These results taken together indicate that the prominence associated with broad focus (in the ‘new’ condition) is characterized by spatial expansion accompanied by an increase in peak velocity, but not in duration, and is distributed almost entirely over the bisyllabic words, whether the final syllable is open or closed.

The effect of Info-Structure also did not consistently interact with the boundary effect. In Figure 3c–d, the boundary effect on DISP and PKVEL was generally greater when the target was produced on the given condition, as differentiated by the line type (solid = given, dashed = new). However, as marked by the gray ovals, the Boundary × Info-Structure interaction effect on DISP and PKVEL was significant only in the final C3-closing gesture in C₁V.C₂VC₃ words. Temporal measures in Figure 3a–b also showed significant interaction effects on C2-opening of C₁V.C₂VC₃, but the direction of the interaction effect was reversed in the following gesture (C3-closing), showing inconsistent interaction effects.

3.2. Relationships between kinematic variations

In this section, we provide further data on kinematic characteristics of preboundary articulation by exploring relationships between individual kinematic parameters associated with gestures in the final syllable. Note that the purpose of these additional analyses was to further illuminate how boundary-related temporal variation as reflected in PBL may be related to variation in displacement and movement velocity that is also observed with preboundary articulation. Understanding these relationships between kinematic parameters will also allow us to speculate on the nature of preboundary articulation in dynamical terms. In particular, if variation in spatial expansion that was found to accompany PBL can be largely accounted for by variation in PBL, one can infer that the temporal modulation of gesture at a prosodic juncture is what underlies spatial expansion at that prosodic juncture. Alternatively, if the relationship analyses reveal that variation in spatial expansion is independent from variation in PBL, one can assume that there is an additional dynamical parameter that may be modulated to give rise to variation in spatial expansion.

Figure 4 visualizes the kinematic relationships in scatter plots with an indication of the boundary condition (IP-final vs. IP-medial) to which each datapoint belongs. However, to examine correlations between the individual parameters within the overall distribution, rather than within each boundary level, the r-coefficients and p-values provided in Figure 4 are based on all data points pooled across boundary conditions. The scatter plots are presented separately for the vowel sequence condition (i.e., whether the final vowel was /i/ or /a/), because the two final vowels are not comparable in terms of the size of oral opening and intrinsic vowel length.

Figure 4

Relationships between kinematic parameters of DUR, PKVEL, and DISP. Data points are plotted by boundary conditions along with separate linear regression lines (black and gray). (Note that the values on the X and Y axes reflect the mean normalized z-scores obtained from each speaker’s z-scores.)

As shown in Figure 4a, it is interesting to see that DUR is only weakly correlated with PKVEL (r-coefficients ranging from 0 to 0.41). Instead, the data points of IP-final are separable from those of IP-medial primarily on the vertical (DUR) dimension, indicating that despite some degree of correlation between the two parameters, the temporal expansion at a larger prosodic boundary (IP-final vs. IP-medial) is largely independent from variation in movement peak velocity. On the other hand, as shown in Figure 4b, DUR is quite highly correlated with DISP (r-coefficients ranging from 0.40 to 0.76), indicating a better correlation between variations in duration and displacement than between variations in duration and movement peak velocity. Accordingly, as can be seen in Figure 4b, the distribution of IP-final versus IP-medial data points is separable diagonally (i.e., on both vertical and horizontal dimensions), indicating that PBL in the final syllable cannot be accounted for by variation in DUR or DISP alone, but by their interdependent relationship. Finally, as shown in Figure 4c, DISP is highly correlated with PKVEL (the larger, the faster), with r-coefficients ranging from 0.73 to 0.95. Though to a lesser degree than Figure 4b, the data points for IP-final are distributed upward to the right relative to those for IP-medial, confirming that the spatial expansion of preboundary (IP-final) articulation accompanies an increase in movement peak velocity.

4.1. Kinematic characteristics of preboundary articulation

One of the basic questions that the present study started off with was how preboundary articulation in Korean as an edge-prominence language could be characterized in kinematic terms, and whether and how it would differ from that in English as a head-prominence language (cf. Jun, 2014). Our results indicate that preboundary articulation at the right edge (in the IP-final position) in Korean is not only longer in duration (preboundary lengthening), but it is also larger in displacement and higher in peak velocity. Preboundary articulation in Korean can therefore be best characterized in kinematic terms as being larger in all directions compared to that in the IP-medial position. Moreover, such an articulatory strengthening effect does not interact with Info-Structure (given/new) at all in CVCV, and for most cases in CVCVC. This indicates that preboundary articulation is modulated in both spatial and temporal dimensions by prosodic constituency (or prosodic phrasing), largely independent of ‘new’ versus ‘given’ conditions of Info-Structure.

This boundary-induced kinematic pattern in Korean is quite different from some of the earlier findings in English, which indicate that preboundary articulation is characterized primarily by temporal expansion (PBL), while spatial expansion and increased movement velocity do not necessarily accompany it (e.g., Beckman & Edwards, 1992; Byrd & Saltzman, 1998; Byrd & Saltzman, 2003; Cho, 2006). Instead, the observed kinematic characteristics of preboundary articulation in Korean appear to be more comparable to those of localized hyperarticulation (prominence/stress-induced articulatory strengthening) in English that can also be characterized by a spatio-temporal expansion (i.e., with articulation under prominence being larger, longer and faster compared to that in the non-prominent context) (Beckman & Edwards, 1992; de Jong, 1995; Fowler, 1995; Cho, 2006; see Mücke & Grice, 2014 for similar effects in German). This holds regardless of the broad focus conditions driven by information structure (‘new’ versus ‘given’). Thus, the preboundary articulation in Korean that reveals spatio-temporal articulatory strengthening effects can be taken to make preboundary articulation prominent (or perceptually salient) in a similar way that prominence-induced hyperarticulation effects do in English.

The right-edge effect on hyperarticulation in the spatio-temporal dimension found in Korean bears some broader implications when compared to the realization of contrastive tones at the lexical level. For instance, DiCanio, Benn, and García (2021) studied tone realization in Yoloxóchitl Mixtec, an endangered Mexican language with fixed stem-final stress and distinct lexical tones. Their acoustic experiments revealed that speakers of Yoloxóchitl Mixtec not only extended the temporal realization of the final syllable, a form of preboundary lengthening comparable to the IP-final position, but also enhanced the contrast of lexical tones. This enhancement included expanding the tonal f0 range, resulting in high tone raising and lowering of both low and falling tones. Emerging evidence from the present study and previous studies, including DiCanio et al. (2021), on different languages suggests that preboundary temporal slowing, initially rooted in low-level phonetics, can manifest as a locus of hyperarticulation at the right edge of prosodic constituents on a phrase level. This phenomenon extends to both spatial and tonal dimensions, and it may be under speaker control, as discussed by Cho (2016) and DiCanio et al. (2021) in the context of maximizing lexical distinction through prominence.

Another basic finding of the present study is that the scope of PBL in Korean was not confined to the proximal gestures in the final syllable, but PBL (as measured by movement duration) was found to extend leftward to the distal consonantal closing gesture (C1-closing) and opening gesture (C1-opening) of the non-final syllable in bisyllabic words. The magnitude of PBL of the two leftmost gestures was, however, drastically attenuated, and time to peak velocity did not show a significant boundary effect in these distal gestures associated with the first syllable. This implies that, while the PBL process operates in a gradient fashion across the two syllables, the effect is asymmetrical and weighted much more strongly on the gestures in the final syllable as has been found with other languages. It is also noticeable that the leftward spreading to the most distal gestures was not observed in displacement and peak velocity as well, indicating that the boundary-induced spatio-temporal expansion is largely localized to those gestures that form the final syllable.

The present study also asked how kinematic characteristics of preboundary articulation under the hypothesized edge-related prominence would be compared with those in the ‘new’ (broad focus) versus ‘given’ (no focus) conditions that are related to prominence associated with information structure that may not be always expressed by phrasing in Korean. Results indicate that the edge-induced strengthening effects on preboundary articulation in Korean are indeed different from those of strengthening that might arise with ‘new’ versus ‘given’ conditions of Info-Structure. Recall that there were significant effects of Info-Structure on kinematic realization primarily in displacement and peak velocity across the board. All the lip closing and opening gestures except for C2-opening gesture in the closed syllable (C₁V.C₂VC₃) condition (which showed a marginal effect) are produced with a significant increase in both displacement and peak velocity in the ‘new’ (broad focus) condition compared to the ‘given’ (unfocused) condition. On the other hand, the ‘new’ condition of Info-Structure barely induced temporal expansion—i.e., the ‘new’ condition showed a lengthening effect only on C2-closing gesture in C₁V.C₂V, and only C3-closing gesture in C₁V.C₂VC₃, whereas all other gestures showed no lengthening effect of Info-Structure. These results suggest that the articulatory strengthening effect of prominence associated with the ‘new’ (broad focus) condition is primarily reflected on the spatial dimension while the articulatory strengthening effect of prominence associated with the right edge is reflected on both the spatial and temporal dimensions.

4.2. Understanding asymmetrical boundary effects due to syllable structure

The present study also explored whether the presence or absence of the coda gesture (C3-closing) in the final syllable would influence the distribution of PBL, especially given that any gesture that precedes it can be assumed to be more distal from the boundary in its presence (C₁V.C₂VC₃) than in its absence (C₁V.C₂V). The results revealed that the magnitude of PBL on the C2-closing gesture was comparable in the two syllable contexts (i.e. 47%-increase when followed by two gestures (C2-opening and C3-closing) in C₁V.C₂VC₃ versus 44%-increase when followed by one gesture (C2-opening) in C₁V.C₂V). This indicates that as far as C2-closing gesture is concerned, the physical distance from the prosodic juncture does not influence the distribution and the magnitude of PBL. Crucially, however, the distribution of PBL was found to differ substantially due to the syllable structure on the gesture(s) that follow the C2-closing gesture. The C2-opening gesture in the absence of the following C3-closing gesture (in the open syllable condition of C₁V.C₂V) showed a 208% increase, whereas it showed a 105% increase in the presence of the following C3-closing gesture (in the closed syllable condition of C₁V.C₂VC₃). On the other hand, the C3-closing gesture itself in CVCVC context showed a 123% increase.

These results have some implications for effects of syllable structure of the final syllable (open versus closed) on which PBL is concentrated. When we consider the temporal distribution of PBL separately in each syllable context, it still shows a progressively attenuating PBL effect from the right edge to the left. But it is noteworthy that the magnitude of PBL on the C2-opening gesture in the open syllable context (C₁V.C₂V) was far greater than that on either the C2-opening gesture or on the C3-closing gesture in the closed syllable (C₁V.C₂VC₃). In fact, the magnitude of PBL on the C2-opening gesture (208%-increase) in the open syllable context was largely comparable to a magnitude of PBL on the C2-opening and C3-closing gestures combined (105%-increase and 123%-increase, respectively) in the closed syllable context. Interestingly, a similar effect has been observed in an acoustic study on Japanese PBL (Seo et al., 2019), showing that the magnitude of PBL in the open syllable with one mora is a combined effect of the final two morae (or a vowel and a nasal coda) that form a rhyme of the closed syllable. The observed effects may be due to PBL being realized on the basis of syllable structure with a similar magnitude on the rhyme (as discussed in Seo et al., 2019), or on the basis of the type of the constriction gesture (closing versus opening) that is aligned with the right edge. While this remains to be further corroborated, what appears to be the case is that the magnitude of PBL on the final syllable is influenced by whether there is a coda constriction or not. On a related point, the boundary effect on other kinematic measures (displacement and peak velocity) was found to be more robust on the final gestures when the syllable was closed (when the final gesture involves a closing movement) than when it was open. Again, it is not clear to us why this asymmetry occurred, except that it was due to the presence/absence of the final closing (coda) gesture. It may be due to the fact that the closed syllable is phonologically heavy, so that the boundary effect that lends prominence is more robustly realized on the spatial dimension in Korean. The fact that the broad focus effect was also found to be most robust on the final coda closing gesture appears to lend further support to the heavy-syllable related possibility. (Recall that the prominence arising with the ‘new’ (broad focus) condition of Info-Structure was primarily reflected on a spatial expansion across the board, but the rightmost C3-closing gesture in the closed syllable condition showed robust effects in both spatial and temporal dimensions.) Alternatively, or in addition, the asymmetry may be due to the fact that the closed syllable involves the constriction formation gesture for occlusion that is not released in Korean, which may cause more pronounced boundary-related articulation of the formation gesture.

4.3. Understanding kinematic characteristics of preboundary articulation of Korean in dynamical terms

Examination of the relationships between kinematic parameters further revealed the nature of the observed boundary-induced articulatory strengthening in Korean. It was found that PBL was only weakly related to variation in movement velocity, while it was more closely related with spatial expansion as reflected in variation in displacement, which in turn was highly correlated with variation in peak velocity. These results elaborate on the strengthening of preboundary articulation in dynamical terms in Korean in relation to the theory of the π-gesture model (Byrd & Saltzman, 2003). As introduced at the outset of the paper, the π-gesture, as a temporal modulation gesture, is assumed to modulate the rate of the clock in a dynamical system that controls articulatory temporal activation in the vicinity of the prosodic juncture. Thus, at first glance, spatial expansion of the sort observed in this study may not be considered to come about as direct consequence of π-gesture as a temporal modulation gesture. But it is still explainable in the π-gesture model, as clearly noted by Byrd and Saltzman (2003). For example, greater activation time of the π-gesture can provide sufficient time for the gestural target to be attained fully without truncation by an upcoming gesture, which allows possible spatial expansion of preboundary articulation. In this regard, Byrd and Saltzman (2003, p. 172) explain that “[i]t seems that transgestural perturbations of clock rate due to a π-gesture that locally slows time flow in an utterance can result in appropriate kinematic changes not only in the temporal domain but also in the spatial domain.” In fact, in the present study, the time-to-peak velocity measure, which is not affected by possible truncation, indicated a clear PBL effect on the final syllable as predicted by the π -gesture model. One might still ask, however, whether such spatial variation, if it occurs purely due to the absence of gestural truncation, can account for an increase in peak velocity because gestural truncation itself does not directly modify movement velocity. We do not have a compelling explanation to offer, but it may be simply due to the natural tendency that the articulatory movement peak velocity is linearly related to the movement displacement (Munhall et al., 1985; Ostry & Munhall, 1985)—i.e., the increased peak velocity may be attributable to this relationship (see Roon, Hoole, Zeroual, Du, & Gafos, 2021 for related discussion on the relatively little role in variation in peak velocity which explains differences in gestural overlap of consonant clusters in Moroccan Arabic).

Now an important question with respect to language specificity arises from the fact that preboundary articulation in Korean is much stronger in terms of spatio-temporal realization (especially reflected in the spatial expansion) than in English, and that it is by and large comparable to the prominence-induced hyperarticulation in English. Thus, while preboundary articulation in both English and Korean can be taken to be modulated by the π-gesture as discussed above, we are left with a question regarding how the Korean-specific strengthening pattern associated with preboundary articulation can be understood in the theoretical framework of the π-gesture. Admittedly, this question cannot be easily answered with the limited data available in the present study, but one possibility is that the π-gesture interacts with another kind of modulation gesture in a language-specific way that regulates articulation in the spatial dimension, which in turn engenders hyperarticulation of the sort observed with preboundary articulation in Korean. In this regard, Saltzman, Nam, Krivokapic, and Goldstein (2008) extended the concept of the π-gesture as a temporal modulation gesture to a more general modulation gesture, the μ-gesture. They considered two kinds of μ-gesture (i.e., a temporal modulation gesture (a “μ_t-gesture”) and a spatial modulation gesture (a “μ_s-gesture”)). They suggested that the prominence-related strengthening pattern (i.e., localized hyperarticulation) can be accounted for by an interaction of two types of the μ-gesture that regulate the spatial variation and the temporal variation of the articulation movement of constriction gestures, respectively. The temporal modulation gesture, the ‘μ_t-gesture,’ shares similarities with the π-gesture, as both control the temporal aspects of constriction gestures. If the observed spatial expansion, accompanied by an increase in peak velocity, exceeds what can be explained by clock slowing alone under the influence of the temporal modulation of the π-gesture, we should consider the introduction of a spatial modulation gesture of some sort that interacts with the temporal modulation of the π-gesture (see Iskarous & Pouplier, 2022, for a related discussion). Just as a general μ-gesture integrates both the spatial and the temporal modulation, it is in principle feasible that the π-gesture can be extended to integrate a spatial modulation gesture all within the theoretical framework of the (‘prosodic’) π-gesture. Such an integration can account for the hyperarticulation effect observed at the right edge in our study, as well as the consistent domain-initial (left-edge) strengthening typically observed in Korean.

But the general progressive pattern of PBL observed in Korean is still predicted by the π-gesture model (Byrd & Saltzman, 2003). As discussed in Cho (2006), activation time can increase and decrease differently across languages, resulting in possible cross-linguistic variation in PBL. The π-gesture activation time in Korean can be considered to be stretched to the extent that it controls temporal realization of the distal gestures in the first (non-final) syllable. Moreover, the activation strength of the π-gesture is assumed to be maximal at the prosodic juncture and then decreases gradually towards the distal gestures in both directions (left and right). This can account for the drastic attenuation of the PBL of the distal gestures. It should be noted, however, that the current finding of the leftward spreading of PBL to the initial syllable is in contrast with a shortening of the penultimate syllable reported in Jang and Katsika (2020). Thus, we should not draw a firm conclusion on this, but we suspect that some differences between the two studies may have resulted in the discrepancy. First, two-syllable target words used in Jang and Katsika occurred as a second noun in a two-word compound noun phrase. Second, the vowel in the penultimate syllable of the target word which showed a shortening effect was an intrinsically short vowel /i/. Third, their target words occurred always in the non-prominent (unfocused) context with contrastive (narrow) focus falling on the first noun of the compound roughly in the half of their tokens analyzed (in their four-syllable-long Accentual Phrase context), or on an earlier word in the other half of the tokens analyzed (in their seven-syllable-long Accentual Phrase context). Thus, their target words occurred in a context of post-focal reduction and dephrasing. One cannot therefore rule out the possibility that their reported small degree of shorting of the non-final syllables has to do with these specific conditions which all appear to contribute some kind of reduction to the non-final syllable. But given that these specific conditions may affect both the phrase and the word boundary conditions tested in their study, how exactly these differences would lead to discrepant results reported in the present study remains to be further investigated. More research on PBL in various other contexts is certainly called for to understand the exact nature of PBL effects on non-final syllables.

1. Model structure

Each of the four dependent variables (DV) was modelled separately for each gesture using the following lme4 syntax in R.

lmer (DV ~ Boundary * Info-Structure + C-type + V-type + Boundary:C-type + Boundary: V-type + (1+Boundary |Subject) + (1 |TestWord))

2. Model output

β-coefficients and p-values for the intercept and the five fixed effects that are not given in Table 2 in the results section are summarized below in Table 3.