Spoken languages convey grammatical and prosodic information through both segmental and suprasegmental cues. Grammatical markers—such as verb inflections in English and sentence-final suffixes in Korean—and prosodic features like intonation, stress, and rhythm collaboratively serve to organize syntactic structures, distinguish sentence types, and highlight prominence (Cho, 2016; Jun, 2006, 2010).

Sign languages similarly encode grammatical and prosodic structure using both manual articulators (e.g., timing, repetition, size, and location of signs) and nonmanual signals (e.g., facial expressions, head movements, eye gaze, and mouth gestures; Meier, 1993; Brentari, 2010; Sandler and Lillo-Martin, 2006; Stokoe, 2005; Nespor and Sandler, 1999). Among these, nonmanual features play a particularly salient role in expressing prosodic prominence—functions typically realized by pitch accent and stress in spoken languages (Sandler, 2010; Dachkovsky et al., 2013). Raised eyebrows, widened eyes, head movements, and directed gaze have been identified as key markers of informational focus in many sign languages (Wilbur, 2000, 2013; Crasborn and Van der Kooij, 2013; Fenlon and Brentari 2021). These cues frequently co-occur in coordinated clusters, generating visually salient markers of prominence. For example, in Sign Language of the Netherlands (NGT) and Italian Sign Language (LIS), prominence is expressed through eyebrow movements, intensified mouthing, and head tilts (Crasborn and Van der Kooij, 2013; Geraci, 2015; Branchini and Mantovan, 2020; Fontana and Caligiore, 2021; Sbranna et al., 2023).

In spoken languages, prominence is typically realized through prosodic modulation of pitch, stress, and duration to signal focus (Beckman and Pierrehumbert, 1986; Crystal, 2011). These modulations enhance the perceptual salience of target elements and often influence the articulation of adjacent segments—a phenomenon known as prosodic strengthening (Cho, 2016; Mücke and Grice, 2014). In Korean, prominence commonly appears on focused elements within a prosodic phrase and is phonetically marked by a combination of pitch rise, increased amplitude, and temporal expansion, particularly through pre-boundary lengthening (Cho, 2016; Jun, 2006). These phonetic cues are closely tied to prosodic structuring: Focus may trigger prosodic restructuring, resulting in the formation of a new Accentual Phrase (AP; Jun, 2006), or it may be realized within an existing AP through localized phonetic enhancement in AP-medial positions (Cho, 2022). This distinction has generated ongoing theoretical debate about the relationship between prominence and prosodic phrasing.

In Korean Sign Language (KSL), however, the prosodic realization of prominence—especially through nonmanual articulations—remains underexplored. Prior studies have largely focused on syntactic strategies such as topicalization and focalization (National Institute of Korean Language, 2021), while empirical investigations of how prominence is marked prosodically, particularly at levels below the Intonational Phrase (IP), are limited. Although nonmanual features such as head nods and eye gaze have been observed at IP boundaries—paralleling patterns in other sign languages—it remains unclear whether these cues also function to mark focused elements within smaller prosodic domains like the AP.

Of particular interest is the question of whether KSL signers use (non)manual cues—such as eyebrow movement and/or head orientation—to mark prominence at the level of the AP. This raises broader theoretical questions about the interaction between boundary and prominence: Does prominence in KSL trigger the formation of a new AP, as in Jun’s (2006) account of spoken Korean, or can prominence be expressed within an existing AP without boundary modification, as suggested by Cho (2022)? Understanding how prominence is encoded at the AP level in KSL not only contributes to our knowledge of sign language prosody but also informs cross-modal comparisons of prosodic systems.

To address these questions, the present study investigates the use of (non)manual features in marking prominence and prosodic boundaries in KSL, focusing specifically on AP boundaries. We analyze several nonmanual cues—eye contact, furrowed eyebrows, squinted eyes, wide eyes, mouthing, etc.—under controlled conditions manipulating both prominence (focused vs. unfocused) and boundary position (AP-initial vs. AP-medial).

This study is guided by the following research questions:

By situating KSL within broader typological and theoretical discussions of prosody, this study contributes to a deeper understanding of how modality shapes the interplay between prominence and prosodic structure.

To investigate the effects of prominence (focused vs. unfocused) and boundary (AP-initial vs. AP-medial) on the frequency of (non)manual features, we conducted statistical analyses using R version 4.0.2 (R Core Team, 2019) with the tidyverse package version 1.3.0 (Wickham et al., 2019). Effect sizes (Cohen’s d) were computed using the effsize package version 0.8.0 (Torchiano, 2020). Bayesian mixed-effects regression models were implemented with brms (version 2.13.3; Bürkner, 2017). The Bayesian mixed models were specified with (Non)manual Features as the dependent variable and boundary (AP-initial vs. AP-medial) and prominence (focused vs. unfocused) as fixed effects, along with their interaction. Random effects included by-Signer intercepts to account for individual variability. Models were run using four Markov chain Monte Carlo (MCMC) chains with 6,000 iterations per chain, including 3,000 warm-up iterations, yielding a total of 12,000 post-warmup draws. Convergence diagnostics were assessed using R̂ values, with all parameters reaching R̂ ≈ 1.00, indicating proper mixing. Effective sample sizes (Bulk_ESS and Tail_ESS) were also examined to confirm model reliability.

Figure 10 presents the frequency of (non)manual features across prominence and boundary conditions. First, Table 4 presents the effects of prominence (focused vs. unfocused). Prominence showed significant and systematic effects on (non)manual features. Several nonmanual features were significantly more frequent in focused conditions, including eye contact (β = −1.54, SE = 0.46, p < 0.01), furrowed eyebrows (β = −1.50, SE = 0.47, p < 0.01), raised eyebrows (β = −2.43, SE = 0.57, p < 0.001), squinted eyes (β = −2.07, SE = 0.70, p < 0.01), wide eyes (β = −2.97, SE = 0.72, p < 0.001), head down (β = −1.24, SE = 0.58, p = 0.033), and leaning left (β = −1.73, SE = 0.56, p < 0.01). In contrast, features such as head up (β = 0.82, SE = 1.29, p > 0.05) and intense signing (β = 0.38, SE = 0.77, p > 0.05) did not differ significantly between focused and unfocused conditions. These results underscore prominence as a key driver of nonmanual feature use in KSL, particularly for eye- and eyebrow-related movements.

Next, the effects of boundary position (AP-initial vs. AP-medial) are summarized in Table 5. Overall, there was limited evidence supporting a significant increase in the frequency of (non)manual features in AP-initial positions compared to AP-medial positions. Moreover, most coefficients associated with AP-initial conditions were close to zero and statistically insignificant. Specifically, eye contact (β = 0.20, SE = 0.48, p > 0.05) and furrowed eyebrows (β = −0.10, SE = 0.48, p > 0.05) showed no significant differences between positions. However, squinted eyes tended to be less frequent in AP- initial positions, but this did not reach significance (β = −2.32, 95% CI [−4.93, 0.29], p = 0.081). Features such as head up (β = 1.93, SE = 1.40, p > 0.05) and intense signing (β = 0.72, SE = 0.89, p > 0.05) displayed non-significant trends toward increased frequency in AP-initial positions, suggesting weak but not robust boundary-marking tendencies.

Finally, the interaction between prominence and boundary conditions was examined (Table 6) to assess whether prominence systematically increased nonmanual feature usage specifically at AP-initial boundaries. At the feature level, all Boundary × Prominence coefficients had 95% confidence intervals that overlapped zero (Table 6), indicating no reliable interactions for individual features. By contrast, the model-level interaction term was statistically significant (β = 1.10, p < 0.05), but this average effect did not translate into consistent, feature-specific patterns. For instance, eye contact (β = −0.12, SE = 0.53, p > 0.05), furrowed eyebrows (β = 0.25, SE = 0.54, p > 0.05), and head up (β = −2.29, SE = 1.49, p > 0.05) were not significantly affected by the interaction between boundary and prominence. Thus, while prominence substantially increased the frequency of (non)manual features, this effect did not uniformly depend on boundary position, suggesting that prominence operates relatively independently from boundary cues in KSL prosody.⁶ Taken together, these results suggest that prominence robustly increases (non)manual frequency irrespective of boundary position, with no systematic amplification at AP-initial.

To support the statistical findings, we provide illustrative examples showing how (non)manual features varied across prominence and boundary conditions. In (5) how nonmanual markers are used for silly under the AP-initial focused condition is presented. In this condition, signers consistently produced a cluster of (non)manual cues associated with prominence—specifically mouthing, furrowed eyebrows, raised eyebrows, head up, and eye contact. These nonmanual signals were frequently accompanied by manual articulatory features, such as a slower signing rate and sign lengthening. Together, these observations suggest that prosodic prominence in KSL is expressed through the coordinated use of both manual and nonmanual cues, particularly when a focused element occurs at the beginning of an Accentual Phrase.

A series of four images showing a person using sign language against a solid blue background. The first image shows the sign for "NO" with the person making a hand gesture. The second and third images depict the sign for "SILLY" with the person employing different hand motions and facial expressions. The fourth image illustrates the sign for "FRONT" with a distinct gesture. Throughout, the person displays mouthing, furrowed eyebrows, raised eyebrows, head up, and eye contact. Text below describes the meaning of the signs.

Next, in the AP-initial unfocused condition, only raised eyebrows and head lean left were observed. Notably, eye contact was not present, as seen in (6). This suggests that, in the absence of prominence, certain nonmanual cues—such as eye contact—may not be triggered, even at AP-initial positions.

Three-panel image showing a person signing in American Sign Language against a blue background. The first panel shows the sign for "NO" with an expression of disagreement. The second panel shows the sign for "SILLY" with a playful expression. The third panel displays the sign for "BEHIND," indicating position. The caption describes the expression: raised eyebrows, head leaning left, indicating the meaning: "No, the tree (card) is behind (the card including the sign) SILLY."

In the AP-medial focused condition, the sign silly was accompanied by raised eyebrows, eye contact, and mouthing, as seen in (7). This indicates that, even in AP-medial positions, eye contact can still emerge as a cue for prominence when the sign is in focus.

A person is demonstrating a series of American Sign Language (ASL) signs against a blue background. The sequence captures signs for "NO," "PURPLE," "SILLY," and "FRONT." Each image shows different facial expressions, including mouthing, raised eyebrows, and eye contact, to emphasize meaning. The context provided describes the phrase: "No, the tree (card) is in front of (the purple colored card including the sign) SILLY."

Finally, in the AP-medial unfocused condition, only mouthing was observed, with minimal use of other nonmanual feature, as seen in (8). Again, prominence-related cues such as eye contact were absent.

Four images show a person using sign language against a blue background. Each image corresponds to words: first for "No," second for "Purple," third for "Silly," and fourth for "Behind." The person gestures and moves their head left, conveying a message in sign language.

These examples are consistent with the quantitative findings, reinforcing the interpretation that prominence plays a more decisive role than boundary position in shaping the distribution of nonmanual features in KSL. Eye-related cues, in particular, were more consistently observed under focused conditions regardless of AP position, whereas postural and head movement cues appeared more frequently at AP-initial boundaries.

Taken together, these patterns suggest that prominence exerts a stronger and more systematic influence on nonmanual articulation than boundary position, with minimal interaction between the two factors.

Our findings clearly demonstrate that nonmanual prosody in KSL is not a random or stylistic choice, but rather a systematic and functionally organized component of the language’s prosodic system. Statistical analyses revealed that both prominence (focused vs. unfocused) and boundary position (AP-initial vs. AP-medial) significantly influenced the frequency and type of nonmanual features observed. Specifically, focused signs consistently exhibited a higher frequency of nonmanual cues—such as eye contact, furrowed eyebrows, raised eyebrows, wide eyes, head down, and leaning left—compared to unfocused signs. These results confirm that nonmanual prosody in KSL plays a critical role in prominence marking, particularly through facial and head articulations.

A feature-specific analysis further revealed that while most nonmanual cues were closely associated with prominence, squinted eyes functioned as a boundary cue, appearing predominantly in AP-initial positions and rarely in AP-medial focused conditions. This fine-grained division of prosodic functions highlights the systematic nature of nonmanual prosody in KSL and underscores the language-specific role of facial and head movements. Unlike previous findings in ISL, where eyebrow movements primarily marked boundaries (Dachkovsky and Sandler, 2009), KSL signers predominantly used eyebrow and eye movements as prominence markers. This suggests that prosodic cues are subject to cross-linguistic and cross-modal variation.

Our findings align with Crasborn and van der Kooij (2013) observations in NGT, where prominence marking commonly involves the co-occurrence of multiple nonmanual cues rather than a single isolated signal. KSL signers similarly used coordinated facial and head articulations, supporting the view that prominence in sign languages is typically realized through multimodal enhancement.

Importantly, our study contributes to the ongoing theoretical debate on whether focus necessarily triggers prosodic restructuring. In spoken Korean, Jun (2006) argued that focus induces the formation of a new AP boundary, whereas Cho (2022) demonstrated that prominence can also be realized without prosodic restructuring. Extending this debate to KSL, our results suggest that in KSL, prominence is primarily marked through nonmanual cues independent of prosodic boundary formation, aligning with Cho (2022) view. This finding indicates that prominence and boundary marking in KSL are functionally more differentiated, and that prosodic restructuring is not a prerequisite for marking prominence.

Taken together, these findings provide empirical evidence that prosodic strategies in KSL are modality-specific, with a unique emphasis on facial and head movements rather than on spatial or temporal adjustments commonly reported in other sign languages. The systematic use of nonmanual cues in KSL reflects both universal prosodic strategies and language-specific adaptations shaped by the visual-manual modality and the cultural-linguistic context of the Korean Deaf community.

Future studies should further explore larger prosodic units (e.g., IP boundaries) and more spontaneous signing to assess the generalizability of the current findings. Perceptual studies could also investigate how KSL users process nonmanual prosodic cues, providing deeper insights into cue salience and cognitive processing. Additionally, cross-linguistic studies across sign languages (e.g., ASL, JSL, and BSL) and research on the acquisition of nonmanual prosody in KSL learners will be valuable for understanding both universal and language-specific aspects of prosody.

In sum, this study provides the first systematic phonetic-prosodic analysis of prominence and boundary marking in KSL. By demonstrating that prominence can be realized independently of prosodic restructuring, we contribute to a growing body of work that emphasizes the flexibility of prosodic systems across modalities. Our findings position KSL within broader typological and theoretical frameworks of prosody, offering new insights into the cross-modal diversity of prosodic organization.