Executive function and Korean children’s stop production^*

Korean has a three-way laryngeal contrast among fortis, lenis and aspirated stops at three places of articulation (i.e., /p’, t’, k’/, /p, t, k/, /p^h, t^h, k^h/). A great body of work has explored the unusual three-way laryngeal contrast of Korean stops in perception and production over several decades (e.g., Cho et al., 2002; Han & Weitzman, 1970; Kang & Guion, 2008; Kim, 2000; Kong et al., 2011; Lisker & Abramson, 1964; Oh, 2011; Silva, 2006; Wright, 2007; see Lee et al. (2020) for a review). Traditionally, VOT was the sole primary cue for all pairs within the contrast (e.g., Han & Weitzman, 1970; Hardcastle, 1973; Lisker & Abramson, 1964). But under the current sound change in progress, F0 has overtaken VOT as a primary cue, particularly for the lenis-aspirated stop pair (e.g., Kang, 2014; Silva, 2006), and this sound change has spread from standard Seoul Korean into regional dialects, such as the Gyeongsang dialects (Lee & Jongman, 2012; Lee et al., 2013). Specifically, F0 values pattern in the order of lenis<fortis<aspirated stops, and VOT patterns in the order of fortis<lenis=aspirated stops. That is, the importance of the two acoustic dimensions depends on the specific laryngeal contrast: VOT plays an important role in distinguishing the fortis stop from the other two stops, while F0 is necessary for the lenis-aspirated contrast. Therefore, effective listeners might establish a perceptual strategy by relying on a useful cue and ignoring a less informative cue to identify the stop correctly.

The changing role of VOT and F0 is also reflected in the development of Korean-speaking children’s stop production and perception (e.g., Kim, 1999; Kim & Stoel-Gammon, 2009; Kong et al., 2011; Son, 2018). In previous literature, children under age 10 showed significantly shorter VOTs for the fortis stop compared to non-fortis stops. However, there was no difference in VOT between the lenis and aspirated stops, and F0 made up for the lack of a VOT distinction between the lenis-aspirated pair. This stop production pattern in children was confirmed by perception data from adult listeners, who reliably used VOT in identifying children’s production of the fortis stop (Kong et al., 2011). In both production and perception studies with 3-year-old children, Son (2018) observed the critical role of F0 for the distinction between lenis and aspirated stops. This previous literature confirmed the reduced role of VOT and enhanced role of F0 based not only on adults’ speech, but also on children’s perception and production.

Group differences have also been observed for such phonetic trade-off relations between the two acoustic dimensions in a way that the degree of reliance on F0 (or VOT) varies across speaker/ listener characteristics such as gender (Kang, 2014) or dialect (e.g., Lee & Jongman, 2012). Focusing on human-internal resources inducing such phonetic variation, a few perception studies have examined correlations between individuals’ cognitive abilities and perceptual strategies related to acoustic cue use (e.g., Kong & Yoo, 2017; Lee & Kong, 2016). Lee & Kong (2016) examined the relationship between individual traits and variability in processing Korean stops by testing 28 adult Korean-speaking listeners. They conducted a three-alternative forced-choice stop perception task (3AFC) in two different conditions: in one condition listeners just completed the stop identification task, and in the other condition listeners also had to complete simple arithmetic problems that served as distractors. In addition to the speech perception tasks, a series of executive function (EF) tasks assessed each participants’ cognitive abilities through a digit n-back task, a flanker task, and a trail-making task for working memory capacity (WM), inhibition, and cognitive flexibility, respectively. In Lee & Kong (2016), a meaningful correlation between individuals’ cognitive traits and the use of multiple cues in perception was observed in that individuals with better executive function were more likely to utilize the less informative VOT in distinguishing between the lenis and aspirated stops. Based on these findings, Lee & Kong (2016) suggested that individual language users’ cognitive traits could be one factor explaining systematic individual variation.

Kong & Yoo (2017) examined how 7- to 8-year-old Korean- speaking children utilized VOT and F0 in the perception of Korean stops, and tested whether and how their cognitive abilities are associated with their perceptual strategies in using multiple acoustic dimensions. 15 children’s cognitive abilities were assessed by testing their working memory, inhibition, and shifting attention abilities. These cognitive abilities were measured by a series of executive tasks including a digit n-back task (WM), a flanker task (inhibition), a Stroop test (inhibition) and a trail-making (attention) test in which accuracy and response times (RT) were obtained. Overall, in Kong & Yoo (2017), the individual observations indicated a meaningful correlation between executive function and the use of acoustic cues in the perception of Korean stops. Based on a series of correlation tests, Kong & Yoo (2017) found a strong correlation between the use of VOT and F0 cues and children’s cognitive abilities in identifying the pairs of stop contrasts. Specifically, child listeners with better WM and inhibition control exhibited more effective perceptual strategies by relying less on the unimportant F0 cue in the identification of the fortis-aspirated stops. Likewise, individuals with better attention reported effective use of an informative cue (i.e., VOT) in identifying the fortis stop from the other two stops. Overall, Kong & Yoo (2017) showed that children with better cognitive abilities showed more effective perceptual strategies by effectively utilizing a more informative cue and ignoring a less informative one. This observation is somewhat opposite from Lee & Kong (2016), in which adult listeners with better cognitive ability tended to utilize an inefficient cue, but may reflect children’s more limited cognitive resources compared to adults’ in processing sub-phonemic information in speech perception.

From the previous studies (Kong & Yoo, 2017; Lee & Kong, 2016), the following questions arise. First, given that the previous observation was limited to speech perception, the present study asks whether and how this relationship between individuals’ perceptual strategies and cognitive resources are reflected in the production of Korean stops, at least in the case of children. Hypothesizing consistency between speech perception (Kong & Yoo, 2017) and production, we might predict that children with better cognitive capacity or EF scores would be more likely to use an informative acoustic cue and suppress a redundant cue (e.g., enhancing VOT, but suppressing F0 for the fortis-aspirated contrast) in producing Korean stops. In a similar vein, we also aim to closely look into the presence or absence of significant correlations considering stop pair types and EF tasks. While it is expected to find a meaningful association between better EF and children’s accurate pronunciation of the phonemes, as existing production studies showed (e.g., Eaton & Ratner, 2016; Netelenbos et al., 2018), the present study uniquely investigates sub-phonemic acoustic properties of the sounds in children’s productions in relation to EF. Finally, the present study will also expand the empirical foundation for exploring individual variation in cognitive factors as they relate to speech science, allowing us to revisit the empirical rift between Kong & Yoo (2017) and Lee & Kong (2016). For these purposes, we examined the production data of forty-one Korean-speaking children, consisting of a subset of the data presented in Holliday et al. (2023).¹ While the statistical analyses presented here consider gender and dialect as factors, the discussion of the present paper mainly focuses on the relationship between individual variation in cue-weighting and cognitive capacities estimated by EF tasks.

Forty-one Seoul (13 F, 10 M) and Gyeongsang Korean (GK, 12 F, 6 M) elementary school children participated in the experiment and were given monetary compensation. The children were recruited in Seoul and Changwon (a city in South Gyeongsang province) and tested either at their home or at the elementary school they attended [see Holliday et al. (2023) for more methodological details]. All were between age 9 and 10 at the time of testing.

Speech production: The children read a list of stop-initial words (Appendix 1, three repetitions of 18 words in a randomized order) presented on a printed paper. They could take their time, with no time restriction, and on average the recording session took about 15 minutes. While researchers did not provide any feedback on speed or accuracy during the session, a few participants self-corrected their mispronunciations, in which case (19 tokens) the corrected productions were used for acoustic analysis. We used digital recorders and condenser microphones for the task, digitizing the recordings at a 44,100 Hz sampling rate and 16-bit quantization.

Executive function: We administered Dimensional Change Card Sorting (DCCS) and digit n-back tasks to assess children’s mental flexibility and WM, respectively (Owen et al., 2005; Zelazo et al., 2003). (It is noted that besides the two EF tasks, a Stroop test was also given to a subset of participants as an inhibition control measure. However, we could not administer the task to every participant due to attention and time constraints.) In each DCCS trial, participants were asked to sort cards (e.g., ‘blue ball’ and ‘yellow truck’) as fast as possible according to either color (blue or yellow) or shape (ball or truck). Both the counts and RT of correct responses were collected when the two cues shifted (e.g., ‘color’ followed by ‘shape’) and stayed (e.g., ‘color’ followed by ‘color’). RT differences between shift and stay conditions and accuracy of the shift condition were regarded as cognitive cost of mental flexibility. In the digit n-back task, children were asked to decide as fast as possible whether the single digit on the current slide is identical (same) to or different (different) from that of the previous slide. In consideration of time and the difficulty of the task, we administered only the 1-back session, in which children were to compare the two numbers one slide apart. Similar to the DCCS, RT difference (RT[same]–RT[different]) and accuracy counts on the different trials were used as numeric indices to represent the participants’ WM. Both tasks were programmed in E-Prime (ver. 3) to automatically record response details.

Acoustic analysis: We measured stop VOT and the F0 of the following vowel using Praat (Boersma & Weenink, 2023). For VOT, we manually pinpointed the stop burst and the voicing onset in the following vowel, using a Praat script only to locate each target word. For F0, the script first computed F0 by averaging the pitch values over the initial 15 ms beginning at the voicing onset. If there was no measurable F0 during the initial 15 ms due to creak, the script moved forward in 5 ms steps until a measurable F0 value was obtained.

Statistical analysis: Three logistic mixed-effects regression models were built to predict the stop category among the three contrasts dependent variable (DV pairs: lenis-aspirated, fortis-aspirated, and fortis-lenis stops) based on fixed effect variables of VOT and F0 along with their interactions with gender (male vs. female) and dialect region (GK vs. Seoul). By-subject intercepts and slopes of VOT and F0 were included as random effects, from which individual speakers’ coefficients for VOT and F0 were estimated.

As preliminary simple correlation tests revealed that the VOT and F0 coefficients were highly correlated [Fortis-Lenis model: r(39)=.84, p<.0001], a series of partial correlation tests were conducted to define the relationship between EF capacities and acoustic cue utilizations in producing the stops. The partial correlation tests take each EF score (RTs and accuracies of DCCS and digit 1-back tasks) as one test variable (X), each acoustic variable coefficient (VOT and F0) as the other test variable (Y), and either F0 or VOT coefficients was taken as a control variable (Z). Because the test yields correlation coefficients between X and Y subtracting Z’s correlations with X and/or Y, we can avoid misleading estimates of association between X and Y that actually come from the confounding variable Z. We used the ppcor package in RStudio (Kim, 2015; Posit Team, 2023).

Figure 1 displays F0 values (converted to semitone) as a function of VOT values (log-transformed) separated by gender and dialect region. Overall, all groups of the children exhibited similar distributions of VOT and F0. Children from both dialects differentiated aspirated stops from fortis stops by having longer VOT, although the two dialect groups differed slightly in their realization of lenis stops: GK children’s lenis stops were more or less between fortis and aspirated in the VOT and F0 dimensions, but those of Seoul children rarely overlapped with fortis stops.

Figure 1. VOT and F0 distributions of the Korean stops (Fortis vs. Lenis vs. Aspirated) separated by dialect region and gender.

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These rough observations were confirmed in the statistical analyses in which the three pairs of the stops were predicted by VOT and F0. Table 1 summarizes the mixed-effects regression models for the three pairs of contrastive stops (i.e., Lenis-Asp., Fortis-Asp., Fortis-Lenis models). For the fortis-aspirated stop model, the fixed effect coefficient of VOT but not that of F0 was statistically meaningful, while both VOT and F0 were significant predictors for the lenis-aspirated and the fortis-lenis stop models. There were no dialect or gender group interactions in the two models, while the lenis-aspirated model yielded a marginally significant interaction of VOT with Region in which a negative interaction coefficient (β_VOT:Region=−.97, SE=.56, p=.08) indicates GK children’s VOT coefficients were greater than those of Seoul children. Figure 2 presents individuals’ VOT and F0 coefficients from the models (random coefficients added to group-averaged fixed effect coefficients), showing a trend that GK children’s VOT coefficients were slightly greater than those of Seoul children. The current result finding no dialect group difference supports recent studies examining GK speakers’ stop productions in which children realized the innovative variants of lenis and aspirated stops similarly to Seoul speakers (Lee, 2020).

Figure 2. Individual children’s VOT and F0 coefficients from the three mixed-effects regression models: Fortis-Asp., Fortis-Lenis, and Lenis-Asp. models.

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Table 1 summarizes the descriptive statistics of children’s performance of digit 1-back and DCCS tasks. While we employed both RTs (i.e., log-transformed time difference between target condition and control condition) and accuracy counts to represent cognitive ability, the two measures were not always correlated. RTs of digit 1-back were negatively correlated with accuracy counts [r(39)=–0.56, p<.0005], suggesting that children with higher accuracy scores responded faster than others. However, RTs and accuracy counts from DCCS were not meaningfully correlated [r(39)=0.24, p=.11].

Based on the results of no significant inter-group differences in production, we performed correlation tests over all children ignoring dialect and gender. Table 2 presents a summary (i.e., correlation coefficients and p-values) of the partial correlation tests over the 24 combinations of EF scores and acoustic variables. After a Bonferroni correction (a conservative adjustment of a significance level for the hypothesis test to countercheck multiple comparisons, 0.05/24=0.002), there was only one variable pair whose correlation coefficient was marginally significant: in the lenis-aspirated model, DCCS accuracy was positively correlated with F0 coefficients when the VOT coefficient was controlled [r(41)=.46, p=.002]. This means that children with better mental flexibility utilized F0 more than others in producing the lenis-aspirated stop contrast. Further analyses over subject subsets also revealed the same relationship: female children [r(24)=.56, p=.005] and Gyeongsang children [r(24)=.69, p=.002] with greater DCCS accuracy used F0 more than those with lower DCCS accuracy in realizing the lenis-aspirated contrast.

Unlike the findings from Kong & Yoo (2017) exploring children’s perception, the production data in the present study did not yield any robust relationship between EF scores and acoustic variables for the fortis-aspirated stop model. Similarly, acoustic coefficients from the fortis-lenis stop model were not significantly associated with EF scores, either. Although greater VOT coefficients used for the fortis-lenis stops were weakly correlated with slower RTs (i.e., poor EF capacity), the partial correlation coefficients between VOT and the RT of digit 1-back and DCCS did not reach the conservatively adjusted level of significance (digit 1-back RT: r=.375, p<.01, DCCS RT: r=.335, p<.05; Table 3).