2.1. Mandarin sibilants and apical vowels

Mandarin (Standard Chinese, Putonghua) has dental sibilants [s ts tsʰ], palatal sibilants [ɕ tɕ tɕʰ], and retroflex sibilants [ȿ tȿ tȿʰ]. The dental sibilants are produced with the tongue tip against the back of the upper incisors, the palatal sibilants with the tongue blade against the hard palate, and the retroflex sibilants with the upper surface of the tongue tip approaching the center of the alveolar ridge (Ladefoged & Wu, 1984; Cao, 1990).¹ In terms of center of gravity (COG, spectral mean), both [s] and [ɕ] have a high frequency peak between 5000Hz and 9000Hz, while the retroflex [ȿ] has a lower COG; in terms of dispersion, [s] has a narrower distribution of energy than [ɕ] and [ȿ] (Svantesson, 1986; Wu & Lin, 1989, p. 133). The fricative components of the affricates are in general similar to the corresponding fricatives (Svantesson, 1986).

In Mandarin, the vowel [i] follows the palatal sibilants and most other consonants but not the dental and retroflex sibilants. As shown in (2), the dental and retroflex sibilants are followed by the homorganic syllabic approximants [ɹ̩] and [ɻ̩] (Lee & Zee, 2003, Lee-Kim, 2014b), which are often referred to as the ‘apical vowels.’² The acoustic analysis in Lee-Kim (2014b) showed that, in producing Mandarin [sɹ̩], [ɕi], and [ȿɻ̩], the tongue positions do not change significantly after the tongue reaches the consonantal targets, which confirms that the consonantal and the vocalic gestures are homorganic. That is, the apical vowels are the ‘vocalized prolongation’ of their preceding consonants (Chao, 1934, p. 374). The two apical vowels [ɹ̩] [ɻ̩] and the vowel [i] differ in their first three formants, particularly F2 (Zee & Lee, 2001; Lee & Li, 2003; Cheung, 2004; Zee & Lee 2004), and there is no obvious consonant-vowel formant transition in the syllables like [sɹ̩], [ɕi], and [ȿɻ̩] due to the homorganic relation between the sibilants and the vocalic segments.

With regard to the formation of the apical vowel, there are two competing views: A place assimilation view and a contrast enhancement view. The place assimilation account treats the apical vowel as the result of the dental/retroflex sibilants spreading their place features to the following /i/. For example, Wang (1985) noted that it is a natural tendency for /i/ to change into [ɹ̩] or [ɻ̩] under the influence of their preceding dental sibilants [s ts tsʰ] or retroflex sibilants [ȿ tȿ tȿʰ]. A similar position was taken in phonological analyses that treat the post-dental or post-retroflex apical vowel as a vocalic slot unspecified for place, and the place value is filled in by feature spreading from the onset sibilants (Lin, 1989; Wang, 1993, 1999; Wiese, 1997).

The contrast enhancement account, on the other hand, regards the apical vowels as an enhancement of contrast, whereby the less distinct contrast /si, ɕi, ȿi/ turns into the more distinct [sɹ̩, ɕi, ȿɻ̩] (Stevens et al., 2004; Lee & Li, 2003, Lee-Kim, 2014a, among others). More specifically, the enhancement analysis assumes that the frication noise of the dental /s/, the palatal /ɕ/, and the retroflex /ȿ/ are acoustically close to each other (Stevens et al., 2004) and that, for a contrast triplet like /si, ɕi, ȿi/, the place distinction among the sibilants is likely to be compromised due to the palatalization from the following vowel [i] (Lee & Li, 2003). With the contrast at risk, an enhancement gesture is introduced to make /s/ and /ȿ/ more distinct from /ɕ/. That is, the apical vowels [ɹ̩] and [ɻ̩] are the continuation of the enhancing gesture on /s/ and /ȿ/ throughout the vowel /i/ (Stevens et al., 2004; Keyser & Stevens, 2006), whereby a shift of vowel helps preserve the contrast /si, ɕi, ȿi/ (Lee & Li, 2003, Lee-Kim, 2014a). As the fricative components of affricates are similar to their fricative counterparts (Svantesson, 1986), the enhancement analysis for Mandarin fricatives could be easily extended to the affricates.

Although the place assimilation account and the contrast enhancement account both work for Mandarin apical vowels, they make different predictions about language typology. The assimilation account assumes sibilant-to-/i/ spreading to be the cause of apical vowel formation. Thus, it predicts that a language may have an apical vowel as long as it has the dental/retroflex sibilants followed by /i/. For example, when there are /si, tsi, tsʰi/ syllables, the place feature of the dental sibilants should be able to spread to the following /i/, presumably for the conservation of articulatory effort as assumed for assimilation in general (Abercrombie, 1967, p. 139; Janson, 1986, among others). In contrast, the enhancement account assumes reduced contrast distinctiveness, e.g., in /si ɕi ȿi/, to be the motivation of apical vowel formation. Thus, it predicts that apical vowels should only emerge in a language when it has phonological contrasts between ‘sibilant+i’ sequences, e.g., /si-ɕi/, /tsi-tɕi/, or /tsʰi-tɕʰi/. That is, the apical [ɹ̩] and [ɻ̩] should not appear in a language if there are no such contrasts.

2.2. The typology of sibilant contrasts across Chinese dialects

To evaluate the two analyses above, we conducted a typological survey of apical vowels by collecting the syllabic inventories of Chinese dialects. We used Fangyan ‘Dialects,’ a Chinese journal specializing in the description of Chinese dialects, as the basis of our survey. We looked through all articles in Fangyan from 1979 to 2012 and identified 155 articles, each describing the syllabic inventory of a Chinese dialect. About two-thirds of the 155 dialects were explicitly described in the original article as belonging to a particular dialect group, i.e., Mandarin (or the northern dialects), Xiang, Yue, Min, Gan, Hakka, Jin, etc. There was no information of dialect grouping of the other dialects, though it is clear that they are distributed across regions of different dialect groups and geographically across different areas of mainland China.

Our focus is on the apical vowels [ɹ̩], [ɻ̩] and the vowel [i], and the relevant consonants are the dental [s ts tsʰ], the retroflex [ȿ tȿ tȿʰ], and the palatal [ɕ tɕ tɕʰ]. We divided the dialects by the number of sibilant places and we observed that 31 of the 155 dialects have sibilants at one place, 78 dialects have sibilants at two places, and 46 dialects have sibilants at three places. The typology of sibilant inventory is listed in Table 1. Table 1 showed that, for the 31 dialects with sibilants at one place, the majority (27/31) have dental sibilants, for the 78 dialects with sibilants at two places, the majority (76/78) have dental vs. palatal sibilants, and all of the 46 dialects with sibilants at three places have dental vs. palatal vs. retroflex sibilants. Within each group, the dialects generally belong to different dialect groups and are geographically distributed across different areas in mainland China.

We checked all of the CV syllables in the 155 dialects whose onsets are the dental, palatal, or retroflex sibilants and whose vowels are [i], [ɹ̩], or [ɻ̩]. That is, we checked five types of syllables: [sɹ̩ tsɹ̩ tsʰɹ̩], [si tsi tsʰi], [ɕi tɕi tɕʰi], [ȿɻ̩ tȿɻ̩ tȿʰɻ̩], and [ȿi tȿi tȿʰi].³ Each of the 155 dialects has at least one of the five types of syllables. For example, Xiamen Chinese (Zhou, 1991) has [si tsi tsʰi] only and no other types, while Mandarin have three types of these syllables, as in (2). It is observed from the 155 dialects that the apical vowel appears in a dialect only when there are phonological contrasts between ‘sibilant+i’ sequences. That is, there is no dialect where apical vowels [ɹ̩] and [ɻ̩] appear after the sibilants in the absence of any contrast. In particular, this holds true in the 27 dialects that are reported to have the dental sibilants only like Xiamen (Zhou, 1991). Significantly, none has the place assimilation pattern as shown in (3).⁴ The absence of cases like (3) is predicted by the contrast enhancement analysis, which takes the existence of potentially weak contrast (e.g., /si, ɕi, ȿi/) to be the cause of apical vowel formation. The place assimilation account, however, predicts the presence of (3), as it assumes that sibilant-to-[i] assimilation should apply without referring to any phonological contrast. The typological survey, therefore, supports the contrast enhancement analysis.

The enhancement analysis of apical vowels (Stevens et al., 2004, Lee & Li, 2003, Lee-Kim, 2014a) was proposed with regard to the sibilant inventory of Mandarin. That is, apical vowels [ɹ̩] and [ɻ̩] are assumed to be formed to enhance the three-way fricative distinction in /si, ɕi, ȿi/. This raises the question of whether apical vowels are only formed when there are three sibilant places in the sound inventory, i.e., in a more crowded sibilant space.⁵ To investigate this issue, we further checked separately the dialects that have three sibilant places and those that have only two sibilant places. Among the 155 dialects in the survey, there were 46 with sibilants at three places (dental vs. palatal vs. retroflex) and 45 of them avoided the three-way contrast in the [_i] context with the introduction of apical vowels, which is consistent with the enhancement analysis (Stevens et al., 2004, Lee & Li, 2003) and the cross-linguistic typology (Lee-Kim, 2014a). In the 76 dialects with the two-way dental vs. palatal sibilant contrast, contrastive dental-[i] vs. palatal-[i] is only allowed in 22 (≈29%), e.g., Jiangyong (4a), and avoided in 54 (≈71%), e.g., Dayü (4b) and Shibei (4c). Moreover, in the 54 dialects that avoid the dental-[i] vs. palatal-[i] contrasts, 52 introduced the apical vowel [ɹ̩] after the dental sibilants, as in Dayü (4b); the other two dialects avoided the contrasts with the non-combination of dental sibilants and the vowel [_i], as in Shibei (4c). Finally, in the two other dialects with two sibilant places—Jinggangshan (dentals vs. retroflexes; Lu, 1995) and Haizhou (palatals vs. retroflexes; Su, 1990)—the apical vowel [ɻ̩] has been developed after the retroflexes, and therefore, the two-way place distinction in the [_i] context is also avoided.

In general, the typological survey shows that a three-way sibilant contrast in the [_i] context is virtually always avoided via the introduction of apical vowels, in support of the contrast enhancement view of apical vowels (Stevens et al., 2004, Lee & Li, 2003). In addition, the typology also shows that a two-way sibilant contrast between dentals and palatals in the [_i] context also tends to be avoided with the introduction of the dental apical vowel [ɹ̩].

2.3. Distinctiveness between dental vs. palatal sibilants

The contrast enhancement analysis (Stevens et al., 2004; Lee & Li, 2003) was proposed primarily on the basis of the acoustic properties of Mandarin fricatives and an essential component of this view is that sibilant place contrasts have reduced distinctiveness in the [_i] context. There have been few studies that directly tested the effect of vowel contexts, e.g., [_i] vs. other vowels, on the perceptual distinctiveness of Mandarin sibilants. In this study, we conducted a perceptual experiment on the distinctiveness of Mandarin sibilants. Given that the typological survey shows that in the [_i] context, the dental vs. palatal sibilants tend to be avoided even when there are no retroflex sibilants in the sound system, we focus on the perceptual distinctiveness of the dental and palatal sibilants in this study. We reserve the distinctiveness among three sibilant places for future research.

Following the enhancement view (Stevens et al., 2004; Lee & Li, 2003), we hypothesize that the dental and palatal sibilants form perceptually ‘weak contrasts’ in the [_i] context, e.g., [si tsi tsʰi] vs. [ɕi tɕi tɕʰi]. As a baseline for comparison, we hypothesize that dental vs. palatal sibilants form more distinct contrast in the other vowel contexts (e.g., [_a] and [_ou]) than in the [_i] context. More specifically, we predict that the sound pair [si-ɕi] is less distinct than (represented by ‘<’) the pairs [sa-ɕa] and [sou-ɕou] and that the same holds for the affricate pairs:

We also hypothesize that the introduction of the apical vowel enhances the contrast. That is, we predict that pairs like [si-ɕi] are less distinct than pairs like [sɹ̩-ɕi]:

The hypotheses above refer to the sibilants in Chinese dialects, yet the vowel effect is by no means assumed to be specific to Chinese, as the avoidance of sibilant place contrast in the [_i] context has been observed crosslinguistically (Lee-Kim, 2014a).

3.2. Predictions

It was hypothesized that, for dental vs. palatal sibilants, the [_i] context would introduce reduced perceptual distinctiveness compared with other vowels. For a speeded AX-discrimination task, we predicted that the [_i] context will lead to a longer response time than other vowels. We also hypothesized that speeded AX discrimination is able to bypass the influence of L1 phonology and elicit psychoacoustic perception. Therefore, the English and Chinese listeners should have no difference in their responses, even though Chinese and English differ in their sibilant inventory.

3.3. Results

For each stimulus pair, we calculated the response time from the onset of the sibilant in the second stimulus, e.g., from the start of the frication noise of [ɕi] in the pair [si-ɕi]. The raw response time was transformed into Log Response Time (LogRT) and we analyzed the listeners’ ‘different’ responses to phonetically different pairs (i.e., the correct responses to different pairs). For each onset pair per listener, the data points outside two standard deviations from the mean of LogRT were trimmed off to exclude outliers. That is, for each listener, we trimmed off the outliers outside two standard deviations separately for the stimulus pairs whose onsets were [s-ɕ], [ts-tɕ], and [tsʰ-tɕʰ]. This was necessary because the onset pairs intrinsically differ in their durations ([s-ɕ] = 125 ms, [ʦ-ʨ]= 50 ms, and [tsʰ-tɕʰ] = 100 ms) and the onset duration was included in the response time. The 30 listeners gave 2865 responses to phonetically different pairs and 2701 (=94%) were correct ones. Out of the 2701 correct responses, a total of 120 tokens (=4.4%) were excluded as outliers, including 38 tokens below two standard deviations and 82 tokens above two standard deviations. The statistical analysis applied to the remaining 2581 tokens. The mean LogRTs for each CV pair are plotted in Figure 3 with separate graphs for Chinese and English listeners.

Figure 3

Mean LogRT for Chinese listeners (upper) and English listeners (lower). The response time was counted from the onset of the second stimulus and transformed into LogRT. (6.10 = 446 ms, 6.15 = 469 ms, 6.20 = 493 ms, 6.25 = 518 ms.) The error bars indicate the standard errors of the mean values.

The whole data set (=the 2581 tokens) was analyzed in the Linear Mixed Effects Models using the lmer function in the R package lme4 (Bates et al., 2015a, b) and the p-values were determined by the R package lmerTest (Kuznetsova et al., 2015). LogRT was taken as the dependent variable, and the predicting variables were Vowel (the four vowel contexts, [_i], [_a], [_ou] and [_ɹ̩_i]), Onset (the three onset pairs, [s-ɕ], [ts-tɕ], [tsʰ-tɕʰ]), and NativeLg (the listeners’ native languages as English vs. Chinese). The baseline for Vowel was [_i] and that for Onset was [s-ɕ] and we used contrastive coding for NativeLg (Chinese = –0.5 and English = 0.5). The random factors were Subject, Subject:Onset, and Subject:Vowel.

The fixed effects in the final model are presented in Table 3 and several steps were taken before arriving at this model. First, a null model with Subject, Subject:Onset, and Subject:Vowel as the random factors was compared separately with three superset models with Vowel, Onset, or NativeLg as the predicting variables. The addition of Vowel and Onset both significantly improved the model (Vowel: X² = 20.055, df = 3, p <.001; Onset: X² = 46.708, df =2, p <.001) and the addition of NativeLg did not significantly improve the model (X² = 0.086, df = 1, p = .769). Second, a model with Vowel, Onset, and NativeLg as the predicting variables and Subject, Subject:Onset, and Subject:Vowel as the random factors was compared separately with three superset models each including a two-way interaction, e.g., Vowel*Onset, Vowel*NativeLg, or Onset*NativeLg. It turned out that the addition of none of these interactions significantly improved the model. Finally, a model with all the two-way interactions, i.e., Vowel*Onset, Vowel*NativeLg, and Onset*NativeLg, was compared with a superset model with the three-way interaction Onset*Vowel*NativeLg. The addition of the three-way interaction did not significantly improve the model. Therefore, the final model in Table 3 included Vowel and Onset as the fixed effects and Subject, Subject:Onset, and Subject:Vowel as the random factors.⁷

From Figure 3, we can see that the effect of Vowel was due to the fact that, for each onset pair, the [_i] context had a longer RT than the contexts [_a], [_ou], and [_ɹ̩_i]; this was also shown by the coefficient estimates in Table 3. There is no evidence for a difference between Chinese and English listeners in terms of the Vowel effect since there was no Vowel*NativeLg interaction or Vowel*Onset*NativeLg interaction. We further checked the difference among the four vowel contexts [_i], [_a], [_ou], and [_ɹ̩_i] using [_i], [_a], [_ou] alternatively as the baseline. There were significant differences between [_i] and the other three vowel contexts and no significant differences among the three contexts [_a], [_ou], and [_ɹ̩_i], as summarized in Table 4.

The effect of Onset was not of interest as the onset pairs had intrinsically different durations ([s-ɕ]= 125 ms, [ʦ-ʨ]= 50 ms, [ʦʰ-ʨʰ]= 100 ms) and the response time was calculated from the onset of the second syllable in each pair. We do not discuss Onset further since there was no interaction between Onset and the other predicting variables.

The overall error rate of the responses for different pairs was 6% and the 30 listeners made a total of 164 discrimination errors (i.e., phonetically different pairs judged as being ‘the same’): 54 from the 10 Chinese listeners and 110 from the English listeners. Figure 4 below provides the number of errors in each CV pair for Chinese and English listeners. Generally, the [_i] context induced a larger number of errors compared with other vowel contexts for both groups of listeners. Due to the small number of data points, no statistics was run on the accuracy data.

Figure 4

Number of discrimination errors by Chinese listeners (upper) and English listeners (lower).

3.4. Discussion

In the response time data, the main effect of vowel context came from the fact that the [_i] context introduced a significantly longer response time than other vowels. Assuming that a longer RT indicates less distinctiveness, the result confirms the hypothesis that the dental vs. palatal sibilants are less distinct in the [_i] context than in other vowel contexts. The absence of Vowel*Onset interaction indicated that the same vowel effect held for all onset pairs. That is, the [_i] context generally induces less distinctiveness to the contrasts [s-ɕ], [ts-tɕ], and [ʦʰ-ʨʰ]. Moreover, the lack of a significant effect of NativeLg and its interactions with Vowel and Onset supported the idea that English and Chinese listeners did not differ in their responses, providing evidence for a language-independent effect of vowel context on the perceived distinctiveness of sibilant contrast. This result is consistent with the typological pattern that contrastive dental vs. palatal sibilants in the [_i] context tend to be avoided across Chinese dialects even when there are only two sibilant places in the sound inventory. Moreover, the dental vs. palatal sibilants are less distinct in the [_i] context than in the [_ɹ̩_i] context (i.e., dentals followed by apical vowel and palatals followed by [i]) and this is consistent with the enhancement account of Mandarin apical vowels (Stevens et al., 2004, Lee & Li, 2003, Lee-Kim, 2014a).

For the English subjects, the directionality of the vowel effect cannot be attributed to their L1 phonology. English has the vowels [_ɑ], [_oʊ] and [_i] but no word-initial contrast between the dental and palatal sibilants. Thus, without the dental vs. palatal contrast in any vowel context, L1 phonology should not have biased English listeners’ perception. On the other hand, even if the English listeners had tried to map the dental vs. palatal contrast to L1 contrast, the observed results still cannot be accounted for by their L1 phonology. More specifically, English has the alveolar vs. postalveolar fricatives ([s] vs. [ʃ]) in the vowel contexts [_ɑ], [_oʊ], and [_i], as in (7).

The English listeners may have tried to map the non-native pair [s-ɕ] to the L1 [s-ʃ], as in Figure 5(a), i.e., a ‘two-category assimilation’ in the Perceptual Assimilation Model (PAM, Best, 1995; Best et al., 2001); alternatively, they may have left the non-native [ɕ] uncategorized, as in Figure 5(b), i.e., an uncategorized-categorized assimilation in PAM. In either case, the mapping should not bias the perceived distinctiveness towards any vowel context, since the English [s-ʃ] contrast is observed in [_ɑ], [_oʊ] as well as [_i], as in (7). Therefore, the observed vowel effect ([_i] being less distinct) must come from factors other than L1 phonology, i.e., the psychoacoustic perception of sibilant distinctiveness in the [_i] context as compared with the other vowel contexts.

Figure 5

Mapping of stimuli onsets to English consonants.

For Chinese listeners, the reduced distinctiveness in the [_i] context agrees with the phonotactics of Mandarin, as Mandarin has the dental vs. palatal contrast in the [_a], [_ou], and [_ɹ̩_i] (i.e., dentals followed by apical vowel and palatals followed by [i]) contexts, but not the [_i] context. But the fact that English listeners did not behave differently from the Mandarin listeners indicates that the observed vowel effect cannot be based on the language-specific phonotactics of Mandarin.

4.1. Phonetic basis of the vowel effect

There are three possibilities for the acoustic basis of the vowel effect observed in our experimental results. First, the vowel effect may have resulted from the acoustic properties of the sibilant consonants. For example, it could be that the dental vs. palatal sibilants were acoustically more similar in the [_i] context, due to coarticulation or palatalization from the vowel [_i] (Lee & Li, 2003). To examine this possibility, we measured the center of gravity (COG) and the intensity of the sibilants. More specifically, COG was measured over the center 80% of the sibilants for the frequency range 0–10 kHz and intenstity was measured over the entire consonant. For the aspirated affricates [tsʰ] and [tɕʰ], the COG was measured on the turbulent noise before the aspiration portion. The results were listed in Table 5. Note that the [_ɹ̩_i] context was not included because the stimulus syllables, e.g., [sɹ̩-ɕi], differ in both consonants and vowels and the comparison of COG and intensity would not capture the acoustic difference between the relevant stimulus pairs.

As shown in Table 5(a), the COG differences (ΔCOG) between the dental and palatal sibilants were larger in the [_i] context than in the [_a] and [_ou] contexts. In Table 5(a), for example, the sibilant COG difference of [si-ɕi] was larger than that of [sa-ɕa], which is opposite to the assumption that the reduced perceptual distinctiveness in the [_i] context came from a smaller acoustic difference between the dentals and the palatals in the [_i] context. There was also no systematic pattern of intensity difference corresponding to the observed reduced distinctiveness in the [_i] context. In Table 5(b), for example, the intensity difference (ΔIntensity) of [si-ɕi] was larger than that of [sou-ɕou] whereas the intensity difference of [tsi-tɕi] was smaller than that of [tsou-tɕou]. Therefore, it is unlikely that the vowel effect ([_i] being less distinct) was rooted in the acoustic similarity of the onset sibilants.

Second, it is possible that the vowel effect came from the acoustic properties of the vowels. That is, it may be that in the stimulus pairs the vowel [_i]s (e.g., in [tsi] vs. [tɕi]) were more similar to each other than the vowel [_a]s (e.g., in [tsa] vs. [tɕa]). To test this possibility, we measured F1 and F2 at the mid point of the steady formant portion of each CV syllable. The results were listed in Table 6. To evaluate the formant difference, ΔF1 and ΔF2 were included in Table 6, which were the differences between the steady formant midpoint values in a stimulus pair. For example, for the pair [si-ɕi], ΔF2 = 2578–2549 = 29.

Previous studies have shown that, for isolated vowel formants, the Just Noticeable Difference (JND) was generally 3%–5% of the reference formant frequency (Flanagan, 1955, Kakusho & Karo, 1968; Mermelstein, 1978; Nord & Sventelius, 1979), but JND as low as 1.5% has also been reported (Kewley-Port & Watson, 1994). For vowels in consonantal contexts, Mermelstein (1978) reported mean difference limens of 60 Hz for F1 and 176 Hz for F2. To examine if the formant difference is perceivable, a ratio (in parentheses in Table 6) was calculated by dividing the formant difference value (ΔF) by the higher formant value on the left. For example, in the first cell, ΔF2 was 29Hz (i.e., 2578–2549) and the reference F2 was that of [si] (2578Hz). Thus, the ratio was 29/2578 = 1%. As shown in Table 6, most of the ratio values were below or close to 5% and only the ΔF1 of [sou-ɕou] and [tsou-tɕou] were above 10%. Thus, most of the formant differnces should not have led to salient perceptual differences. Put simply, in each stimulus pair, the two vowels are close to each other in terms of F1 and F2, and it is unlikely for the observed vowel effect to have come from the difference in the steady-state vowels.

Third, the vowel effect might have come from the properties of consonant-vowel formant transition in different vowels. Formant transition has been shown to be important in the place identification of consonants (Delattre et al., 1955; Whalen, 1981, 1991; Nowak, 2006; Babel & McGuire, 2013) and Lee-Kim (2014a) argued that vowel effects on consonant distinctiveness can be reduced to the relative magnitude of formant transitions specific to each vowel. Regarding the perception of palatal fricative, for example, a low/back vowel may provide a greater palatal transition and thus a more robust perceptual cue while a high/front vowel may provide smaller palatal transition and thus a less robust perceptual cue (Lee-Kim, 2014a). To investigate this possibility, we evaluated the transitonal difference between the dental and palatal sibilants in each CV pair, e.g., [si-ɕi], and compared this difference across the three vowel contexts. In Table 7 below, F2_onset and F2_offset indicate the formant values at the beginning and end of the consonant-vowel transition, i.e., the vocalic portion before the steady formant structures of the following vowels.⁸ We calculated ΔF2_onset, which was the F2 difference between the dental and palatal sibilants at the beginning of the CV transitions, where a larger value indicates a larger onset F2 difference. The same held for ΔF2_offset.

The values of ΔF2_offset were generally small and therefore the transitional difference between the two syllables in a stimulus pair was mostly determined by the value of ΔF2_onset. As shown in Table 7, ΔF2_onset for the same sibilant pair was always smaller in the [_i] context than in the [_a] and [_ou] contexts. In other words, the transitional difference between the dentals and palatals was the smallest in the [_i] context, which is consistent with the observation that the [_i] context induced reduced distinctiveness between the dental and palatal sibilants compared with the other vowel contexts.

To summarize, based on the measurements of onset COG and intensity, the vowel effect is unlikely to be rooted in the acoustic differences in the sibilants, nor is it likely to come from the acoustic difference in the steady vowel formants. Rather, the vowel effect is most likely to come from the fact that the formant transitions of the dentals and palatals are acoustically more similar in the [_i] context than in other vowel contexts. This is schematized in Figure 6. Given that the COG difference in the sibilants is in fact larger in the [_i] context, our results also suggest that in the listeners’ discrimination of the CV pairs, the difference in the CV transition has generally overidden the difference in the COG of the sibilants. However, we must recognize the following caveats to our conclusion: (a) Our sibilant measurements were restricted to COG and intensity, and it is possible that the onset pairs were more similar (or more distinct) in other acoustic aspects; and (b) these conclusions were drawn from the specific stimuli use in our experiment, and it is possible that the realization of the dental-i sequence in a different language is different (e.g., with more palatalization on the sibilant).

Figure 6

Schematic illustration of the vowel effect on the consonant place distinction: [_i] vs. [_a].

4.2. Contrast distinctiveness and sound systems

The linguistic literature generally agrees that human languages prefer sounds that are more distinct from each other. The distinctiveness has been discussed in terms of consonant and vowel inventories (Martinet, 1952; Lindblom, 1986, 1990; Flemming, 2002, 2004, 2005, etc.) as well as the effect of neighboring sounds on the perception of consonant contrasts (Steriade, 1997, 1999, 2001, 2008). Our study falls into the second category and the experimental results suggest that the dental and palatal sibilants are less distinct in the [_i] context than in the [_a] and [_ou] contexts where the place distinction can be better cued by larger formant transition differences between the dental and palatal sounds. This suggests that CV combination could potentially be taken as a unit on which a language configures its contrast distinctiveness. The motivation to consider units larger than segments is that the perceptual information for a segment is usually distributed over its neighboring segments (Liberman et al., 1967; Sereno et al., 1987). While it is certainly possible to discuss contrast distinctiveness in a context-neutral way (e.g., a vowel system like /i-a-u/ is generally preferred cross-linguistically), taking into account the following vowel allows a more nuanced understanding of the perceptual distinctiveness between consonants.

This perspective is compatible with the proposal of Licensing by Cue (Steriade, 1997, 1999) or P-Map (Steriade, 2001, 2008), which posits a greater likelihood of contrast in the phonetic environments where the contrasting cues can be better recovered by the listener. The experimental study in this paper showed that, regarding consonant place contrast, different vocalic contexts may differ in cue recoverability. For the dental vs. palatal sibilant contrast, the transitional cues in the [_i] context tend to be less recoverable due to the smaller transition difference in this context. Such contrasts are shown to be dispreferred in the typology of Chinese dialects. This is similar to the observation in Lee-Kim (2014a) that crosslinguistically, sibilant place contrasts in the [_i] context tend to be avoided. That is, the less recoverability of place cues in the [_i] context showed its effect in language typology.

Regarding the sound system of a particular language, evaluating distinctiveness in a unit larger than a segment would make different claims about the sound inventory. In the case of Mandarin, for example, the less distinct contrasts between the dentals and the palatals in the [_i] context (e.g., [tsi-tɕi]) are avoided with the introduction of an apical vowel after the dental sibilants (e.g., [tsɹ̩-tɕi]).¹⁰ This introduced one more vocalic sound into the vowel system, and it will inevitably make the vowel space ‘more crowded’ under a theory that evaluates the density of the vowel space with the number of vowels in the F1 and F2 dimensions (e.g., the introduction of [ɹ̩] as an allophone of /i/ makes the /i-ɤ/ contrast more crowded in the dental context). In other words, the phonological change may be deemed as an enhancment of a consonantal contrast at the cost of undermining vowel distinctiveness. However, if the CV combination is adopted as a unit to evaluate distinctiveness, it is in principle possible to compare the perceptual distinctiveness among these larger units in a unified space (e.g., [tsi-tɕi-tsɤ] vs. [tsɹ̩-tɕi-tsɤ]).