1. Introduction

Vowel epenthesis is a common repair strategy used by speakers to adapt loanwords that contain unfamiliar phonological structure in the borrowing language (e.g., Davidson, 2006; Fleischhacker, 2001; Hall, 2011; Kabak & Idsardi, 2007; Kang, 2011; Uffmann, 2006). Such is the case in Japanese where vowel epenthesis serves to make non-native structures more native-like (e.g., Hirayama, 2003; Itô, 1989; Smith, 2006; Kubozono, 2015). For example, the English word ‘pipe’ [paɪp] is commonly pronounced as [paɪpɯ] with [ɯ] occurring in word-final position, as consonants other than [ɴ] do not occur word-finally in Japanese (Kubozono, 2015). The adaptation of unfamiliar consonant sequences by epenthesis in Japanese has served as a test case for studying the influence of native speech experience on speech perception and production (e.g., Dupoux, Kakehi, Hirose, Pallier, & Mehler, 1999; Dupoux, Pallier, Kakehi, & Mehler, 2001; Dupoux, Parlato, Frota, Hirose, & Peperkamp, 2011; Monahan, Takahashi, Nakao, & Idsardi, 2009; Peperkamp & Dupoux, 2003; Shoji & Shoji, 2014; Sperbeck, 2012; Yazawa, Konishi, Hanzawa, Short, & Kondo, 2015). For example, Dupoux et al. (1999) found that native Japanese listeners perceive an illusory vowel, [ɯ],¹ between sequences of consonants that are illicit in Japanese. This finding is consistent with research showing that speech perception is constrained by a listener’s phonotactic knowledge; that is, non-native sound sequences are generally assimilated perceptually to licit sequences in the listener’s native language (e.g., Best, 1994, 1995; Best & Strange, 1992; Dupoux et al., 2001; Dupoux et al., 2011; Hallé, Segui, Frauenfelder, & Meunier, 1998; Kabak, 2003).

Japanese epenthesis has also received attention in the literature because three separate epenthetic vowels have been observed, [i, o, ɯ], with the choice among them being dependent on the quality of the preceding consonant (e.g., Hirayama, 2003; Irwin, 2011; Katayama, 1998; Kubozono, 2001; Lovins, 1975; Otaki, 2012). In nativized loanwords, the vowel [ɯ] has been shown to occur after labial, alveolar (except alveolar stops), and velar² consonants and is generally considered to be the default epenthetic vowel (Hirayama, 2003; Shoji & Shoji, 2014; Kubozono, 2015). Meanwhile, the vowel [i] is epenthesized after the palatal affricates [tɕ] and [dʑ], and [o] is used after the alveolar stops [t] and [d] (e.g., Kaneko, 2006; Kubozono, 2015).

This basic pattern of the distribution of the epenthetic vowel is corroborated by the findings of Yazawa et al. (2015). They investigated whether patterns of English speech production by Japanese learners of English are similar to the phonology of loanword epenthesis in Japanese, considered in relation to the level of English proficiency of the speakers. They analyzed speech corpus data of Japanese participants reading the Aesop fable “The North Wind and the Sun” in English, recorded in 2009 as part of the J-AESOP (Asian English Speech cOrpus Project) Corpus. They used a combination of an automatic annotator and manual/visual inspection of the formant space in comparison to ‘typical’ lexical vowels of Japanese to identify the quality of the epenthetic vowels. Their results showed that irrespective of learners’ proficiency level, the quality of epenthetic vowels is similar to the patterns in loanword phonology. That is, an epenthetic vowel has a quality close to [o] after [t] and [d], [i] after [tɕ] and [dʑ], and [ɯ] when it follows any other consonant.

Typically, the explanation for this basic distributional pattern has relied on a combination of perceptual and phonotactic observations. The occurrence of [ɯ] as the ‘default’ epenthetic vowel is consistent with the view that the epenthetic vowel is the perceptually least salient in the language (Byarushengo, 1976; Fleischhacker, 2001; Kang, 2003; Kenstowicz, 2007; Shinohara, 1997; Steriade, 2001, 2008), as [ɯ] is considered to be the shortest vowel and the most susceptible to weakening and deletion in Japanese, common properties of perceptually weak vowels (e.g., Hirayama, 2003; Sagisaka & Tokuhara, 1984 as cited in Irwin, 2011; Shoji & Shoji, 2014; Kubozono, 2015). Meanwhile, the use of [i] after palatals is arguably because the front vowel [i] shares similar articulatory and perceptual properties with these consonants (Kubozono, 2015). Finally, the use of [o] after alveolar stops is presumably because neither of the other two vowels ([ɯ] or [i]) is phonotactically licit in this position in native Japanese (i.e., *[tɯ], *[dɯ], *[ti], and *[di]) (Kaneko, 2006; Kubozono, 2015). Kubozono (2015) suggests that the choice of [o] is also associated with perceptual properties; inserting [o] after alveolar stops keeps the original consonants, while inserting [ɯ] after alveolar stops could result in the alveolars changing to affricates [ts] and [dz] due to an allophonic rule in Japanese (Hirayama, 2003; Irwin, 2011; Kubozono, 2015).

There are, however, several potential problems with these explanations of the choice of epenthetic vowel. First, recent perceptual epenthesis studies suggest that the distribution of epenthetic vowels in perception differs from that observed in nativized loanwords and the Yazawa et al. (2015) corpus study. Mattingley, Hume, and Hall (2015) asked Japanese-speaking listeners to identify what vowel, if any, occurred between the two Cs in VCCV forms, where the place of articulation of the first C varied. In the labial, alveolar stop, and velar contexts, they perceived an epenthetic vowel between the two Cs 60–70% of the time, and in the palatal context, they perceived an epenthetic vowel 98% of the time. The choice of epenthetic vowel, however, did not always match the distribution described above. In the palatal context and the velar context, the perceptual epenthetic vowels were largely as expected; of the tokens where an epenthetic vowel was perceived at all, 94% were the expected [i] in the palatal context and 94% were the expected [ɯ] in the velar context. In the labial context, however, only 84% of perceived vowels were the expected [ɯ], with the rest being fairly equally distributed among [a], [e], and [i]. In the alveolar stop context, the discrepancy was even more extreme, with only 16% of perceived vowels being the expected [o], and 71% being [ɯ] instead, even though *[dɯ] is an illicit phonotactic sequence in native Japanese. It should be noted that there was no control group for language in this study; thus it is possible that these perceptual effects could be driven not by Japanese-specific patterns but rather by more general acoustic characteristics of the stimuli. Interestingly, Monahan et al. (2009) found that while Japanese-speaking listeners do perceive an illusory [ɯ] in velar VCCV contexts, they were able to discriminate alveolar VCCV sequences from similar sequences with either a medial [ɯ] or [o], suggesting that they perceived these as VCCV sequences without an epenthetic vowel (they did not test labial or palatal contexts).

There is also evidence from a different type of production study than that used in Yazawa et al. (2015) that suggests that the productive choice of epenthetic vowel may be different from that in lexicalized loanwords. Shoji and Shoji (2014) used a writing production experiment to examine patterns of vowel epenthesis in hypothetical loanwords from nonce words spelled in orthographic Latin script; native Japanese speakers transcribed the nonce words in Japanese characters, which forced them to either delete or epenthesize in every case. They found that in palatal contexts, where [i] might be expected, [i] occurred only 34% of the time in word-initial clusters, with [ɯ] occurring another 23.7% of the time; in word-final clusters, [i] occurred 85.6% of the time, while [ɯ] occurred 12.2% of the time. They also found that in alveolar-stop contexts, where [o] might be expected, [o] occurred only 45.6% of the time and [ɯ] occurred 32.32% of the time in word-initial clusters; [o] occurred 91.1% of the time and [ɯ] 5.6% of the time in word-final clusters. In velar contexts, where [ɯ] might be expected, [ɯ] did in fact occur 71% of the time in word-initial clusters, while [i] was used 15.6% of the time, and [ɯ] was used 95.6% of the time in word-final clusters. They do not report the results of labial contexts separately, but do find the expected [ɯ] used in their ‘other’ contexts, which included labials, >90% of the time in both word-initial and word-final clusters. Thus, while the traditionally expected vowels were the most frequent in any given context, in several contexts, they accounted for fewer than half of the actual tokens.

Third, there is separate evidence that the phonotactic constraints governing loanwords are changing in non-epenthetic contexts. Pintér (2015, p. 121–122) points out that while older loanwords with /ti/ sequences were typically adapted as [tɕi] (e.g., ‘team’ adapted as [tɕi:mɯ]), more recent loanwords are adapted more faithfully with [ti] (e.g., ‘party’ adapted as [pa:ti:]). Kubozono (2015, p. 325) notes that a single loanword can also have multiple, age-related adaptations (e.g., ‘tissue’ being adapted as either [tʃitʃʃɯ] or [tetʃʃɯ] by older speakers but as [tiʃʃɯ] by younger speakers). That said, [t] and [d] have been observed to occur before [i] in at least some loanwords since at least 1950 (Bloch, 1950 cites examples like vanity case [vaniti] and caddy [kjadi:]). A similar change is reported for /tu/ sequences; despite the traditional constraint against [tɯ] in Japanese, Pintér (2015) reports that this sequence is in fact possible in more recent loanwords, e.g., ‘Bantu’ adapted as [bantɯ:]. As Pintér (2008, p. 112) also points out, even the official stance of the Japanese National Language Committee changed between 1954 and 1991; in 1954, [tɯ] was not acknowledged as a possible written syllable of Japanese, but in 1991, it was acknowledged though not officially supported. Thus, the phonotactic motivations for epenthesizing only [o] after alveolars and [i] after only palatals may be eroding. Indeed, an examination of the Balanced Corpus of Contemporary Written Japanese (National Institute for Japanese Language and Linguistics, 2011) indicates that all five lexical vowels can occur after both [d] and [dʑ] in loanwords (defined as words of foreign origin other than Chinese); see also discussion in Hall (2009, 2013) showing that loanwords are eroding the predictability of several pairs of Japanese phonemes.

These results combine to raise questions about the current state of epenthesis in Japanese. Specifically, it would appear that the use of [o] after alveolar stops and [i] after palatals, i.e., the typical pattern in loanword epenthesis, is not currently a clear-cut pattern in perception, production, and other areas of loanword adaptation. This suggests that either perception and production factors are not the explanatory causes of the lexicalized loanword epenthesis patterns or that there may similarly now be a change in the epenthesis adaptation patterns themselves. The current study is designed to be a first step in understanding the current state of loanword epenthesis patterns from a production perspective. It involves a production task that tests the full range of epenthetic vowels used to break up VCCV sequences in Japanese; in order to maintain maximal control over the stimuli, however, these sequences are simply presented as nonce words, rather than being loanwords in any real sense (i.e., they are not associated with meaning or claimed to come from a particular foreign source language); we note, however that it has generally been claimed that loanwords and nonce words in Japanese tend to follow similar grammatical patterns. As Kawahara (2012, p. 1194) points out, “both loanwords and nonce words show default accentuation patterns in Japanese (e.g., Katayama, 1998; Kawahara & Kao, 2012; Kubozono, 1996, 2006, 2008; Labrune, 2012; McCawley, 1968; Shinohara, 2000), [and] neither native words nor Sino-Japanese words allow voiced geminates, while both loanwords and nonce words allow them (Itô & Mester, 1995, 1999, 2008).” Kawahara goes on to demonstrate that Japanese speakers judge both loanwords and nonce words as being more natural when they follow Lyman’s Law, even though Lyman’s Law does not hold categorically outside of the native vocabulary. Thus, we have reason to believe that nonce words might be a reasonable proxy for loanwords.

To anticipate the results, some patterns were consistent with earlier studies, while there was considerable individual speaker variability in other cases. In particular, [ɯ] was consistently used by all 14 participants after both labials and velars, which is in line with reported patterns of loanword adaptation. However, after alveolar stops, only two participants used the expected [o], with the other 12 speakers either using [ɯ] or using some combination of both [o] and [ɯ], suggesting that the loosening of the phonotactic constraint against [dɯ] is possibly extending to epenthetic contexts. Perhaps most surprisingly, the palatal context induced a large range of epenthetic vowels; again, only two speakers consistently used the expected vowel, [i], in this context, while four used [ɯ], and the rest used some combination of almost every lexical vowel, including [e] and [a], which are not generally reported as being used epenthetically at all.

3. Results

3.1. Overall acoustic characteristics of the production tokens

3.1.1. Characteristics of the control (lexical) vowels

We start by considering the acoustic characteristics of the vowels in the control condition, to establish a baseline of the vowel characteristics against which the epenthetic vowels can be compared. First, we consider vowel duration. Table 5 shows that the ranking of vowels produced by speakers, from longest to shortest mean duration, is [a], [e], [o], [i], and [ɯ], consistent with earlier vowel duration studies (Campbell, 1992; Han, 1962, cited in Shoji & Shoji, 2014). The standard deviation of vowel durations ranges from 17 to 19 ms, and is consistent across vowels. Overall, [i] and [ɯ] are the shortest vowels, approximately the same as each other, and shorter than all of the others. An ANOVA on a linear mixed effect model with random slopes for speaker indicates that there is a significant effect of vowel quality on duration [F(4, 52) = 78.471, p < 2.2e-16]. A series of subsequent post-hoc t-tests indicate that [i] and [ɯ] are each significantly shorter than either [a] or [e] (p < 0.02), but that there are no other significant differences in vowel durations.

Table 5

Mean durations and standard deviations (SD), in milliseconds, for each vowel, from 14 speaker averages.

	Vowel Duration (ms)
	Mean	SD
[a]	104	19
[e]	98	18
[o]	93	19
[i]	81	17
[ɯ]	81	19

Next, we consider the quality of each vowel. Figure 3 shows the overall normalized F1/F2 spaces for each lexical vowel across all 14 speakers and all four consonantal contexts, which on the whole are fairly well separated. It can be seen that [i] slightly overlaps with [e], and [ɯ] slightly overlaps with each of [e] and [o]. The high vowel [i] is higher and more fronted than any other vowel. The vowels [e] and [o] are similar in height, and the high vowel [ɯ] and low vowel [a] are similar in backness. Note that both of these latter vowels are actually more central than back. An articulatory study (Nogita, Yamane, & Bird, 2013) reported that the vowel conventionally described as the high back vowel in Japanese is, in fact, a rounded high central vowel [ʉ] in younger speakers. Participants in the current study were mostly under 35 years old, except for two participants in their 40s. Although the vowel [ɯ] is phonetically central, for readers’ convenience, we use [ɯ] for this vowel, following the usual convention.

Figure 3

Lobanov normalized mean F1 and F2 values for each lexical vowel from 14 speakers. Ellipses represent 95% data ellipses.

3.2. Epenthetic vowels in the labial context

3.2.1. F1 and F2 analyses

We begin by looking at the quality of the epenthetic vowel in the labial context where, according to the traditional statement of the distributional pattern of epenthesis, we would expect it to be similar to the lexical vowel [ɯ]. The current results largely support that expectation. Figure 4 shows lexical vowels from [bVC]-forms and epenthetic vowels from [bC]-forms for all speakers; ellipses are 95% data ellipses. For this context, the epenthetic vowel /V/ (represented by black circles) largely overlaps with the lexical vowel /ɯ/ (pink triangles), though both /V/ and /ɯ/ also slightly overlap with the space of /o/ (dark blue crossed squares). The number of tokens of each vowel are: [a] = 77, [e] = 80, [i] = 78, [o] = 74, [ɯ] = 76, and V = 107. As can be seen, a few productions of the epenthetic vowel are outside of the bound of [ɯ]; specifically, there are three tokens that seem to clearly fall in the space of [i] and two that seem to clearly fall in the space of [a]. Overall, however, most of the tokens are clearly in the high-back vowel space, and overlap entirely with the space for [ɯ].

Figure 4

Normalized mean F1 and F2 values for each lexical vowel and the epenthetic vowel from 14 speakers in the labial context. Ellipses are 95% data ellipses. The number of tokens of each vowel are: [a] = 77, [e] = 80, [i] = 78, [o] = 74, [ɯ] = 76, and V = 107. The epenthetic vowel /V/ largely overlaps with the lexical vowel /ɯ/, though some productions occur outside of the [ɯ] space.

Two linear mixed-effect models, to predict either F1 or F2 values from vowel quality, were fit to the 492 tokens of lexical and epenthetic vowels in the labial context, shown in Tables 6 (F1) and 7 (F2). The intercept in each case was set to be the epenthetic vowel V. Thus, we can see for each vowel whether the model predicts its formant values to be significantly different from those of the epenthetic vowel. For the F1 model (Table 6), each of the other vowels is predicted to have a significantly different F1 value than the epenthetic vowel V, with the notable exception of [ɯ] (t = –0.919, p = 0.379). The vowels [a], [e], and [o] are predicted to have statistically significant F1 values that are higher than that of the epenthetic vowel (i.e., to be significantly lower vowels), while [i] is predicted to have a statistically significant F1 value that is lower than that of the epenthetic vowel (i.e., to be significantly higher). For F2 (Table 7), the results also show a significant effect of most vowels except for [ɯ] (t = –0.657, p = .523). That is, no significant difference in formants was found between the epenthetic vowel and [ɯ] in this context, while the epenthetic vowel is decidedly different from most of the other vowels.⁶ The vowels [i] and [e] are predicted to have significantly higher F2 values than the epenthetic vowel (i.e., to be significantly more fronted vowels), while [o] (and to some extent, [a]) is predicted to have a lower F2 value than the epenthetic vowel (i.e., to be a more backed vowel). In short, the quality of the epenthetic vowel is extremely similar to [ɯ] in both height and backness, and not particularly similar to any other vowel.

Table 6

Effect estimates and p-values on predictors for the F1 value of vowels in the labial context. Vowels with a significantly different F1 value from the epenthetic vowel V are marked with asterisks.

	Estimate	SE	t value	Pr(>|t|)
(Intercept)	–0.469	0.045	–10.536	6.73e-06	***
Vowel a	2.52	0.067	37.492	2.96e-12	***
Vowel e	0.580	0.067	8.707	5.19e-06	***
Vowel i	–0.644	0.067	–9.603	2.19e-06	***
Vowel o	0.556	0.068	8.197	6.71e-05	***
Vowel ɯ	–0.062	0.067	–0.919	0.379

Table 7

Effect estimates and p-values on predictors for the F2 value of vowels in the labial context. Vowels with a significantly different F2 value from the epenthetic vowel V are marked with asterisks.

	Estimate	SE	t value	Pr(>|t|)
(Intercept)	–0.502	0.115	–4.381	0.0006	***
Vowel a	–0.282	0.156	–1.813	0.094	.
Vowel e	1.443	0.152	9.491	2.82e-07	***
Vowel i	2.195	0.156	14.093	4.90e-09	***
Vowel o	–1.052	0.156	–6.747	1.64e-05	***
Vowel ɯ	–0.102	0.156	–0.657	0.523

3.2.2. Duration analyses

Figure 5 presents a set of box plots for the vowel durations of the lexical vowels (in [bVC]-forms) and epenthetic vowels (V in [bC]-forms) for all 14 speakers. The epenthetic vowel tends to be shorter than the other vowels; in raw terms, then, it is most similar to [ɯ]. The results of a linear mixed-effect model predicting the vowel duration from vowel quality are shown in Table 8. The intercept was again set to be the epenthetic vowel V. Thus, we can see for each vowel whether the model predicts its duration to be significantly different from that of the epenthetic vowel. The results show that [a], [e], and [o] are predicted to have significantly different values than the epenthetic vowel (p < 0.05). The epenthetic vowel V is not quite significantly different from [i] assuming an alpha value of 0.05 (t = 1.96, p = 0.076), and it is also not significantly different from [ɯ] (t = –0.02, p = 0.98). Thus, it can be seen that the vowel inserted after a labial consonant is most similar in duration to the shortest vowel, which is [ɯ].

Figure 5

Boxplots of vowel durations for all speakers in the labial context.

Table 8

Effect estimates and p-values on predictors for vowel duration in the labial context. Vowels with a significantly different duration value from the epenthetic vowel V are marked with asterisks.

	Estimate	SE	t value	Pr(>|t|)
(Intercept)	82.54	5.88	14.03	1.90e-11	***
Vowel a	25.98	4.15	6.27	5.75e-05	***
Vowel e	19.70	4.10	4.80	0.0005	***
Vowel i	8.11	4.14	1.96	0.076
Vowel o	13.42	4.16	3.22	0.008	**
Vowel ɯ	–0.09	4.15	–0.02	0.98

3.3. Epenthetic vowels in the velar context

3.3.1. F1 and F2 Analyses

As is the case in the labial context, we expect from prior descriptions that the quality of the epenthetic vowel in the velar context [ɡ] will be similar to that of the lexical vowel [ɯ], and once again, the current results largely corroborate that expectation. Figure 6 shows the overall vowel space for the lexical vowels from [ɡVC]-forms and epenthetic vowels from [ɡC]-forms for all 14 speakers. The number of tokens are: [a] = 79, [e] = 52,⁷ [i] = 61, [o] = 84, [ɯ] = 77, and V = 121. As can be seen in Figure 6, the epenthetic vowel space V (again represented by black circles) is almost entirely overlapping with the vowel space of [ɯ] (pink triangles). Again, however, there are a few productions of the epenthetic vowel in both the [o] space and the [i] space.

Figure 6

Normalized mean F1 and F2 values for each lexical vowel and the epenthetic vowel from 14 speakers in the velar context. Ellipses are 95% data ellipses. The number of tokens of each vowel are: [a] = 79, [e] = 52, [i] = 61, [o] = 84, [ɯ] = 77, and V = 121. The epenthetic vowel space V is almost entirely overlapping with the vowel space of [ɯ], though some productions are scattered outside of the [ɯ] boundary.

As before, linear mixed-effect models predicting F1 or F2 from vowel quality were fit to the 474 tokens of lexical and epenthetic vowels in the velar context. Results are shown in Tables 9 (F1) and 10 (F2). The intercept is the epenthetic vowel V. For F1 (Table 9), each vowel was significantly different from the epenthetic V, except for [ɯ] (t = –0.255, p = .805). The vowels [a], [e], and [o] are predicted to have significantly higher F1 than the epenthetic vowel, while [i] is predicted to have a significantly lower F1 value than the epenthetic vowel. For F2 (Table 10), as in the labial context, the results show a significant effect of each vowel except for [a] (t = –1.063, p = 0.308) and [ɯ] (t = –0.620, p = 0.546), with [e] and [i] being significantly fronter and [o] being significantly backer. Overall, there is no significant difference between the epenthetic vowel and [ɯ] for F1 and F2 values in the velar context, while the epenthetic vowel is significantly different from all other vowels in at least one dimension.

Table 9

Effect estimates and p-values on predictors for the F1 value of vowels in the velar context. Vowels with a significantly different F1 value from the epenthetic vowel V are marked with asterisks.

	Estimate	SE	t value	Pr(>|t|)
(Intercept)	–0.534	0.058	–9.152	3.94e-06	***
Vowel a	2.633	0.079	33.450	2.36e-10	***
Vowel e	0.488	0.084	5.826	0.0001	***
Vowel i	–0.632	0.082	–7.739	1.73e-05	***
Vowel o	0.612	0.078	7.873	3.45e-05	***
Vowel ɯ	–0.020	0.079	–0.255	0.805

Table 10

Effect estimates and p-values on predictors for the F2 value of vowels in the velar context. Vowels with a significantly different F2 value from the epenthetic vowel V are marked with asterisks.

	Estimate	SE	t value	Pr(>|t|)
(Intercept)	–0.348	0.122	–2.851	0.013	*
Vowel a	–0.180	0.170	–1.063	0.308
Vowel e	1.518	0.171	8.856	7.17e-07	***
Vowel i	2.049	0.171	12.011	2.51e-08	***
Vowel o	–1.159	0.165	–7.019	8.41e-06	***
Vowel ɯ	–0.105	0.170	–0.620	0.546

3.3.2. Duration analyses

Figure 7 presents a set of box plots showing the durations for lexical vowels (in [ɡVC]-forms) and epenthetic vowels (V in [ɡC]-forms) for all 14 speakers. Again, the epenthetic V in this context is quite short, and appears to be most similar to [i] or [ɯ]. The linear mixed-effect model results for duration in Table 11 confirm this observation; the values for [a], [e], and [o] are all significantly different from that of the epenthetic vowel (p < 0.01), while [i] (t = 0.57, p = 0.58) and [ɯ] (t = 0.89, p = 0.40) were not significantly different.

Figure 7

Vowel durations in the velar context for all speakers.

Table 11

Effect estimates and p-values on predictors for vowel durations in the velar context. Vowels with a significantly different duration value from the epenthetic vowel V are marked with asterisks.

	Estimate	SE	t value	Pr(>|t|)
(Intercept)	73.78	6.11	12.08	6.33e-10	***
Vowel a	25.85	3.98	6.49	0.0001	***
Vowel e	26.70	4.23	6.31	6.35e-05	***
Vowel i	2.37	4.12	0.57	0.58
Vowel o	16.58	3.93	4.22	0.003	**
Vowel ɯ	3.55	4.00	0.89	0.40

When taken together, the results based on the duration and quality of the epenthetic vowel in the velar context indicate that it is most similar to [ɯ].

3.4. Epenthetic vowels in the alveolar context

3.4.1. F1 and F2 analyses

Figure 8 presents a vowel plot of both the lexical vowels from [dVC]-forms and the epenthetic vowels from [dC]-forms for all 14 speakers. Recall that based on previous accounts, the epenthetic vowel is predicted to be most similar to [o] in this context. The number of tokens of each vowel are: [a] = 76, [e] = 74, [i] = 78, [o] = 81, [ɯ] = 67, and V = 114.

Figure 8

Normalized mean F1 and F2 values for each lexical vowel and the epenthetic vowel, averaged across 14 speakers, in the alveolar context. The number of tokens of each vowel are: [a] = 76, [e] = 74, [i] = 78, [o] = 81, [ɯ] = 67, V = 114. Ellipses are 95% data ellipses. (b) Individual F1 and F2 plots for each speaker in the alveolar contact. The first two speakers appear to use epenthetic [o]; the next seven appear to use [ɯ]; and the final five appear to use either both [o] and [ɯ] or a vowel that is somewhere between these two lexical vowels.

As can be seen in Figure 8a, the vowel space of the epenthetic vowel overlaps substantially with both the high vowel [ɯ] and the mid vowel [o], indicating that there may be variability as to which vowel is epenthesized, a result that is strikingly different from that seen in the labial and velar contexts.

A closer look at the results reveals that much of this variability can be attributed to variability across speakers (see Figure 8b). Only two speakers (M1 and M3) had epenthetic vowels that clearly matched the expected pattern of being [o] (see Figure 9a). Another seven (F1, F4, F9, F10, F13, F15, and M2) had a clear [ɯ] in this context instead (see Figure 9b). Finally, five speakers (F5, F7, F12, F16, and M4) had epenthetic vowels that either alternated between the two categories or spanned both categories.⁸ Impressionistically, two speakers, F7 and F16, seem to sometimes use [o] and sometimes use [ɯ]; their lexical vowels are clearly separated in acoustic space, and they have epenthetic vowels that sometimes fall into one category and sometimes into the other. Another two speakers, F5 and F12, have lexical [o] and [ɯ] categories that are quite similar to each other. Their epenthetic vowels span this entire space, making it harder to determine whether they are sometimes using [o] and sometimes [ɯ], or simply have one epenthetic vowel category that is somewhere in between. Finally, M4 is particularly difficult to categorize; his lexical vowels are fairly distinct, and most of his epenthetic vowels seem to be within the [o] category, but there are three tokens that are closer to his [ɯ] category; all three come from the word [addʑa], suggesting that the following context may be influencing the quality of the epenthetic vowel as well.

Figure 9

(a) Normalized mean F1 and F2 values for each lexical vowel and the epenthetic vowel in the alveolar context from (a) two speakers [M1, M3] who appear to primarily use [o] as the epenthetic vowel and (b) seven speakers [F1, F4, F9, F10, F13, F15, M2] who appear to primarily use [ɯ] as the epenthetic vowel. Ellipses are 95% data ellipses. In (a), the number of tokens for each vowel is: [a] = 9, [e] = 10, [i] = 10, [o] = 11, [ɯ] = 10, and V = 18. In (b), the number of tokens for each vowel is: [a] = 39, [e] = 38, [i] = 40, [o] = 40, [ɯ] = 35, and V = 56.

This division of the speakers into subsets was based on both visual inspection of their individual vowel plots and statistical analyses of each individual speaker (the complete results of which are shown in Appendix B). That said, we acknowledge that the amount of intra-speaker replication is quite small, and the experiment was not designed to examine this kind of individual variability (see also discussion in Senn, 2014); we are not trying to make strong claims about the relative strength or frequency of any of these sub-patterns. Instead, the key point here is that the alveolar-stop context resulted in a considerable degree of both intra- and inter-speaker variability in a way that is quite different from that of the labial and velar contexts discussed above.

3.4.2. Duration analyses

We now turn to an examination of the durations of the epenthetic vowel in the alveolar-stop context. For the two sub-groups of participants that seemed to consistently produce an epenthetic vowel quality similar to one of the lexical vowels, the duration results are also consistent with their producing that same lexical vowel. In particular, it is notable that the epenthetic vowel is not simply similar to the shortest vowel (regardless of quality), but rather seems to match the duration of the lexical vowel that it is similar in quality to. That is, for the two speakers for whom the epenthetic vowel is similar in quality to [o] in the alveolar context, the duration of the epenthetic vowel is most similar to [e] or [o], and is in fact both significantly longer than [i] and [ɯ], and significantly shorter than [a] (see Figure 10a and Table 12a). For the seven speakers for whom the epenthetic vowel is similar in quality to [ɯ], the duration is also short and consistent with both [i] and [ɯ], although also not quite statistically significantly different from [o] (see Figure 10b and Table 12b).

Figure 10

Boxplots of vowel durations in the alveolar context for (a) two speakers who produced [o] as the epenthetic vowel and (b) seven speakers who produced [ɯ] as the epenthetic vowel.

Table 12

Effect estimates and p-values on predictors for vowel duration in the alveolar context for two groups. Vowels with a significantly different duration value from the epenthetic vowel V are marked with asterisks.

(a) Two speakers who produced [o]
	Estimate	SE	t value	Pr(>|t|)
(Intercept)	86.94	5.71	15.23	0.011	*
Vowel a	18.35	5.15	3.57	0.0007	***
Vowel e	7.66	4.97	1.54	0.129
Vowel i	–9.97	4.98	–2.00	0.05	*
Vowel o	–5.07	4.82	–1.05	0.30
Vowel ɯ	–13.44	4.97	–2.71	0.009	**
(b) Seven speakers who produced [ɯ]
	Estimate	SE	t value	Pr(>|t|)
(Intercept)	85.35	10.35	8.24	3.94e-05	***
Vowel a	23.29	5.65	4.12	0.002	**
Vowel e	18.26	5.67	3.22	0.008	**
Vowel i	5.30	5.57	0.95	0.360
Vowel o	11.01	5.64	1.95	0.076
Vowel ɯ	5.65	5.72	0.99	0.342

3.5. Epenthetic vowels in the palatal context

3.5.1. F1 and F2 analyses

Finally, we turn to the palatal context, [dʑ]. Recall that the expected epenthetic vowel based on earlier studies would be similar to the lexical vowel [i]. However, this was not found to be the case for the majority of speakers in this study. In fact, only two speakers produced a vowel similar to [i], while four produced epenthetic vowels similar to [ɯ], and for the others there was a great deal of variability. As above, we describe the results in terms of two factors, formant frequency and vowel duration. Vowel quality results are described in terms of observable trends in the data; they were not analyzed statistically since the dataset for each pattern is small. Statistical results for each individual speaker in this context are given in Appendix C.

Figure 11a shows the individual epenthetic and lexical vowels across all speakers in the palatal context. As can be seen, the epenthetic vowels are broadly distributed across the entire vowel space, overlapping with each of the other five vowel categories.⁹ Based on a combination of visual inspection of the individual plots in Figure 11b and the statistical analyses in Appendix C, speakers generally fall into one of three groups, as discussed just below: (a) epenthetic-[i]; (b) epenthetic-[ɯ]; or (c) variable.

Figure 11

Normalized mean F1 and F2 values for each lexical vowel and individual epenthetic vowels from (a) all 14 speakers in the palatal context and (b) for each individual speaker in the palatal context.

Of the 14 speakers, two (F7, F10) appeared to consistently produce an epenthetic [i] in this context, the vowel observed after palatals in prior studies. As shown in Figure 12, the acoustic space of the epenthetic vowel for these two speakers completely overlaps with that of lexical vowel [i].

Figure 12

Normalized mean F1 and F2 values for each lexical vowel and the epenthetic vowel from two speakers (F7, F10) who appear to epenthesize [i] in the palatal context. Number of vowel tokens displayed: [a] = 11, [e] = 11, [i] = 10, [o] = 10, [ɯ] = 12, V = 17. Ellipses are 95% data ellipses.

Four speakers (F1, F13, F16, M2) consistently produced an epenthetic vowel that is most consistent with their lexical [ɯ] in this context, as shown in Figure 13. Although the epenthetic vowel ellipsis extends slightly into the region of [i] and [e], it mostly overlaps with the acoustic space of [ɯ]. Notably, the lexical [ɯ] vowel in this context appears to be fronted as compared to its production in the other contexts, such that it also overlaps with lexical [i] and [e].

Figure 13

Normalized mean F1 and F2 values for each lexical vowel and the epenthetic vowel from four speakers (F1, F13, F16, M2) who appear to epenthesize [ɯ] in the palatal context. Number of vowel tokens: [a] = 23, [e] = 24, [i] = 23, [o] = 23, [ɯ] = 23, V = 36. Ellipses represent 95% confidence intervals.

Finally, the remaining eight speakers (F4, F5, F9, F12, F15, M1, M3, M4) were not sorted into either of the two groups above, since each of these speakers produced epenthetic vowels of multiple different qualities. Statistical analysis did not support that the speakers produced any specific vowel in the palatal context. Vowel quality results are described in terms of observable trends in the data in Figure 11 and are summarized in Table 13. Again, these results are not meant to be definitive descriptions of what these speakers might always do, but instead are intended to highlight the wide range of variability in this context, including the use of [a] and [e].

Table 13

A list of the results in palatal context, by speaker within the ‘variable’ group.

Speaker	Epenthetic Vowel in Context
	[dʑ]
F4	[e], [i], [ɯ]
F5	[a], [e], [o], [ɯ]
F9	[a], [e], [i], [ɯ]
F12	[a], [e], [ɯ]
F15	[a], [i], [ɯ]
M1	[a], [e], [i], [o]
M3	[a], [e], [i]
M4	[a], [e], [ɯ]

3.5.2. Duration analyses

Turning to duration, the vowel durations for the epenthetic vowel in the palatal context overall were also quite variable—epenthetic vowels were as short and as long as the shortest and longest lexical vowels, which isn’t surprising given that there were tokens of the epenthetic vowel in this context that matched each possible lexical vowel quality. Figure 14 provides boxplots and Table 14 provides a summary of vowel durations in the palatal context for the two groups of speakers who were relatively consistent in their productions in this context, i.e., the two speakers who produced [i] and the four speakers who produced [ɯ]. These groups of participants were also more consistent with their vowel durations; both groups produced consistently short vowels, similar in duration to the lexical vowel they seemed to be producing.

Figure 14

Boxplots of vowel durations for (a) the two speakers who appear to epenthesize [i] in the palatal context and (b) the four speakers who appear to epenthesize [ɯ] in the palatal context.

Table 14

Effect estimates and p-values on predictors for vowel duration in the palatal context for (a) the group who produced [i] and (b) the group who produced [ɯ]. Vowels with a significantly different duration value from the epenthetic vowel V are marked with asterisks.

(a) Two speakers who produced [i]
	Estimate	SE	t value	Pr(>|t|)
(Intercept)	54.12	7.69	7.04	7.09e-05	***
Vowel a	37.12	10.17	3.68	0.003	**
Vowel e	35.14	10.17	3.45	0.005	**
Vowel i	–1.26	10.30	–0.12	0.90
Vowel o	27.63	10.30	2.68	0.02	*
Vowel ɯ	19.89	10.10	1.97	0.07
(b) Four speakers who produced [ɯ]
	Estimate	SE	t value	Pr(>|t|)
(Intercept)	63.08	7.57	8.34	7.26e-05	***
Vowel a	37.93	7.16	5.30	0.0002	***
Vowel e	21.79	7.12	3.06	0.01	*
Vowel i	–2.18	7.16	–0.30	0.77
Vowel o	24.63	7.16	3.44	0.006	**
Vowel ɯ	4.64	7.16	0.65	0.53

For speakers who appear to use the vowel [i] as epenthetic after the palatal [dʑ], the plot in Figure 14(a) and Table 14(a) show that their epenthetic vowel is quite short, similar to their lexical [i]. A linear mixed effect model showed that [a], [e], and [o] are predicted to have significantly larger duration values than the epenthetic vowel (p < 0.01). Both [i] (t = –0.12, p = 0.90) and [ɯ] (t = 1.97, p = 0.07) were not significantly different from V, though [ɯ] was close.

For speakers who appear to use the vowel [ɯ] as epenthetic after the palatal [dʑ], the plot in Figure 14(b) and Table 14(b) show that the shortest vowel of this group is the epenthetic vowel [i]. A linear mixed effect model showed [a], [e], [o] are predicted to have significantly different duration values than the epenthetic vowel (p < 0.01). Both [i] (t = –0.33, p = 0.77) and [ɯ] (t = 0.65, p = 0.53) were not significantly different from V.

In summary, for the two groups that produced a single epenthetic vowel quality in the palatal context, their epenthetic vowel was consistently short, and not significantly different in either case from the duration of the lexical vowel they seemed to use. In both cases, though, this meant that the epenthetic vowel was not significantly different from either [i] or [ɯ].

4. Discussion

The current study was designed to directly test the nature of epenthesis as a strategy to break up unfamiliar consonant clusters in Japanese using orthographic input. The experiment revealed two primary results of interest. First, the production of consonant clusters consistently yielded epenthetic vowels when [aCCa] pseudo-word stimuli were presented (all 504 target tokens included epenthetic vowels, although some tokens were not included in the analysis because the accompanying consonants were misread). This suggests that the general Japanese phonotactic restriction against consonant clusters influences the production of such clusters, as expected. Second, the results are only partially consistent with previous studies in terms of which epenthetic vowel is used. The baseline hypothesis would be that the quality of epenthetic vowels would be similar to patterns found in lexicalized loanword phonology, i.e., [ɯ] after labials and velars, [o] after alveolar stops, and [i] after palatals. After labial and velar consonants, the present study did indeed find that the quality of the epenthetic vowel was quite consistently [ɯ], as has been found in studies of Japanese loanword adaptation (Hirayama, 2003; Kubozono, 2015; Yazawa et al., 2015). However, the current results diverge from those predicted from loanword studies and those found in Yazawa et al. (2015) in the alveolar and palatal contexts. We observed that while some speakers produced [o] in the alveolar context and [i] in the palatal context, such speakers were actually in the minority. Instead, the epenthetic vowel [ɯ] was commonly used in each of these contexts, although there was also a great degree of variability among individuals in the type of vowel that was inserted, as we discuss further below.

The epenthetic vowels produced by each speaker in each context are summarized in Table 15. Note that only one speaker, F7, was close to producing all and only the expected vowel qualities in each context, and even she produced unexpected tokens of [ɯ] in the alveolar context. Interestingly, F7 is also the oldest speaker among the participants (age = 46), which might be suggestive that older speakers tend to have a more conservative pattern. However, the next oldest speaker, M2 (age = 40), consistently produced [ɯ] in all contexts. In fact, three speakers in the current study (F1, F13, and M2) consistently used [ɯ] as the epenthetic vowel in all contexts; F1 and F13 were among the younger speakers overall (ages 23 and 22, respectively). The remaining eleven speakers had varying patterns. As above, we present this summary not to put too much weight on the ‘pattern’ produced by any given speaker, but rather to showcase the striking contrasts between (1) the regularity of the expected epenthetic [ɯ] in the labial and velar contexts, (2) the tendency for both expected [o] and unexpected [ɯ] to be used in the alveolar context, and (3) the striking irregularity in the palatal context (though note that even in this highly variable context, 10 of the 14 speakers did use at least some instances of [ɯ]).

Table 15

A summary of the results for each speaker, by context (Shading marks vowels that were produced as predicted given prior literature. Note that speakers F15 and M3 in the labial context used unpredicted [a] only once each, and similarly, and speakers F13 and F15 in the velar context used unpredicted [o] only once each. Speaker F9 used [i] three times in the labial and one time in the velar context).

Recall from Section 1 that other recent studies have also investigated these epenthesis patterns, and the conflicting results across these prior studies were in part what motivated the current study. Interestingly, the current results do not match any of those other results, although there are some similarities.

Yazawa et al. (2015) conducted a production study where the expected tri-partite pattern was in fact largely found. One difference between that study and the current one is that Yazawa et al. used a text-reading task in which participants read the Aesop fable “The North Wind and the Sun” in English, whereas pseudo-words were used in the current study. Many of the contexts in this passage where epenthesis would be expected were word final rather than word medial, as in the current study, and it is possible that the epenthesis patterns are simply not the same across these two kinds of contexts.

Additionally, the Yazawa et al. (2015) participants (who were all English learners) were likely in ‘English mode,’ given the task, whereas the current participants were specifically instructed to produce the words as if they were Japanese. A priori, this difference might be expected to have biased the results in the opposite direction—that is, one might think that Japanese nonce words would be more likely to follow the traditional tri-partite pattern than English words being produced in English. That said, there are at least two possible reasons for the actual results. One would be that the specific English words that were produced with epenthetic vowels might have lexicalized loanword counterparts that happen to follow the more traditional pattern. Alternatively, the epenthesis in the Yazawa et al. study might in some sense have been more ‘naturalistic,’ in that it happened while participants were reading out a full passage, with their attention presumably directed toward fluent English production more generally. In the current study, on the other hand, attention was focused on illicit consonant clusters in individual nonce words. It is possible that in the more naturalistic setting, epenthesis patterns similar to those in lexicalized Japanese loanwords were more likely to arise spontaneously, while in the more targeted setting, participants were more likely to apply meta-awareness of some sort (e.g., trying to be consistent across their productions of multiple nonce words with different consonantal contexts).

It is also potentially important to note that, while most of the Yazawa et al. (2015) participants “produced at least one epenthetic vowel,” there were only 518 total epenthetic tokens in their data, despite having 71 participants each reading a passage with more than 60 likely opportunities for epenthesis to occur; that is, epenthesis occurred in only around 12% of the places where it might have, suggesting that their participants were very much English-like in their productions. The current study had approximately the same number of actual tokens of epenthetic vowels (463), but these were concentrated in the productions of only 14 speakers, and our participants did in fact epenthesize in 100% of the expected contexts. Thus, the apparent conformity of the tripartite pattern in Yazawa et al. is diluted across a wide set of speakers and contexts. Of the four contexts tested here, all fourteen speakers produced the expected vowel at least some of the time in the labial and velar contexts, and more than half of them produced the expected vowel at least some of the time in the alveolar and palatal contexts. If the rate of epenthesis had been lower in the current study, it is possible that the ‘aberrant’ instances would have been more sparsely represented, and the epenthesis patterns have looked more as expected.

Shoji and Shoji (2014) also conducted a production study whose results do not match the Yazawa et al. (2015) ones, but their results only partially line up with the current ones. Recall that the production task in Shoji and Shoji was orthographically based: Their participants were provided with nonsense words written in Latin script (e.g., consuch, zod, bkautu) and had to re-write them in Japanese characters. Interestingly, they did find that the tri-partite pattern held for the vast majority of their word-final epenthetic contexts, but that it broke down in the word-initial contexts (again suggesting that epenthesis patterns depend at least partially on word position). Of course, in the current study, the consonant clusters targeted by epenthesis were word-medial, making it harder to directly compare the results. That said, in both the palatal and the alveolar stop word-initial contexts, Shoji and Shoji did find, as do we here, that [ɯ] is an extremely common epenthetic vowel, being used around 25% of the time in each context (more than any other vowel except the traditionally expected ones, which occurred 35–45% of the time). Thus, there is at least some converging evidence that Japanese speakers treat [ɯ] as a good candidate epenthetic vowel in the alveolar-stop context in nonce word production.

In terms of perception, both Monahan et al. (2009) and Mattingley et al. (2015) used VCCV stimuli, analogous to those used in the current study. It should be noted that the VCCV stimuli in both studies contained stop release bursts. The presence of the stop release has been known to influence speech perception and facilitate non-native speakers to perceive illusory vowels (e.g., Daland, Oh, & Davidson, 2019). Monahan et al. tested only alveolar-stop and velar contexts, using an AX discrimination task. For the velar contexts, they did indeed find that Japanese listeners seemed to perceive an illusory epenthetic vowel, specifically, [ɯ], as would be expected. But they found that in the alveolar-stop contexts, neither [o] (as would be expected if perception mirrored traditional production) nor [ɯ] (as would be expected if the illusory vowel were always the ‘default’ vowel) was perceived as an illusory vowel. Instead, Japanese listeners seem to have perceived these sequences faithfully as VCCV sequences, on par with English listeners. Mattingley et al., on the other hand, did test the same full set of contexts as the current study, using an identification task. They found that in the labial, velar, and palatal contexts, an illusory vowel was heard the majority of the time (though that varied from 70% for the labial and velar contexts to 98% for the palatal context), and that the illusory vowel usually matched the traditional prediction (i.e., [ɯ] for labial and velar and [i] for palatal). In the alveolar-stop context, they found that 60% of tokens were identified with an illusory epenthetic vowel (echoing Monahan et al.’s finding that illusory vowels are somewhat less likely in this context), but that within that 60%, the majority (70%) were [ɯ] and only 16% were [o]. Thus, again, at least for the alveolar-stop context, there seems to be evidence from perception as well that [ɯ] is a viable epenthetic vowel.

The current study was intended to describe what the production patterns are, rather than to try to explain why the patterns might differ from those traditionally reported. Especially given the varying results across the various recent production and perception studies, it would be premature to try to come up with an explanation for any given set of results. That said, the independent observations that the typical phonotactic constraints in loanword adaptation are loosening (e.g., Pintér, 2008, 2015; Hall, 2013; Kubozono, 2015) suggest at least one pathway of change. Specifically, if there is no longer a phonotactic constraint against the sequence [dɯ] (at least in loanwords), then there is no reason not to use the default epenthetic vowel [ɯ] in this context just as in the labial and velar contexts, exactly as seen in the current results.

Interestingly, even in the older and more conservative stages of Modern Japanese, there has not been a phonotactic restriction against [dʑɯ] sequences; the only historical phonotactic restriction after the palatals was that [e] could not occur. The ostensible reason for the use of epenthetic [i] after palatals simply comes from an articulatory or perceptual closeness between the palatals and [i] (Kubozono, 2015).¹⁰ But, as Kubozono (2015, p. 330) points out, “[t]his raises the interesting question of why the palatoalveolar fricative [ɕ]¹¹ usually takes /ɯ/¹² rather than /i/.” Perhaps, then, the use of [ɯ] in the other three contexts has enabled it to be used after palatal affricates as well. This possibility would not, of course, explain the huge amount of intra- and inter-speaker variability in terms of other vowels used epenthetically in this context (such as [e] and [a]), but we are somewhat reluctant to theorize too much about their use given that other studies have not found similar trends.

That said, one potential explanation for the variety in the palatal context comes from the influence of English orthography; see, e.g., Vendelin and Peperkamp (2006) for general discussion. In English orthography, the letter <j> is most often followed by <u> (247 words in the IPHOD corpus, Vaden, Halpin, & Hickok, 2009); next by <a>, <o>, or <e> (around 180 words each in the IPHOD corpus), and least often by <i> (only 46 words in the IPHOD corpus). Given that (1) the current participants were in an English-speaking country at the time of participation, (2) the target words were written using Romanization instead of Japanese characters, and (3) most of the participants showed at least some influence of English orthography by misproducing at least some of the <ge> or <gi> sequences as starting with [dʑ], it is not outside the realm of possibility that they could have chosen an epenthetic vowel in the <j> context based on what they thought was likely given English orthography. Interestingly, however, in terms of English phonology, [dʒ] is most often followed by [i] or [ɪ] (613 words in the IPHOD corpus),¹³ followed by [e] or [ɛ] (304 words), or another vowel (fewer than 100 words each). And, the current participants did certainly produce the orthographic <j> as [dʑ] (indeed, it was the most accurately produced of the consonantal contexts, with 94% of possible tokens being produced with the correct consonant; the next most accurate was the labial context, with 90%). Furthermore, there’s no particular evidence that the current participants relied on English orthographic frequency patterns for any of the other contexts; in the IPHOD corpus, <b> is most frequently followed by <a>, while both <d> and <g> are most frequently followed by <e> (and in fact all three are least often followed by <u>), so if English orthography is playing a role here, it is unclear as to why it would do so only for <j>. Other potential explanations for the unusual behaviour in the palatal context (the following consonant, priming from adjacent stimuli, the age or gender of speakers, etc.) would similarly be limited in their ability to uniquely predict variability in this context, because such factors were consistent across all contexts.

Another possibility to consider is that the apparent epenthetic vowels that are seen in the current study might in fact not be truly epenthetic but rather the result of gestural mistiming between the production of the first and second consonant in the sequence.¹⁴ If this were the case, then it would make sense that the quality of many of the vowels across the contexts was similar, as it would not be phonologically governed, but rather based on articulatory facts. Davidson (2010) considers the difference between full epenthetic vowels and transitional vocoids in cases where a schwa was inserted between two consonants word-initially by English and Catalan speakers. To diagnose the difference between these scenarios, she examines duration, F1, and F2. A transitional vocoid would likely be shorter than a lexical vowel, have a lower F1 value (because it would be produced not with a tongue height target but rather as a temporary lowering between the surrounding stop closures), and have a lower F2 value in this context (because it would be likely to anticipate the upcoming [a] “even more than for normal vowel-to-vowel coarticulation,” as the [a] would be the next upcoming vowel target; Davidson, 2010, p. 283). As noted in the above results sections, however, none of these characteristics were found. That is, the epenthetic vowels were never significantly different from the lexical vowel they were most similar to in any of these three dimensions. Thus, we believe that the epenthetic vowels found in our tokens are indeed inserted vowels, with accompanying vowel targets, and not simply insufficient attempts to produce CC clusters.

In terms of the larger implications of this work, while the task in the current paper was a non-word production task intended to provide insight into how illicit consonant clusters might be adapted by native Japanese speakers in a loanword situation, the stimuli in the experiment were not themselves loanwords, and so we cannot claim that loanword adaptation patterns are changing. That is, our stimuli were not associated with meaning and did not originate from some foreign source that might be known to the participants. Thus, some factors that likely influence the way that loanwords are adapted were absent from the current study—in particular, participants did not have acoustic models of the words, nor did they have any knowledge of ways in which they might be related to other related loanwords, and there was no larger social context that might influence the adaptation. This is why we cannot claim that loanwords would necessarily follow the same trends as those seen here, but we can speculate that loanword adaptation may be undergoing changes such that loanwords with medial CC clusters in which the first member is a (voiced) alveolar stop or a palatal may no longer follow the clear tri-partite pattern traditionally reported in the literature. Supporting this speculation is the fact that, as was noted in Section 1, Japanese loanwords and nonce words have been shown to follow similar phonological patterns (Kawahara, 2012 and references therein). And in fact, at least one online English/Japanese dictionary tool¹⁵ uses ドゥ [dɯ] instead of ド [do] for English loanwords whose final consonant is [d] in Kana. For example, ‘food’ /fu:d/ is represented as ‘フードゥ’ and ‘read’ /ri:d/ is represented as ‘リードゥ’. All English words that end with [d] or [t] are written in katakana with [dɯ] ドゥ and [tɯ] トゥ instead of using [do] ド and [to] ト.

5. Conclusion

The current paper reports on a production experiment that directly tests the nature of epenthesis as a strategy to break up unfamiliar consonant clusters in Japanese. All fourteen of the speakers in the experiment consistently produced an epenthetic vowel in VCCV sequences, following the expected phonotactic patterns of syllable structure in Japanese. The choice of epenthetic vowel, however, was not always as expected. The expected vowel [ɯ] was used in the labial and velar contexts, but also by many speakers in many tokens in both the alveolar-stop and the palatal contexts, where [o] and [i] would have been expected, respectively. Although a full explanation for the variability of the results is beyond the scope of the paper, the results do suggest that the independent loosening of the phonotactic constraint against [dɯ] sequences may be affecting epenthesis strategies typically assumed to be governed by this constraint. We thus predict that in new loanwords with consonant clusters (at least, word-medial clusters with voiced obstruents), we may increasingly see [ɯ] being used as an across-the-board epenthetic vowel, rather than the quality continuing to be governed by preceding context.

Competing Interests

The authors have no competing interests to declare.