1.1. Listener-based sound change processes

The phonetic influence of sounds that are adjacent to one another in the temporal speech chain is generally referred to as co-articulation. Sound change processes that arise due to these influences are typologically common and are easy to explain on the level of production. For example, the fronting of a velar tongue closure before a front vowel—e.g., Latin cīvitātem [kiːwitaːtem] ‘city’ > Italian città [t͡ʃitːa], French cité [site]—is a common sound change pattern among the world’s languages, and it is readily explained on an articulatory basis: The context of the palatal constriction for [i] gives rise to an articulatory constraint on the velar constriction for [k], resulting in palatalization of the velar consonant. The change is considered to be dependent upon the conditioning environment and it is generally assumed to originate from the speaker herself—after all, it is the speaker’s own vocal tract that provides the context for the articulatory constraint (cf. Guion, 1998).

However, some sound changes are not articulatorily motivated in this way. Take, for example, the change from labio-velar stops in Proto-Indo-European to velar stops in Classical Greek, as shown in 1.1 (Meillet, 1967), and from velar stops in Proto-Bantu to labial stops in West Teke, as shown in 1.2 (Guthrie, 1967–1970).¹

Changes such as these are not considered to originate from the speaker’s own articulatory constraints, since the lips and the tongue dorsum are under independent articulatory control and are not expected to influence one another. Another example involves changes between nasalization and glottalization (laryngealization), and between glottalization and nasalization, e.g., certain dialects of British English [hɑ̃ːf] ‘half’ (Matisoff, 1970). The link between these articulatorily unconnected features (termed rhinoglottophilia by Matisoff, 1975) is not considered to arise from co-articulatory influence, since articulatory control of velum opening/closing is independent of articulatory control of vocal fold abduction/adduction. Similarly, cases of ‘spontaneous nasalization’ have been observed diachronically, wherein phonologically distinctive nasalization has emerged in environments that were once adjacent to high-airflow, spread-glottis consonants (Bloch, 1920, 1965; Grierson, 1922; Ohala, 1975; Ohala & Ohala, 1993; Turner, 1921)—e.g., Sanskrit sarpa ‘snake’ > Hindi [sɑ̃p]. Ohala (1975) argued a perceptual basis for these changes: When the glottis is spread, the lowered amplitude and increased bandwidth of F1 created by the coupling of the oral and subglottal cavities can ‘mock’ comparable acoustic effects due to coupling of the oral and nasal cavities, and listeners can misperceive and lexically re-analyze these effects accordingly. Ohala and Busa (1995) later argued that these same perceptual factors can explain the diachronic ‘inverse’ of spontaneous nasalization: sound changes involving the loss of nasals before voiceless fricatives.

In a similar manner to the link between nasalization and glottalization, sound change patterns involving the link between nasalization and tongue height modification have been observed. Diachronically, systematic changes in vowel height have been documented typologically as nasal vowel systems develop: High nasal vowels have a tendency to be produced with a lower tongue position and low nasal vowels have a tendency to be produced with a higher tongue position as vowel nasality becomes phonologized (Beddor, 1982; Hajek, 1997; Sampson, 1999). A notable example comes from the evolution of nasal vowels in French, e.g., the lowering of [i] in Latin vinum [winum] ‘wine’ > Old French vin [vĩn] > Modern French vin [vɛ̃] (Sampson, 1999). Although there does exist an intrinsic muscular connection between the soft palate and the tongue via the palatoglossus muscle (Kuehn & Azzam, 1978; Zemlin, 1998), this connection is argued to affect velic height when the tongue lowers, but not tongue height when the velum lowers. When considering a possible effect of velum lowering on tongue height, the two articulations are generally considered to be under independent control.

1.2. Listener-based sound change mechanisms

Since sound changes like these do not have any apparent articulatory motivation, they are considered to arise from the listener rather than the speaker. But what are possible mechanisms by which listener-based sound change could occur? Two prominent theories of listener-based sound change mechanisms are discussed in the current section: listeners’ misperception of the source of acoustic effects (Ohala, 1981, 1993) and listeners’ re-weighting of co-varying articulatory properties (Beddor, 2009, 2012).

According to Ohala (1981, 1993), listener-based sound change can be accounted for in terms of listener misperception of the articulatory source of acoustic modulations. Under normal circumstances, listeners are able to correctly attribute the effects of temporally overlapping gestures to their respective articulatory sources (i.e., perceptual compensation; Mann & Repp, 1980; Yu & Lee, 2014; Zellou, 2017). However, in some cases, the listener may fail to compensate for this temporal overlap and, thus, fail to attribute various effects to their proper sources. In these cases, the listener misperceives the effects that arise from one articulatory source as being due to a different articulatory source. Thus, according to Ohala, the change itself first happens at the level of speech perception, when the listener has failed to properly contextualize the various properties of the speech signal (Harrington, Kleber, Reubold, Schiel, & Stevens, in press). When the listener becomes the speaker, this re-analysis of the articulatory source is passed to the level of production, and the listener-turned-speaker subsequently produces the novel articulatory variant in her own speech.

Beddor (2009, 2012) posits a slightly different mechanism for listener-based sound change, one which also assumes that the acoustic consequences of co-articulated speech variants provide the necessary ‘raw material’ (Beddor, 2009, p. 787) for the types of sound change discussed in this paper. The difference in Beddor’s model is that listeners are sensitive to the acoustic consequences of temporally overlapping articulatory gestures, and they do not need to misperceive these consequences in order for sound change to occur. In other words, whereas Ohala argues that listeners’ access to phonetic details is typically due to misperception of the articulatory source, Beddor argues that listeners regularly access these details in order to facilitate perception. However, a given listener may nevertheless differ from a given speaker with respect to the co-articulatory source and effect that is represented in their respective grammars. According to Beddor, an important first stage of sound change that may result from this process is when an articulatory source and its phonetic effect begin to exhibit a trading relationship with one another. When this happens, a listener may correctly perceive a phonetic effect, but she may attribute it to a segment other than its original source. An example given by Beddor involves phonetically nasalized [ṼN] sequences, wherein the listener may correctly perceive the phonetic effect [Ṽ], but fail to parse all of the effect with the speaker source /N/, and instead partially contribute the effect to the adjacent segment /V/. In this way, Beddor’s theory of listener-based sound change does not exclude the possibility of listener misperception—i.e., misperceptions may happen, but sound change can occur even when speech is accurately perceived. Instead, listener-based sound change in Beddor’s model occurs when listeners perceive a speech utterance with a different weighting between co-varying articulatory properties than was originally produced by the speaker. In other words, co-varying articulations aren’t necessarily misperceived, they are simply weighted in a different manner by the listener than by the speaker.

1.3. Testing listener-based sound change mechanisms

The current study has been created to observe possible evidences of these listener-based sound change mechanisms in the laboratory, using naïve listener imitations of native speaker productions. Specifically, Australian English naïve listeners were recruited to imitate phonemic nasal-oral vowel pair tokens produced by Southern French native speakers.

These two particular languages were chosen because vowel nasality is phonologically contrastive in (Southern) French (i.e., /Ṽ/), while it is phonetically contextual in (Australian) English (i.e., [ṼN]). Thus, when imitating the native French tokens, the English listeners would be unable to perceptually compensate and associate nasality with a specific source, N, because there is no context for contextual normalization to occur (i.e., no final N in such French words). If articulatory re-analysis is indeed involved in listener-based sound change mechanisms in some manner, removing the source of a specific effect will help ensure that such re-analysis might be observed in an experimental setting. Furthermore, when imitating productions of phonemic nasal vowels, English speakers would not be biased by their own phonological representation of vowel nasality in their native language, as opposed to, e.g., Hindi or Portuguese speakers.

Naïve listeners were recruited for the imitation task instead of native speakers (e.g., speakers from a different variety of French) or L2 learners of French, since both of these speaker groups would be influenced by their knowledge of the language. Furthermore, a specific prediction from Ohala (1993) is that less experienced listeners are likely to be the primary drivers of sound change, given that they are presumed to have a lesser ability to perceptually compensate for the articulatory source of acoustic effects. Included in this ‘less experienced’ group are listeners such as children or language learners; I contend that naïve listeners are even less experienced than these, and that the use of naïve listeners in an imitation task will further help to ensure that the participants fail to perceptually compensate in this way.

Vowel nasalization, specifically, has been chosen for this study due to the inherently ambiguous nature associated with its production. In the following section, ambiguities associated with the overlapping effects of nasalization, tongue height change, and breathy voicing on F1 frequency are detailed. These ambiguities make vowel nasalization an ideal test case for investigating listener-based sound change: The possibility of multiple many-to-one articulatory-to-acoustic mappings creates the possibility of multiple sources of articulatory re-analysis by listeners.

1.3.1. The multi-dimensional nature of vowel nasality

In the production of vowel nasality, the velum is lowered in order to couple the oral and nasal cavities via the velo-pharyngeal port—known as VP coupling or, simply, nasalization. Due to the coupling of the two cavities, nasalization results in a wide range of acoustic effects in comparison to non-VP-coupled oral vowels. While these include reduced acoustic energy and increased formant bandwidths in low frequencies (Stevens, 2000, p. 193), variation in F1 frequency is also observed due to the introduction of additional pole-zero pairs to the acoustic spectrum (Maeda, 1993). The direction of the shift of F1 frequency depends on the respective frequencies of the oral poles, nasal poles, and nasal zeros, as well as the degree of VP coupling itself. Accordingly, it is not always practical to attempt to determine F1 of the oral cavity alone for VP-coupled vowels. Therefore, the designation F1′ will be used in this manuscript to refer to the most prominent low-frequency spectral peak of a VP-coupled vowel, regardless of whether it arises from the oral tract, the nasal tract, or the combination of the two. Recent articulatory and acoustic evidence has shown that the most pervasive F1′-frequency-related consequence of VP coupling is lowering of F1′ for non-front, non-high vowels (Carignan, 2018). Figure 1 displays the direction of shift in F1/F2 frequencies due to the independent effects of VP coupling, after correction for effects due to oral configuration; the arrows display the average direction of the independent effects on F1/F2 frequencies for each vowel category.

Figure 1

Independent effects of nasalization on acoustic vowel quality. Arrows indicate the direction of the effects in F1/F2 space. Replicated from Carignan (2018).

However, VP coupling is not the only possible source of F1′ frequency modulation in VP-coupled vowels. Perhaps the most familiar source of F1 modulation for vowels (be they oral or nasal) is variation in tongue height. Generally, a higher tongue body is correlated with a lower F1 frequency, while a lower tongue body is correlated with a higher F1 frequency (Johnson, 2003; Stevens, 2000). Thus, differences in tongue height are predicted to modify F1′ frequency of VP-coupled vowels in a manner that is independent of VP coupling itself. A spread glottis can also have an independent effect on the spectral characteristics of F1′, due to the coupling of the oral cavity to the subglottal cavity, as previously mentioned. For example, breathy voice is differentiated from modal voice by a more prominent first harmonic (H1) and a lower-amplitude, wider-bandwidth F1 (Bickley, 1982; Garellek, 2014; Garellek & Keating, 2011; Gordon & Ladefoged, 2001; Hanson, K. N. Stevens, Kuo, Chen, & Slifka, 2001; Kreiman et al., 2012; Lotto, Holt, & Kluender, 1997). Since H1 has, by definition, a low frequency, the combination of a higher-amplitude H1 and a lower-amplitude F1 is generally predicted to lower F1 center of gravity, which can subsequently affect F1′ frequency. In particular, the increased energy from H1 is expected to decrease F1′ frequency, especially for non-high vowels, for which the oral F1 is higher in frequency than H1. Likewise, VP coupling has the effect of increasing the prominence of the first harmonic, decreasing the amplitude of F1, and increasing the bandwidth of F1 for nasal vowels compared to oral vowels (Chen, 1997; Maeda, 1993; Styler, 2015, 2017), as well as lowering F1′ frequency for non-high vowels (Carignan, 2018).

Thus, in the production of vowel nasalization, there exists the possibility for VP coupling, tongue height, and breathiness to affect F1′ frequency in ways that are independent but nevertheless similar to one another. In this manner, F1′ frequency modulation is unique among the acoustic characteristics of nasalization, which is why focus has been placed on F1′ in the current study, rather than on other acoustic characteristics of nasalization (e.g., changes in tongue height are not predicted to affect F1 bandwidth). Vowel nasalization thus creates the potential for interaction between these articulatory variables in the perceptual domain, i.e., due to misperception of the articulatory source of the F1′ modulation: If F1′ is lowered for a given nasal vowel, can listeners ascertain whether the F1′-lowering is due to tongue height change, nasalization, or breathiness? Evidence of this perceptual interaction is described in the following section.

1.4. Hypotheses

In summary, it has been shown in previous research that systematic relationships between nasalization, breathiness, tongue height, and F1′ exist at the levels of both production and perception. As such, the current study has been designed to examine the co-variation of these three articulatory variables and their relation to F1′ frequency in the productions of the phonemic nasal-oral vowel distinction in Southern French, and in the imitation of these productions by Australian English naïve imitators. Comparison between the native productions and the naïve imitations will serve as a way to test for possible evidence to support the theoretical frameworks for listener-based sound change described in Section 1.2.

Since the relationship between these three articulatory variables and F1′ represents a classic many-to-one problem of articulation-to-acoustics (i.e., there are multiple articulations that can affect the singular acoustic characteristic), evidence of a one-to-many relationship is not expected. In other words, no plausible hypotheses can be made concerning an outcome in which the Australian English (AE) imitators produce the same articulatory distinctions as the Southern French (SF) speakers, yet the F1′ distinctions are different for the two groups. Such an outcome would suggest that one (or both) of the groups modify F1′ using articulatory distinctions that are not monitored in this study (e.g., lip configuration or pharyngeal aperture), and it is therefore outside of the scope of the particular research questions pursued here. However, a number of hypotheses can be made for a many-to-one relationship resulting from the AE imitators producing the same F1′ distinctions as the SF speakers but not the same articulatory distinctions. These hypotheses are related to the two theoretical frameworks as follows:

1. Evidence in support of listener misperception (Ohala):

   1. The SF speakers produced articulatory distinctions that the AE imitators fail to replicate.

   2. The AE imitators produce articulatory distinctions that the SF speakers had not originally produced.

2. Evidence in support of perceptual cue re-weighting (Beddor):

   The AE imitators produce the same articulatory distinctions as the SF speakers but to different degrees.

Alternatively, it may be the case that the AE imitators produce the same F1′ distinctions as the SF speakers, as well as all of the same articulatory distinctions. This outcome would suggest that no mechanism of listener-based change is represented in the listener-turned-speaker imitations and, thus, would fail to provide evidence in support of either of the two theoretical frameworks.

3.1. Articulatory and acoustic measurements

F1′, nasalance, tongue height, and EGG measurements were made at time points relating to 25%, 50%, and 75% of the vowel interval for each token. Since the ultrasound data were recorded at the lowest sampling rate (i.e., 30 Hz compared to 44.1 kHz for all other data), the measurements for all signals were carried out at time points associated with the ultrasound frames: The ultrasound frames nearest 25%, 50%, and 75% of each target vowel interval were identified automatically, and the time points related to these three frames were used for the measurements of all four signals. Some tokens were produced with such a short duration that three ultrasound frames could not be identified within the vowel interval. These tokens, all of which contained oral vowels, were excluded from analysis. This occurred for two native speakers and four naïve imitators, but only for a very small set of tokens in each case: as few as one experimental item for naïve imitator AE01 (0.7% of total) and as many as six experimental items for native speaker SF04 (4.17% of total).

3.2. Nasal-oral token matching

The measurements outlined above were used to ascertain the overall articulatory and acoustic distinctions between oral and nasal vowel counterparts. However, in order to compare the differences between the SF native speakers and the AE naïve imitators with respect to these distinctions, as well as with respect to how the articulatory distinctions relate to F1′ distinction for the two groups, a method similar to bootstrap sampling was used to create randomized matched comparisons between oral and nasal tokens for each participant (i.e., each SF speaker and each AE imitator), according to the following step-wise process. Before creating nasal-oral matched comparisons, the data were normalized by calculating z-scores for each variable, for each participant. After normalization, for each AE imitator’s data set, an oral item was first selected at random. Subsequently, a nasal item was selected at random from among the total set of nasal tokens that matched the selected oral token in terms of vowel pair category, onset consonant, and the SF speaker who produced the token. For example, if the random oral item selected for imitator AE05 was a token of beau [bo] “beautiful” produced by speaker SF02, then the random matched nasal item was a token of bon [bɔ̃] “good” that was also produced by speaker SF02 and presented during imitator AE05’s data collection session. Once the matched selection was made, the speaker-normalized articulatory and acoustic z-score values for the oral token were subtracted from the same z-score values for the nasal token for both the AE imitator and the SF speaker. These z-score differences were logged for both the imitator and the speaker, and the two matched tokens were removed from the data set before the iterative process repeated for a new pair of tokens. This randomized matching process continued until each oral token in the data set was matched with a corresponding nasal token. Since the randomized stimuli sets that were presented to the AE imitators were balanced for both word and SF speaker, this step-wise randomized matching process also resulted in a balanced set of nasal-oral matched tokens for each imitator and speaker.

Using this method, the nasal-oral distinctions for a given variable can be compared between the AE imitators and the SF speakers, in order to observe whether the naïve imitators make the same nasal-oral F1′, nasalance, tongue height, and CQ distinctions as the native speakers, as well as whether they make these distinctions to the same degree that the native speakers originally had made. Additionally, each of the articulatory nasal-oral distinctions can be compared to the F1′ nasal-oral distinctions, in order to observe how the articulatory variables contribute to variation in F1′, as well as whether these articulatory-acoustic relationships differ between the two groups.

3.3. Statistical analyses

Linear mixed-effects (LME) models were created in R using the lmer function in the lme4 package (Bates, Mächler, Bolker, & Walker, 2015). Three sets of models were created: one set to test the F1′ and articulatory differences between oral and nasal vowel counterparts (an example of the model structure is provided in 2.1 for the effect of NASALITY on CQ), and two sets to test the nasal-oral differences between the two groups, using the nasal-oral matched measurements described above. Within these latter two sets, one set was created to test the effect of GROUP on the nasal-oral variable distinctions (an example of the model structure is provided in 2.2 for the tongue HEIGHT distinction), and a separate set was created to test the effect of GROUP on the relation between the articulatory variables and F1′ (an example of the model structure is provided in 2.3 for the relation between the NASALANCE distinction and the F1′ distinction). Separate models for each variable and vowel pair combination were created within each set, resulting in 12 models each for the first and second sets (3 vowel pairs × 4 variables) and 9 models for the third set (3 vowel pairs × 3 variables).

In each model, random intercepts were included for naïve IMITATOR, native SPEAKER, token ORDER, and ONSET consonant. Estimates for degrees of freedom, t-statistics, and p-values were generated using Satterthwaite approximation under the lmerTest package (Kuznetsova, Brockhoff, & Christensen, 2016). For all models, the alpha level was adjusted using Bonferroni correction for comparison across the 12 models in the first and second sets (α = 0.004) or across the 9 models in the third set (α = 0.006). The complete results are provided in Appendix A: Table C for the first set of models, Table D for the second set, and Table E for the third set.

4.1. Articulatory and F1ʹ nasal-oral distinctions

4.1.1. /ɑ̃/ vs. /a/ distinctions

Box plots of /ɑ̃/-/a/ z-score differences for F1′, nasalance, tongue height, and CQ values are shown in Figure 3; individual box plots created for each speaker and imitator are provided in Appendix B. For any given box plot, the horizontal line is the median of the distribution, the center notch pointing inward displays the 95% confidence interval around the median, the box displays the interquartile range (i.e., the middle 50% of the data), and the vertical lines extending from the box demarcate 1.5 × the interquartile range. Outliers have been removed to aid visualization. With regard to F1′, the first LME model reveals that /ɑ̃/ was produced with lower F1′ values than /a/ throughout the entire vowel (evidenced also by the negative values in the box plots), and the box plots suggest an increase in this frequency difference throughout the vowel interval (i.e., the box plots are positioned at increasingly lower values across the three time points). No significant effect of GROUP was observed in the second LME model, revealing that the speaker-normalized F1′ distinction between /ɑ̃/ and /a/ was the same for both the naïve imitators and the native speakers. These results suggest that the AE imitators were generally successful in replicating the F1′-frequency-related acoustic pattern produced by the SF speakers.

Figure 3

/ɑ̃/-/a/ z-score differences for F1ʹ, nasalance, tongue height, and CQ, as produced by the SF native speakers (dark box plots) and the AE naïve imitators (light box plots). Separate box plots are displayed for measurements taken at 25%, 50%, and 75% of the vowel duration.

With regard to nasalance, the first LME model reveals that /ɑ̃/ was produced with a greater degree of nasalance than /a/ (evidenced also by the positive values in the box plots). The box plots suggest that the nasalance distinction increases in a linear manner throughout the vowel interval for both groups, which is generally characteristic of phonetically co-articulated vowel nasalization (Cohn, 1990). This observation is not surprising for the Australian English imitators, since vowel nasalization is a phonetic phenomenon in English. However, the same pattern observed for the Southern French native speakers is in contrast to the more constant, plateau-like pattern that is characteristic of nasal vowels in Northern Metropolitan French (Cohn, 1990; Delvaux, 2012). The significant (negative) effect of GROUP in the second LME model reveals that the AE imitators produced a smaller nasalance distinction between /ɑ̃/ and /a/, in comparison to the SF speakers.

With regard to tongue height, although the first LME model reveals that /ɑ̃/ and /a/ were produced with the same overall tongue height, the box plots suggest slightly different patterns for the two groups: Whereas the native speakers produced /ɑ̃/ with a slightly lower tongue position than /a/, the naïve imitators produced /ɑ̃/ with a slightly higher tongue position than /a/, especially in the latter part of the vowel interval. The second main LME model reveals that this difference between the two groups is indeed significant: The AE naïve imitators produced a higher overall /ɑ̃/-/a/ tongue height distinction in comparison to the SF native speakers. Moreover, separate post-hoc LME models with the same random structure as the main models reveal that these separate group-wise patterns are significant for both the SF speakers (t(1,1186.5) = –6.35, p < 0.001) and the AE imitators (t(1,394.6) = 4.97, p < 0.001).

With regard to CQ, the first LME model reveals that /ɑ̃/ was produced with lower CQ values compared to /a/; this is especially apparent in the box plots in the latter part of the vowel interval, suggesting that /ɑ̃/ becomes even breathier towards the end of the vowel. No significant effect of GROUP was observed in the second LME model, revealing that the speaker-normalized CQ distinction between /ɑ̃/ and /a/ was the same for both the naïve imitators and the native speakers. Taking CQ as an indicator of breathiness, these results suggest that /ɑ̃/ was produced with more breathiness than /a/, and that this articulatory difference was similar for the AE naïve imitators and the SF native speakers.

4.1.2. /ɛ̃/ vs. /ɛ/ distinctions

Box plots of /ɛ̃/-/ɛ/ z-score differences for F1′, nasalance, tongue height, and CQ values are shown in Figure 4. With regard to F1′, the first LME model reveals that /ɛ̃/ was produced with higher F1′ compared to /ɛ/, but the box plots suggest that this distinction decreases throughout the vowel interval. No significant effect of GROUP was observed in the second LME model, revealing that the speaker-normalized F1′ distinction between /ɛ̃/ and /ɛ/ was the same for both the naïve imitators and the native speakers. These results suggest that the AE naïve imitators were generally successful in replicating the F1′-frequency-related acoustic pattern produced by the SF native speakers.

Figure 4

/ɛ̃/-/ɛ/ z-score differences for F1ʹ, nasalance, tongue height, and CQ, as produced by the SF native speakers (dark box plots) and the AE naïve imitators (light box plots). Separate box plots are displayed for measurements taken at 25%, 50%, and 75% of the vowel duration.

With regard to nasalance, the same pattern can be observed for the vowel pair /ɛ̃/-/ɛ/ as was observed for the vowel pair /ɑ̃/-/a/: /ɛ̃/ was produced with significantly more nasalance than /ɛ/, and the nasalance distinction increased in linear manner throughout the vowel interval. The significant (negative) effect of GROUP in the second LME model reveals that the AE imitators produced a smaller nasalance distinction between /ɛ̃/ and /ɛ/, in comparison to the SF speakers.

With regard to tongue height, the first LME model reveals that /ɛ̃/ was produced with a lower tongue position than /ɛ/, and the box plots suggest that this distinction increased slightly throughout the vowel interval. The significant (negative) effect of GROUP in the second LME model reveals that the AE imitators produced a larger tongue height distinction between /ɛ̃/ and /ɛ/, in comparison to the SF speakers. In other words, while both groups produced /ɛ̃/ with a lower tongue position than /ɛ/, this (negative) difference in tongue height between the two vowels was larger for the AE naïve imitators than for the SF native speakers.

With regard to CQ, the first LME model reveals that /ɛ̃/ was produced with lower CQ values than /ɛ/, although the box plots suggest that this difference is slight. No significant effect of GROUP was observed in the second LME model, revealing that the speaker-normalized CQ distinction between /ɛ̃/ and /ɛ/ was the same for both groups. Taking CQ as an indicator of breathiness, these results suggest that /ɛ̃/ was produced with more breathiness than /ɛ/, and that this articulatory difference was similar for the AE naïve imitators and the SF native speakers.

4.1.3. /ɔ̃/ vs. /o/ distinctions

Box plots of /ɔ̃/-/o/ z-score differences for F1′, nasalance, tongue height, and CQ values are shown in Figure 5. With regard to F1′, the first LME model reveals that /ɔ̃/ was produced with no overall difference in F1′ compared to /o/. However, the box plots suggest that the dynamic nature of the F1′ distinction is the likely cause for the overall non-significant result: In comparison to /o/, /ɔ̃/ was produced with higher F1′ values in the first part of the vowel interval, no difference at the vowel midpoint, and lower F1′ values in the latter part of the vowel interval. As with /ɑ̃/-/a/ and /ɛ̃/-/ɛ/, this pattern also indicates a decline in frequency throughout the vowel interval; in the case of /ɔ̃/-/o/, however, the pattern results in no overall F1′ distinction between the two vowels. No significant effect of GROUP was observed in the second LME model, revealing that the speaker-normalized F1′ distinction between /ɔ̃/ and /o/ was the same for both the naïve imitators and the native speakers. These results suggest that the AE naïve imitators were generally successful in replicating the F1′-frequency-related acoustic pattern produced by the SF native speakers, as was also observed for /ɑ̃/-/a/ and /ɛ̃/-/ɛ/.

Figure 5

/ɔ̃/-/o/ z-score differences for F1ʹ, nasalance, tongue height, and CQ, as produced by the SF native speakers (dark box plots) and the AE naïve imitators (light box plots). Separate box plots are displayed for measurements taken at 25%, 50%, and 75% of the vowel duration.

Although no overall F1′ distinction was observed between /ɔ̃/ and /o/, there were significant differences in the articulatory parameters for the two vowels. With regard to nasalance, the first LME model and the box plots reveal the same pattern as was observed for /ɑ̃/-/a/ and /ɛ̃/-/ɛ/: /ɔ̃/ was produced with more nasalance than /o/, and the nasalance distinction increases in linear manner throughout the vowel interval. The significant (negative) effect of GROUP in the second LME model reveals that the AE imitators produced a smaller nasalance distinction between /ɔ̃/ and /o/, in comparison to the SF speakers.

With regard to tongue height, although the two vowels were realized with similar F1′ values, the first LME model reveals that /ɔ̃/ was produced with a significantly lower tongue position than /o/. The significant (positive) effect of GROUP in the second LME model reveals that the AE imitators produced a smaller tongue height distinction between /ɔ̃/ and /o/, in comparison to the SF speakers. In other words, while both groups produced /ɔ̃/ with a lower tongue position than /o/, this (negative) difference in tongue height between the two vowels was not as large for the AE naïve imitators as for the SF native speakers.

With regard to CQ, the first LME model reveals that /ɔ̃/ was produced with lower CQ values compared to /o/. However, no significant effect of GROUP was observed in the second LME model, revealing that the speaker-normalized CQ distinction between /ɔ̃/ and /o/ was the same for both the naïve imitators and the native speakers. Taking CQ as an indicator of breathiness, these results suggest that /ɔ̃/ was produced with more breathiness than /o/, and that this articulatory difference was similar for the AE naïve imitators and the SF native speakers.

4.2. Relationship between articulatory distinctions and F1ʹ distinction

Since no difference between the naïve imitators and the native speakers was observed for the F1′ distinction in any of the three nasal-oral vowel pairs, but significant differences were observed for both the nasalance distinction and the tongue height distinction for all three vowel pairs, we now turn to whether the relationship between the articulatory distinctions and the F1′ distinction is different between the two groups. The results from the LME models constructed to investigate these possible differences are detailed in this section; the complete model results are provided in Table E of Appendix A.

5.1. Summary and discussion of results

Table 1 provides a summary of the results for the F1′ and articulatory nasal-oral distinctions (Nasality), the differences between the AE naïve imitators and the SF native speakers with regard to these distinctions (Group), and the relationship between the articulatory variables and F1′ as well as the differences between the AE imitators and the SF speakers with regard to these relationships (F1′ ∼ IV). For the Nasality results, up and down arrows correspond to whether the given variable was significantly higher (↑) or lower (↓) for the nasal vowel compared to its oral counterpart; differences that were not significant are denoted as equal. For the Group results, nasal-oral distinctions that were higher (i.e., positive distinctions that were more positive and negative distinctions that were less negative) for the AE imitators vs. the SF speakers are denoted by AE > SF; nasal-oral distinctions that were lower (i.e., positive distinctions that were less positive and negative distinctions that were more negative) for the AE imitators vs. the SF speakers are denoted by AE < SF; nasal-oral distinctions that were the same for both groups are denoted by AE = SF. For the F1′ ∼ IV results, articulatory variables that positively predicted F1′ are denoted by +, and articulatory variables that negatively predicted F1′ are denoted by –; articulatory-F1′ mappings that yielded a more positive relationship for the AE imitators vs. the SF speakers are denoted by AE > SF; articulatory-F1′ mappings that yielded a more negative relationship for the AE imitators vs. the SF speakers are denoted by AE < SF; articulatory-F1′ mappings that yielded the same relationship between the two groups are denoted by AE = SF. By way of example, we take the nasalance results for the vowel pair /ɑ̃/-/a/: Nasalance was greater for /ɑ̃/ vs. /a/ (↑), but this distinction was smaller for the AE imitators vs. the SF speakers (AE < SF); nasalance negatively predicted F1′ (–), but this relationship was less strong/negative (i.e., more positive slope) for the AE imitators vs. the SF speakers (AE > SF).

With regard to F1′, /ɑ̃/ was realized with lower F1′ values than /a/, /ɛ̃/ was realized with higher F1′ values than /ɛ/, and /ɔ̃/ and /o/ were realized with the same overall F1′ values. The naïve imitators reproduced the same nasal-oral distinctions as the native speakers for every vowel pair. This can be taken as an indication of the imitators faithfully replicating the acoustic output produced by the native speakers—at least with regard to the F1′ frequency distinction, which was the specific focus of the current study—and, thus, successfully completing the experimental task. However, the results for the articulatory variables suggest that the imitators sometimes reached these acoustic goals via different articulatory strategies than those used by the native speakers. Moreover, differences between the AE imitators and the SF speakers were observed for the relationship between the articulatory variables and F1′ frequency.

With regard to nasalance, the AE imitators produced the same distinctions as the SF native speakers: greater nasalance for all of the nasal vowels compared to their oral counterparts. However, this distinction was smaller for the imitators compared to the native speakers in every case. It could be argued that the difference in magnitude between the two groups was due to the AE imitators having had no exposure to a language with phonemic vowel nasality and, thus, being unable to produce distinctive nasality as part of their productions. However, the fact that the naïve imitators reliably produced a distinction between the oral and nasal vowels throughout the entire vowel interval (at least, from 25% of the vowel interval, as measured in this study) suggests that this is not the case: Naïve speakers of a language without contrastive vowel nasality are indeed able to produce contrastive vowel nasality when imitating contrastive nasal vowels. With regard to the relationship between nasalance and F1′, nasalance negatively predicted F1′ frequency for all three vowel pairs, as expected for all of the non-high vowels in this study (see Section 1.3.1). However, this relationship was less strong for the AE imitators compared to the SF speakers in every case (i.e., more positive slope for the AE vs. SF GROUP interaction). In other words, while the AE imitators produced a nasal-oral distinction using degree of nasalization, the distinction was smaller for the imitators compared to the native speakers, and the lowering effect on F1′ was less apparent. These results suggest that, while both groups used degree of nasalization in a way that affected F1′ frequency for all three nasal-oral vowel pairs, the AE naïve imitators employed this articulatory-acoustic relationship to a lesser degree than the SF native speakers had originally used.

With regard to tongue height, the results were different for the three vowel pairs. For /ɑ̃/-/a/, although there was no overall difference in height between the two vowels, this aggregate result was likely due to different patterns for the two groups: The SF native speakers produced /ɑ̃/ with a lower tongue position than /a/, overall, while the AE naïve imitators produced /ɑ̃/ with a higher tongue position than /a/, overall. Indeed, the LME model tested on the randomized matched nasal-oral comparisons revealed that the AE imitators produced a more positive /ɑ̃/-/a/ tongue height distinction compared to the SF speakers. Moreover, separate post-hoc LME models revealed that the two group-wise patterns (i.e., lower tongue position for /ɑ̃/ vs. /a/ for the SF speakers and higher tongue position for /ɑ̃/ vs. /a/ for the AE imitators) were both significant. For /ɛ̃/-/ɛ/, /ɛ̃/ was produced with a lower tongue position than /ɛ/, overall. However, this distinction was even greater for the AE imitators than for the SF speakers: The imitators produced /ɛ̃/ with an even lower tongue position vs. /ɛ/, in comparison to the native speakers. For /ɔ̃/-/o/, /ɔ̃/ was produced with a lower tongue position than /o/, overall. However, this distinction was not as great for the AE imitators compared to the SF speakers. With regard to the relationship between tongue height and F1′, although tongue height negatively predicted F1′ frequency when the data for all three vowel pairs were included together (i.e., the expected inverse relationship between tongue height and F1), this relationship was not observed for any of the three nasal-oral vowel pairs, individually. In fact, an overall positive relationship between tongue height and F1′ was observed for the vowel pair /ɛ̃/-/ɛ/. However, in comparing the AE imitator tongue height/F1′ relationships to those produced by the SF speakers, the AE imitators had a significantly more negative relationship in every case. In other words, although the expected inverse relationship between tongue height and F1′ was not observed for individual vowel pairs, this inverse relationship was revealed in the difference between the AE imitators and the SF speakers for each of the three vowel pairs. These results suggest that the AE naïve imitators employed this articulatory-acoustic relationship to a greater degree than the SF native speakers had originally used.

With regard to breathiness, the AE imitators produced the same distinctions as the SF native speakers: lower CQ values (i.e., greater breathiness) for all of the nasal vowels compared to their oral vowel counterparts. Furthermore, the AE imitators produced this distinction to the same degree as the SF native speakers in every case. With regard to the relationship between breathiness and F1′, CQ positively predicted F1′ frequency for all three vowel pairs. In other words, since CQ is inversely related to breathiness, breathiness negatively predicted F1′ frequency for all three vowel pairs, as predicted for all of the non-high vowels in this study (see Section 1.3.1). This articulatory-acoustic relationship was the same between the two groups for the vowel pairs /ɛ̃/-/ɛ/ and /ɔ̃/-/o/, but the relationship was less strong for the AE imitators vs. the SF speakers for the vowel pair /ɑ̃/-/a/. These results suggest that both groups used breathiness in a way that affects F1′ frequency for all three nasal-oral vowel pairs, and that the AE naïve imitators employed this articulatory-acoustic relationship to the same degree as the SF native speakers for two of the three vowel pairs, but to a lesser degree than the SF speakers had originally used for /ɑ̃/-/a/.

5.2. Interpretations for mechanisms of listener-based sound change

The hypotheses for the two theoretical frameworks for listener-based sound change detailed in Section 1.2 were as follows:

1. Evidence in support of listener misperception (Ohala):

   1. The SF speakers produced articulatory distinctions that the AE imitators fail to replicate.

   2. The AE imitators produce articulatory distinctions that the SF speakers had not originally produced.

2. Evidence in support of perceptual cue re-weighting (Beddor):

   The AE imitators produce the same articulatory distinctions as the SF speakers but to different degrees.

No direct evidence for hypotheses 1a or 1b, supporting listener misperception (Ohala, 1981, 1993), was observed in the current study: Of the three articulatory variables that were monitored in this study (degree of nasalization, tongue height, breathiness) no cases were observed of the imitators employing an articulator that was never originally used by the native speakers, or vice versa.⁴ However, the results for tongue height and F1′ for the vowel pair /ɑ̃/-/a/ can be argued to support a misperception-based account in a manner that synthesizes these two hypotheses. Although the SF native speakers produced /ɑ̃/ with a lower tongue position than /a/ (predicted to raise F1′ for /ɑ̃/ vs. /a/), /ɑ̃/ was nonetheless realized with a lower F1′ frequency than /a/. Given that /ɑ̃/ was found to be produced by the SF speakers with greater degrees of nasalization and breathiness compared to /a/, and that these articulatory distinctions were found to be significantly predictive of lower F1′ values, it is likely that the lower F1′ frequency observed for /ɑ̃/ vs. /a/ arose due to these articulatory distinctions, and not to tongue height. In comparison, although the AE naïve imitators produced the same F1′ distinction that the SF speakers had produced (and to the same degree), they did so by using a different articulatory strategy than the SF speakers had used. In particular, the AE imitators produced /ɑ̃/ with a higher tongue position than /a/, which is expected to lower F1′ frequency. Thus, these results suggest that the SF native speakers had produced an articulatory distinction that the AE naïve imitators failed to replicate (i.e., a lower tongue position for /ɑ̃/ vs. /a/), while the AE imitators produced instead an articulatory distinction that the SF speakers had not originally made (i.e., a higher tongue position for /ɑ̃/ vs. /a/). This result indicates that the AE imitators may have misperceived the lower F1′ frequency for /ɑ̃/ vs. /a/ as being due to a higher tongue position, and subsequently produced this articulatory distinction in their imitations.

Evidence for hypothesis 2, supporting perceptual cue re-weighting of co-varying articulatory properties (Beddor, 2009, 2012), was observed in the current study. This evidence was observed primarily in the differences between the SF speakers and the AE imitators with regard to how nasalance and tongue height related to F1′ frequency for the two groups. Crucially, the AE imitators successfully produced the same F1′ nasal-oral distinctions that the SF speakers had originally produced for each of the three vowel pairs, and they produced these distinctions to the same degree. However, the articulatory evidence suggests that the AE imitators reached these F1′ targets using a different weighting of the various articulatory distinctions compared to the SF native speakers. On the one hand, both groups used breathiness in the same manner (i.e., all nasal vowels were breathier than their oral counterparts), and increased breathiness was predictive of decreased F1′ for both groups (although this relation was less strong for the AE imitators’ productions of /ɑ̃/-/a/ compared to the SF speakers). On the other hand, evidence was observed of a trading relation between nasalance and tongue height with regard to how the two groups employed these articulatory variables to affect F1′ frequency. Overall, an increase in nasalance was found to significantly predict a decrease in F1′ frequency; however, the AE imitators produced a smaller nasal-oral nasalance distinction for all of the vowel pairs and, accordingly, the relationship between nasalance and F1′ was less strong for the AE imitators in comparison to the SF speakers for all three vowel pairs. Increased tongue height was found to be inversely correlated with F1′ for all of the vowel data combined (although this relationship was not observed for any of the individual vowel pairs); however, this inverse relationship between tongue height and F1′ was stronger for the AE imitators in comparison to the SF speakers for all three vowel pairs. Taken together, these results suggest that, in comparison to the SF native speakers, the AE naïve imitators employed nasalance less, and tongue height more, to affect F1′ frequency in their nasal-oral vowel distinctions. Since the F1′ frequency distinctions were the same for the two groups, this indicates that the AE imitators may have re-weighted the relative magnitudes of the nasalance and tongue height distinctions in the SF speaker productions, assigning different importance to these variables (i.e., less to nasalance and more to tongue height) in order to obtain a similar F1′ frequency output as the SF speakers.

An alternative explanation for the trading relation between the tongue height/F1′ relationship and the nasalization/F1′ relationship concerns the respective phonological representations of these two articulations in French and English. When a phonetic effect (such as nasalization) becomes phonologized in a given language, the articulatory distinction is sometimes enhanced beyond what is observed in purely phonetic variation (Hyman, 2013; Solé, 2007). Thus, since vowel nasalization is phonemic in (Southern) French, but phonetic in (Australian) English, the observation that the AE imitators produced a smaller nasalance distinction than the SF speakers can be explained by the different phonological representations of vowel nasality in the two languages.⁵ Likewise, the fact that the AE imitators realized a stronger inverse relationship between tongue height and F1′ could be explained by their native phonology—more precisely, since neither vowel nasality nor breathy voice are phonologically contrastive in English, the imitators may have relied more strongly on tongue height in order to realize F1′ distinctions between oral and nasal vowel counterparts. However, there are two observations from this study that provide possible counter-evidence to this interpretation. Firstly, the AE imitators (re-)produced the same F1′ nasal-oral distinctions that the SF speakers had originally produced for each of the three vowel pairs. If the trading relation between nasalance and tongue height was due simply to differences in the phonological representations of these articulations in the two languages, there is no a priori reason to expect that the acoustic realizations should be the same for the two groups; rather, the results from this study suggest that the AE imitators re-weighted these two articulatory variables in order to achieve the same acoustic output (here, F1′ frequency) as the SF speakers. Secondly, breathy voicing is not phonologically contrastive for vowels in either French or English; nonetheless, both groups produced all three nasal vowels with a greater degree of breathiness compared to their oral counterparts. If the phonetic differences in nasalization and tongue height between the two groups were due simply to differences in the phonological representation of these articulations in the two languages, then we should not necessarily expect either group to produce phonetically distinctive breathiness as part of the nasal-oral contrast, yet this is what has been observed here. Ultimately, the trading relation between the tongue height/F1′ relationship and the nasalization/F1′ relationship may also be compatible with a hybrid account of cue re-weighting and language-specific phonology: English phonology may contribute to the AE imitators’ preferred cue weighting, biasing their productions. In other words, it may not necessarily be the case that the AE imitators re-weighted the cues present in the speech signal in an independent, unconstrained manner. Rather, the cue weights of their native phonology—i.e., more weight given to the tongue height/F1′ relationship and less weight given to the nasalization/F1′ relationship—may have influenced how the re-weighting strategy was manifested in their imitations.⁶