## 1 Introduction

Many languages in Africa are reported to have vowels that contrast for the feature [Advanced Tongue Root] (ATR). This contrast is realized articulatorily as a distinction between an advanced position of the tongue root, along with pharyngeal expansion, in opposition to a neutral (Hudu, 2014) or retracted position (Lindau, 1978).1 It is also commonly accompanied or identified by the presence of vowel harmony for this feature. Indeed, ATR vowel harmony is especially common within the Nilo-Saharan and Niger Congo phyla (Casali, 2008; Clements, 2007). Rolle, Lionnet, and Faytak (2020) report that out of 681 languages in their Areal Linguistic Features of Africa database, 358, or 53%, exhibit ATR vowel harmony. ATR vowel harmony has long contributed to the development of phonological theoretical accounts of vowel harmony (Stewart, 1967, 1971, 1983; Schacter, 1969; Clements, 1981, 1984, 1985; Bakovic, 2000; Casali, 2008). Phoneticians have also studied the articulation of ATR contrasts in African languages using a variety of methods, including X-rays and ultrasound. Acoustic properties of the vowel contrasts have also been extensively studied. Yet, despite such a large focus on ATR systems, and anecdotal reports of difficulty perceiving distinctions between certain vowels, surprisingly little attention has been paid to the perception of ATR contrasts, especially by speakers of languages with ATR harmony.

### 1.1 Articulation

Figure 1

Lindau (1978, p. 551) cineradiographic tracings for one Akyem dialect speaker. Solid lines indicate +ATR vowels; dashed lines indicate –ATR vowels.

Tiede (1996) used MRI to map Akan vowels for one male speaker of the Asante Twi dialect. He confirmed the tongue root advancement and lower larynx height for +ATR vowels of earlier work, and also showed that +ATR vowels have a larger pharyngeal expansion. The –ATR vowels showed pharyngeal constriction, not just a narrowing of the pharyngeal space due to tongue root and larynx position. Ultrasound has also proven to be an effective imaging technique for ATR contrasts. Studies on Kinande (Bantu, DR Congo) (Gick, Pulleyblank, Campbell, & Mutaka, 2006), Dagbani (Gur, Ghana) (Hudu, 2010, 2014), Lopit (Nilotic, South Sudan) (Billington, 2014), Yoruba (Benue-Congo, Nigeria) (Allen, Pulleyblank, & Ajíbóyè, 2013), and Akan (Kirkham & Nance, 2017) confirm distinct tongue root positions for +ATR and –ATR vowels. Finally, Edmondson, Padayodi, Hassan, and Esling (2007) used laryngoscopy to examine the production of Kabiyè (Gur, Togo) and Akan vowels. They determined that there is an “open epilaryngeal space, a less retracted tongue and neutral larynx height” for [+constricted] (or +ATR vowels), whereas [–constricted] (–ATR vowels) exhibit “a flatter forward-bending laryngeal sphincter angle, a more retracted tongue, and raised laryngeal structures.” Edmondson and Esling (2006) also report compression of the arytenoids and aryepiglottic folds forwards and upwards for –ATR vowels. Most articulatory studies involve a small number of speakers; the largest number is eight in Jacobsen’s (1978) study of DhoLuo.

### 1.2 Acoustics

Numerous phonetic studies have attempted to determine the acoustic properties of ATR vowel distinctions. Halle and Stevens (1969, p. 211) report that “The clearest and most consistent acoustic consequence of widening the vocal tract in the vicinity of the tongue root is a lowering of the first-formant.” This has been confirmed in numerous studies since. Lindau’s (1978) examination of Akan vowels reported that F1 is the main distinguishing acoustic parameter. [+ATR] vowels have lower F1 than their [–ATR] counterparts. The F1/F2 vowel plot is shown below. The chart in Figure 2 shows that the +ATR vowels /i u e o/ are well-distinguished from the –ATR counterparts /ɪ ʊ ɛ ɔ a/, but other vowels such as [e] and [ɪ] appear to be acoustically similar, though not identical.

Figure 2

Lindau (1978, p. 552) formant chart for four speakers (three Akyem, one Ashanti) dialect.

Hess (1992) is a detailed study of the acoustic properties of Akan vowels, based on data from a speaker of the Kwawu dialect (similar to Asante Twi). She examined four acoustic parameters: Formant frequency, formant bandwidth, vowel duration, and relative amplitude of spectral components. The two measures most strongly correlated with [ATR] were F1 and F1 bandwidth. But since F1 is also a correlate of vowel height distinctions, she deemed F1 bandwidth as the more reliable measurement for distinguishing ATR, at least for vowels that have similar formant frequencies. [+ATR] vowels had narrower first formant bandwidths than [–ATR] vowels, correlated with the third harmonic. [+ATR] vowels had a weaker third harmonic than [–ATR] vowels. The F1 bandwidth distinction was useful for distinguishing [+ATR] [e, o] compared to [–ATR] /ɪ, ʊ/, as their F1 values are similar, as seen on the following vowel plot in Figure 3. This plot also includes the [æ], which is similar to [ɛ], but with higher F1.

Figure 3

Hess (1992, p. 480) formant chart for one speaker of Kwawu dialect.

Appiah-Padi (1994) analyzed data from nine male speakers of the Asante Twi dialect. She found that besides [+ATR] vowels having lower F1 than [–ATR] vowels, [+ATR] vowels were also longer, except for the [ɛ]/[e] pair. Unlike Hess (1992), F1 bandwidth was not found to distinguish ATR pairs of vowels. Furthermore, a discriminant analysis showed that all ATR pairs were well distinguished using formants, bandwidth, duration, and amplitude, but, in contrast, [ɪ]/[e] and [ʊ][o] showed much poorer discrimination ratios.

Kirkham and Nance (2017) provided the F1/F2 vowel space for six speakers of the Akuapem dialect of Akan. They reported similar patterns to previous studies in terms of the similarity of [ɪ] and [e], as well as [ʊ] and [o]. They also showed that speakers differ in the position of the vowel [ɜ]. For three male speakers, it is positioned close to [ɛ] or [ɪ], whereas for two female speakers, it is close to [e]. For one female speaker, it is close to [a].

Other notable acoustic studies of ATR distinctions include Jacobson (1978) on Dholuo (Nilotic, Kenya) and Guion, Post, and Payne (2004) on Maa (Nilotic, Kenya). Guion et al. found that [+ATR] vowels have lower first formant values and relatively less energy in the higher frequency regions than [–ATR] counterparts. Fulop, Kari, and Ladefoged (1998) analyzed data from six speakers of Degema (Edoid, Nigeria). They found that all [+ATR]/[–ATR] vowel pairs were distinguished by F1, except the low vowel pair. Starwalt (2008) investigated 11 African languages with ATR contrasts from the Kwa and Bantu families, with five speakers of each language. She found considerable variation among languages and speakers, but the most consistently reliable acoustic measurement distinguishing the contrast was F1. [+ATR] vowels tended to have lower F1 than their [–ATR] pairs. F1 bandwidth and normalized A1–A2 measurements were only marginally different in some languages. [+ATR] vowels also tended to have lower center of gravity measurements.

There are reports in the literature of ‘breathy’ or ‘hollow’ phonation associated with +ATR vowels (Berry, 1957; Casali, 2008). Meanwhile –ATR vowels are described as ‘brighter’ (Fulop et al., 1998), ‘creaky,’ or ‘tense’ (see Denning, 1989 for an overview), but several acoustic studies cast doubt on voice quality as a distinguishing feature. Hess (1992) finds no evidence for breathy phonation in Akan as measured by the difference in amplitude between H2 and F0. Guion et al. (2004) used electroglottographic data from one Maa speaker and found a slightly less constricted glottis for [+ATR] than [–ATR] vowels, which may be indicative of a phonatory difference, but they could find no discernible voice quality distinctions in recordings. Local and Lodge (2004) found that for a speaker of the Tugen dialect of Kalenjin, breathiness was associated with –ATR vowels, auditorily perceptible as well as established through measurements of contact quotient of the glottal cycle made from electrolaryngographic recordings. They otherwise found that F1 was the primary acoustic measure distinguishing the vowels.

Finally, it is worth noting the acoustic similarity reported between certain vowels that are distinguished by both ATR and height: [ɪ]/[e] and [ʊ]/[o]. Lindau (1987, p. 51) states that “…there may be a case for concluding that for practical purposes, these two pairs of vowels have merged phonetically in Akan.” However, the tongue root positions are different, and the height of the larynx is distinct between the +ATR and –ATR pairs, so articulatorily, they are produced differently, despite being acoustically similar. Stewart (1971, p. 200) reports that in some Akan dialects these vowels have merged, but it is not clear if that means an acoustic/perceptual merger or a merger in articulation. Hess (1992) also noted an acoustic merging of front –ATR /ɪ/ and front +ATR /e/, but states that “They remain phonetically distinct in that they are produced in characteristic ways,” so articulatorily they are different even if their acoustics makes them similar or merged. Hess further reports they are distinguished by F1 bandwidth measurements. Nevertheless, the acoustic similarity may have repercussions for their perception. Anecdotal evidence points to perceptual difficulty. Casali (2017) reports that the high –ATR [ɪ] and mid +ATR [e], as well as the [ʊ] and [o], are frequently confused by fieldworkers and this has led to mischaracterizations of inventories. The term “fieldworkers” implies people who do not speak languages with ATR contrasts. Therefore, they may be guided by their own first language perceptual system, and this does not mean that speakers of languages with ATR contrasts cannot distinguish them (Casali, 2017, p. 84). As for the [a]/[ɜ] pair, Casali (2012) reports that “The difference between [a] and [æ] was also clearly perceptible to our Akuapem language consultant, who described [æ] as sounding like the vowel [ɛ]” (where the transcription [æ] corresponds to [ɜ]). In the acoustic F1/F2 vowel space for many Akan speakers, [ɜ] is closer to [ɛ] than it is to [a]. He does not report on the perception of the other similar vowels for this consultant. Kpogo (2022) reports that there is an urban/rural distinction in the current Asante Twi dialect with respect to the [+ATR] harmonic counterpart of /a/. While rural speakers have a distinct tenth vowel ([æ] or [ɜ]), urban speakers produce [e] instead, exhibiting merger with the [e] corresponding to phonemic /e/. In addition to Akan, there are reports of acoustic similarity between certain vowels in other ATR systems, particularly the high [–ATR] and mid [+ATR] pairs [ɪ e] and [ʊ o] (e.g., Omamor, 1988 on Okpẹ and Uvwiẹ; Koffi, 2018 on Anyi; Casali, 2003, 2008 for general discussion). It should be noted that with the exception of Kpogo (2022), who recorded data from 62 speakers, most acoustic studies are based on a small number of speakers, often less than 10.

### 1.3 Perception

Despite the plethora of phonetic studies on articulation and acoustics in languages with ATR harmony, there is very little research that examines perception, particularly perception by speakers of ATR languages.

Lindau (1975) conducted a study of the perception of Akan vowels by four non-Akan speaking phoneticians. Tokens of each vowel were produced by an Akyem dialect speaker (who apparently had both –ATR [æ] and +ATR [ɜ]) in a monosyllabic CV word in a carrier phrase and then the vowel was repeated three times (e.g., mesee bʊ ʊ ʊ ʊ). The task required the four listeners to position a vowel produced in isolation on an F1/F2 chart. The composite results show that [o] and [ʊ] were perceived as very similar but still distinct, but there was a perceptual merger for [e] and [ɪ], and for [ɜ] and [ɛ]. Still, Lindau notes that the native speaker who produced the vowels was able to perceptually distinguish between [o] and [ʊ], and between [e] and [ɪ] on two separate occasions, once after a three-week interval and once after a four-week interval.

Hess (1992) conducted a small perception study as part of her acoustic assessment of Akan vowels. Specifically, she tested two Akan speakers on an identification task of single CV syllables in isolation containing [ɪ] [e] [ʊ] [o]. It is not stated if the syllables were real words or not. These vowels were chosen because each front or back pair are acoustically similar, especially for F1, suggestive of a merger. Speakers were tested on 40 items, with 10 tokens of each vowel. One speaker made two errors and the other speaker eight errors (87.5% accuracy), but it is not reported on which vowels or what kinds of errors. Hess concluded that despite the acoustic similarity of some of the vowels, they are perceptually distinct.

Fulop et al. (1998) report a perception study on Degema (Edoid, Nigeria) which has a 10 vowel ATR system (+ATR i e u o ə and –ATR ɪ ɛ ʊ ɔ a). Five Degema listeners performed a vowel identification task on synthesized vowels that manipulated F1, F2, and F3 in order to test how formants are used to distinguish vowels. Other elements such as fundamental frequency and phonation type (relative formant amplitudes) were kept constant. Listeners were given a written prompt of a particular vowel and asked to identify that vowel among a selection of tokens. Results showed the –ATR mid vowels ɛ ɔ as distinct from all other vowels, but considerable overlap within three groups of vowels: [i ~ ɪ ~ e], [u ~ ʊ ~ o], and [a ~ ə]; the study reports that listeners “do not behave alike when trying to synthesize any of the other vowels” (p. 96).

Kingston, Macmillan, Walsh Dickey, Thorburn and Bartels (1997) also used synthesized tokens designed to replicate variation in tongue root position and accompanying voice quality. This was done by manipulating F1, and the percentage or quotient of the glottal cycle in which the glottis was open and the overall tilt of the source spectrum. The experimental subjects were American English listeners. They report that F1 integrates perceptually with spectral effects of extreme lax or tense phonation.

A recent study (Ozburn, Giovio Canavesi, & Akinbo, 2022) examines perception of ATR distinctions in Dàgáárè, a Mabia language of Ghana, and is the only other study besides the current one to test perception with a large number of speakers (over 20) of a language with ATR contrasts. Stimuli were kV and kVkV nonce words using front vowels [i ɪ e ɛ] in an ABX task. Results showed low to medium rates of accuracy for three types of vowel contrasts (ATR i ~ ɪ, e ~ ɛ, and ATR/height ɪ ~ e), ranging from lowest performance with the ɪ ~ e contrast to highest with e ~ ɛ contrast. Disharmonic bisyllabic tokens resulted in lower accuracy rates than monosyllables, but harmonic bisyllabic forms did not improve accuracy compared to monosyllables.4

Thus, while a few studies have examined perception of ATR vowel qualities in listeners who do not speak languages with ATR distinctions, or have examined synthetic vowel quality in listeners who do, there are very few perceptual tests of natural speech of ATR vowel qualities with listeners who speak languages with ATR distinctions. There are a few anecdotal reports that certain vowel distinctions may pose difficulties. The current study aims to fill this gap.

### 1.4. Akan vowel system and ATR harmony

Akan is a central Tano language of the Kwa language family. It has several dialects including Asante Twi, Akuapem, Akyem, and Fante. Specific descriptions of ATR vowel harmony in Akan date back as early as Christaller (1875), taken up again in Berry (1957), Stewart (1967, 1971, 1983), Schachter and Fromkin (1968), Clements (1981, 1984), Dolphyne (1988), and Casali (2012), among others. Recent papers by Akan-speaking linguists include Owusu (2014) and Abakah (2016).

Berry (1957) divided the vowels into two sets and noted that prefixes alternate between these sets according to the vowel of the root. Akan has nine phonemic vowels that contrast for ATR. The low –ATR vowel /a/ is unpaired with a phonemic +ATR low vowel, but does have an allophonic counterpart, transcribed as [ɜ] or [æ], and produced via vowel harmony.5

1. (1)
1.
2. high
3.
4. mid
5.
6. low
1.
2. +ATR
3. –ATR
4. +ATR
5. –ATR
6. –ATR
1. front
2. i
3. ɪ
4. e
5. ɛ
6.
1.
2.
3.
4.
5.
6. a
1. back
2. u
3. ʊ
4. o
5. ɔ
6.

In general, all vowels in a word match for ATR:

1. (2)
1.
2. a.
3. b.
4. c.
5.
1. –ATR
2. ɔ̀wʊ́
3. ɛ̀fɪ́ɛ̀
4. bɔ̀nɪ́
5. *bɔni
1.
2. ‘she delivers a baby’
3. ‘vomit’
4. ‘evil’
5.
1.
2. d.
3. e.
4. f.
5.
1. +ATR
2. òwú
3. èfíé
4. bòsómè
5. *ɛfie
1.
2. ‘death’
3. ‘home’
4. ‘moon’
5.

In addition, prefixes and vowel-initial suffixes harmonize with the root for ATR, and therefore exhibit alternations between –ATR and +ATR:

1. (3)
1.
2. a.
3. b.
1. –ATR
2. wʊ́-dà
3. ɔ̀-tɔ́-ɪ̀
1.
2. ‘you sleep’
3. ‘s/he bought it’
1.
2. c.
3. d.
1. +ATR
2. wú-dìʔ
3. ò-tú-ì
1.
2. ‘you eat’
3. ‘s/he dug it up’

Asymmetry in the system is noted by various authors. When /a/ precedes a +ATR vowel, it is realized as +ATR [ɜ] (or [æ] depending on transcription): e.g., kɜri ‘weigh’. However, when /a/ follows a +ATR vowel, it is realized as the –ATR [a]: bisa ‘ask’ or èdʑá ‘fire’, which can be construed as disharmonic forms. The vowel [ɜ] only occurs preceding a +ATR vowel and never alone in a monosyllable. This asymmetry prompted Casali (2012) to argue that Akan vowel harmony operates in a regressive direction. This argument is bolstered by two additional properties. Consonant-initial –ATR suffixes do not harmonize with a +ATR root (4a,c) and +ATR consonant-initial suffixes can trigger harmony leftwards, as shown by the agentive suffix -ni (4d). See also Schacter and Fromkin (1968, p. 62) on this point.

1. (4)
1. a.
2. b.
1. kúnú-nʊ̀m̀
2. sìká
1. ‘husbands’
2. ‘money’
1. c.
2. d.
1. ò-sìsì-fʊ́
2. sìkɜ́-ní
1. ‘cheater’
2. ‘rich person’

Dolphyne (1988) treats [ɜ] as a [+ATR] allophonic variant of /a/. Clements (1981) argues that it is actually gradient coarticulation that raises /a/ to [ɜ] due to a following high [+ATR] vowel, but Hess (1992) and Casali (2012) dispute this analysis with acoustic evidence. Nevertheless, this vowel does show different distribution and behavior from the phonemic vowels. Although /a/ can become [+ATR] [ɜ], the harmonized vowel fails to trigger [+ATR] harmony to its left: wò-bé-díʔ ‘they will eat’ vs. ̀-́-kɜ́rì ‘they will weigh’. Therefore, in sequences of two /a/ preceding a +ATR high vowel, only the first one undergoes harmony. There are also reports that while /a/ becomes [ɜ] before high vowels, it does not do so when in a prefix before mid vowels in verbs (Stewart, 1967; Hess, 1992): wà-bétú ‘he has come and pulled it out’ vs. ̀-bísá ‘he has asked’. We verified that this is the case with the Asante dialect. However, [ɜ] can occur before mid vowels in nouns, eg., ɜ̀kóò ‘parrot’. Finally, there are reports of some dialects having other restrictions on mid +ATR vowels. Schacter and Fromkin (1968) report that Asante does not allow monosyllabic verb roots with just [e] or [o], and where the other dialects have these vowels in verb roots, it has [ɪ] and [ʊ].

### 1.5 Hypotheses on vowel perception in Akan

In order to test perception of ATR contrasts in Akan, we put forward two main hypotheses:

Hypothesis 1: Phonological contrast

Speakers of a language with ATR contrasts will have more difficulty distinguishing vowel pairs that have an allophonic relationship than those that have a phonemic relationship.

Hypothesis 2: Acoustic similarity

Speakers of a language with ATR contrasts will have difficulty perceiving vowel pairs that are acoustically similar, even if contrastive.

In general, phones that belong to separate phonemes are expected to be easier to distinguish than allophones of the same phoneme. Boomershine, Hall, Hume, and Johnson (2008) tested the perception of consonants that had either a phonemic or an allophonic relationship in English and Spanish. English speakers were better at perceiving the distinction between d ~ ð in a VCV context than Spanish speakers; this is a phonemic distinction in English but allophonic in Spanish. Similarly, Spanish speakers were better at perceiving the distinction between d ~ ɾ in a VCV context than English speakers, which is phonemic in Spanish but allophonic in English. However, other researchers have found that the phonological environment influences perception. Peperkamp, Pettinato, and Dupoux (2003) report that French speakers perform poorly at distinguishing the allophones [ʁ] and [χ] of /ʁ/ preceding voiceless consonants. This is a context in which the voiceless allophone [χ] is expected. However, they perform well at distinguishing the same sounds in VC isolation contexts where the more widespread [ʁ] is expected. Overall, the results of these two studies suggest that allophonic pairs should be perceived as more similar to each other than phonemic pairs, but that context may impact the results if one allophone has a more restricted distribution. Applying this to the Akan case, the Phonological contrast hypothesis predicts that pairs that exhibit phonemic contrast for ATR are expected to be well distinguished. This includes the high vowel pairs i ~ ɪ and u ~ ʊ, and the mid vowel pairs e ~ ɛ, o ~ ɔ. It also includes the acoustically similar pairs ɪ ~ e and ʊ ~ o. Applying the general principle of phonemic contrast, the low vowel pair ɜ ~ a, where [ɜ] is allophonic and only occurs preceding +ATR vowels, is expected to be poorly distinguished. However, given that [ɜ] is a contextual +ATR vowel harmonic allophone of /a/, it is possible that better perception between [ɜ] and [a] will occur in an isolation context where the default [a] is expected, following the results of Peperkamp et al. (2003).

The Acoustic similarity hypothesis, in contrast, predicts that the main driving factor in perceptual similarity will be acoustic similarity (Dubno & Levitt, 1981). That is, among phonologically contrastive categories, there may be differences in acoustic similarity which impact perception. This could also override allophonic relationships between segment pairs. Furthermore, distribution and frequency in the lexicon can impact how segments in a phonemic contrast relationship are perceived. Hall, Letawsky, Turner, Allen, and McMullin (2015) found that phonemic fricatives in contexts where only one of the sounds is highly likely to occur are perceived as more similar than the same sounds in contexts where either is likely to occur. With respect to ATR, in some languages the high vowels are acoustically similar in the F1/F2 vowel space (e.g., Ikposo (Starwalt, 2008), Masaai (Quinn-Wriedt, 2013), Lopit (Billington, 2017)). In other languages, like Akan, they are more separated, and the mid +ATR vowels have lower F1 than the high [–ATR] vowels, what Casali (2012, p. 49) terms a ‘leapfrog’ pattern or what can be called a ‘flip’ in the vowel space. Therefore, in terms of acoustic similarity, high ATR pairs may not pose difficulty.

As reported above, however, certain vowels are very similar acoustically, even if distinguished by phonological properties of ATR and vowel height: [ɪ] / [e] and [ʊ] / [o]. There are anecdotal reports of fieldworkers having difficulty distinguishing these vowels (Casali, 2017), although it is not well known if speakers of languages with ATR contrasts can distinguish among them, except for the results of the Degema perceptual study (Fulop et al., 1998) and the Dàgáárè study (Ozburn et al., 2022), and anecdotal reports in Omamor (1988) and Koffi (2018). Koffi (2018) reports that Anyi speakers confused /e/ and /ɪ/ in literacy training. Omamor, a speaker of Uvwiẹ, notes that acoustic measurements helped reveal that Uvwiẹ had the vowels /ɪ/ and /ʊ/, which were hard to distinguish from other vowels. Notwithstanding, speakers of an ATR language are expected to perform better than speakers of a non–ATR language in detecting ATR contrasts because they have acquired a language that employs these contrasts, and so we assume their perceptual system is attuned to the perceptual cues of ATR contrasts. Although we do not directly test the ATR vs. non–ATR language speaker hypothesis in the study, this is the reason speakers of an ATR language were tested.

The acoustic similarity hypothesis would predict that the distinction between such vowels would be hard to perceive, despite their phonological feature contrast. As for the vowel [ɜ], its F1/F2 position suggests that it will be more likely to be confused with [ɛ] rather than [a] based on acoustic similarity. This is so even if [ɜ] maps to the phoneme /a/ and /a/ and /ɛ/ are differentiated by frontness and vowel height (in phonological features [low] and [back]). In order to calculate acoustic similarity more precisely, we measured Euclidean distance of vowel pairs using the F1/F2 measurements of vowel productions used in the stimuli in the experiments (Table 1). Each formant was z-scored prior to calculation so that F2, with its higher raw Hz values, did not dominate distances. The precise distances are provided in Appendix C, but the results reveal three broad categories of distance, with vowel pairs distinguished by both ATR and height features having the smallest distance. It is assumed that the smaller the Euclidean distance measurement, the greater the similarity between vowels.

Table 1

Acoustic similarity: z-scored Euclidean distances between vowels. See Appendix C for details.

 Acoustic similarity Vowel pairs Category Low similarity, dist. ≥1.00 i ~ ɪ, e ~ ɛ, u ~ ʊ, o ~ ɔ, a ~ [ɜ], ɛ ~ a ATR or Height Medium, dist. 0.61–0.99 i ~ e ɪ ~ ɛ u ~ o ʊ ~ ɔ Height High similarity, dist. ≤0.60 ɪ ~ e ʊ ~ o ɛ ~ [ɜ] ATR+Height

The Phonological contrast hypothesis does not predict a difference in perception based on the number of phonological features that two vowels may share or not share. Nevertheless, it is instructive to examine how phonological similarity calculations differ from acoustic similarity. Frisch, Broe and Pierrehumbert (2004) propose a phonological similarity metric using a formula based on distinctive phonological features and shared and unshared natural classes. Similarity = shared natural classes / shared natural classes +unshared natural classes. Using this method, the lower the number, the less phonologically similar the vowels are. The least phonologically similar phonemic vowels are the ATR+Height vowels ɪ ~ e and ʊ ~ o, while the other vowels have higher similarity (Table 2). See Appendix D for details. Note that because this metric of similarity is based on distinctive phonological features, only phonemes are included for assessment.

Table 2

Phonological similarity based on natural classes.

 Similarity Vowel pairs Category Low similarity, 0.16 ɪ ~ e ʊ ~ o ATR+Height High similarity, >0.30 i ~ ɪ u ~ ʊ e ~ ɛ o ~ ɔi ~ e u ~ o ɪ ~ ɛ ʊ ~ ɔ ɛ ~ a ATR or Height

These two measures of similarity produce contradictory results for the ATR+Height vowels compared to the other vowels. The ATR+Height vowels have high acoustic similarity (that is, low Euclidean distance), but low phonological similarity. Acoustic similarity measurements suggest that distinguishing ATR+Height vowels will pose perception difficulties, while phonological similarity suggests they should be well-differentiated.

The predictions of the two hypotheses with respect to ATR differences are presented below (Table 3) with respect to predicted performance on a perceptual discrimination task. Both hypotheses predict that the non-low ATR vowels will show good discrimination in Akan, based on phonological contrast or the separation in acoustic space. The two hypotheses differ with respect to the other vowel pairs. The Phonological hypothesis predicts that those pairs distinguished by both ATR and vowel height will be easily discriminated, but the Acoustic similarity hypothesis predicts difficulty. The opposite predictions are made for the low ATR pair. As the relationship is allophonic, perceptual difficulty is predicted under the Phonological hypothesis, whereas the Acoustic hypothesis predicts that, as they are not phonetically similar, good discrimination should result.

Table 3

Predictions of Phonological and Acoustic similarity hypotheses.

 Label Vowel pairs Phonological Acoustic Non-low ATR i ~ ɪ e ~ ɛ u ~ ʊ o ~ ɔ ✔ ✔ Height i ~ e ɪ ~ ɛ u ~ o ʊ ~ ɔ ✔ ✔ ATR+Height ɪ ~ e ʊ ~ o ɛ ~ ɜ ✔ ✖ Low ATR a ~ ɜ ✖ ✔

## 2. Experiment 1

### 2. 1 Methods

#### 3.1.2 Stimuli

Experiment 2 consisted of 272 trials. Each trial consisted of three words, two of which were different tokens of the same word, whereas the other contained vowels that mismatched along one or more dimensions. Two consonant frames were used – gVbV and tVkV – and all tokens had low tone. These frames did not correspond to any actual Akan words, as verified by two Akan speakers. In order to include [ɜ] in the tokens, this vowel was produced preceding the vowel [i] in tokens such as [tɜki] or [kɜti], and then the syllable in which it appeared was spliced with another syllable. No other tokens were spliced in order to keep the tokens sounding as natural as possible.

Set A consisted of 96 trials where each token presented contained two identical vowels, e.g., A. gùbù B. gʊ̀bʊ̀ X. gùbù. The vowel pairs compared were the same as used in Experiment 1, encompassing ATR, Height, and ATR+Height differences, except point vowels were not used, and we dropped the [ɛ] ~ [a] height combination to reduce the number of trials. There were eight trials per combination so that the order of A and B were switched and X was matched to either A or B, and both consonant frames were employed. All of the bisyllables in these trials were necessarily harmonic given that the vowels in each word are identical.

Set B consisted of 64 harmonic non-identical vowel trials, where harmonic means that within a word, the vowels agreed for ATR, and non-identical means that within a word, the two vowels were not identical. These consisted of trials where either A or B had non-identical vowels, the other word had identical vowels and the X always had identical vowels (e.g., A. gubu B. gubo X. gubu or gobo). This meant that there was only one vowel across all three words that was different. The trials contained words in which both orders internal to the word (gubo and gobu) were used, and they also varied whether the non-identical word was A or B, resulting in eight combinations. Adding in the two different consonant frames, there were 16 trials per pair. The harmonic restriction meant that only height differences were employed in these trials: i ~ e u ~ o ɪ ~ ɛ ʊ ~ ɔ.

Set C consisted of 112 disharmonic non-identical vowels. They followed the same format as Set B, except that the vowels within the word mismatched for ATR, and either had identical height (e.g., A. gʊbu B. gubu X. gubu), or mismatched for both ATR and height (e.g., A. gʊbo B. gobo X. gobo). We did not test the [ɛ] ~ [ɜ] distinction in this set to reduce the number of trials, due to concern over the length of the experiment. Note that disharmonic within a word does not necessarily mean phonotactically illegal in Akan. As harmony is regressive and +ATR dominant, sequences of –ATR+ATR are phonotactically illegal within a word, but the reverse sequence of +ATR –ATR is disharmonic but phonotactically legal, as in the examples in (4). Not all vowel combinations occur in this order though. The stimuli are summarized in Table 6:

Table 6

Experiment 2 list of stimuli.

 Vowel pairs Trials Set A Harmonic Identical gʊbʊ/gubu ATR i ~ ɪ u ~ ʊ e ~ ɛ o ~ ɔ [ɜ] ~ a 40 Height i ~ e u ~ o ɪ ~ ɛ ʊ ~ ɔ 32 ATR+Height ɪ ~ e ʊ ~ o ɛ ~ [ɜ] 24 Sub-Total 96 Set B Harmonic non-identical gobu / gubu Height i ~ e u ~ o ɪ ~ ɛ ʊ ~ ɔ 64 Set C Disharmonic non-identical gʊbu / gubu or gʊbo / gobo ATR i ~ ɪ u ~ ʊ e ~ ɛ o ~ ɔ [ɜ] ~ a 80 ATR+Height ɪ ~ e ʊ ~ o 32 Sub-Total 112 Total 272

Stimuli were recorded by a multilingual speaker (Michael Obiri-Yeboah) of Gua (Kwa, Ghana) and Akan (Akuapem dialect), pronouncing the tokens as if speaking Akan. This change in the language of the stimuli from Experiment 1 ensured that the subjects knew they were processing Akan. Results should therefore reflect Akan L1 processing and not processing of an unfamiliar language with a similar vowel system. The acoustic F1/F2 vowel space of the tokens is provided below in Figure 8, measured from both syllables of the identical vowel tokens in the experiment. As expected, there is overlap between [ɪ] and [e], and between [ɛ] and [ɜ]. There is very little overlap between [ʊ] and [o]. This is in line with other reports that the acoustically similar front vowels are closer than the back ones in Akan.

Figure 8

F1/F2 measurements of vowels in identical tokens in Experiment 2; Ellipses show 85% Confidence Interval.

#### 3.1.3 Procedure

An auditory ABX discrimination task was presented to participants using PsychoPy on a laptop through headphones. The interstimulus interval (ISI) was set at 700 ms between the first and second and between the second and third stimuli. This longer ISI was aimed to encourage phonological processing (Piccinini & Arvaniti, 2019). Participants were told that they would hear new, made-up words in Akan. They were asked if the last word sounded the same as the first or the second. They pressed button ‘b’ on the keyboard for ‘first’ (baako in Akan) or ‘m’ for ‘second’ (mmienu in Akan). Items were presented in random order to the participants. There was a set of four practice trials, half answer ‘first’ and half answer ‘second’, using distinctly-different words (i.e., teke-gubu-teke) and providing accuracy feedback to acclimate participants to the procedure. If participants missed one of these four easy items, the practice trials were repeated. After one repetition of the practice trials, participants continued to the main experiment regardless of score.

### 3.2 Predictions

Based on the results of Experiment 1, we make several predictions for the results of Experiment 2 in accordance with the Acoustic Hypothesis (Table 7). With respect to the identical vowel pairs (Set A), we expect to see excellent discrimination for ATR contrasts and for Height contrasts, but poor performance for vowels contrasting in both ATR and Height. Given the outcome of Experiment 1, we also expect that ATR pairs will be discriminated slightly better than Height pairs. Compared to Experiment 1, the bisyllables may also afford speakers more opportunity to detect distinctions, since there is not reliance on a single vowel, possibly leading to high accuracy.

Table 7

Experiment 2, Acoustic hypothesis vs. Phonological hypothesis predictions compared to results.

 Vowel pairs Acoustic Phonological d' Accuracy Identical vowels Non-low ATR i ~ ɪ e ~ ɛ u ~ ʊ o ~ ɔ ✔ ✔ 2.73 (1.07) 87.3% Height i ~ e u ~ o ɪ ~ ɛʊ ~ ɔ ɛ ~ a ✔ ✔ 2.35 (0.85) 84.9% ATR+Height ɪ ~ e ʊ ~ o ɛ ~ ɜ ✖ ✔ 0.67 (0.98) 60.0% Low ATR a ~ [ɜ] ✔ ✖ 3.85 (1.38) 92.0%

As for the non-identical vowel tokens (Sets B and C), we expect an overall poorer discrimination rate for these sets than identical (Set A), as there are two opportunities to hear the vowel distinctions (gibi vs. gebe) with identical vowels, but only one opportunity with the non-identical categories (gibi vs. gibe). At a more detailed level, the role of vowel harmony should impact the results. The Harmony hypothesis (Table 8) predicts that Set B, where all vowels match for ATR and are harmonic, should show better discrimination than disharmonic forms (that is, Set C).

Table 8

Experiment 2, Harmony hypothesis predictions compared to results.

 Non-identical vowels Harmony hypothesis d' Accuracy Non-low ATR i ~ ɪ e ~ ɛ u ~ ʊ o ~ ɔ ✖ 1.74 (0.68) 78.9% Height i ~ e u ~ o ɪ ~ ɛʊ ~ ɔ ɛ ~ a ✔ 1.74 (0.75) 78.7% ATR+Height ɪ ~ e ʊ ~ o ✖ 0.56 (0.61) 60.1% Low ATR a ~ [ɜ] ✖ 2.01 (1.20) 78.8%

### 3.3 Results

As with Experiment 1, the acoustically similar pairs had lower rates of accurate perception compared to the ATR pairs and the Height pairs (Figure 9). Detection rates for ATR pairs and Height pairs in the Identical stimuli (Set A) were high. For completeness, accuracy for individual sound pairs are shown in Appendix B.

Figure 9

Experiment 2 accuracy values, with standard errors. Dashed line indicates chance performance (.50). Identical-vowel trials (e.g., gubu-gʊbʊ-gubu) showed high accuracy for ATR+Height contrasts, with lower overall accuracy for ATR+Height-differing items. Non-identical vowels (e.g., gubu-gubʊ-gubu; Sets B and C) showed similar patterns, again with Height (harmonic) and ATR (disharmonic) accuracy higher than ATR+Height (disharmonic).

For maximum parallelism to Experiment 1, we computed d' for this experiment as well. The d' values for ABX tasks are computed slightly differently, as follows. Correct trials where the expected response was the A (first) item were counted as hits, while correct trials where the expected response was the B (second) item were counted as correct rejections. Correct rejections were converted to false alarms (FA = 1-CR) and then hits and FAs were used to compute d'.

We conducted an ANOVA on d' values with two within-subjects predictors: Vowel Identity within the differing word (identical vowels, nonidentical vowels) and vowel Contrast Type (ATR, Height, or ATR+Height). Trials with ɜ-ɛ were dropped as these did not have a counterpart in the differing-vowel case. Recall that Bonferroni correction for three comparisons yields a p-value of .0083. There was an effect of Contrast Type (F(2,76) = 102.9, p < .0001), such that ATR pairs showed marginally higher d' than Height pairs (t(38) = 2.72, p = .01; marginal after Bonferroni correction), and both exceeded the ATR+Height pairs (vs. ATR: t(38) = 11.58, p < .0001; vs. Height: t(38) = 10.08, p < .0001). There was also an effect of Vowel Identity (F(1,38) = 84.7, p < .0001), such that d' values were greater when the test word contained two identical vowels than when the test word contained two different vowels. Finally, the main effects were qualified by a two-way interaction (F(2,76) = 15.04, p < .0001). To understand the nature of the interaction, we compared the contrast types for each Vowel Identity. For identical vowels, ATR changes were better identified than Height changes (t(38) = 3.16, p = .003), and both were better identified than ATR+Height changes; (vs. Height: t(38) = 8.82, p < .0001; vs. ATR: t(38) = 10.81, p < .0001). The results for nonidentical vowels inform the Harmony Hypothesis, that harmonic nonidentical vowel stimuli would be easier to process than disharmonic stimuli. While both Height-changing stimuli (e.g., gubo; t(38) = 8.64, p < .0001) and ATR-changing stimuli (e.g., gubʊ; t(38) = 8.88, p < .0001) were better identified than ATR+Height-changing stimuli (e.g., gʊbo), the Height stimuli were no better detected than the (disharmonic) ATR stimuli (t(38) = 0.06, p = .95). While this might seem to suggest that disharmonic stimuli are not overall at a perceptual disadvantage, it is worth noting that the disharmonic ATR stimuli show a larger drop from the identical-vowel stimuli than the Height stimuli do. A follow-up ANOVA compared the size of this drop by testing the interaction of Contrast Type (ATR, Height; leaving out ATR+Height) and Vowel Identity. The interaction term was significant (F(1,38) = 6.82, p = .01), suggesting that the drop in performance in two-vowel stimuli was larger for ATR stimuli than for Height stimuli, which is consistent with a relative disadvantage of disharmony.8 This may be seen as partly consistent with the Harmony Hypothesis. Despite all of these differences, and low d' values in some cases, all six values shown in Figure 10 exceeded chance (t(38) ≥ 4.28, p ≤ .0001), meaning that all vowel classes show better discriminability than chance.

Figure 10

d' values for identical-vowel and non-identical vowel trials, collapsed across vowel pairs. The ɜ-ɛ trials have been omitted as they were only present in the identical-vowel condition.

As in Experiment 1, we continue to find that ATR+Height-differing vowels are difficult to distinguish perceptually. Among the ATR+Height contrasts, ɜ-ɛ appeared to be distinguished more accurately. To verify this statistically, we computed individual per-vowel d' scores for identical-vowel ATR+Height stimuli. The ɜ-ɛ contrast outperformed the other two ATR+Height contrasts (F(2,76) = 19.01, p < .0001; individually, it outscored e-ɪ, t(38) = 5.26, p < .0001; and o-ʊ, t(38) = 5.50, p < .0001).9

As in Experiment 1, to provide further support for the Acoustic Hypothesis, we assessed the significance of the correlation between the vowel pairs’ Euclidean distances and their accuracy (identical trials only). As in Experiment 1, there was a positive linear relationship between the two measures (t(10) = 3.60, r = .751, p = .005), as shown in Figure 11.

Figure 11

Experiment 2, accuracy x acoustic Euclidean distance scores.

As in Experiment 1, we wanted to know how well the only allophonic ATR contrast, [ɜ]-a, fared. Did it show poor discrimination (in keeping with the Phonological hypothesis) or good discrimination (in keeping with the Acoustic hypothesis)? To test this, we computed per-vowel d' scores for identical-vowel stimuli differing in ATR. A t-test comparing the [ɜ]-a pair (d' = 3.85, SE = 1.38) to the combination of the other four pairs (d' = 2.73, SE = 1.07) showed that [ɜ]-a had greater d' (t(38) = 4.95, p < .0001), suggesting that its allophonic status did not make it more difficult to discriminate. This is consistent with the Acoustic hypothesis—and our findings in Experiment 1—but not the Phonological hypothesis.

### 3.4 Discussion

As with Experiment 1, the acoustically similar phonemic pairs ɪ ~ e and ʊ ~ o had lower rates of accurate perception compared to ATR and Height pairs. This was true for both Identical and Non-identical vowel trials. Further, the [ɜ] ~ a ATR pair was again perceived well, even though this is an allophonic contrast. Like Experiment 1, these results are consistent with the Acoustic hypothesis whereby similar acoustic vowels exhibit lower perceptual accuracy. This is especially striking for the [o] ~ [ʊ] contrast, as these vowels are acoustically similar but do not show as great an overlap in the acoustic vowel space. This is the case even if the vowels are produced in a different articulatory manner and even if they are distinguished by two phonological features. While degree of phonological similarity was not part of the hypotheses tested, being distinguished by two phonological features contributes to a low phonological similarity rating, which is not congruent with the perceptual results for ɪ ~ e and ʊ ~ o pairs. As for the Harmony hypothesis, it was not the case that bisyllabic harmonic Height tokens were better perceived than bisyllabic disharmonic ATR tokens, as predicted. Performance on the non-identical tokens were similar for all ATR pairs and Height pairs, but the ATR+Height pairs again showed the weakest performance. However, there was a stronger drop in performance for the ATR pairs than the Height pairs, suggesting that disharmony did negatively impact perception for ATR distinctions.

However, unlike in Experiment 1, the ɜ ~ ɛ pair showed much better perceptual accuracy than the other ATR+Height pairs in Experiment 2. We do not believe this is due to the vowels being significantly different between the two experiments, as they were pronounced similarly in both with a large overlap in F1/F2. This difference may be due to two aspects of Experiment 2. First, [ɜ] does not occur in monosyllables in Akan words, but [ɜ] can occur in initial position in bisyllables (albeit followed by +ATR vowels other than [ɜ]), so the bisyllabic environment aids perception. However, it is also possible that the spliced bisyllabic gɜbɜ/tɜkɜ tokens may not have sounded as natural as other tokens, so when gɜbɜ was compared to gɛbɛ, it may have been easier to detect different vowels. In Experiment 1, we cut out the first syllable [kɜ] from a bisyllabic word, but in this experiment, we had to splice two halves of two words together. Regardless, it is clear that overall, the ATR+Height pairs are more difficult to perceive than ATR differences or Height differences.

## 4. General Discussion

Overall, both experiments provided evidence inconsistent with the Phonological hypothesis and consistent with the Acoustic hypothesis. Both confirmed that the acoustically similar vowels [ɪ] / [e] and [ʊ] / [o] are poorly discriminated by Akan listeners, even if they are differentiated by two distinctive phonological features. Conversely, both experiments also confirmed that the acoustically distinct vowels [ɜ] / [a] are well discriminated, despite being in an allophonic rather than contrastive relationship.

Recall that there were several differences between the two experiments, which make their relative agreement all the more striking. They varied in number of syllables, monosyllabic (Experiment 1) vs. bisyllabic (Experiment 2); interstimulus interval, with 500 ms (Experiment 1), vs. a more “phonological” 700 ms in Experiment 2; words in the phonologically-similar language Gua in Experiment 1, vs. nonce words in Akan in Experiment 2; an AX design that allowed acoustic discrimination in Experiment 1 vs. an ABX design that required a more phonological level of comparison in Experiment 2. Despite these differences, acoustically similar (but phonologically distinct) pairs in both experiments were significantly harder to distinguish than the other pairs, and the allophonic (but acoustically distinct) pair were at least as easy to distinguish as the phonemic counterparts.

The Ozburn et al. (2022) study on Dàgáárè also found that the ATR+Height pair [ɪ ~ e] was poorly discriminated, at least compared to [e ~ ɛ], in keeping with the overlap in the acoustic space of that pair, and similar to our results. However, discrimination was not significantly different for [i ~ɪ] compared to either of the other two pairs. They also found that Dàgáárè listeners fared worse with bisyllable disharmonic stimuli than with monosyllables, but did not find that harmonic stimuli improved perception compared to monosyllables. The experiments employed an ABX design and had monosyllable and bisyllable stimuli in the same experiment, allowing for direct comparison between the two kinds of stimuli. Our Experiment 2 results did show lower accuracy for disharmonic bisyllables than harmonic identical forms, but not for harmonic non-identical forms. However, we are not able to make the same comparison between monosyllables and bisyllables due to the multiple different parameters outlined above.

## 5. Implications for the phonology of Akan

Akan appears to present a case of near-merger – the situation whereby speakers differentiate sounds in production, but report that they are ‘the same’ in perception tests (Labov et al., 1991). Indeed, phoneticians in earlier work often described the situation in Akan as a merger, particularly for the vowels [ɪ] ~ [e] due to the close F1/F2 acoustic measurements. However, X-ray and MRI data from the 1970s and 1990s, albeit based on a few speakers and different generations of speakers, confirm distinct tongue root productions for Akan vowels despite the acoustic similarity; recent ultrasound data (Kirkham & Nance, 2017) confirm this for most vowel pairs (i/ɪ u/ʊ o/ɔ o/ʊ), but were not able to confirm it for the e/ɛ or e/ɪ distinctions due to speakers adopting different articulatory strategies.10

This apparent near-merger raises the question of learnability. If the vowels are so hard to perceive as different, how do Akan speakers learn to produce them differently? A recent study of first language acquisition by nine Akan-speaking children (Asante-Twi dialect) aged three to five years (Amoako, 2020) shows that these particular vowel distinctions may not necessarily be the ones posing problems. Her results indicate that all +ATR vowels have a lower production accuracy than –ATR vowels across the age groups and children. In particular, [ɜ] (she transcribes this as [æ]) has the lowest accuracy rate of all vowels and is still not fully acquired by age five. It is frequently substituted with either [ɛ], which fits with acoustic similarity, or with [a], its harmonic counterpart, which may reflect lack of vowel harmony application for this vowel (as noted in Section 1.4, the contexts where /a/ is harmonized to [ɜ] are restricted). This shows that the acquisition of the allophonic vowel is delayed compared to the phonemic contrasts, which may be a reflection of its more limited contextual environment and possible ongoing merger with [e] from /e/ (Kpogo, 2022), rather than difficulty in perception. Interestingly, the other [+ATR] vowels are commonly substituted with their [–ATR] counterparts, not the acoustically similar but height-differing [–ATR] counterparts. For example, the word èdʑá ‘fire’ is produced as [ɛ̀dʑá] rather than [ɪ̀dʑá]. Although there are some sporadic cases of /o/ being pronounced as [ʊ], or [e] as [ɪ], given what we know about adult perception, we might have expected a far stronger tendency for these vowels to be merged in children’s productions. Amoako (2020, p. 130) notes that the children have acquired vowel harmony, and vowel substitutions tend to respect it “even when there were errors in their production.” This provides some supporting evidence that vowel harmony is learned at a young age and appears to reinforce the distinction between vowels by keeping them separated within words. We hypothesize that vowel harmony may aid speakers in acquiring phonemic contrasts between acoustically similar vowels, such as ATR+Height pairs /ɪ e/ and /ʊ o/. Nevertheless, if the author, as an adult Akan speaker, transcribed the vowels based on her own perception, substitution errors between [e] and [ɪ] may not have been as readily perceived as between other vowels. Future acquisition work may benefit from using acoustic measurements.

## 6. Conclusion

This is a large-scale study of the perception of Advanced Tongue Root vowel contrasts by speakers of a language with ATR vowels and vowel harmony. Two experiments were conducted to test perception of ATR contrasts in Akan: An AX task and an ABX task. The AX task (Experiment 1) tested perception of monosyllables drawn from a related language (Gua), and participants were told it was a different language than Akan. The ABX task (Experiment 2) tested perception of bisyllables in the Akan language and participants were told it consisted of nonce Akan words. The results show that participants were able to accurately perceive all vowel pairs that differed based on only ATR in both experiments, even the pair a ~ ɜ that was not phonologically contrastive. However, acoustically similar vowels ɪ ~ e, ʊ ~ o, and ɜ ~ ɛ in Experiment 1, were poorly perceived. This is so despite these vowel pairs being phonologically contrastive and differing for two phonological features, vowel height and ATR. Similar results obtain for ɪ ~ e, ʊ ~ o in Experiment 2.

The results show that acoustic similarity in Akan vowels drives perception, rather than phonological similarity or phonemic status. It provides supporting perceptual evidence that some vowel pairs are so acoustically similar as to be practically merged, despite being produced differently based on tongue root and larynx position. This constitutes a case of near merger where production is different but perception shows a merger. The role of ATR vowel harmony is important in Akan for maintaining the distinction between similar vowels. These vowels commonly occur in words with other vowels that match them for ATR, respecting vowel harmony. There are thus few instances where distinguishing, say, [e] from [ɪ], must occur in the absence of other vowel cues. Nevertheless, we know from historical evidence in other languages that vowels may exhibit merger despite the presence of vowel harmony, reducing nine vowel ATR systems to seven (Elugbe, 1989). It remains to be seen whether Akan will move definitively in this direction.

## Notes

1. The phonological feature Retracted Tongue Root [RTR] is used in opposition to Advanced Tongue Root [ATR] by some researchers instead of binary values of a single [ATR] feature (e.g., Pulleyblank, 2002). In addition, [RTR] has been used for vowel distinctions in languages outside Africa to refer to tongue root distinctions, such as in Altaic languages (Li, 1996; Kang & Ko, 2012; Kang, 2018). [^]
2. We employ the vowel transcriptions [i ɪ e ɛ u ʊ o ɔ] instead of the IPA symbols [i̘ i̙ e̘ e̙ u̘ u̙ o̘ o̙] as the diacritics to indicate ATR distinctions are difficult to discern. [^]
3. Hess (1992) reports that Lindau (1975) examined the low vowel /a/ and its harmonic counterpart [æ], and determined that in the Akyem and Asante Twi dialects, [æ] does not have an advanced tongue root position. But this leaves out the fact that the Akyem speaker also had [ɜ] in words preceding +ATR vowels. Lindau describes [ɜ] as +ATR and [æ] as –ATR. She states (1975, p. 50) that ‘the [ɜ] is articulated with a more advanced tongue root and lower larynx than [æ] and [a]. It is also interesting to note that the low [æ] has the highest larynx position of all eleven vowels in Akyem.” We examined the word list Lindau used (1978, p. 138) and found that while the [ɜ] preceded high vowels, the [æ] was in the word sá [sæ] ‘to dance,’ which purportedly contrasts with sá [sa] ‘to cure.’ However, these actually have the same vowel but different tone: ‘to cure’ is [sà]. This may explain the difference in larynx height. Furthermore, the word ‘to dance’ is shown with an optional glottal stop. Although glottal stops can occur pre-pausally in Akan, perhaps the tokens this speaker produced did contain glottal stops, which could have affected the vowel quality. It seems the [æ] for this one speaker are –ATR fronted allophones of /a/. [^]
4. The Ozburn et al. (2022) study was conducted online in 2021, whereas the current study was run prior to the COVID-19 pandemic in 2018 (Experiment 1) and early 2020 (Experiment 2), and was therefore able to be conducted in person in Ghana. [^]
5. Stewart claims that the a/ɜ contrast is phonemic based on pairs such as ntam ‘oath’ vs. ntɜm ‘between.’ However, ǹtɜ́m̀ derives from ǹtá ‘twin/pair’ + mù ‘inside,’ suggesting that the /ù/ harmonized the /á/ and then was deleted, leaving its low tone on the nasal. See also Appiah-Padi (1994) on this point. Even though Stewart acknowledges this derivation, he still views the distinction as phonemic. Nevertheless, the view that these two vowels are phonemic is a minority opinion. Kirkham and Nance (2017) state that they are phonemic in the Akuapem dialect based on earlier reports (Ladefoged, 1968; Dolphyne, 1988), but those reports do not distinguish phonemic from allophonic status clearly, indicating only the presence of [ɜ]. [^]
6. Euclidean distance measures are i ~ u = 1.888, i ~ a = 3.67 and u ~ a = 3.074, all greater distance measures than other vowel pairs in the experiment. [^]
7. Akan does have cross-word vowel harmony (Schachter, 1989; Dolphyne, 1988; Hess, 1992; Kügler, 2015) that applies regressively and affects the last vowel of the preceding word, so disharmonic words can arise in sentential contexts due to this process. [^]
8. A reviewer asks if there are differences in performance on the disharmonic sequences depending on the order of the vowels, due to the regressive nature of vowel harmony. Poorer perception might be expected if the vowel order is –ATR +ATR (illicit) compared to +ATR –ATR (licit). This is indeed the case if the standard X is +ATR+ATR, but not when it is –ATR –ATR, suggesting that speakers may be perceptually compensating for regressive harmony. However, the order +ATR –ATR was harder to differentiate when X was –ATR –ATR than when it was +ATR +ATR, which does not appear to be due to directionality. We do not include directionality results for reasons of space, and due to the difficulty of making clear hypotheses and predictions. For example, if participants misperceive [ɪ] as [e] and [ʊ] as [o], or vice versa, there are a range of possible perceptual directionality outcomes. [^]
9. While accuracy values appear different for ATR+Height between the two studies, this is primarily because Experiment 1 just shows accuracy on the “different” trials, while Experiment 2 shows accuracy on all trials. The d' values for the two studies are similar. While d' values are somewhat lower for Height-only and ATR-only in Experiment 2, given the higher memory load in Experiment 2 (having to remember four syllables before getting to X (A+B = 4 syllables), over a longer ISI, vs. Experiment 1’s one syllable and shorter ISI), as well as the change in language, we are reluctant to ascribe too much practical significance to this difference. [^]
10. Kirkham and Nance (2017) describe the situation as follows: “An examination of individual speaker data shows two divergent patterns amongst the Twi speakers: GF01 and GF02 produce /e/ slightly more advanced than /ɛ/, whereas GM01 and GM02 produce /e/ slightly more retracted than /ɛ/. These patterns remain the same whether we use values extracted from the vowel midpoint or values extracted 80% into the vowel. There is also no clear correspondence between the use of these two articulatory strategies and any patterns in tongue height, which suggests that the Twi speakers may be particularly variable in the articulation of the /e ɛ/ contrast, despite greater acoustic consistency among speakers.” [^]

## Acknowledgements

We are grateful to Associate Editor James Kirby and two anonymous reviewers for helpful comments and critiques on this paper. We thank audiences at the UC San Diego Center for Research in Language, the Southern California Workshop on Phonology at California State University Long Beach, and the Annual Conference on African Linguistics 50 at the University of British Columbia for feedback. We are grateful to Aaron Braver for useful tips on experiment design, and Monica Apenteng Obiri-Yeboah for nonce word assessment and recording help. We thank the Department of Applied Linguistics and Prof. Avea Nsoh of the Colleges of Languages Education at the University of Education, Winneba in Ghana, who provided logistical support for this research. We thank all the Akan speakers who participated in the study. This research was supported by a grant from the UC San Diego Academic Senate.

## Competing interests

The authors have no competing interests to declare.

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