PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Discrimination of mono-syllables in sentence context: the case ofJapanese listeners’ perception of /ba/-/da/ continuum
Kanako TOMARU 1, 2
1

Mejiro University, Japan

2

Sophia University, Japan

ABSTRACT
It is widely known that native speech sounds are perceived according to the phonetic categories of the native
language: the process is called categorical perception. The hypothesis of categorical perception assumes that
two stimuli are perceived to be different only when these are identified as different. Therefore, the best
discrimination performance (discrimination peak) is observed when the paired stimuli are members ofdifferent categories. Traditionally, categorical perception has been tested using monosyllables presented inisolation. However, my recent research has shown that the discrimination peak is not observed when the
mono-syllabic stimuli are embedded in a sentence. This recent research investigated the categoricalperception in a sentence using an AXB paradigm with a stimulus interval of 300 ms: The relatively shortinterval may be one of the factors that suppressed the discrimination peak. To clarify this point, the current
study conducted a perceptual experiment on Japanese listeners using an AXB paradigm with a stimulusinterval of 1 s. The results suggest that for the perception of a Japanese /ba/-/da/ continuum by Japaneselisteners, the length of the interval has only a small impact.
Keywords: Categorical perception, Sentence context, Stimulus interval

1. INTRODUCTION
The process of speech perception involves two common abilities, namely the ability to identify
phonemes and the ability to discriminate one phoneme from another. The combination of theseabilities leads to categorical perception (1). Strictly speaking, categorical perception assumes that
listeners can discriminate two sounds only when these sounds are identified as different phonemes. For
example, when native speakers of English perceive a sound continuum that gradually changes fromEnglish /r/ to /l/ in ten steps, they divide the continuous stimuli into two categories, namely /r/ (Step 1
through Step 4) and /l/ (Step 6 through Step 10), placing a boundary in the middle (Step 5). Whendiscriminating stimuli along the continuum, listeners use the category to make discrimination
judgments. In other words, the discrimination performance for two sounds can be predicted fromlisteners’ identification results. Therefore, the hypothesis of categorical perception predicts that two
stimuli from the same category will be heard as being the same. In contrast, the best discrimination
performance (discrimination peak (2)) is observed when the paired stimuli are members of different
phonemic categories (cross-boundary pairs). The fundamental principle that discrimination
performance can be predicted from identification results can be adopted for cross-language speech
perception. For native speakers of Japanese, English /r/ and /l/ are heard as the single Japanesephoneme /4/ (e.g., (3,4)); therefore, there appears to be no categorical boundary along the /ra/-/la/continuum. Thus, no discrimination peak is observed when native speakers of Japanese perceptuallydiscriminate these English phonemes.
Traditionally, categorical perception is observed through identification and discrimination
experiments using monosyllabic stimuli presented in isolation. However, recent research has shownthat the characteristics of categorical perception, especially a discrimination peak, disappear when themonosyllabic stimuli are embedded in a sentence. For instance, Tomaru and Arai (5,6) reported thatwhen native speakers of English discriminate English /ra/ syllables from /la/ syllables, discrimination
1
2

ka-tomaru@sophia.ac.jp
k.tomaru@mejiro.ac.jp

6823

(a)

(b)

performance does not increase at the cross-boundary pairs. For their experiments, synthetic stimuli
that continuously change from English /ra/ to /la/ were synthesized. The experimental tasks included
identification and discrimination tests under three sound conditions: (i) in isolation (5,6), (ii) between
pure tones (6), and (iii) within a sentence (5,6). Their experiments revealed that discrimination
performance under conditions (i) and (ii) was consistent with the traditional prediction of categoricalperception, i.e., discrimination was highly accurate when comparing stimulus pairs from different
phoneme categories. However, under condition (iii), discrimination accuracy did not increase even for
a comparison of cross-boundary pairs. Subsequent research conducted by Tomaru and Arai (7) foundthe same tendency for Japanese listeners’ perception of voiced stops, i.e., /ba/ and /da/.
The purpose of the current research is to replicate the preceding findings (i.e., the characteristics
of categorical perception are not found under the sentence condition) using an AXB paradigm with alonger inter-stimulus interval (ISI). In the previous research (5–7), listeners’ discrimination
performance was assessed using an AXB paradigm with an ISI of 300 ms, which is adequate for
examining the categorical perception of monosyllables in isolation (8). However, for sentence
perception, it is possible that a short ISI increases the perceptual burden. Under the sentence
condition, English syllables, for example /ra/-/la/, are embedded into an English sentence, forexample “Clear /ra/ (or /la/) is appreciated” (5, 6). During an AXB discrimination task, listeners hear
the sentence three times consecutively, and judge whether X matches A or B. Although speech
perception is a relatively fast automatic process (9), it may be difficult for listeners to complete two
steps, namely identify the stimuli in question and compare them, during a short period of time. If
discrimination performance is negatively affected by a short ISI, then a longer ISI should yield adiscrimination peak. The current research investigates Japanese listeners’ categorical perception of/ba/-/da/ syllables under two conditions: (i) in isolation and (ii) within a sentence. The results of aperceptual experiment suggest that the length of an ISI does not affect listeners’ discrimination
performance under the sentence condition.

2. Method
2.1 Stimuli
In the current study, a nine-step continuum of /ba/-/da/ syllables is employed. Under the isolation
condition, the continuum of syllables was presented to participants in isolation. Under the sentence
condition, the same syllables were embedded into a sentence, e.g., sorekara /ba/ ga aruto omoimasu(/ba/ was embedded), “And, I think there is /ba/.” The target syllable, the embedded /ba/ in thisexample, changes in nine steps along the continuum.
The nine-step /ba/-/da/ continuum originally created for previous experiments was used here (7).
The continuum was created using the cascade-formant synthesizer designed by Klatt and Klatt (10,11).
The parameter values for creating syllables were decided on the basis of a /ba/ utterance by a malenative speaker of Japanesesorekara /ba/ ga aruto omoimasu “And I think that there is /ba/.” Themean values of formants 1 through 5 were obtained from the token and used as the parameter values. In
addition to the formant frequencies, the bandwidth of each formant, extra tilt of the voicing spectrum,and quotient of vocal-fold opening were manipulated on the synthesizer to imitate the speaker. To
create a continuum, the formant trajectories of F1 and F2 were varied in nine steps from canonical /ba/
(Step 1) to /da/ (Step 9). The values of the fundamental frequency (F0) were decided so that the createdsyllables would naturally fit the original sentence when presented within a sentence under the sentence
condition. Please refer to (7) for further information on stimulus synthesis.
2.2 Participants
Twenty native speakers of Japanese (7 males and 13 females) ranging in age from 19 to 32 years(mean age = 24.1 years) participated in the experiment. The participants reported no hearing
difficulties at the time of the experiment.
2.3 Experimental Methods
The participants performed identification and discrimination tasks under the isolation condition
and the sentence condition. Each participant had a familiarization session before experimental
sessions. All sessions were conducted using Praat software (12) and headphones connected to acomputer via a USB-connected audio interface.

6824

Percentage of Correct Responses

Percentage of /ba/ Responses

100
90
80
70
60
50
40
30
20
10
0
1

2

(a)

3

4

5

6

7

8

100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
1–3

9

(b)

Stimulus Step

2–4

3–5

4–6

5–7

6–8

7–9

Stimulus Pair

Figure 1 – Results of identification (a) and discrimination (b) tasks under the isolation condition.
2.3.1. Familiarization
Before the experimental sessions, participants were exposed to the synthesized syllables tobecome familiarized with artificial sounds. First, the participants heard five repetitions of the edge
stimuli, i.e., Step 1 and Step 9. Then, the continuum was presented twice. The first time, theparticipants heard the continuum from Step 1 to Step 9 in ascending order; the second time, they
heard the same continuum in descending order. During the familiarization session, the participants
were asked to make self-judgments about whether the stimuli they heard were /ba/ or /da/. The
participants were able to adjust the sound to a comfortable listening level during this session. They
were asked not to change the volume once the main experiments had begun.
2.3.2. Identification Task
The two-alternative forced-choice method (2AFC) was utilized. Participants were asked to judge
whether the syllable they heard was “ࡤ /ba/ ” or “ࡔ /da/” by clicking on the appropriate button in
the computer interface. The participants performed the task under the isolation condition and thesentence condition in separate sessions. For both conditions, each stimulus (syllable or sentence)along the continuum was repeated five times in random order (9 stimuli × 5 repetitions = 45
judgments). Practice sessions preceded experimental sessions.
2.3.3. Discrimination Task
The AXB paradigm, where participants were asked to judge whether X matches A or B, wasemployed. As done in the identification session, the participants performed the AXB discrimination
task under the isolation condition as a separate session from that for the sentence condition. For bothconditions, the stimuli were paired such that each pair differed by two steps along the continuum(e.g., Step 1 and Step 3). In addition, the edge stimuli (i.e., Step 1 and Step 9), were paired andpresented to participants as fillers. Each pair was repeated three times in random order (8 pairs × 3
repetitions). For both conditions, the ISI was 1 s. Practice sessions were conducted before
experimental sessions.
Under the isolation condition, participants were instructed to compare syllables and to judge
whether the second syllable (X) matched the first (A) or third (B) syllable. Similarly, under thesentence condition, the participants were told to concentrate on the syllables in the middle of asentence, and to compare three sentences. Then, they were asked to judge whether the second
sentence matched the first or third sentence.

3. RESULTS
3.1

Isolation Condition

3.1.1. Identification Task
The identification results obtained under the isolation condition are summarized in Fig. 1-(a). The
participants’ responses are averaged and shown by the dotted line. The solid line shows the percentage
of /ba/ responses fitted using the following logistic model.
= 100 1 +

(

)

(1)

6825

100%

Percentage of Correct Responses

Percentage of /ba/ Responses

100
90
80
70
60
50
40
30
20
10
0
1

(a)

2

3

4

5

6

7

Stimulus Step

8

90%
80%
70%
60%
50%
40%
30%
20%
10%
0%

9

(b)

3–5

4–6

5–7

6–8

Stimulus Pair

Figure 2 – Results of the identification (a) and discrimination (b) tasks under the Sentence Condition.
,where y is the percentage of /ba/ responses and x is the step number on the continuum. The
parameters a and b represent the slope of the curve and the 50% crossover point, respectively. In thiscase, the slope of the curve is 2.0 and the 50% crossover point is 5.5. Based on the identification
results, we can predict a discrimination peak at the pair Step 4 and Step 6, which crosses thecategory boundary.
3.1.2. Discrimination Task
The percentage of correct responses was calculated for each listener. Figure 1-(b) shows theaverage percentage of correct responses. The identification results indicate that the categoryboundary is at 5.5. Therefore, the discrimination peak is predicted for the pairs 4–6, 5–7. Assumingthat these pairs are cross-boundary pairs, the stimulus pairs were divided into three groups: (i) thewithin /ba/ pairs, i.e., pairs 1–3, 2–4, and 3–5, (ii) the cross-boundary pairs, i.e., pairs 4–6 and 5–7,(iii) the within /da/ pairs, i.e., pairs 6–8 and 7–9. A one-way analysis of variance (ANOVA) revealed
a main effect of the group (F(2, 137) = 23.46, p < 0.001). A post hoc multiple comparison with
Bonferroni correction found a significant difference between the groups: the percentage of correct
responses for the within /ba/ pairs was different from that for the cross-boundary pairs (p < 0.01).
Similarly, the percentage of correct responses for the within /da/ pairs was different from that for the
cross-boundary pairs (p < 0.01). In addition, the difference between the within /ba/ group and thewithin /da/ group was significant (p < 0.05). The results indicate that the observed characteristics of
categorical perception are as predicted under the isolation condition.
3.2

Sentence Condition

3.2.1. Identification Task
The identification results obtained under the sentence condition are shown in Fig. 2-(a). The dottedline indicates the average responses of all participants. The average responses were fitted using themodel in Eq. (1). The solid line indicates the fitted function. Under the sentence condition, the slope ofthe curve is 1.5 and the 50% crossover point is 5.9. Thus, if the characteristics of categoricalperception are to be observed under this condition, a discrimination peak should appear at thestimulus pairs 4–6 and 5–7.
3.2.2. Discrimination Task
The discrimination results are summarized in Fig. 2-(b). Under this condition, more than sevenlisteners’ responses for some within-category pairs, i.e., the pairs 1–3, 2–4 and 7–9, were below achance level, ranging from 17% to 42%. This may suggest that listeners intentionally gave responses
that were opposite to those they were expected to provide. Thus, the results for these pairs were
eliminated from the analysis.
As shown in Fig. 2-(b), no discrimination peak was observed under the sentence condition. As
done in the analysis under the isolation condition, the stimulus pairs were classified into threegroups: (i) the within /ba/ group, i.e., the pair 3–5, (ii) the cross-boundary group, i.e., the pairs 4–6,
5–7, and (iii) the within /da/ group, i.e., the pair 6–8. A one-way ANOVA found no main effect of thegroup (F(2, 77) = 2.07, p = 0.13). No significant difference between the groups was observed.

4. Discussion and Conclusion
The purpose of the research was to investigate whether a discrimination peak appears under the6826

sentence condition with an ISI of 1 s. The results of the perceptual experiments show that even with a
1-s ISI, the sentence context hinders discrimination performance. That is, the lack of a discrimination
peak under the sentence condition is not the result of a perceptual burden caused by a short ISI.
The hypothesis of categorical perception predicts a discrimination peak because it assumes that
listeners can discriminate two sounds only when they have different phonemic labels. If an AXB
paradigm with a short ISI, e.g., 300 ms (5–7), causes an excessive burden for listeners, preventing
them from completing the labeling process and making discrimination judgments at the same time,then the same paradigm with a relatively long ISI (e.g., 1 s), should reduce this burden; as a result,discrimination results should reflect labeling results. However, this was not the case. Therefore, theresults of the current experiment support the idea that listeners do not use labeling information during
sentence discrimination. This is consistent with previous studies (5–7). Further, the lack of adiscrimination peak suggests that the ability to discriminate sounds based on phonemic categories islimited to the perception of sounds in small units (e.g., syllables). When perceiving sentences,
listeners either do not discriminate sounds at all or rely on information other than phonemic labels.
In addition, it is worth mentioning that listeners’ discrimination performance is unstable under thesentence condition compared to that under the isolation condition. The results of the current
experiment showed that the percentage of correct responses did not reach a chance level in manycases for within-category pairs. A previous study (7) reported similar results, where several listeners’
percentage of correct responses for the pairs 1–3 and 2–4 was remarkably low, ranging from 8% to42%. These outcomes may imply that sentence discrimination and syllable discrimination should betreated differently. Remaining issues will be investigated in future research.

ACKNOWLEDGEMENTS
I would like to express sincere gratitude to Professor Takayuki Arai and the members of the Arai
Laboratory at Sophia University for providing helpful comments and feedback on the current
experiment.
This research was supported by a Japan Society for the Promotion of Science (JSPS) KAKENHI
grant (JP26884051, JP18K12378).

REFERENCES
1. Liberman AM, Harris KS, Hoffman HS, Griffith BC. The discrimination of speech sounds within andacross phoneme boundaries. J Exp Psychol. 1957;54:358-368.
2. Howell P, Rosen S. Natural auditory sensitivities as universal determiners of phonemic contrasts.
Linguistics. 1983;21:205-236.
3. Guinon SG, Flege JE, Akahane-Yamada R, Pruitt JC. An investigation of current models of second
language perception: The case of Japanese adults' perception of English consonants. J Acoust Soc Am.
2000;107(5):2711-2724.
4. MacKain KS, Best CT, Strange W. Categorical perception of English /r/ and /l/ by Japanese bilinguals.
Appl Psycholinguist. 1981;2:369-390.
5. Tomaru K, Arai T. Perception of /ra/-/la/ contrast in different contexts: mono-syllable vs. sentence.
Proc Meet Acoust; 2-7 June 2013; Montreal, Canada 2013. p. 1-9.
6. Tomaru K, Arai T. Discrimination of /ra/-/la/ speech continuum by native speakers of English under
nonisolated conditions. Acoust Sci Tech. 2014;35(5):251-259.
7. Tomaru K, Arai T. Role of labeling mediation in speech perception: Evidence from a voiced stop
continuum perceived in different surrounding sound contexts. Acoust Sci Tech. 2016;37(6):303-314.
8. Pisoni DB. Auditory and phonetic memory codes in the discrimination of consonants and vowels.
Percept Psychophys. 1973;13:235-260.
9. Johnson K, Ralston JV. Automaticity in speech perception: Some speech/nonspeech comparisons.
Phonetica. 1994;51:195-209.
10. Klatt DH. The new MIT speech VAX computer facility. In: Research Laboratory of Electronics ed.
Speech Communication Group Working Papers IV. MIT: Cambridge; 1984. p. 73-82.
11.Klatt DH, Klatt LC. Analysis, synthesis, and perception of voice quality variation among female andmale talkers. J Acoust Soc Am. 1990;87:820-857.
12.Boersma P, Weenink D. Praat, a system for doing phonetics by computer. Glot Int. 2001;5(9):341-347.

6827