Japanese speakers who learn English as a second language later than childhood often have difficulty in accurately hearing and producing the consonants /r/ and /l/. While Japanese has one liquid phoneme, /r/, usually realized as an apico-alveolar tap [ɾ] and sometimes as an alveolar lateral approximant [l], English has two: rhotic /r/ and lateral /l/, with varying phonetic realizations centered on the postalveolar approximant [ɹ̠] and on the alveolar lateral approximant [l], respectively.
Phonetic differences
The Japanese liquid is most often realized as an alveolar tap [ɾ], though there is some variation depending on phonetic context. /r/ of American English (the dialect Japanese speakers are typically exposed to) is most commonly a postalveolar median approximant with simultaneous secondary pharyngeal constriction [ɹ̠ˤ] or less commonly a retroflex approximant [ɻ]. /l/ involves contact with the alveolar ridge as well as some raising of the tongue dorsum (velarization), especially when syllable-final.
Perception Evidence from Best & Strange (1992) and Yamada & Tohkura (1992) suggests that Japanese speakers perceive English /r/ as somewhat like the compressed-lip velar approximant [w͍] and other studies have shown speakers to hear it more as an ill-formed Japanese /r/. Goto (1971) reports that native speakers of Japanese who have learned English as adults have difficulty perceiving the acoustic differences between English /r/ and /l/, even if the speakers are comfortable with conversational English, have lived in an English-speaking country for extended periods, and can articulate the two sounds when speaking English. Japanese speakers can, however, perceive the difference between English /r/ and /l/ when these sounds are not mentally processed as speech sounds. Miyawaki et al. (1975) found that Japanese speakers could distinguish /r/ and /l/ just as well as native English speakers if the sounds were acoustically manipulated in a way that made them sound less like speech (by removal of all acoustic information except the F3 component). Lively et al. (1994) found that speakers' ability to distinguish between the two sounds depended on where the sound occurred. Word-final /l/ and /r/ with a preceding vowel were distinguished the best, followed by word-initial /r/ and /l/. Those that occurred in initial consonant clusters or between vowels were the most difficult to distinguish accurately. Bradlow et al. (1997) provide evidence that there is a link between perception and production to the extent that perceptual learning generally transferred to improved production. However, there may be little correlation between degrees of learning in perception and production after training in perception, due to the wide range of individual variation in learning strategies.
Production Goto (1971) reports that Japanese speakers who cannot hear the difference between /r/ and /l/ may still learn to produce the difference, presumably through articulatory training in which they learn the correct places and manners of articulation required for the production of the two sounds. In this sense, they learn to produce /r/ and /l/ in much the same way a deaf person would. Although they have only a single acoustic image corresponding to a single phoneme intermediary between /r/ and /l/, they can determine they are producing the correct sound based on the tactile sensations of the speech articulators (i.e. tongue, alveolar ridge, etc.) coming into contact with each other without any auditory feedback or confirmation that they are indeed producing the sound correctly.
Variations in acquisition There is some indication that Japanese speakers tend to improve more on the perception and production of /r/ than /l/. Aoyama et al. (2004) conducted a longitudinal study that examined the perception and production of English /l/, /r/, and /w/ by adults and children who were native speakers of Japanese but living in the United States. Over time, the children improved more on English /r/ than English /l/. Similarly, Guion et al. (2000) found that Japanese speakers who received training in distinguishing English sounds improved more on /r/ than on /l/. They suggest that English /l/ is perceived as more similar to Japanese /r/ than English /r/ is, and hence it is harder for Japanese speakers to distinguish Japanese /r/ from English /l/ than Japanese /r/ from English /r/. Kuzniak & Zapf (2004) found differences between the second and third formants in /r/ and /l/ of a native Japanese speaker and a native English speaker. The results showed that the Japanese speaker had a hard time producing an English-like third formant, especially that which is required to produce an /l/.
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