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The Motor Theory of Speech Perception is a proposed explanation of the fundamental relationship between the way speech is produced and the way it is perceived. Associated primarily with the work of Liberman and colleagues, it posited the active participation of the motor system in the perception of speech. Early versions of the theory contained elements that later proved untenable, such as the expectation that the neural commands to the muscles (as seen in electromyography) would be more invariant than the acoustics. Support drawn from categorical perception (in which discrimination is quite poor within linguistic categories but excellent across boundaries) was called into question by studies showing means of improving within-category discrimination and finding similar results for nonspeech sounds and for animals perceiving speech. Evidence for motor involvement in perceptual processes nonetheless continued to accrue, and related motor theories have been proposed. Neurological and neuroimaging results have yielded a great deal of evidence consistent with variants of the theory, but they highlight the issue that there is no single “motor system,” and so different components appear in different contexts. Assigning the appropriate amount of effort to the various systems that interact to result in the perception of speech is an ongoing process, but it is clear that some of the systems will reflect the motor control of speech.

The Japanese psycholinguistics research field is moving rapidly in many different directions as it includes various sub-linguistics fields (e.g., phonetics/phonology, syntax, semantics, pragmatics, discourse studies). Naturally, diverse studies have reported intriguing findings that shed light on our language mechanism. This article presents a brief overview of some of the notable early 21st century studies mainly from the language acquisition and processing perspectives. The topics are divided into various sections: the sound system, the script forms, reading and writing, morpho-syntactic studies, word and sentential meanings, and pragmatics and discourse studies sections. Studies on special populations are also mentioned. Studies on the Japanese sound system have advanced our understanding of L1 and L2 (first and second language) acquisition and processing. For instance, more evidence is provided that infants form adult-like phonological grammar by 14 months in L1, and disassociation of prosody is reported from one’s comprehension in L2. Various cognitive factors as well as L1 influence the L2 acquisition process. As the Japanese language users employ three script forms (hiragana, katakana, and kanji) in a single sentence, orthographic processing research reveal multiple pathways to process information and the influence of memory. Adult script decoding and lexical processing has been well studied and research data from special populations further helps us to understand our vision-to-language mapping mechanism. Morpho-syntactic and semantic studies include a long debate on the nativist (generative) and statistical learning approaches in L1 acquisition. In particular, inflectional morphology and quantificational scope interaction in L1 acquisition bring pros and cons of both approaches as a single approach. Investigating processing mechanisms means studying cognitive/perceptual devices. Relative clause processing has been well-discussed in Japanese because Japanese has a different word order (SOV) from English (SVO), allows unpronounced pronouns and pre-verbal word permutations, and has no relative clause marking at the verbal ending (i.e., morphologically the same as the matrix ending). Behavioral and neurolinguistic data increasingly support incremental processing like SVO languages and an expectancy-driven processor in our L1 brain. L2 processing, however, requires more study to uncover its mechanism, as the literature is scarce in both L2 English by Japanese speakers and L2 Japanese by non-Japanese speakers. Pragmatic and discourse processing is also an area that needs to be explored further. Despite the typological difference between English and Japanese, the studies cited here indicate that our acquisition and processing devices seem to adjust locally while maintaining the universal mechanism.