Role of the motor system in language knowledge

Abstract

All spoken languages express words by sound patterns, and certain patterns (e.g., blog) are systematically preferred to others (e.g., lbog). What principles account for such preferences: does the language system encode abstract rules banning syllables like lbog, or does their dislike reflect the increased motor demands associated with speech production? More generally, we ask whether linguistic knowledge is fully embodied or whether some linguistic principles could potentially be abstract. To address this question, here we gauge the sensitivity of English speakers to the putative universal syllable hierarchy (e.g., blif≻bnif≻bdif≻lbif) while undergoing transcranial magnetic stimulation (TMS) over the cortical motor representation of the left orbicularis oris muscle. If syllable preferences reflect motor simulation, then worse-formed syllables (e.g., lbif) should (i) elicit more errors; (ii) engage more strongly motor brain areas; and (iii) elicit stronger effects of TMS on these motor regions. In line with the motor account, we found that repetitive TMS pulses impaired participants' global sensitivity to the number of syllables, and functional MRI confirmed that the cortical stimulation site was sensitive to the syllable hierarchy. Contrary to the motor account, however, ill-formed syllables were least likely to engage the lip sensorimotor area and they were least impaired by TMS. Results suggest that speech perception automatically triggers motor action, but this effect is not causally linked to the computation of linguistic structure. We conclude that the language and motor systems are intimately linked, yet distinct. Language is designed to optimize motor action, but its knowledge includes principles that are disembodied and potentially abstract.any animal species communicate using vocal patterns, and humans are no exception. Every hearing human community preferentially expresses words by oral patterns (1). Speech sounds, such as d,o,g give rise to contrasting patterns (e.g., dog vs. god), and certain speech patterns are systematically preferred to others. Syllables like blog, for instance, are more frequent across languages than lbog (2). Behavioral experiments further demonstrate similar preferences among individual speakers despite no experience with either syllable type (3)(4)(5)(6).Although such facts demonstrate that the sound patterns of language are systematically constrained, the nature of such constraints remains unknown. One explanation invokes universal linguistic constraints on the sound structure of language (7). However, in an alternative account, these patterns are thought to reflect motor, rather than linguistic, constraints caused by their embodiment in the motor system of speech (8-11). Indeed, the speech patterns that are attested across spoken languages are not arbitrary, and frequent patterns tend to optimize speech production (12). Such observations open up the possibility that the so-called "language universals" are action based. In this view, the encoding of a speech stimulus engages ...

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