Collaborative research: role of the motor system in phonological and phonetic processing

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Across the world's languages, certain sound combinations occur more frequently than others. However, the basis of these regularities is unknown. Some researchers have suggested that combinations such as "lbog" are rare because they require more complex articulatory gestures than combinations like "blog." Another possibility is that combinations such as "lbog" tend to be avoided because they violate much more abstract linguistic constraints on allowable syllable structures. The investigators will evaluate these possibilities by examining the role of the articulatory motor system in speech perception using both noninvasive brain stimulation and behavioral methods. Clarifying the relative contributions of speech motor processes and linguistic knowledge is critical to the diagnosis and treatment of speech language disorders, to first- and second-language acquisition, and to reading. This research explores the role of embodiment and abstraction in speech perception. It proposes that speech is represented at multiple levels (embodied phonetics and abstract phonological rules) with different susceptibility to motor simulation, depending on (a) the level of analysis and (b) a speaker's linguistic experience. The proposed experiments test this hypothesis by combining brain stimulation and behavioral experiments. Using MRI-guided Transcranial Magnetic Stimulation (TMS), the research team will disrupt activity in brain regions linked to motor action (the cortical representation of the left orbicularis oris muscle in BA4) and brain regions linked to combinatorial phonological operations (left pars triangularis, PTr, BA 45), and assess the impact on two tasks that rely differentially on phonetic processing or the putatively abstract phonological computation of syllable structure. While it is unlikely that either task or brain region is selective to a single level of analysis, these experiments gauge whether they differ in their degree of participation. If phonetic categorization requires motor simulation, then disruption of BA4 should affect phonetic categorization more than it affects the computation of syllable structure. Alternatively, if the computation of syllable structure relies on disembodied processes effected by the PTr, then the disruption of BA45 should produce a stronger effect on syllable structure than on phonetic categorization. Projects 1 and 2 examine these predictions with speakers of English and Russian (languages that contrast in their syllabic inventory). The group comparison evaluates experience-dependent plasticity or uniformity in the engagement of the motor system across tasks. Projects 3 and 4 suppress articulation mechanically. If the greater contribution of BA4 to phonetic categorization reflects its role in motor simulation, then results from disruption of the orbicularis oris muscle by TMS and mechanical suppression should converge.