Attention Fine-Tunes Auditory–Motor Processing of Speech Sounds

Abstract

The earliest stages of cortical processing of speech sounds take place in the auditory cortex. Transcranial magnetic stimulation (TMS) studies have provided evidence that the human articulatory motor cortex contributes also to speech processing. For example, stimulation of the motor lip representation influences specifically discrimination of lip-articulated speech sounds. However, the timing of the neural mechanisms underlying these articulator-specific motor contributions to speech processing is unknown. Furthermore, it is unclear whether they depend on attention. Here, we used magnetoencephalography and TMS to investigate the effect of attention on specificity and timing of interactions between the auditory and motor cortex during processing of speech sounds. We found that TMS-induced disruption of the motor lip representation modulated specifically the early auditory-cortex responses to lip-articulated speech sounds when they were attended. These articulator-specific modulations were left-lateralized and remarkably early, occurring 60 -100 ms after sound onset. When speech sounds were ignored, the effect of this motor disruption on auditory-cortex responses was nonspecific and bilateral, and it started later, 170 ms after sound onset. The findings indicate that articulatory motor cortex can contribute to auditory processing of speech sounds even in the absence of behavioral tasks and when the sounds are not in the focus of attention. Importantly, the findings also show that attention can selectively facilitate the interaction of the auditory cortex with specific articulator representations during speech processing.

Role of the motor system in language knowledge

“…In line with the embodiment hypothesis, experiments 1-3 found that the identification of spoken syllables engages the articulatory motor system. These findings agree with previous studies (14,(16)(17)(18)(19)(20), demonstrating that speech perception automatically triggers action. However, our findings challenge the causal role of motor simulation in the computation of linguistic structure (29).…”

“…Many previous studies have suggested that the identification of individual speech sounds (e.g., b vs. p) triggers motor simulation (14,(16)(17)(18)(19)(20)28). These findings leave open the possibility that the combinatorial process that forms sound patterns (e.g., syllables such as blog vs. lbog) relies on algebraic rules, i.e., principles that are disembodied and abstract.…”

“…Across languages, certain syllable types (e.g., blog) are overrepresented relative to others (e.g., lbog), and preferred syllables are more readily identified by individual speakers-both adults (14,(16)(17)(18)(19)(20) and infants (27). However, whether such preferences Fig.…”

“…Converging evidence is presented by studies using transcranial magnetic stimulation (TMS)-a noninvasive technique that induces focal cortical current via electro-magnetic induction to modulate (inhibit or facilitate) specific brain regions (15). Results show that TMS to the lip and tongue motor regions selectively affects the identification of corresponding speech sounds (16)(17)(18)(19)(20). However, although the motor system is demonstrably linked to the identification of isolated speech categories (e.g., of b), it is unclear whether it might further constrain their patterning into syllables (e.g., b+l+a vs. l+b+a).…”

No evidence of somatotopic place of articulation feature mapping in motor cortex during passive speech perception

“…They asked participants to make either a semantic or phonological judgement and concluded that PMC is involved in phonological, but not semantic, judgements, demonstrating a lack of participation of PMC in speech comprehension. More recently, Möttönen, Van de Ven, and Watkins (2014) used TMS and MEG to measure the impact of attention on auditory-motor processing. They found that in the absence of a task, TMS to motor cortex resulted in nonspecific modulations of auditory cortex; articulator-specific effects of TMS were only seen during a working memory task.…”