The role of hearing ability and speech distortion in the facilitation of articulatory motor cortex

Abstract

Excitability of articulatory motor cortex is facilitated when listening to speech in challenging conditions. Beyond this, however, we have little knowledge of what listener-specific and speech-specific factors engage articulatory facilitation during speech perception. For example, it is unknown whether speech motor activity is independent or dependent on the form of distortion in the speech signal. It is also unknown if speech motor facilitation is moderated by hearing ability. We investigated these questions in two experiments. We applied transcranial magnetic stimulation (TMS) to the lip area of primary motor cortex (M1) in young, normally hearing participants to test if lip M1 is sensitive to the quality (Experiment 1) or quantity (Experiment 2) of distortion in the speech signal, and if lip M1 facilitation relates to the hearing ability of the listener. Experiment 1 found that lip motor evoked potentials (MEPs) were larger during perception of motor-distorted speech that had been produced using a tongue depressor, and during perception of speech presented in background noise, relative to natural speech in quiet. Experiment 2 did not find evidence of motor system facilitation when speech was presented in noise at signal-to-noise ratios where speech intelligibility was at 50% or 75%, which were significantly less severe noise levels than used in Experiment 1. However, there was a significant interaction between noise condition and hearing ability, which indicated that when speech stimuli were correctly classified at 50%, speech motor facilitation was observed in individuals with better hearing, whereas individuals with relatively worse but still normal hearing showed more activation during perception of clear speech. These findings indicate that the motor system may be sensitive to the quantity, but not quality, of degradation in the speech signal. Data support the notion that motor cortex complements auditory cortex during speech perception, and point to a role for the motor cortex in compensating for differences in hearing ability.

Modulation of intra- and inter-hemispheric connectivity between primary and premotor cortex during speech perception

“…This is in keeping with previous reports of behavioural changes post-TMS, which predominantly manifest in a change in response time (Devlin, Matthews, & Rushworth, 2003;Krieger-Redwood, Gaskell, Lindsay, & Jefferies, 2013;Pobric, Jefferies, & Ralph, 2007;Whitney, Kirk, O'Sullivan, Lambon Ralph, & Jefferies, 2011). Surprisingly, MEPs were not modulated by distorted sentences, despite this form of distortion modulating MEPs to pre-lexical stimuli (Nuttall, Kennedy-Higgins, Devlin, & Adank, 2017;Nuttall et al, 2016). MEPs were not affected by cTBS when considered at group level.…”

Shared neural and cognitive mechanisms in action and language: The multiscale information transfer framework

“…Based on research into the relationship between action and language, the view that both phonetic and semantic processing shares cognitive and neural resources with the sensorimotor system has gained increasing acceptance (Barsalou, ; Fischer & Zwaan, ; Willems & Hagoort, ). For example, comprehending the meanings of words semantically related to body parts (e.g., eat , mouth ) activates brain regions also engaged in moving these body parts(Andrews, Frank, & Vigliocco, ; Meteyard, Cuadrado, Bahrami, & Vigliocco, ; Pulvermüller, ), just like decoding sounds recruits regions responsible for the production of these sounds (Nuttall, Kennedy‐Higgins, Devlin, & Adank, ; Skipper, Devlin, & Lametti, ).…”

Facilitation of motor excitability during listening to spoken sentences is not modulated by noise or semantic coherence

“…Changes in the size of MEPs reflect changes in the excitability of the motor pathways connecting the cortical representations with the corresponding muscles. Using this technique, several studies have demonstrated that the excitability of the primary motor cortex, which controls articulatory gestures to produce speech, is enhanced during listening to speech ( Fadiga et al., 2002 , Murakami et al., 2011 , Murakami et al., 2013 , Nuttall et al., 2017 , Nuttall et al., 2016 , Watkins et al., 2003 ).…”