Dissociating Contributions of the Motor Cortex to Speech Perception and Response Bias by Using Transcranial Magnetic Stimulation.

Eleonore H M Smalle,Jack Rogers,Riikka Möttönen

doi:10.1093/cercor/bhu218

Abstract

Recent studies using repetitive transcranial magnetic stimulation (TMS) have demonstrated that disruptions of the articulatory motor cortex impair performance in demanding speech perception tasks. These findings have been interpreted as support for the idea that the motor cortex is critically involved in speech perception. However, the validity of this interpretation has been called into question, because it is unknown whether the TMS-induced disruptions in the motor cortex affect speech perception or rather response bias. In the present TMS study, we addressed this question by using signal detection theory to calculate sensitivity (i.e., d′) and response bias (i.e., criterion c). We used repetitive TMS to temporarily disrupt the lip or hand representation in the left motor cortex. Participants discriminated pairs of sounds from a “ba”–“da” continuum before TMS, immediately after TMS (i.e., during the period of motor disruption), and after a 30-min break. We found that the sensitivity for between-category pairs was reduced during the disruption of the lip representation. In contrast, disruption of the hand representation temporarily reduced response bias. This double dissociation indicates that the hand motor cortex contributes to response bias during demanding discrimination tasks, whereas the articulatory motor cortex contributes to perception of speech sounds.

Highlights

It is under debate whether the motor regions that control movements of the articulators during speech production contribute to speech perception (Scott et al 2009; Pulvermüller and Fadiga 2010; Hickok et al 2011a)
The key question is whether the articulatory motor cortex is only involved in production of speech sounds or whether it supports speech perception by generating motor models of speech sounds produced by others (Liberman et al 1967; Stevens and Halle 1967; Liberman and Mattingly 1985)
We have shown that transcranial magnetic stimulation (TMS)-induced disruption of the lip representation in the primary motor cortex impairs discrimination of synthetic speech sounds that are close to the phonetic category boundary (e.g., “ba” vs. “da”), whereas disruption of the motor hand representation does not affect discrimination performance (Möttönen and Watkins 2009)

Summary

Introduction

It is under debate whether the motor regions that control movements of the articulators during speech production contribute to speech perception (Scott et al 2009; Pulvermüller and Fadiga 2010; Hickok et al 2011a). We have shown that TMS-induced disruption of the lip representation in the primary motor cortex impairs discrimination of synthetic speech sounds that are close to the phonetic category boundary (e.g., “ba” vs “da”), whereas disruption of the motor hand representation does not affect discrimination performance (Möttönen and Watkins 2009) The findings of this and other TMS studies have been interpreted as support for the idea that the articulatory motor cortex contributes to speech perception, consistent with the weak version of the motor theory of speech perception (Liberman et al 1967; Liberman and Mattingly 1985) and the concept of analysis by synthesis (Stevens and Halle 1967)

Methods

Results

Discussion

Conclusion