We are hosting Dr Patti Adank from UCL (Twitter, web page) who will present a seminar entitled ”How does the brain process variation in speech?’ on Wednesday, 6 May 2015 from 3-4.30pm. You can register to attend here. The seminar is sponsored by the Leeds Beckett Centre for Applied Social Research (@CeASR_Leeds).
I will give a short overview of the current research and dominant models of speech perception in challenging listening conditions, and will present the results from two TMS (Transcranial Magnetic Stimulation) studies that have been run in our lab. Recent functional Magnetic Resonance Imaging (fMRI) studies have implicated speech motor areas in perception of, as well as in perceptual adaptation to, distorted speech signals. The involvement of speech motor areas during speech perception has been verified in a series of TMS experiments. Nonetheless, exactly how motor regions support perception and the mechanisms involved in this process remains unclear.
There are two views on how the motor system could support perception. Both posit that listeners mentally imitate/simulate others’ actions during speech perception to aid understanding, thus explaining why motor cortex is active during perception. However, these views differ in their predictions on which conditions maximally recruit motor cortex. One predicts that there is greater recruitment when speech perception is challenging, such as when speech is distorted. The other predicts that motor recruitment is greatest when perception is easiest. We aimed to disambiguate between these predictions in two TMS experiments in which we measured Motor Evoked Potentials (MEPs), as they can probe excitability of the primary motor cortex (M1).
In the first experiment, we stimulated M1 lip and hand areas and recorded MEPs in the associated peripheral muscles, to test if MEPs were greater when listening to undistorted versus distorted VCV (vowel-consonant-vowel) stimuli. We also used a place-of-articulation contrast to confirm if the effect was modulated in an articulator-specific manner. Finally, we compared individual differences in MEPs to a behavioural measure (identification) of speech perception. In the second experiment, we evaluated MEPs in lip M1 in response to listening to two types of distorted speech signals: a motor distortion (unclearly articulated speech) and an energetic masking distortion (added background noise) and compared these MEPs with those recorded for listening to undistorted speech. Consistent with the first view of motor involvement in speech perception, we found that lip motor excitability was increased when listening to distorted relative to clear speech. The second study showed that motor cortex excitability was not selective to any particular source of degradation, and acts via a distortion-general mechanism. These findings indicate that increased activity of speech motor cortex serves to support and facilitate speech perception under challenging conditions, and this relationship may be enhanced in those who are better able to perceive speech.