Can we decode phonetic features in inner speech using surface electromyography?

Area of Science:

• Experimental Psychology
• Neuroscience
• Speech Motor Control

Background:

• The link between inner speech (covert speech) and speech muscle activity remains a debated topic in experimental psychology.
• Previous research has yielded conflicting evidence regarding the engagement of the speech motor system during inner speech.

Purpose of the Study:

• To investigate the electromyographic correlates of inner speech production.
• To determine if specific speech muscle activity accompanies covert speech.
• To explore the potential for automatic classification of phonetic content during inner speech.

Main Methods:

• A preregistered experiment was conducted using electromyography (EMG) to record speech muscle activity.
• Participants produced nonwords with two distinct phonetic classes in both overt and inner speech conditions.
• An automatic classification approach was applied to EMG signals to discriminate between articulatory features.

Main Results:

• The automatic classification approach achieved reasonable accuracy for overt speech but failed to discriminate phonetic content during inner speech.
• Surface electromyography signals did not reliably differentiate the phonetic content of covertly produced nonwords.
• Exploratory analyses suggested potential for distinguishing between rounded and spread nonwords in covert speech for two participants.

Conclusions:

• The study's findings suggest that overt speech is accompanied by distinct muscle activity, while inner speech may not be.
• Current electromyography methods may not be sensitive enough to detect subtle speech motor engagement during inner speech.
• Further research is needed to explore alternative methods for investigating the speech motor system's role in covert speech production.