Abstract
Efference copies refer to internal duplicates of movement-producing neural signals. Their primary function is to predict, and often suppress, the sensory consequences of willed movements. Efference copies have been almost exclusively investigated in the context of overt movements. The current electrophysiological study employed a novel design to show that inner speech - the silent production of words in one's mind - is also associated with an efference copy. Participants produced an inner phoneme at a precisely specified time, at which an audible phoneme was concurrently presented. The production of the inner phoneme resulted in electrophysiological suppression, but only if the content of the inner phoneme matched the content of the audible phoneme. These results demonstrate that inner speech - a purely mental action - is associated with an efference copy with detailed auditory properties. These findings suggest that inner speech may ultimately reflect a special type of overt speech.
Highlights
Sensory attenuation – known as self-suppression – refers to the phenomenon that self-generated sensations feel less salient, and evoke a smaller neurophysiological response, than externallygenerated sensations which are physically identical (Hughes et al, 2013; Cardoso-Leite et al, 2010)
These results suggest that inner speech production is associated with a time-locked and content-specific internal forward model, similar to the one that operates in the production of overt speech
The present study used a novel experimental protocol to demonstrate that the production of an inner phoneme resulted in sensory attenuation of the auditory-evoked potential elicited by a simultaneously-presented audible phoneme, in the absence of any overt motor action
Summary
Sensory attenuation – known as self-suppression – refers to the phenomenon that self-generated sensations feel less salient, and evoke a smaller neurophysiological response, than externallygenerated sensations which are physically identical (Hughes et al, 2013; Cardoso-Leite et al, 2010). Sensory attenuation is believed to result from the action of an internal forward model, or IFM (Blakemore et al, 2000a; Wolpert and Miall, 1996) According to this account, the sensory consequences of self-generated movements are predicted based on a copy of the outgoing motor command, known as an efference copy. In the case of a self-generated movement, the internal prediction is able to account for, and ‘explain away’, much of the resulting sensation, which is why self-initiated sensations typically feel less salient, and evoke a smaller neurophysiological response, than externally-initiated sensations (Blakemore et al, 1998). Auditory stimuli elicit an electrophysiological brain response (the auditory-evoked potential) with a characteristic N1 component. Numerous electroencephalographic (EEG) and magnetoencephalographic (MEG) studies have found that self-generated vocalizations elicit an N1 component
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