A little more conversation, a little less action — candidate roles for the motor cortex in speech perception

Abstract

The motor theory of speech perception assumes that activation of the motor system is essential in the perception of speech. However, deficits in speech perception and comprehension do not arise from damage that is restricted to the motor cortex, few functional imaging studies reveal activity in motor cortex during speech perception, and the motor cortex is strongly activated by many different sound categories. Here, we evaluate alternative roles for the motor cortex in spoken communication and suggest a specific role in sensorimotor processing in conversation. We argue that motor-cortex activation it is essential in joint speech, particularly for the timing of turn-taking.

Interaction between auditory and motor systems in speech perception

“…It has been suggested that one of the functions of motor activation is tracking the talker's speed and rhythm over time, and provides the timing signals to the auditory cortex. Particularly in a conversation, the monitoring role of the motor system in interacting with the auditory system over time can induce fl uent conversation [53] .…”

Activation of articulatory information in speech perception

“…Finally, recent work using repetitive TMS has revealed that disruption to regions of the premotor cortex impacts on perceptual discrimination of speech sounds (5) in a somatotopic manner (6). These studies are all consistent with the proposal that the motor system is activated (and perhaps even essential) in the perception of speech.However, although there is agreement that motor regions can be activated in speech perception studies, recent reviews of the literature have raised two important challenges over precisely what drives this activation (7,8). The first challenge stems from the fact that neuroimaging data have been inconsistent, with relatively few studies showing motor activity at a whole-brain corrected level of significance compared with matched nonspeech conditions (7).…”

“…However, although there is agreement that motor regions can be activated in speech perception studies, recent reviews of the literature have raised two important challenges over precisely what drives this activation (7,8). The first challenge stems from the fact that neuroimaging data have been inconsistent, with relatively few studies showing motor activity at a whole-brain corrected level of significance compared with matched nonspeech conditions (7).…”

“…Some investigators have attributed this inconsistency to the baselines used across studies, claiming that those studies using a complex acoustic baseline (e.g., musical rain, spectrally rotated speech) are less likely to yield motor activity unique to speech perception than those studies using a silent baseline (7). The implication of this first challenge is that the motor activity sometimes observed in speech perception studies has nothing to do with the phonetic content of speech but rather is driven by some acoustic event common to speech and nonspeech sounds (7,8). The second challenge concerns the possibility that motor activation arises not as a result of basic processes underlying speech perception but as a result of the requirements of certain tasks or listening situations (7)(8)(9).…”

“…The first challenge stems from the fact that neuroimaging data have been inconsistent, with relatively few studies showing motor activity at a whole-brain corrected level of significance compared with matched nonspeech conditions (7). Some investigators have attributed this inconsistency to the baselines used across studies, claiming that those studies using a complex acoustic baseline (e.g., musical rain, spectrally rotated speech) are less likely to yield motor activity unique to speech perception than those studies using a silent baseline (7). The implication of this first challenge is that the motor activity sometimes observed in speech perception studies has nothing to do with the phonetic content of speech but rather is driven by some acoustic event common to speech and nonspeech sounds (7,8).…”