Typical lipreading and audiovisual speech perception without motor simulation

ABSTRACT All it takes is a face to face conversation in a noisy environment to realize that viewing a speaker’s lip movements contributes to speech comprehension. Following the finding that brain areas that control speech production are also recruited during lip reading, the received explanation is that lipreading operates through a covert unconscious imitation of the observed speech movements in the observer’s own speech motor system – a motor simulation. However, motor effects during lipreading do not necessarily imply simulation or a causal role in perception. In line with this alternative, we report here that some individuals born with lip paralysis, who are therefore unable to covertly imitate observed lip movements, have typical lipreading abilities and audiovisual speech perception. This constitutes existence proof that typically efficient lipreading abilities can be achieved without motor simulation. Although it remains an open question whether this conclusion generalizes to typically developed participants, these findings demonstrate that alternatives to motor simulation theories are plausible and invite the conclusion that lip-reading does not involve motor simulation. Beyond its theoretical significance in the field of speech perception, this finding also calls for a re-examination of the more general hypothesis that motor simulation underlies action perception and interpretation developed in the frameworks of the motor simulation and mirror neuron hypotheses.