Inhibitory TMS over visual area V5/MT disrupts visual speech recognition.

During face-to-face communication, the perception and recognition of facial movements can facilitate individuals' understanding of what is said. Facial movements are a form of complex, biological motion. Separate neural pathways are thought to processing (i) simple, non-biological motion with an obligatory waypoint in the motion-sensitive middle temporal area (V5/MT) and (ii) complex, biological motion. Here, we present findings that challenge this dichotomy. Neuronavigated offline TMS over V5/MT on 24 participants (17 female and 7 male) led to increased response times in the recognition of simple, non-biological motion as well as visual speech recognition compared to TMS over the vertex, an active control region. TMS of area V5/MT also reduced practise effects on response times, that are typically observed in both visual speech and motion recognition tasks over time. Our findings provide first indication that area V5/MT causally influences the recognition of visual speech. Significance Statement In everyday face-to-face communication, speech comprehension is often facilitated by viewing a speaker's facial movements. Several brain areas contribute to the recognition of visual speech. One area of interest is the visual medial temporal area V5/MT, which has been associated with the perception of simple, non-biological motion such as moving dots, as well as more complex, biological motion such as visual speech. Here, we demonstrate using non-invasive brain stimulation that area V5/MT is causally relevant in recognizing visual speech. This finding provides new insights into the neural mechanisms that support the perception of human communication signals, which will help guide future research in typically developed individuals and populations with communication difficulties.