Hemiface contributions to hemispheric dominance in visual speech perception

Lateralized displays are used widely to investigate hemispheric asymmetry in language perception. However, few studies have used lateralized displays to investigate hemispheric asymmetry in visual speech perception, and those that have yielded mixed results. This issue was investigated in the current study by presenting visual speech to either the left hemisphere (LH) or the right hemisphere (RH) using the face as recorded (normal), a mirror image of the normal face (reversed), and chimeric displays constructed by duplicating and reversing just one hemiface (left or right) to form symmetrical images (left-duplicated, right-duplicated). The projection of displays to each hemisphere was controlled precisely by an automated eye-tracking technique. Visual speech perception showed the same, clear LH advantage for normal and reversed displays, a greater LH advantage for right-duplicated displays, and no hemispheric difference for left-duplicated displays. Of particular note is that perception of LH displays was affected greatly by the presence of right-hemiface information, whereas perception of RH displays was unaffected by changes in hemiface content. Thus, when investigated under precise viewing conditions, the indications are not only that the dominant processes of visual speech perception are located in the LH but that these processes are uniquely sensitive to right-hemiface information.