Effects of horizontal viewing angle on visual and audiovisual speech perception
Discovering how humans extract visual speech from faces viewed at different horizontal angles remains an outstanding goal for understanding brain function and for implementing human speech perception in machines. We report on two experiments which investigated effects of different horizontal viewing angles on visual speech perception by normal seeing and hearing adults using a talking face presented at three different perspectives: 0 degrees (full-face), 45 degrees (three-quarter) and 90 degrees (profile). Auditory and visual forms of the syllables /ba/, /bi/, /ga/, and /gi/ were used to produce the following stimulus types: auditory; visual; congruent audiovisual; and incongruent audiovisual. Identification of unimodal visual speech and identification of the auditory components of congruent and incongruent audiovisual speech were unaffected by changes in viewing angle when clearly audible auditory signals were used and when auditory signals were presented in noise. The findings suggest that human visual speech processing can be viewpoint independent.