Visually-biased perception in cochlear implant users: a study of the McGurk and sound-induced flash illusions

The reduction in spectral resolution by cochlear implants oftentimes requires complementary visual speech cues to aid in understanding. Despite substantial clinical characterization of auditory-only speech outcome measures, relatively little is known about the audiovisual integrative abilities that most cochlear implant (CI) users rely on for daily speech comprehension. In this study, we tested audiovisual integration in 63 CI users and 69 normal-hearing (NH) controls using the McGurk and sound-induced flash illusions. This study is the largest to-date measuring the McGurk effect in this population and the first to test the sound-induced flash illusion. When presented with conflicting audiovisual speech stimuli (i.e., the phoneme "ba" dubbed onto the viseme "ga"), we found that 55 CI users (87%) reported a fused percept of "da" or "tha" on at least one trial. However, overall, we found that CI users experienced the McGurk effect less often than controls--a result that was concordant with results with the sound-induced flash illusion where the pairing of a single circle flashing on the screen with multiple beeps resulted in fewer illusory flashes for CI users. While illusion perception in these two tasks appears to be uncorrelated among CI users, we identified a negative correlation in the NH group. Because neither illusion appears to provide further explanation of variability in CI outcome measures, further research is needed to determine how these findings relate to CI users speech understanding, particularly in ecological listening conditions that are naturally multisensory.