Rethinking the Mechanisms Underlying the McGurk Illusion

Abstract

The McGurk illusion occurs when listeners hear an illusory percept (i.e., "da"), resulting from mismatched pairings of audiovisual (AV) speech stimuli (i.e., auditory/ba/paired with visual/ga/). Hearing a third percept-distinct from both the auditory and visual input-has been used as evidence of AV fusion. We examined whether the McGurk illusion is instead driven by visual dominance, whereby the third percept, e.g., "da," represents a default percept for visemes with an ambiguous place of articulation (POA), like/ga/. Participants watched videos of a talker uttering various consonant vowels (CVs) with (AV) and without (V-only) audios of/ba/. Individuals transcribed the CV they saw (V-only) or heard (AV). In the V-only condition, individuals predominantly saw "da"/"ta" when viewing CVs with indiscernible POAs. Likewise, in the AV condition, upon perceiving an illusion, they predominantly heard "da"/"ta" for CVs with indiscernible POAs. The illusion was stronger in individuals who exhibited weak/ba/auditory encoding (examined using a control auditory-only task). In Experiment2, we attempted to replicate these findings using stimuli recorded from a different talker. The V-only results were not replicated, but again individuals predominately heard "da"/"ta"/"tha" as an illusory percept for various AV combinations, and the illusion was stronger in individuals who exhibited weak/ba/auditory encoding. These results demonstrate that when visual CVs with indiscernible POAs are paired with a weakly encoded auditory/ba/, listeners default to hearing "da"/"ta"/"tha"-thus, tempering the AV fusion account, and favoring a default mechanism triggered when both AV stimuli are ambiguous.

Keywords: McGurk illusion; audiovisual fusion; cross-modal phonetic encoding; multisensory integration; phonemic representations.

FIGURE 1

Gray-scale matrix depicting percentages of response percepts to visual-only (V-only) CV utterances for Experiment1 (A) and Experiment2 (B). The percentage for each percept was calculated as the percentage of responses of that percept relative to all other responses within a stimulus type (i.e., for each viseme). (C) For the visual-only (V-only) CV utterances in Experiment1 (left) and Experiment2 (right), the percentage of responses for the “da”/“ta” or “ga”/“ka” percept relative to all other responses across all visemes, except/ba/. The boxplots indicate the median (the horizontal line inside of the box), the 25 and 75th percentiles (the box’s bottom and top edges, respectively), and the whiskers indicate the range of the individual data points. (Note that there were no outliers).

FIGURE 2

Gray-scale matrix depicting percentages of percepts in response to congruent/ba/AV utterances and incongruent AV utterances of various visual CVs combined with audio/ba/for Experiment1 (A) and Experiment2 (B). The percentage for each percept was calculated as the percentage of responses of the percept relative to all other responses within a stimulus type.

FIGURE 3

Plots depicting Experiment1 correlations between percentages of the “ba”/“pa” percept of the A-only/ba–da/stimuli (superimposed/ba/and/da/CVs) and the overall response percentages of the “da”/“ta”/“tha” percept (across all stimuli except/ba/) for the V-only condition (A), the AV condition (B), and the mean of the V-only and AV conditions (C).

FIGURE 4

Gray-scale matrix depicting percentages of percepts in response to A-only superimposed stimuli for Experiment1 (A) and Experiment2 (B).

FIGURE 5

A plot depicting the correlation between percentages of “da”/“ta”/“tha” responses to the A-only/ba–ba/stimuli (superimposed/ba/and/ba/CVs) and the overall response percentages of the “da”/“ta”/“tha” percept during the AV condition. These data are from Experiment1.