Variability and stability in the McGurk effect: contributions of participants, stimuli, time, and response type

Abstract

In the McGurk effect, pairing incongruent auditory and visual syllables produces a percept different from the component syllables. Although it is a popular assay of audiovisual speech integration, little is known about the distribution of responses to the McGurk effect in the population. In our first experiment, we measured McGurk perception using 12 different McGurk stimuli in a sample of 165 English-speaking adults, 40 of whom were retested following a one-year interval. We observed dramatic differences both in how frequently different individuals perceived the illusion (from 0 % to 100 %) and in how frequently the illusion was perceived across different stimuli (17 % to 58 %). For individual stimuli, the distributions of response frequencies deviated strongly from normality, with 77 % of participants almost never or almost always perceiving the effect (≤10 % or ≥90 %). This deviation suggests that the mean response frequency, the most commonly reported measure of the McGurk effect, is a poor measure of individual participants' responses, and that the assumptions made by parametric statistical tests are invalid. Despite the substantial variability across individuals and stimuli, there was little change in the frequency of the effect between initial testing and a one-year retest (mean change in frequency = 2 %; test-retest correlation, r = 0.91). In a second experiment, we replicated our findings of high variability using eight new McGurk stimuli and tested the effects of open-choice versus forced-choice responding. Forced-choice responding resulted in an estimated 18 % greater frequency of the McGurk effect but similar levels of interindividual variability. Our results highlight the importance of examining individual differences in McGurk perception instead of relying on summary statistics averaged across a population. However, individual variability in the McGurk effect does not preclude its use as a stable measure of audiovisual integration.

Keywords: Audiovisual integration; Individual differences; McGurk effect; Speech perception.

Fig. 1

Responses to McGurk stimuli during Session 1 of Experiment 1. (a) Mean frequencies of McGurk responses for 165 participants (one symbol per participant, ordered by increasing frequency; data from Exp. 1, Session 1). Fusion responses ranged from 0 % to 100 %. The black symbol shows Participant 108. (b) Frequencies of McGurk responses of Participant 108 to each of the 12 McGurk stimuli. The black line shows the mean frequency across stimuli for this participant; most stimuli are far above or below the mean.

Fig. 2

Distributions of McGurk response percentages across stimuli. (a) Mean percentages of McGurk responses for 12 different stimuli (error bars show SEMs; data from Exp. 1, Session 1). (b) Percentage of McGurk responses for Stimulus # 9 in each of the 165 participants (one symbol per participant, ordered by increasing percentages). Dashed horizontal lines show the extremes of the distribution (≤10 %, ≥90 %). (c) Percentages of McGurk responses for Stimulus 9, plotted as the percentage of participants within each 10 % frequency bin. Dark bars highlight participants in the extremes of the distribution. (d) Average percentages of participants across stimuli in the extremes of the distribution (EXT, dark bar) and the middle (>10 % and <90 %) of the distribution (MID, light bar). Error bars indicate SEMs. (e) Percentages of participants in the extremes and in the middle of the distribution for each individual stimulus.

Fig. 3

Stability of McGurk responses after a one-year interval: Mean percentages of McGurk responses for each participant for the initial test (Exp. 1, Session 1) and at retest (Exp. 1, Session 2) one year later. Each point represents one participant.

Fig. 4

Data from Experiment 2, comparing open-choice and forced-choice responding. (a) Percentages of fusion responses averaged across stimuli for open-choice (Open) and three-alternative forced-choice (Forced) responding, tested in two different groups of participants. Error bars indicate SEMs. (b) Percentages of fusion responses for each individual stimulus. (c) Percentage of fusion responses for each individual participant in the open-choice group. (d) Percentage of fusion responses for each individual participant in the forced-choice group. (e) Average percentages of participants across stimuli at the extremes (EXT, dark bar) and in the middle (>10 % and <90 %; MID, light bar) of the distribution. Error bars show SEMs. (f) Percentages of participants in the extremes and in the middle of the distribution for each individual stimulus.