Spatial Frequency Requirements and Gaze Strategy in Visual-Only and Audiovisual Speech Perception

Purpose: The aim of this article is to examine the effects of visual image degradation on performance and gaze behavior in audiovisual and visual-only speech perception tasks.

Method: We presented vowel-consonant-vowel utterances visually filtered at a range of frequencies in visual-only, audiovisual congruent, and audiovisual incongruent conditions (Experiment 1; N = 66). In Experiment 2 (N = 20), participants performed a visual-only speech perception task and in Experiment 3 (N = 20) an audiovisual task while having their gaze behavior monitored using eye-tracking equipment.

Results: In the visual-only condition, increasing image resolution led to monotonic increases in performance, and proficient speechreaders were more affected by the removal of high spatial information than were poor speechreaders. The McGurk effect also increased with increasing visual resolution, although it was less affected by the removal of high-frequency information. Observers tended to fixate on the mouth more in visual-only perception, but gaze toward the mouth did not correlate with accuracy of silent speechreading or the magnitude of the McGurk effect.

Conclusions: The results suggest that individual differences in silent speechreading and the McGurk effect are not related. This conclusion is supported by differential influences of high-resolution visual information on the 2 tasks and differences in the pattern of gaze.