The effects of spatial frequency filtering and central vision loss on performance and gaze behaviour in speech perception

Understanding speech is a vital part of our daily life, and although audition plays an important role in perceiving speech information, vision is also an integral part of this process. The three experiments presented in this study address the visual requirements of speech perception by examining the effects of visual degradation on gaze behaviour while observing speech and the accuracy of speech perception. Experiments 1 and 2 used eye-tracking to test the effect of spatial frequency filtering on gaze behaviour during a visual-only consonant-recognition task and an audiovisual McGurk task. In Experiment 3, patients with unilateral central vision loss performed a McGurk task and a consonant-recognition task with degraded auditory information.;The results suggest that degrading the resolution of the visual speech signal affects speech perception. Both the McGurk and visual-only tasks were affected by spatial frequency filtering, however perception of the McGurk effect was very robust even when the high spatial frequency information was removed. Similarly, central vision loss did not diminish McGurk perception considerably in participants with small central vision loss. Measurements of gaze in Experiments 1 and 2 also suggest that speech perception is not dependent on foveal vision. The results show that gaze towards the mouth does not correlate with increased lipreading performance or increased perception of the McGurk effect.;Gaze measurements across the spatial frequency filter conditions suggest that participants look increasingly away from the mouth and eyes when high spatial frequency information is removed. In Experiment 3, central vision loss leads participants to shift their gaze in a fairly consistent manner when using their affected eye.;In summary, these results support the idea that high acuity vision is not essential for speech perception. The results also suggest that perceivers adapt their gaze strategy to the available visual information in the image, moving away from the mouth and eyes when visual information in these regions is degraded. Results from Experiment 3 provide insight into how patients with central vision loss gather and process visual information, and suggest new directions for analyzing the effects of visual impairments on communication.