Disruptive effects of auditory signal delay on speech perception with lipreading

The effect of auditory signal delay on audio-visual perception of videotaped sentence lists by Ss with normal hearing was measured. The test material consisted of sentences read by a man, with or without a picture representing as context one of the key words in each sentence. In Exper. I, 12 Ss (6M, 6F), inexperienced with lipreading, were presented test material in the audio-only (A), visual-only (V) and audio-visual (AV) modes. Speech signals were presented at 0 or -10 db S/N re a multitalker babble fixed at 60 dbA. The audio signal was delayed (re visual signal) in 6 steps from 0-300 msec. Exper. II repeated Exper. I with 6 Ss (2M, 4F) familiar with lipreading; the speech signal was presented at -5 db S/N and 4 audio delays from 0-240 msec. While these Ss performed better in the V mode than those in Exper. I, the main effects of context and delays were similar. The disruptive effect of the audio delay on speech perception with lipreading (AV mode) was a function of S/N, being relatively more disruptive at the worse S/N, but it was not significant for delays up to a range of 80-120 msec. The results are in agreement with the theory (McGrath and Summerfield, J. Acoust. Soc. Am., 1985, 77, 678-685) that sensitivity to audio-visual desynchrony is significant only at a syllabic level in connected speech. The results further imply that moderate delays of up to 80 msec introduced by speech-processing aids for lipreading, as for cochlear-implanted patients, will not interfere with the advantages of providing the auditory information.