Quantifying the contribution of vision to speech perception in noise.

The intelligibility of sentences presented in noise improves when the listener can view the talker's face. Our aims were to quantify this benefit, and to relate it to individual differences among subjects in lipreading ability and among sentences in lipreading difficulty. Auditory and audiovisual speech-reception thresholds (SRTs) were measured in 20 listeners with normal hearing. Sixty sentences, selected to range in the difficulty with which they could be lipread (with vision alone) from easy to hard, were presented for identification in white noise. Using the ascending method of limits, the SRT was defined as the lowest signal-to-noise ratio at which all three 'key words' in each sentence could be identified correctly. Measured as the difference in dB between auditory-alone and audiovisual SRTs, 'audiovisual benefit' averaged 11 dB, ranging from 6 to 15 dB among subjects, and from 3 to 22 dB among sentences. As predicted, audiovisual benefit is a measure of lipreading ability. It was highly correlated with visual-alone performance (n = 20, r = 0.86, P less than 0.01). Likewise, those sentences which were easiest to lipread gave a higher measure of benefit from vision in audiovisual conditions than did sentences that were hard to lipread (n = 60, r = 0.92, P less than 0.01). The results establish the basis of an efficient test of speech-reception disability in which measures are freed from the floor and ceiling effects encountered when percentage correct is used as the dependent variable.