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.

The strategy for measuring speech-reception thresholds for sentences in noise advocated by Plomp and Mimpen (Audiology, 18, 43-52, 1979) was modified to create a reliable test for measuring the difficulty which listeners have in speech reception, both auditorily and audio-visually. The test materials consist of 10 lists of 15 short sentences of homogeneous intelligibility when presented acoustically, and of different, but still homogeneous, intelligibility when presented audio-visually, in white noise. Homogeneity was achieved by applying phonetic and linguistic principles at the stage of compilation, followed by pilot testing and balancing of properties. To run the test, lists are presented at signal-to-noise ratios (SNRs) determined by an up-down psychophysical rule so as to estimate auditory and audio-visual speech-reception thresholds, defined as the SNRs at which the three content words in each sentence are identified correctly on 50% of trials. These thresholds provide measures of a subject's speech-reception abilities. The difference between them provides a measure of the benefit received from vision. It is shown that this measure is closely related to the accuracy with which subjects lip-read words in sentences with no acoustical information. In data from normally hearing adults, the standard deviations (s.d.s) of estimates of auditory speech reception threshold in noise (SRTN), audio-visual SRTN, and visual benefit are 1.2, 2.0, and 2.3 dB, respectively. Graphs are provided with which to estimate the trade-off between reliability and the number of lists presented, and to assess the significance of deviant scores from individual subjects.

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