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      Variations in the Slope of the Psychometric Functions for Speech Intelligibility: A Systematic Survey

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          Abstract

          Although many studies have looked at the effects of different listening conditions on the intelligibility of speech, their analyses have often concentrated on changes to a single value on the psychometric function, namely, the threshold. Far less commonly has the slope of the psychometric function, that is, the rate at which intelligibility changes with level, been considered. The slope of the function is crucial because it is the slope, rather than the threshold, that determines the improvement in intelligibility caused by any given improvement in signal-to-noise ratio by, for instance, a hearing aid. The aim of the current study was to systematically survey and reanalyze the psychometric function data available in the literature in an attempt to quantify the range of slope changes across studies and to identify listening conditions that affect the slope of the psychometric function. The data for 885 individual psychometric functions, taken from 139 different studies, were fitted with a common logistic equation from which the slope was calculated. Large variations in slope across studies were found, with slope values ranging from as shallow as 1% per dB to as steep as 44% per dB (median = 6.6% per dB), suggesting that the perceptual benefit offered by an improvement in signal-to-noise ratio depends greatly on listening environment. The type and number of maskers used were found to be major factors on the value of the slope of the psychometric function while other minor effects of target predictability, target corpus, and target/masker similarity were also found.

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          Most cited references122

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          Development of the Hearing in Noise Test for the measurement of speech reception thresholds in quiet and in noise.

          A large set of sentence materials, chosen for their uniformity in length and representation of natural speech, has been developed for the measurement of sentence speech reception thresholds (sSRTs). The mean-squared level of each digitally recorded sentence was adjusted to equate intelligibility when presented in spectrally matched noise to normal-hearing listeners. These materials were cast into 25 phonemically balanced lists of ten sentences for adaptive measurement of sentence sSRTs. The 95% confidence interval for these measurements is +/- 2.98 dB for sSRTs in quiet and +/- 2.41 dB for sSRTs in noise, as defined by the variability of repeated measures with different lists. Average sSRTs in quiet were 23.91 dB(A). Average sSRTs in 72 dB(A) noise were 69.08 dB(A), or -2.92 dB signal/noise ratio. Low-pass filtering increased sSRTs slightly in quiet and noise as the 4- and 8-kHz octave bands were eliminated. Much larger increases in SRT occurred when the 2-kHz octave band was eliminated, and bandwidth dropped below 2.5 kHz. Reliability was not degraded substantially until bandwidth dropped below 2.5 kHz. The statistical reliability and efficiency of the test suit it to practical applications in which measures of speech intelligibility are required.
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            Speech recognition in noise as a function of the number of spectral channels: comparison of acoustic hearing and cochlear implants.

            Speech recognition was measured as a function of spectral resolution (number of spectral channels) and speech-to-noise ratio in normal-hearing (NH) and cochlear-implant (CI) listeners. Vowel, consonant, word, and sentence recognition were measured in five normal-hearing listeners, ten listeners with the Nucleus-22 cochlear implant, and nine listeners with the Advanced Bionics Clarion cochlear implant. Recognition was measured as a function of the number of spectral channels (noise bands or electrodes) at signal-to-noise ratios of + 15, + 10, +5, 0 dB, and in quiet. Performance with three different speech processing strategies (SPEAK, CIS, and SAS) was similar across all conditions, and improved as the number of electrodes increased (up to seven or eight) for all conditions. For all noise levels, vowel and consonant recognition with the SPEAK speech processor did not improve with more than seven electrodes, while for normal-hearing listeners, performance continued to increase up to at least 20 channels. Speech recognition on more difficult speech materials (word and sentence recognition) showed a marginally significant increase in Nucleus-22 listeners from seven to ten electrodes. The average implant score on all processing strategies was poorer than scores of NH listeners with similar processing. However, the best CI scores were similar to the normal-hearing scores for that condition (up to seven channels). CI listeners with the highest performance level increased in performance as the number of electrodes increased up to seven, while CI listeners with low levels of speech recognition did not increase in performance as the number of electrodes was increased beyond four. These results quantify the effect of number of spectral channels on speech recognition in noise and demonstrate that most CI subjects are not able to fully utilize the spectral information provided by the number of electrodes used in their implant.
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              The BKB (Bamford-Kowal-Bench) sentence lists for partially-hearing children.

              Linguistic guidelines for the design of sentences for speech audiometry with children are described, and new lists of test sentences which are based on such guidelines--the Bamford-Kowal-Bench Sentence Lists for Children--are introduced. Audiometric data relating to the use of the new lists are presented and discussed.
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                Author and article information

                Journal
                Trends Hear
                Trends Hear
                TIA
                sptia
                Trends in Hearing
                SAGE Publications (Sage CA: Los Angeles, CA )
                2331-2165
                27 May 2014
                2014
                : 18
                : 2331216514537722
                Affiliations
                [1 ]MRC/CSO Institute of Hearing Research—Scottish Section, Glasgow Royal Infirmary, Glasgow, UK
                [2 ]School of Psychological Sciences & Health, University of Strathclyde, Glasgow, UK
                Author notes
                [*]Alexandra MacPherson, MRC/CSO Institute of Hearing Research—Scottish Section, New Lister Building, Glasgow Royal Infirmary, 10-16 Alexandra Parade, Glasgow G31 2ER, UK. Email: alex@ 123456ihr.gla.ac.uk
                Article
                10.1177_2331216514537722
                10.1177/2331216514537722
                4227668
                24906905
                ab53ce4d-37ed-471a-97c4-bba1920b9e45
                © The Author(s) 2014

                This article is distributed under the terms of the Creative Commons Attribution-NonCommercial 3.0 License ( http://www.creativecommons.org/licenses/by-nc/3.0/) which permits non-commercial use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access page( http://www.uk.sagepub.com/aboutus/openaccess.htm).

                History
                Categories
                Review
                Custom metadata
                January - December 2014

                speech-in-noise understanding,psychometric functions,perceptual benefit

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