Bashford, Jr. et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4916793]
Published Online 9 April 2015
How broadband speech may avoid neural firing rate saturation at high intensities and maintain intelligibility James A. Bashford, Jr.,a) Richard M. Warren, and Peter W. Lenz Department of Psychology, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, Wisconsin 53201 [email protected], [email protected], [email protected]
Abstract: Three experiments examined the intelligibility enhancement produced when noise bands flank high intensity rectangular band speech. When white noise flankers were added to the speech individually at a low spectrum level (30 dB relative to the speech) only the higher frequency flanker produced a significant intelligibility increase (i.e., recovery from intelligibility rollover). However, the lower-frequency flanking noise did produce an equivalent intelligibility increase when its spectrum level was increased by 10 dB. This asymmetrical intensity requirement, and other results, support previous suggestions that intelligibility loss at high intensities is reduced by lateral inhibition in the cochlear nuclei. C 2015 Acoustical Society of America V
[JH] Date Received: December 18, 2014
Date Accepted: March 16, 2015
1. Introduction Intensity coding in the auditory system is highly effective over a broad range of levels, with a difference limen (DL) of only about 1 dB for signal levels up to 100 dB sound pressure level (SPL) and a DL of only about 1.5 dB at a level of 120 dB SPL (for a review, see Viemeister1). Interestingly, the majority of auditory nerve (AN) fibers have much narrower ranges (20 to 40 dB) over which their firing rates can vary with signal level before reaching rate-saturation. This interesting “dynamic range problem” (Ref. 1) is also apparent in the perception of speech, which may remain nearly perfectly intelligible at levels exceeding 90 dB,2 despite the fact that most AN fibers reach their firing-rate limits at conversational speech levels of about 65 dB SPL, and at higher intensities are unable to provide a firing-rate-based encoding of either the fine spectral details3 or amplitude envelope fluctuations4 that encode the critical features of speech. Viemeister5 employed conditions that excluded both neural synchrony and spread-of-excitation as likely cues for intensity discrimination, and obtained strong behavioral evidence that the dynamic range of rate-based discrimination does indeed extend to 100 dB SPL, with acuity of 1 dB, despite the limited ranges of most individual AN fibers. He also provided a theoretical account of discrimination at high signal intensities1,5 that relies on firing-rate information provided by the small population of AN fibers known to have high thresholds and wide dynamic ranges. He argued that these fibers are sufficient to account for rate-based processing at the upper end of the dynamic range, if input from the larger population of readily saturated low-threshold fibers is excluded from analysis at high signal intensities. Physiological models have been proposed6,7 that attribute this exclusionary process to mechanisms of lateral suppression, which reduce input from AN fibers when sufficient stimulation occurs in spectral regions adjacent to their best frequencies. These mechanisms of mutual suppression a)
Author to whom correspondence should be addressed.
EL340 J. Acoust. Soc. Am. 137 (4), April 2015
C 2015 Acoustical Society of America V
Bashford, Jr. et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4916793]
Published Online 9 April 2015
may include both mechanical (two-tone) suppression within the cochlea8 and a substantially more effective neural inhibition of AN input to cells of the cochlear nucleus (CN).9 Eriksson and Roberts6 and Winslow et al.7 also have proposed that, at high intensities, lateral inhibition within the cochlear nuclei selectively attenuates input from low-threshold, readily saturated AN fibers, producing a shift in the weighting of intensity analysis to favor input from high-threshold unsaturated fibers. In order to test behavioral predictions from this lateral inhibition hypothesis, Bashford et al.10 used a speech stimulus consisting of a steeply filtered, effectively rectangular 2/3-octave band of “everyday” sentences centered at 1500 Hz. Rectangular band (RB) filtering was employed to eliminate contributions from filter slopes, which might interfere with accurate measurement of the decline, or “rollover” of intelligibility within the passband at high intensities (see Warren et al.11). The RB speech proved to be much more vulnerable to rollover than broadband speech: A statistically significant intelligibility loss was obtained when speech intensity reached 65 dB SPL, a level at which, as discussed above, most AN neurons cannot provide rate-based encoding of speech cues. This intelligibility rollover at moderate signal levels was considered to be due to the absence of lateral suppression that would normally be evoked by speech components spectrally adjacent to the 2/3-octave RB. Bashford et al.10 then tested the conjoint prediction that adding flanking bands of white noise would restore intelligibility of the speech band by producing lateral inhibition, but only at a speech intensity sufficient to produce rollover. Listeners were presented with the RB sentences, without flanking noise, at either 45 or 75 dBA slow-peak SPL, which yielded average intelligibility scores of about 80% vs 60%, respectively, indicating a substantial rollover effect. When flanking noise bands were added to the 75-dB speech at relative spectrum levels ranging from 40 to 20 dB, intelligibility increased, with a maximum recovery of 13% obtained with noise presented at a level of 30 dB. As predicted, however, flanking noise did not increase the intelligibility of the sentences presented at 45 dB SPL. In a subsequent study, Bashford et al.12 conducted a series of experiments designed to better specify the likely mechanism and site in the auditory pathway responsible for the noise-induced recovery of intelligibility from rollover. They reasoned that if recovery is produced by lateral interactions in the cochlear nucleus, then the effectiveness of the flanking noise bands should not be influenced by binaural lateralization cues processed more centrally in the superior olivary complex. Their test employed a masking level difference paradigm in which the 75 dB 2/3-octave RB speech was accompanied either by RB noise matching the spectral limits of the speech (and producing within-band masking), or by broadband noise (producing both withinband masking and out-of-band reduction of rollover). The noise was presented at a relative spectrum level of 20 dB to ensure that some masking would occur. On half of the trials the noise added to the diotic (interaurally identical) sentences was also diotic, and on the remaining trials was interaurally uncorrelated. Decorrelation of the noise was found to produce a binaural release from masking exerted by the within-band noise components in both the narrowband and broadband noise conditions: Intelligibility was about 5% higher in the uncorrelated conditions regardless of noise bandwidth. Crucially, however, intelligibility was about 13% higher in the broadband noise conditions, regardless of interaural noise correlation, strongly suggesting that the processing responsible for the observed protection against rollover occurs below the level of the superior olivary complex. Bashford et al.12 also used the contrasting narrowband and broadband noise conditions to determine whether an effect peripheral to the CN might contribute to the reduction of rollover. It was possible that the spread of excitation from the flanking noise bands into the frequency range of the speech band produced classical firing-rate adaptation in AN fibers, shifting their operating ranges to include higher speech levels.13 To test this hypothesis, 75-dB rectangular 2/3-octave speech was again accompanied by either spectrally matching narrowband noise or broadband noise at 20 dB
J. Acoust. Soc. Am. 137 (4), April 2015
Bashford, Jr. et al.: Defeating intelligibility rollover EL341
Bashford, Jr. et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4916793]
Published Online 9 April 2015
relative spectrum level, and on half of the trials the noise was continuous while on the remaining trials it was gated on and off with individual sentences. As expected, an “overshoot” of within-band masking did occur with the gated noise: Intelligibility was about 5% lower in that condition, whether the noise was spectrally matching or broadband. Importantly, however, intelligibility was about 11% higher with broadband rather than narrowband noise, regardless of gating, which would appear to rule out peripheral adaptation as a contributor to the effect of flanking noise on rollover. In a final experiment, Bashford et al.12 presented the RB speech monaurally and found that intelligibility enhancement required that the flanking noise bands be delivered to the same ear. This finding ruled out two additional mechanisms as potential contributors to enhancement: Contralateral input can evoke aural reflex contractions of the tensor tympani and stapedius muscles14 as well as the medial olivocochlear reflex.15 With a number of possible alternative mechanisms now eliminated from consideration, the current study focuses on another specific prediction from the CN lateral inhibition hypothesis. Lateral interactions observed between cells of the cochlear nucleus show a striking asymmetry: Higher-frequency inhibitors are generally effective at substantially lower intensities (often by 20 dB or more) than lower-frequency inhibitors.9 The present study tested for a corresponding asymmetry in the effectiveness of high-frequency vs low-frequency flanking bands in reducing the rollover of intelligibility for narrowband speech presented at 75 dB. In experiment 1, flanking bands of white noise were presented at a relative spectrum level of 30 dB. It was expected that this low noise level would reveal the higher activation level required for suppression by the low frequency noise band. In experiment 2, the spectrum level of the noise required to reduce rollover was determined separately for the low- and high-pass flanking noise bands, using separate groups of listeners. Experiment 3 employed flankers derived from speech-shaped [United States of America Standards Institute (USASI)] noise, which boosted the noise-power per unit bandwidth of the low frequency noise band by roughly 20 dB relative to the higher noise band, and permitted an evaluation of the compatibility between the asymmetrical thresholds of activation for lateral inhibition and the spectral tilt of broadband speech. 2. Method 2.1 Listeners The 88 listeners in this study (24 each in Experiments 1, and 3, and 2 groups of 20 in Experiment 2) were undergraduate students at the University of Wisconsin-Milwaukee who were paid for their participation. They ranged in age from 18 to 31 and were native monolingual English speakers who reported having no hearing problems and had normal bilateral hearing, with pure tone thresholds of 20 dB hearing level or better at octave frequencies from 0.25 to 8 kHz. 2.2 Stimuli The formal test stimuli were the 100 (10 lists of 10) Central Institute of the Deaf (CID) Everyday sentences,16 which contain 500 keywords (50 keywords per list) that are used for scoring. An additional 25 practice sentences were drawn from the high predictability sub-list of the revised Speech Perception in Noise test.17 The original broadband recordings (44.1 kHz sampling with 16-bit quantization) were produced by the same male speaker, who has no evident regional accent and has an average voicing frequency of about 100 Hz. Prior to filtering, the sentences were transduced by a Sennheiser HD 250 Linear II headphone (Sennheiser Electronics Corp., Old Lyme, CT) and their slow rootmean-square (rms) peak levels were digitally matched to within 0.2 dBA in slow-rms peak level, using a flat-plate coupler in conjunction with a Br€ uel and Kjær model 2230 digital sound-level meter (Br€ uel and Kjaer North American Inc., Norcross, GA) set at Ascale weighting (as were all level measurements reported). The sentences were then passed through two successive stages of 4000-order bandpass finite impulse response (FIR) filtering (producing slopes of approximately 3.9 dB/Hz), using the fir1 function in MATLAB
EL342 J. Acoust. Soc. Am. 137 (4), April 2015
Bashford, Jr. et al.: Defeating intelligibility rollover
Bashford, Jr. et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4916793]
Published Online 9 April 2015
(Mathworks, Natick, MA), to produce a 2/3-octave narrowband of speech, centered at 1500 Hz (passband from 1191 to 1890 Hz), with effectively vertical filter slopes.11 Bandpass filtering of the sentences reduced their average slow-rms peak level by 5.4 dB. Hence, for the 75-dB peak level employed for the narrowband sentences in the present study, the corresponding nominal broadband speech level would be 80.4 dB. White noise, low-pass filtered at 20 kHz, was used to produce the low- and high-pass flanking noise bands employed in experiments 1 and 2 and a band of USASI speech noise, also lowpass filtered at 20 kHz was used to derive the flankers presented in Experiment 3. The low- and high-pass flankers were each produced through two successive stages of 4000order FIR filtering, as with the speech band. The low-frequency limit of the lower flanking band and the high-frequency limit of the higher flanking band matched those of the broadband noise. Separate pairs of flanking noise bands were prepared for each noise spectrum-level employed in the present study (10, 20, 30, and 40 dB relative to the speech spectrum-level). 3. General procedure The design of each experiment incorporated repeated measures. Before receiving a given stimulus condition listeners were presented with five practice sentences, which were first presented broadband and then presented bandpass filtered, along with noise when appropriate, in the same manner as the test sentences that followed. The different experimental conditions presented to a given group of listeners were assigned to separate sets of the everyday test sentences. This assignment varied pseudorandomly, so that, across listeners in a group, each condition was applied an equal number of times to each set of sentences. Testing was performed in a sound attenuating chamber with the stimuli delivered through Sennheiser HD 250 Linear II headphones. Listeners were instructed to call out what the voice was saying as best they could and were encouraged to guess when unsure. Their responses were recorded and scored online by the experimenter, who sat with them in the chamber during the experiment. 3.1 Experiment 1: Extent of recovery from intelligibility rollover produced by low-pass vs high-pass flanking bands of white noise presented at matched levels Experiment 1 examined the relative effectiveness of low- and high-pass flanking bands of white noise when presented at a spectrum level of 30 dB relative to the speech, a level previously found optimal for level-matched flankers by Bashford et al.10 The 24 listeners in this experiment were presented with separate sets of 25 everyday sentences (containing approximately 125 keywords each) in each of 4 flanking noise conditions: (1) A no-noise baseline condition; (2) a low-pass white noise condition; (3) a high-pass white noise condition; and (4) a condition in which both flanking noise bands were added to the 75-dB speech. The order of presentation for conditions was pseudo-randomized with the restriction that each condition was presented four times in each serial position across listeners. 3.2 Results for experiment 1 The proportions of keywords correctly reported for the separate sets of 25 sentences (approximately 125 keywords) presented in each of the 4 experimental conditions are presented in Table 1. The data were arcsine transformed and subjected to a repeated measures analysis of variance, which yielded a significant effect of noise condition Table 1. Mean percent intelligibility scores and standard errors (N ¼ 24) for 2/3-octave narrowband sentences presented at a slow-rms peak level of 75 dBA along with either: No flanking noise bands; low-pass flanking white noise; high-pass flanking white noise, or both low- and high-pass flanking bands of white noise presented at 30 dB spectrum levels relative to the speech. No noise
Low-pass noise
High-pass noise
Both noise bands
64.0 (1.9)
66.0 (2.7)
70.1 (2.2)
73.2 (1.8)
J. Acoust. Soc. Am. 137 (4), April 2015
Bashford, Jr. et al.: Defeating intelligibility rollover EL343
Bashford, Jr. et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4916793]
Published Online 9 April 2015
[F(3,69) ¼ 5.1, p < 0.004]. A set of orthogonal, single-df contrasts comparing the “nonoise” baseline condition with the remaining conditions indicated that the high-pass noise flanker (F ¼ 4.6, p < 0.04) and the combination of high- and low-pass flankers (F ¼ 23.4, p < 0.001) did produce a significant recovery from rollover, whereas the low-pass flanker alone did not (F ¼ 0.46, p > 0.5). These results provide preliminary evidence of a difference in the level requirements for rollover reduction produced by low-pass vs high-pass flanking noise that is generally consistent with the asymmetry in activation thresholds for lateral inhibition evoked by lower vs higher frequency flanking signals. Experiment 2 extended this finding by providing an examination of the range of levels over which the low- and high-pass flanking bands of white noise effectively reduce rollover. 3.3 Experiment 2: Effects of level on the effectiveness of low-pass vs high-pass flanking bands of white noise in reducing intelligibility rollover In this experiment 2 separate groups of 20 listeners each were employed to determine the spectrum levels required for low-pass vs high-pass flanking white noise bands to reduce intelligibility rollover of the 75 dB 2/3-octave sentences. Both the low- and highpass flanker groups were presented with 20 sentences (100 keywords) in each of 5 conditions, which included a no-noise baseline condition and 4 conditions presenting the flanking white noise band at spectrum levels of 40, 30, 20, and 10 dB relative to the speech band. 3.4 Results for experiment 2 Table 2 presents the mean percent repetition accuracy for the sentence keywords in each condition, along with their standard errors. Analysis of variance for listeners’ arcsine transformed proportions of correct responses yielded significant main effects of level for both the low-pass [F(4,76) ¼ 3.2, p < 0.02] and high-pass [F(4,76) ¼ 3.0, p < 0.03] flanking noise bands. For the low-pass flanker, orthogonal contrasts comparing the no-noise baseline intelligibility with that obtained with noise present indicated that only the 20 dB spectrum level low-pass flanker condition produced a significant recovery from rollover [F ¼ 10.1, p < 0.005]. In contrast, the high-pass flanking noise produced a reliable recovery at 30 dB [F ¼ 7.4, p < 0.02] and 20 dB [F ¼ 11.8, p < 0.003], as well as a marginally significant recovery of intelligibility at 40 dB [F ¼ 4.2, p < 0.06], which provides tentative behavioral evidence of a roughly 20-dB difference in minimal level requirements for rollover reduction by the low-pass vs highpass flankers. This difference is consistent with typical physiologically measured differences in activation thresholds for low-side vs high-side inhibition in the cochlear nucleus.9 However, given the pronounced spectral tilt of normal broadband speech, results obtained with white noise flankers may not provide the best estimate of potential benefits from lateral inhibition evoked by adjacent speech components. In particular, the contribution of low-side inhibition to speech intelligibility might be compromised by a greater upward spread of masking. To evaluate this possibility, experiment 3 employed flanking bands derived from USASI speech-noise, with the bands adjusted in level such that their spectral components that were directly Table 2. Mean percent intelligibility scores and standard errors for 2/3-octave narrowband sentences presented at a slow-rms peak level of 75 dBA along with no noise, and with either low- or high-pass flanking bands of white noise at each of four spectrum levels relative to the speech: 40, 30, 20, or 10 dB. Separate groups of 20 listeners received the low- and high-pass flanking white noise band conditions. Flanking noise
No noise
40 dB noise
30 dB noise
20 dB noise
10 dB noise
High-pass group Low-pass group
64.5 (1.7) 65.0 (1.5)
69.4 (1.5) 65.7 (1.1)
71.4 (2.1) 67.0 (1.7)
71.2 (1.1) 71.8 (1.8)
70.8 (2.1) 64.7 (1.8)
EL344 J. Acoust. Soc. Am. 137 (4), April 2015
Bashford, Jr. et al.: Defeating intelligibility rollover
Bashford, Jr. et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4916793]
Published Online 9 April 2015
Table 3. Mean percent intelligibility scores and standard errors (N ¼ 24) for 2/3-octave narrowband sentences presented at a slow-rms peak level of 75 dBA along with either: No flanking noise bands; low-pass flanking speech-shaped noise; high-pass flanking speech-shaped noise, or both low- and high-pass flanking bands of speech-shaped noise presented at 20 dB spectrum level relative to the speech. No noise
Low-pass noise
High-pass noise
Both noise bands
65.1 (2.2)
72.7 (1.8)
70.5 (2.2)
76.0 (2.1)
contiguous with the speech band were at the 20 dB relative spectrum level found effective for both bands in experiment 2. 3.5 Experiment 3: Extent of rollover recovery produced by low-pass vs high-pass flanking bands of speech-shaped noise Experiment 3 examined the effect of speech-shaped noise flankers, using the same design as that employed in experiment 1, with listeners presented 25 sentences (approximately 125 keywords) in each of the 4 conditions: The no-noise baseline condition, low-pass noise alone, high-pass noise alone, and both flanking bands. 3.6 Results for experiment 3 Mean percent repetition accuracy and standard errors are presented in Table 3. Analysis of variance for arcsine transformed proportions of correct responses yielded a significant effect of noise condition [F(3,69) ¼ 12.1, p < 0.001], and orthogonal contrasts indicated that intelligibility increased significantly with the addition of low-pass flanking noise alone [F ¼ 17.4, p < 0.001], high-pass flanking noise alone [F ¼ 6.3, p < 0.02], and also when both noise bands were added [F ¼ 38.4, p < 0.001]. Intelligibility in the low- and high-pass flanker conditions did not differ by Tukey test, but the dual flanker condition produced higher intelligibility than the high-pass flanker alone (p < 0.05). 4. General discussion The results of these experiments are consistent with established characteristics of lateral inhibition and its proposed function in reducing rate-saturation, and hence reducing rollover of intelligibility for high-intensity speech.6,7 Moreover, the results of experiment 3, using speech-shaped rather than white noise flankers, suggest that the asymmetrical thresholds for low- and high-side inhibition in the cochlear nucleus may compensate for the spectral tilt of speech, providing roughly equivalent effects of low- and high-side inhibition for broadband speech, and hence equivalent reduction of rollover, at least in the spectral midrange as examined thus far. The effectiveness of flanking noise in reducing rollover when presented at a spectrum level 40 dB below the speech signal, as found in experiment 2 of this study and earlier by Bashford et al.,10 is consistent with the wide dynamic ranges of CN interneurons, such as onset-choppers, that are thought to mediate lateral inhibition.18 These neurons are activated by broadband stimulation, and are comprised of two separate populations, one of which is activated only by ipsilateral input,19 as is the reduction of intelligibility rollover produced by flanking noise.12 Acknowledgments The project described was supported by Award No. R01DC000208 from the National Institute on Deafness and Other Communication Disorders. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute on Deafness and Other Communication Disorders or the National Institutes of Health.
J. Acoust. Soc. Am. 137 (4), April 2015
Bashford, Jr. et al.: Defeating intelligibility rollover EL345
Bashford, Jr. et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4916793]
Published Online 9 April 2015
References and links 1
N. F. Viemeister, “Intensity coding and the dynamic-range problem,” Hear. Res. 34, 267–274 (1988). G. A. Studebaker, R. L. Sherbecoe, D. M. McDaniel, and C. A. Gwaltney, “Monosyllabic word recognition at higher-than-normal speech and noise levels,” J. Acoust. Soc. Am. 105, 2431–2444 (1999). 3 M. B. Sachs and E. D. Young, “Encoding of steady-state vowels,” J. Acoust. Soc. Am. 66, 470–479 (1979). 4 A. R. Palmer and E. F. Evans, “On the peripheral coding of the level of individual frequency components of complex sounds at high levels,” Exp. Brain Res. 2, 19–26 (1979). 5 N. F. Viemeister, “Auditory intensity discrimination at high frequencies in the presence of noise,” Science 221, 1206–1207 (1983). 6 J. L. Eriksson and A. Robert, “The representation of pure tones and noise in a model of cochlear nucleus neurons,” J. Acoust. Soc. Am. 106, 1865–1879 (1999). 7 R. L. Winslow, P. Barta, and M. B. Sachs, “Rate coding in the auditory nerve,” in Auditory Processing of Complex Sounds, edited by W. A. Yost and C. S. Watson (Erlbaum, Hillsdale, NJ, 1987), pp. 212–224. 8 W. S. Rhode, “Observations of the basilar membrane in the squirrel monkey using the Mossbauer technique,” J. Acoust. Soc. Am. 49, 1218–1231 (1971). 9 W. S. Rhode and S. Greenberg, “Lateral suppression and inhibition in the cochlear nucleus of the cat,” J. Neurophysiol. 71, 493–514 (1994). 10 J. A. Bashford, Jr. R. M. Warren, and P. W. Lenz, “Enhancing intelligibility of narrowband speech with out-of-band noise: Evidence for lateral suppression at high-normal intensity,” J. Acoust. Soc. Am. 117, 365–369 (2005). 11 R. M. Warren, J. A. Bashford, Jr., and P. W. Lenz, “Intelligibility of bandpass filtered speech: Steepness of slopes required to eliminate transition band contributions,” J. Acoust. Soc. Am. 115, 1292–1295 (2004). 12 J. A. Bashford, Jr. R. M. Warren, and P. W. Lenz, “Maintaining intelligibility at high speech intensities: Evidence of lateral inhibition in the lower auditory pathway,” J. Acoust. Soc. Am. 134, EL119–EL125 (2013). 13 D. J. Gibson, E. D. Young, and J. A. Costalupes, “Similarity of dynamic range adjustment in auditory nerve and cochlear nuclei,” J. Neurophysiol. 53, 940–958 (1985). 14 J. D. Durrant and J. H. Lovrinic, Bases of Hearing Science, 3rd ed. (Williams and Wilkins, Baltimore, MD, 1995), pp. 170–172. 15 J. J. Guinan, Jr., “Olivocochlear efferents: Anatomy, physiology, function, and the measurement of efferent effects in humans,” Ear Hear. 27, 589–607 (2006). 16 S. R. Silverman and I. J. Hirsh, “Problems related to the use of speech in clinical audiometry,” Ann. Otol. Rhinol. Laryngol. 64, 1234–1244 (1955). 17 R. C. Bilger, J. M. Nuetzel, W. M. Rabinowitz, and C. Rzeczkowski, “Standardization of a test of speech perception in noise,” J. Speech Lang. Hear. Res. 27, 32–48 (1984). 18 I. M. Winter and A. R. Palmer, “Level dependence of cochlear nucleus onset unit responses and facilitation by second tones or broadband noise,” J. Neurophysiol. 73, 141–159 (1995). 19 N. J. Ingham, S. Bleeck, and I. M. Winter, “Contralateral inhibitory and excitatory frequency response maps in the mammalian cochlear nucleus,” Eur. J. Neurosci. 24, 2515–2529 (2006). 2
EL346 J. Acoust. Soc. Am. 137 (4), April 2015
Bashford, Jr. et al.: Defeating intelligibility rollover
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[http://dx.doi.org/10.1121/1.4916793]
Published Online 9 April 2015
How broadband speech may avoid neural firing rate saturation at high intensities and maintain intelligibility James A. Bashford, Jr.,a) Richard M. Warren, and Peter W. Lenz Department of Psychology, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, Wisconsin 53201 [email protected], [email protected], [email protected]
Abstract: Three experiments examined the intelligibility enhancement produced when noise bands flank high intensity rectangular band speech. When white noise flankers were added to the speech individually at a low spectrum level (30 dB relative to the speech) only the higher frequency flanker produced a significant intelligibility increase (i.e., recovery from intelligibility rollover). However, the lower-frequency flanking noise did produce an equivalent intelligibility increase when its spectrum level was increased by 10 dB. This asymmetrical intensity requirement, and other results, support previous suggestions that intelligibility loss at high intensities is reduced by lateral inhibition in the cochlear nuclei. C 2015 Acoustical Society of America V
[JH] Date Received: December 18, 2014
Date Accepted: March 16, 2015
1. Introduction Intensity coding in the auditory system is highly effective over a broad range of levels, with a difference limen (DL) of only about 1 dB for signal levels up to 100 dB sound pressure level (SPL) and a DL of only about 1.5 dB at a level of 120 dB SPL (for a review, see Viemeister1). Interestingly, the majority of auditory nerve (AN) fibers have much narrower ranges (20 to 40 dB) over which their firing rates can vary with signal level before reaching rate-saturation. This interesting “dynamic range problem” (Ref. 1) is also apparent in the perception of speech, which may remain nearly perfectly intelligible at levels exceeding 90 dB,2 despite the fact that most AN fibers reach their firing-rate limits at conversational speech levels of about 65 dB SPL, and at higher intensities are unable to provide a firing-rate-based encoding of either the fine spectral details3 or amplitude envelope fluctuations4 that encode the critical features of speech. Viemeister5 employed conditions that excluded both neural synchrony and spread-of-excitation as likely cues for intensity discrimination, and obtained strong behavioral evidence that the dynamic range of rate-based discrimination does indeed extend to 100 dB SPL, with acuity of 1 dB, despite the limited ranges of most individual AN fibers. He also provided a theoretical account of discrimination at high signal intensities1,5 that relies on firing-rate information provided by the small population of AN fibers known to have high thresholds and wide dynamic ranges. He argued that these fibers are sufficient to account for rate-based processing at the upper end of the dynamic range, if input from the larger population of readily saturated low-threshold fibers is excluded from analysis at high signal intensities. Physiological models have been proposed6,7 that attribute this exclusionary process to mechanisms of lateral suppression, which reduce input from AN fibers when sufficient stimulation occurs in spectral regions adjacent to their best frequencies. These mechanisms of mutual suppression a)
Author to whom correspondence should be addressed.
EL340 J. Acoust. Soc. Am. 137 (4), April 2015
C 2015 Acoustical Society of America V
Bashford, Jr. et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4916793]
Published Online 9 April 2015
may include both mechanical (two-tone) suppression within the cochlea8 and a substantially more effective neural inhibition of AN input to cells of the cochlear nucleus (CN).9 Eriksson and Roberts6 and Winslow et al.7 also have proposed that, at high intensities, lateral inhibition within the cochlear nuclei selectively attenuates input from low-threshold, readily saturated AN fibers, producing a shift in the weighting of intensity analysis to favor input from high-threshold unsaturated fibers. In order to test behavioral predictions from this lateral inhibition hypothesis, Bashford et al.10 used a speech stimulus consisting of a steeply filtered, effectively rectangular 2/3-octave band of “everyday” sentences centered at 1500 Hz. Rectangular band (RB) filtering was employed to eliminate contributions from filter slopes, which might interfere with accurate measurement of the decline, or “rollover” of intelligibility within the passband at high intensities (see Warren et al.11). The RB speech proved to be much more vulnerable to rollover than broadband speech: A statistically significant intelligibility loss was obtained when speech intensity reached 65 dB SPL, a level at which, as discussed above, most AN neurons cannot provide rate-based encoding of speech cues. This intelligibility rollover at moderate signal levels was considered to be due to the absence of lateral suppression that would normally be evoked by speech components spectrally adjacent to the 2/3-octave RB. Bashford et al.10 then tested the conjoint prediction that adding flanking bands of white noise would restore intelligibility of the speech band by producing lateral inhibition, but only at a speech intensity sufficient to produce rollover. Listeners were presented with the RB sentences, without flanking noise, at either 45 or 75 dBA slow-peak SPL, which yielded average intelligibility scores of about 80% vs 60%, respectively, indicating a substantial rollover effect. When flanking noise bands were added to the 75-dB speech at relative spectrum levels ranging from 40 to 20 dB, intelligibility increased, with a maximum recovery of 13% obtained with noise presented at a level of 30 dB. As predicted, however, flanking noise did not increase the intelligibility of the sentences presented at 45 dB SPL. In a subsequent study, Bashford et al.12 conducted a series of experiments designed to better specify the likely mechanism and site in the auditory pathway responsible for the noise-induced recovery of intelligibility from rollover. They reasoned that if recovery is produced by lateral interactions in the cochlear nucleus, then the effectiveness of the flanking noise bands should not be influenced by binaural lateralization cues processed more centrally in the superior olivary complex. Their test employed a masking level difference paradigm in which the 75 dB 2/3-octave RB speech was accompanied either by RB noise matching the spectral limits of the speech (and producing within-band masking), or by broadband noise (producing both withinband masking and out-of-band reduction of rollover). The noise was presented at a relative spectrum level of 20 dB to ensure that some masking would occur. On half of the trials the noise added to the diotic (interaurally identical) sentences was also diotic, and on the remaining trials was interaurally uncorrelated. Decorrelation of the noise was found to produce a binaural release from masking exerted by the within-band noise components in both the narrowband and broadband noise conditions: Intelligibility was about 5% higher in the uncorrelated conditions regardless of noise bandwidth. Crucially, however, intelligibility was about 13% higher in the broadband noise conditions, regardless of interaural noise correlation, strongly suggesting that the processing responsible for the observed protection against rollover occurs below the level of the superior olivary complex. Bashford et al.12 also used the contrasting narrowband and broadband noise conditions to determine whether an effect peripheral to the CN might contribute to the reduction of rollover. It was possible that the spread of excitation from the flanking noise bands into the frequency range of the speech band produced classical firing-rate adaptation in AN fibers, shifting their operating ranges to include higher speech levels.13 To test this hypothesis, 75-dB rectangular 2/3-octave speech was again accompanied by either spectrally matching narrowband noise or broadband noise at 20 dB
J. Acoust. Soc. Am. 137 (4), April 2015
Bashford, Jr. et al.: Defeating intelligibility rollover EL341
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Published Online 9 April 2015
relative spectrum level, and on half of the trials the noise was continuous while on the remaining trials it was gated on and off with individual sentences. As expected, an “overshoot” of within-band masking did occur with the gated noise: Intelligibility was about 5% lower in that condition, whether the noise was spectrally matching or broadband. Importantly, however, intelligibility was about 11% higher with broadband rather than narrowband noise, regardless of gating, which would appear to rule out peripheral adaptation as a contributor to the effect of flanking noise on rollover. In a final experiment, Bashford et al.12 presented the RB speech monaurally and found that intelligibility enhancement required that the flanking noise bands be delivered to the same ear. This finding ruled out two additional mechanisms as potential contributors to enhancement: Contralateral input can evoke aural reflex contractions of the tensor tympani and stapedius muscles14 as well as the medial olivocochlear reflex.15 With a number of possible alternative mechanisms now eliminated from consideration, the current study focuses on another specific prediction from the CN lateral inhibition hypothesis. Lateral interactions observed between cells of the cochlear nucleus show a striking asymmetry: Higher-frequency inhibitors are generally effective at substantially lower intensities (often by 20 dB or more) than lower-frequency inhibitors.9 The present study tested for a corresponding asymmetry in the effectiveness of high-frequency vs low-frequency flanking bands in reducing the rollover of intelligibility for narrowband speech presented at 75 dB. In experiment 1, flanking bands of white noise were presented at a relative spectrum level of 30 dB. It was expected that this low noise level would reveal the higher activation level required for suppression by the low frequency noise band. In experiment 2, the spectrum level of the noise required to reduce rollover was determined separately for the low- and high-pass flanking noise bands, using separate groups of listeners. Experiment 3 employed flankers derived from speech-shaped [United States of America Standards Institute (USASI)] noise, which boosted the noise-power per unit bandwidth of the low frequency noise band by roughly 20 dB relative to the higher noise band, and permitted an evaluation of the compatibility between the asymmetrical thresholds of activation for lateral inhibition and the spectral tilt of broadband speech. 2. Method 2.1 Listeners The 88 listeners in this study (24 each in Experiments 1, and 3, and 2 groups of 20 in Experiment 2) were undergraduate students at the University of Wisconsin-Milwaukee who were paid for their participation. They ranged in age from 18 to 31 and were native monolingual English speakers who reported having no hearing problems and had normal bilateral hearing, with pure tone thresholds of 20 dB hearing level or better at octave frequencies from 0.25 to 8 kHz. 2.2 Stimuli The formal test stimuli were the 100 (10 lists of 10) Central Institute of the Deaf (CID) Everyday sentences,16 which contain 500 keywords (50 keywords per list) that are used for scoring. An additional 25 practice sentences were drawn from the high predictability sub-list of the revised Speech Perception in Noise test.17 The original broadband recordings (44.1 kHz sampling with 16-bit quantization) were produced by the same male speaker, who has no evident regional accent and has an average voicing frequency of about 100 Hz. Prior to filtering, the sentences were transduced by a Sennheiser HD 250 Linear II headphone (Sennheiser Electronics Corp., Old Lyme, CT) and their slow rootmean-square (rms) peak levels were digitally matched to within 0.2 dBA in slow-rms peak level, using a flat-plate coupler in conjunction with a Br€ uel and Kjær model 2230 digital sound-level meter (Br€ uel and Kjaer North American Inc., Norcross, GA) set at Ascale weighting (as were all level measurements reported). The sentences were then passed through two successive stages of 4000-order bandpass finite impulse response (FIR) filtering (producing slopes of approximately 3.9 dB/Hz), using the fir1 function in MATLAB
EL342 J. Acoust. Soc. Am. 137 (4), April 2015
Bashford, Jr. et al.: Defeating intelligibility rollover
Bashford, Jr. et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4916793]
Published Online 9 April 2015
(Mathworks, Natick, MA), to produce a 2/3-octave narrowband of speech, centered at 1500 Hz (passband from 1191 to 1890 Hz), with effectively vertical filter slopes.11 Bandpass filtering of the sentences reduced their average slow-rms peak level by 5.4 dB. Hence, for the 75-dB peak level employed for the narrowband sentences in the present study, the corresponding nominal broadband speech level would be 80.4 dB. White noise, low-pass filtered at 20 kHz, was used to produce the low- and high-pass flanking noise bands employed in experiments 1 and 2 and a band of USASI speech noise, also lowpass filtered at 20 kHz was used to derive the flankers presented in Experiment 3. The low- and high-pass flankers were each produced through two successive stages of 4000order FIR filtering, as with the speech band. The low-frequency limit of the lower flanking band and the high-frequency limit of the higher flanking band matched those of the broadband noise. Separate pairs of flanking noise bands were prepared for each noise spectrum-level employed in the present study (10, 20, 30, and 40 dB relative to the speech spectrum-level). 3. General procedure The design of each experiment incorporated repeated measures. Before receiving a given stimulus condition listeners were presented with five practice sentences, which were first presented broadband and then presented bandpass filtered, along with noise when appropriate, in the same manner as the test sentences that followed. The different experimental conditions presented to a given group of listeners were assigned to separate sets of the everyday test sentences. This assignment varied pseudorandomly, so that, across listeners in a group, each condition was applied an equal number of times to each set of sentences. Testing was performed in a sound attenuating chamber with the stimuli delivered through Sennheiser HD 250 Linear II headphones. Listeners were instructed to call out what the voice was saying as best they could and were encouraged to guess when unsure. Their responses were recorded and scored online by the experimenter, who sat with them in the chamber during the experiment. 3.1 Experiment 1: Extent of recovery from intelligibility rollover produced by low-pass vs high-pass flanking bands of white noise presented at matched levels Experiment 1 examined the relative effectiveness of low- and high-pass flanking bands of white noise when presented at a spectrum level of 30 dB relative to the speech, a level previously found optimal for level-matched flankers by Bashford et al.10 The 24 listeners in this experiment were presented with separate sets of 25 everyday sentences (containing approximately 125 keywords each) in each of 4 flanking noise conditions: (1) A no-noise baseline condition; (2) a low-pass white noise condition; (3) a high-pass white noise condition; and (4) a condition in which both flanking noise bands were added to the 75-dB speech. The order of presentation for conditions was pseudo-randomized with the restriction that each condition was presented four times in each serial position across listeners. 3.2 Results for experiment 1 The proportions of keywords correctly reported for the separate sets of 25 sentences (approximately 125 keywords) presented in each of the 4 experimental conditions are presented in Table 1. The data were arcsine transformed and subjected to a repeated measures analysis of variance, which yielded a significant effect of noise condition Table 1. Mean percent intelligibility scores and standard errors (N ¼ 24) for 2/3-octave narrowband sentences presented at a slow-rms peak level of 75 dBA along with either: No flanking noise bands; low-pass flanking white noise; high-pass flanking white noise, or both low- and high-pass flanking bands of white noise presented at 30 dB spectrum levels relative to the speech. No noise
Low-pass noise
High-pass noise
Both noise bands
64.0 (1.9)
66.0 (2.7)
70.1 (2.2)
73.2 (1.8)
J. Acoust. Soc. Am. 137 (4), April 2015
Bashford, Jr. et al.: Defeating intelligibility rollover EL343
Bashford, Jr. et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4916793]
Published Online 9 April 2015
[F(3,69) ¼ 5.1, p < 0.004]. A set of orthogonal, single-df contrasts comparing the “nonoise” baseline condition with the remaining conditions indicated that the high-pass noise flanker (F ¼ 4.6, p < 0.04) and the combination of high- and low-pass flankers (F ¼ 23.4, p < 0.001) did produce a significant recovery from rollover, whereas the low-pass flanker alone did not (F ¼ 0.46, p > 0.5). These results provide preliminary evidence of a difference in the level requirements for rollover reduction produced by low-pass vs high-pass flanking noise that is generally consistent with the asymmetry in activation thresholds for lateral inhibition evoked by lower vs higher frequency flanking signals. Experiment 2 extended this finding by providing an examination of the range of levels over which the low- and high-pass flanking bands of white noise effectively reduce rollover. 3.3 Experiment 2: Effects of level on the effectiveness of low-pass vs high-pass flanking bands of white noise in reducing intelligibility rollover In this experiment 2 separate groups of 20 listeners each were employed to determine the spectrum levels required for low-pass vs high-pass flanking white noise bands to reduce intelligibility rollover of the 75 dB 2/3-octave sentences. Both the low- and highpass flanker groups were presented with 20 sentences (100 keywords) in each of 5 conditions, which included a no-noise baseline condition and 4 conditions presenting the flanking white noise band at spectrum levels of 40, 30, 20, and 10 dB relative to the speech band. 3.4 Results for experiment 2 Table 2 presents the mean percent repetition accuracy for the sentence keywords in each condition, along with their standard errors. Analysis of variance for listeners’ arcsine transformed proportions of correct responses yielded significant main effects of level for both the low-pass [F(4,76) ¼ 3.2, p < 0.02] and high-pass [F(4,76) ¼ 3.0, p < 0.03] flanking noise bands. For the low-pass flanker, orthogonal contrasts comparing the no-noise baseline intelligibility with that obtained with noise present indicated that only the 20 dB spectrum level low-pass flanker condition produced a significant recovery from rollover [F ¼ 10.1, p < 0.005]. In contrast, the high-pass flanking noise produced a reliable recovery at 30 dB [F ¼ 7.4, p < 0.02] and 20 dB [F ¼ 11.8, p < 0.003], as well as a marginally significant recovery of intelligibility at 40 dB [F ¼ 4.2, p < 0.06], which provides tentative behavioral evidence of a roughly 20-dB difference in minimal level requirements for rollover reduction by the low-pass vs highpass flankers. This difference is consistent with typical physiologically measured differences in activation thresholds for low-side vs high-side inhibition in the cochlear nucleus.9 However, given the pronounced spectral tilt of normal broadband speech, results obtained with white noise flankers may not provide the best estimate of potential benefits from lateral inhibition evoked by adjacent speech components. In particular, the contribution of low-side inhibition to speech intelligibility might be compromised by a greater upward spread of masking. To evaluate this possibility, experiment 3 employed flanking bands derived from USASI speech-noise, with the bands adjusted in level such that their spectral components that were directly Table 2. Mean percent intelligibility scores and standard errors for 2/3-octave narrowband sentences presented at a slow-rms peak level of 75 dBA along with no noise, and with either low- or high-pass flanking bands of white noise at each of four spectrum levels relative to the speech: 40, 30, 20, or 10 dB. Separate groups of 20 listeners received the low- and high-pass flanking white noise band conditions. Flanking noise
No noise
40 dB noise
30 dB noise
20 dB noise
10 dB noise
High-pass group Low-pass group
64.5 (1.7) 65.0 (1.5)
69.4 (1.5) 65.7 (1.1)
71.4 (2.1) 67.0 (1.7)
71.2 (1.1) 71.8 (1.8)
70.8 (2.1) 64.7 (1.8)
EL344 J. Acoust. Soc. Am. 137 (4), April 2015
Bashford, Jr. et al.: Defeating intelligibility rollover
Bashford, Jr. et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4916793]
Published Online 9 April 2015
Table 3. Mean percent intelligibility scores and standard errors (N ¼ 24) for 2/3-octave narrowband sentences presented at a slow-rms peak level of 75 dBA along with either: No flanking noise bands; low-pass flanking speech-shaped noise; high-pass flanking speech-shaped noise, or both low- and high-pass flanking bands of speech-shaped noise presented at 20 dB spectrum level relative to the speech. No noise
Low-pass noise
High-pass noise
Both noise bands
65.1 (2.2)
72.7 (1.8)
70.5 (2.2)
76.0 (2.1)
contiguous with the speech band were at the 20 dB relative spectrum level found effective for both bands in experiment 2. 3.5 Experiment 3: Extent of rollover recovery produced by low-pass vs high-pass flanking bands of speech-shaped noise Experiment 3 examined the effect of speech-shaped noise flankers, using the same design as that employed in experiment 1, with listeners presented 25 sentences (approximately 125 keywords) in each of the 4 conditions: The no-noise baseline condition, low-pass noise alone, high-pass noise alone, and both flanking bands. 3.6 Results for experiment 3 Mean percent repetition accuracy and standard errors are presented in Table 3. Analysis of variance for arcsine transformed proportions of correct responses yielded a significant effect of noise condition [F(3,69) ¼ 12.1, p < 0.001], and orthogonal contrasts indicated that intelligibility increased significantly with the addition of low-pass flanking noise alone [F ¼ 17.4, p < 0.001], high-pass flanking noise alone [F ¼ 6.3, p < 0.02], and also when both noise bands were added [F ¼ 38.4, p < 0.001]. Intelligibility in the low- and high-pass flanker conditions did not differ by Tukey test, but the dual flanker condition produced higher intelligibility than the high-pass flanker alone (p < 0.05). 4. General discussion The results of these experiments are consistent with established characteristics of lateral inhibition and its proposed function in reducing rate-saturation, and hence reducing rollover of intelligibility for high-intensity speech.6,7 Moreover, the results of experiment 3, using speech-shaped rather than white noise flankers, suggest that the asymmetrical thresholds for low- and high-side inhibition in the cochlear nucleus may compensate for the spectral tilt of speech, providing roughly equivalent effects of low- and high-side inhibition for broadband speech, and hence equivalent reduction of rollover, at least in the spectral midrange as examined thus far. The effectiveness of flanking noise in reducing rollover when presented at a spectrum level 40 dB below the speech signal, as found in experiment 2 of this study and earlier by Bashford et al.,10 is consistent with the wide dynamic ranges of CN interneurons, such as onset-choppers, that are thought to mediate lateral inhibition.18 These neurons are activated by broadband stimulation, and are comprised of two separate populations, one of which is activated only by ipsilateral input,19 as is the reduction of intelligibility rollover produced by flanking noise.12 Acknowledgments The project described was supported by Award No. R01DC000208 from the National Institute on Deafness and Other Communication Disorders. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute on Deafness and Other Communication Disorders or the National Institutes of Health.
J. Acoust. Soc. Am. 137 (4), April 2015
Bashford, Jr. et al.: Defeating intelligibility rollover EL345
Bashford, Jr. et al.: JASA Express Letters
[http://dx.doi.org/10.1121/1.4916793]
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References and links 1
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Bashford, Jr. et al.: Defeating intelligibility rollover
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