The Contribution of Individual Differences in Memory Span and Language Ability to Spatial Release From Masking in Young Children.

Purpose Working memory capacity and language ability modulate speech reception; however, the respective roles of peripheral and cognitive processing are unclear. The contribution of individual differences in these abilities to utilization of spatial cues when separating speech from informational and energetic masking backgrounds in children has not yet been determined. Therefore, this study explored whether speech reception in children is modulated by environmental factors, such as the type of background noise and spatial configuration of target and noise sources, and individual differences in the cognitive and linguistic abilities of listeners. Method Speech reception thresholds were assessed in 39 children aged 5-7 years in simulated school listening environments. Speech reception thresholds of target sentences spoken by an adult male consisting of number and color combinations were measured using an adaptive procedure, with speech-shaped white noise and single-talker backgrounds that were either collocated (target and back-ground at 0°) or spatially separated (target at 0°, background noise at 90° to the right). Spatial release from masking was assessed alongside memory span and expressive language. Results and Conclusion Significant main effect results showed that speech reception thresholds were highest for informational maskers and collocated conditions. Significant interactions indicated that individual differences in memory span and language ability were related to spatial release from masking advantages. Specifically, individual differences in memory span and language were related to the utilization of spatial cues in separated conditions. Language differences were related to auditory stream segregation abilities in collocated conditions that lack helpful spatial cues, pointing to the utilization of language processes to make up for losses in spatial information.

Development of a method for determining binaural sensitivity to temporal fine structure.

OBJECTIVE: To develop and evaluate a test of the ability to process binaural temporal-fine-structure (TFS) information. The test was intended to provide a graded measure of TFS sensitivity for all listeners. DESIGN: Sensitivity to TFS was assessed at a sensation level of 30 dB using the established TFS-LF test at centre frequencies of 250, 500 and 750 Hz, and using the new TFS-AF test, in which the interaural phase difference (IPD) was fixed and the frequency was adaptively varied. IPDs varied from 30 to 180°. STUDY SAMPLE: Nine young (19-25 years) and 23 older (47-84 years) listeners with normal hearing over the tested frequency range. RESULTS: For the young listeners, thresholds on the TFS-AF test did not improve significantly with repeated testing. The rank-ordering of performance across listeners was independent of the size of the IPD, and moderate-to-strong correlations were observed between scores for the TFS-LF and TFS-AF tests. Older listeners who were unable to complete the TFS-LF test were all able to complete the TFS-AF test. CONCLUSIONS: No practice effects and strong correlations with an established test of binaural TFS sensitivity make the TFS-AF test a good candidate for the assessment of supra-threshold binaural processing.

Effects of age and hearing loss on stream segregation based on interaural time differences.

The effect of interaural time differences (ITDs) on obligatory stream segregation for successive tone bursts was investigated for older listeners with normal hearing (ONH) and hearing loss (OHL), by measuring the threshold for detecting a rhythmic irregularity in an otherwise isochronous sequence of interleaved "A" and "B" tones. The A and B tones had equal but opposite ITDs from 0 to 0.5 ms. For some of the ONH listeners, the threshold increased with increasing ITD, but no OHL listener showed an effect of ITD. It is concluded that hearing loss reduces the potency of ITDs in inducing obligatory stream segregation.

Objective and subjective measures of pure-tone stream segregation based on interaural time differences.

The effect of interaural time differences (ITDs) on stream segregation for successive tone bursts was investigated. Obligatory stream segregation was inferred from the threshold for detecting a rhythmic irregularity in an otherwise isochronous sequence of interleaved "A" and "B" tones (task 1). Subjective stream segregation was evaluated by requiring listeners to indicate whether they heard one or two streams during presentation of a 30-s long sequence (task 2). The A and B tones had equal but opposite ITDs and had the same or different frequencies of 500 and/or 707 Hz. The ITDs ranged from 0 to 2 ms in study 1, and from 0 to 0.5 ms in study 2. Sensitivity on task 1 was poor in both studies when A and B had different frequencies, and was little affected by ITD. Thresholds for the same-frequency conditions worsened somewhat with increasing ITD up to 0.5 ms and then (for study 1) flattened off. There was a small increase in subjective streaming as the ITD was increased up to 0.5 ms, but little streaming for larger ITDs (study 1). We conclude that ITD, at most, has weak effects in producing obligatory and subjective stream segregation.

The importance for speech intelligibility of random fluctuations in "steady" background noise.

Spectrally shaped steady noise is commonly used as a masker of speech. The effects of inherent random fluctuations in amplitude of such a noise are typically ignored. Here, the importance of these random fluctuations was assessed by comparing two cases. For one, speech was mixed with steady speech-shaped noise and N-channel tone vocoded, a process referred to as signal-domain mixing (SDM); this preserved the random fluctuations of the noise. For the second, the envelope of speech alone was extracted for each vocoder channel and a constant was added corresponding to the root-mean-square value of the noise envelope for that channel. This is referred to as envelope-domain mixing (EDM); it removed the random fluctuations of the noise. Sinusoidally modulated noise and a single talker were also used as backgrounds, with both SDM and EDM. Speech intelligibility was measured for N = 12, 19, and 30, with the target-to-background ratio fixed at -7 dB. For SDM, performance was best for the speech background and worst for the steady noise. For EDM, this pattern was reversed. Intelligibility with steady noise was consistently very poor for SDM, but near-ceiling for EDM, demonstrating that the random fluctuations in steady noise have a large effect.

The dynamic range of useful temporal fine structure cues for speech in the presence of a competing talker.

Within an auditory channel, the speech waveform contains both temporal envelope (E(O)) and temporal fine structure (TFS) information. Vocoder processing extracts a modified version of the temporal envelope (E') within each channel and uses it to modulate a channel carrier. The resulting signal, E'(Carr), has reduced information content compared to the original "E(O) + TFS" signal. The dynamic range over which listeners make additional use of E(O) + TFS over E'(Carr) cues was investigated in a competing-speech task. The target-and-background mixture was processed using a 30-channel vocoder. In each channel, E(O) + TFS replaced E'(Carr) at either the peaks or the valleys of the signal. The replacement decision was based on comparing the short-term channel level to a parametrically varied "switching threshold," expressed relative to the long-term channel level. Intelligibility was measured as a function of switching threshold, carrier type, target-to-background ratio, and replacement method. Scores showed a dependence on all four parameters. Derived intensity-importance functions (IIFs) showed that E(O) + TFS information from 8-13 dB below to 10 dB above the channel long-term level was important. When E(O) + TFS information was added at the peaks, IIFs peaked around -2 dB, but when E(O) + TFS information was added at the valleys, the peaks lay around +1 dB.

Sequential streaming due to manipulation of interaural time differences.

The effect of apparent spatial location on sequential streaming was investigated by manipulating interaural time differences (ITDs). The degree of obligatory stream segregation was inferred indirectly from the threshold for detecting a rhythmic irregularity in an otherwise isochronous sequence of interleaved "A" and "B" tones. Stimuli were bandpass-filtered harmonic complexes with a 100-Hz fundamental. The A and B tones had equal but opposite ITDs of 0, 0.25, 0.5, 1, or 2 ms and had the same or different passbands. The passband ranges were 1250-2500 Hz and 1768-3536 Hz in experiment 1, and 353-707 Hz and 500-1000 Hz in experiment 2. In both experiments, increases in ITD led to increases in threshold, mainly when the passbands of A and B were the same. The effects were largest for ITDs above 0.5 ms, for which rhythmic irregularities in the timing of the A or B tones alone may have disrupted performance. It is concluded that the differences in apparent spatial location produced by ITD have only weak effects on obligatory streaming.

Determination of preferred parameters for multichannel compression using individually fitted simulated hearing AIDS and paired comparisons.

OBJECTIVE: To determine preferred parameters of multichannel compression using individually fitted simulated hearing aids and a method of paired comparisons. DESIGN: Fourteen participants with mild to moderate hearing loss listened via a simulated five-channel compression hearing aid fitted using the CAMEQ2-HF method to pairs of speech sounds (a male talker and a female talker) and musical sounds (a percussion instrument, orchestral classical music, and a jazz trio) presented sequentially and indicated which sound of the pair was preferred and by how much. The sounds in each pair were derived from the same token and differed along a single dimension in the type of processing applied. For the speech sounds, participants judged either pleasantness or clarity; in the latter case, the speech was presented in noise at a 2-dB signal-to-noise ratio. For musical sounds, they judged pleasantness. The parameters explored were time delay of the audio signal relative to the gain control signal (the alignment delay), compression speed (attack and release times), bandwidth (5, 7.5, or 10 kHz), and gain at high frequencies relative to that prescribed by CAMEQ2-HF. RESULTS: Pleasantness increased with increasing alignment delay only for the percussive musical sound. Clarity was not affected by alignment delay. There was a trend for pleasantness to decrease slightly with increasing bandwidth, but this was significant only for female speech with fast compression. Judged clarity was significantly higher for the 7.5- and 10-kHz bandwidths than for the 5-kHz bandwidth for both slow and fast compression and for both talker genders. Compression speed had little effect on pleasantness for 50- or 65-dB SPL input levels, but slow compression was generally judged as slightly more pleasant than fast compression for an 80-dB SPL input level. Clarity was higher for slow than for fast compression for input levels of 80 and 65 dB SPL but not for a level of 50 dB SPL. Preferences for pleasantness were approximately equal with CAMEQ2-HF gains and with gains slightly reduced at high frequencies and were lower when gains were slightly increased at high frequencies. Speech clarity was not affected by changing the gain at high frequencies. CONCLUSIONS: Effects of alignment delay were small except for the percussive sound. A wider bandwidth was slightly preferred for speech clarity. Speech clarity was slightly greater with slow compression, especially at high levels. Preferred high-frequency gains were close to or a little below those prescribed by CAMEQ2-HF.

Relative contribution to speech intelligibility of different envelope modulation rates within the speech dynamic range.

The contribution of envelope cues at different rates to intelligibility in a competing-speech task was measured as a function of the short-term envelope level. The target and background mixture was processed using tone vocoders. Envelope signals for each vocoder channel were simultaneously extracted with two low-pass filters, the cutoff frequency of one filter (L) being two octaves below that of the other (H). The envelope from the H filter was used at the peaks and that from the L filter at valleys, or vice versa. This was achieved by cross-fading between the two envelope signals based on a "switching threshold" that was parametrically varied relative to the long-term RMS level of the channel signal. When the cutoff frequencies of the H and L filters were 50 and 12.5 Hz, changes in speech intelligibility occurred mainly when the switching threshold was between -18 and +10 dB. The range was slightly narrower when the cutoff frequencies of the H and L filters were 200 and 50 Hz. Intensity-importance functions for higher-rate envelope modulations suggested that levels ranging from 20 dB below to about 10 dB above the channel RMS level were important, with maximum importance for levels around -5 dB.

Preliminary evaluation of a method for fitting hearing aids with extended bandwidth.

This paper describes a preliminary laboratory-based evaluation of a method for fitting hearing aids with an extended high-frequency response, called CAMEQ2-HF. Linear filtering was used to implement the CAMEQ2-HF-prescribed gains for speech with an input level of 65 dB SPL. The results obtained using four normal-hearing (NH) and fifteen hearing-impaired (HI) listeners showed: (1) The gains were sufficient to make components above 5 kHz audible when those components were presented alone, and when they were presented together with the lower-frequency components; (2) NH listeners preferred a wider bandwidth (10 or 7.5 kHz versus 5 kHz) for both pleasantness and speech clarity, while HI listeners usually preferred a narrower bandwidth for pleasantness but a wider bandwidth for clarity; (3) HI listeners performed better on the 'S-test (detection of word-final /s/ or /z/) with a wider than with a narrower bandwidth (7.5 versus 5 kHz); (4) Identification of vowel-consonant-vowel nonsense syllables improved with increasing bandwidth from 5 to 7.5 kHz for the NH but not for the HI listeners.

Effect of spatial separation, extended bandwidth, and compression speed on intelligibility in a competing-speech task.

The benefit for speech intelligibility of extending the bandwidth of hearing aids was assessed when the target speech (sentences) and background (two talkers) were co-located or spatially separated. Also, the relative benefits of slow and fast compression were assessed. Sixteen hearing-impaired (HI) subjects with mild-to-moderate high-frequency hearing loss and eight normal-hearing (NH) subjects were tested. The target and interfering sounds were recorded using a KEMAR manikin and were located at +/-60 degrees azimuth, either co-located or spatially separated. Simulated binaural hearing-aid processing using five-channel slow or fast compression was performed offline, with gains set individually for each HI subject. Upper cutoff frequencies were 5, 7.5, or 10 kHz. Processed stimuli were presented via headphones. For both NH (unaided) and HI subjects, there was no significant effect of cutoff frequency for the co-located condition, but a small but significant benefit from increasing the cutoff frequency from 5 to 7.5 kHz for the spatially separated condition. For the HI subjects, slow compression gave slightly but significantly higher scores than fast compression for the spatially separated but not for the co-located condition. There were marked individual differences both in the benefit from extended bandwidth and in the relative benefit of slow and fast compression.

Relative importance of different spectral bands to consonant identification: relevance for frequency transposition in hearing aids.

Listeners with high-frequency dead regions (DRs) benefit from amplification of frequencies up to 1.7 times the edge frequency, f(e), of the DR. Better consonant identification might be achieved by replacing the band from f(e) to 1.7f(e) with a higher spectral band. We aimed to identify the optimal band, using simulations with normal-hearing listeners. In experiment 1, nonsense syllables were lowpass filtered to simulate DRs with f(e) of 0.5, 0.75, and 1.0 kHz. Identification was measured for each of these base bands alone and with a bandpass-filtered band added (but not transposed). The added band either extended from f(e) to 1.7f(e) or its center frequency was increased, keeping bandwidth fixed in ERB(N)-number. Performance improved with increasing center frequency and then reached an asymptote or declined. Experiment 2 used a mid-frequency base band, and a lower-frequency added band. The results also showed a beneficial effect of frequency separation of the added and base bands. Experiment 3 resembled experiment 1, but with bandwidth fixed in Hertz. For higher-frequency added bands, the benefit was lower than for experiment 1.

Estimation of the center frequency of the highest modulation filter.

For high-frequency sinusoidal carriers, the threshold for detecting sinusoidal amplitude modulation increases when the signal modulation frequency increases above about 120 Hz. Using the concept of a modulation filter bank, this effect might be explained by (1) a decreasing sensitivity or greater internal noise for modulation filters with center frequencies above 120 Hz; and (2) a limited span of center frequencies of the modulation filters, the top filter being tuned to about 120 Hz. The second possibility was tested by measuring modulation masking in forward masking using an 8 kHz sinusoidal carrier. The signal modulation frequency was 80, 120, or 180 Hz and the masker modulation frequencies covered a range above and below each signal frequency. Four highly trained listeners were tested. For the 80-Hz signal, the signal threshold was usually maximal when the masker frequency equaled the signal frequency. For the 180-Hz signal, the signal threshold was maximal when the masker frequency was below the signal frequency. For the 120-Hz signal, two listeners showed the former pattern, and two showed the latter pattern. The results support the idea that the highest modulation filter has a center frequency in the range 100-120 Hz.

Benefit of high-rate envelope cues in vocoder processing: effect of number of channels and spectral region.

In cochlear implants, or vocoder simulations of cochlear implants, the transmission of envelope cues at high rates (related to voice fundamental frequency, f0) may be limited by the widths of the filters used to form the channels and/or by the cutoff frequency, f(lp), of the low-pass filters used for envelope extraction. The effect of varying f(lp) in tone and noise vocoders was investigated for channel numbers, N, from 6 to 18. As N increased, the widths of the channels decreased. The value of f(lp) was 45 Hz (envelope or "E" filter), or 180 Hz (pitch or "P" filter). The following combinations of cutoff frequencies were used for channels below and above 1500 Hz, respectively: EE, PE, EP, and PP. Results from a competing-talker task showed that the tone vocoder led to better intelligibility than the noise vocoder. The PP condition led to the best intelligibility and the EE condition to the worst. For N=6, intelligibility was better for condition PE than for condition EP. For N=18, the reverse was true. The results indicate that the channel bandwidths can compromise the transmission of f0-related envelope information, and suggest that vocoder simulations of cochlear-implant processing have limitations.

Detection of 1st- and 2nd-order temporal-envelope cues in a patient with left superior cortical damage.

This psychophysical study explores the extent to which the auditory cortex is necessary for various aspects of temporal-envelope perception, that is, perception of the slow temporal modulations in amplitude known to be crucial for sound identification. The ability to detect 1st- and 2nd-order sinusoidal amplitude modulation (AM) is evaluated in a single patient showing left-hemisphere damage encroaching the primary and secondary auditory cortices. Here, 1st- and 2nd-order AM refer to (1) sinusoidal variation in the amplitude of a 2 kHz pure tone, and (2) sinusoidal variation in the depth of a 64 Hz AM applied to the 2 kHz pure tone, respectively. The results replicate previous findings by showing that damage to the left auditory cortex results in a selective deficit in auditory sensitivity to the lowest 1St-order AM (i.e., 1st-order AM frequencies < 16 Hz). Moreover, a dissociation is apparent between the ability to detect 1st- and 2nd-order temporal-envelope cues. The patient shows poorer than normal ability to detect 2nd-order AM at low frequencies ranging from 4-23 Hz, but normal ability to detect the high (64 Hz) 1st-order AM carrying these 2nd-order modulations. This result indicates that damage to the left primary and secondary auditory cortices affects the ability to detect temporal variations in the local properties of sounds(such as AM depth). It is also consistent with the idea that, as in vision, central nonlinear mechanisms are involved in the computation of such local (or 2nd-order) temporal properties.

Effect of cochlear damage on the detection of complex temporal envelopes.

Recent studies have demonstrated that the detection of complex temporal envelopes relies - at least partially - on the perception of a distortion component generated by a peripheral (cochlear) and/or central (post-cochlear) non-linearity. In the present study, first- and second-order amplitude modulation (AM) detection thresholds were obtained in normally hearing (NH) and hearing-impaired (HI) listeners using a 2-kHz pure-tone carrier. In both groups of listeners, first-order AM detection thresholds were measured for AM rates fm ranging between 4 and 87 Hz, and second-order AM detection thresholds were measured for second-order AM rates fm' ranging between 4 and 23 Hz, using a fixed first-order 'carrier' AM rate fm of 64 Hz. When the sound pressure level was adjusted in order to yield equal detectability in both groups for the 64-Hz first-order carrier modulation, (i) first-order AM detection thresholds for the HI listeners were normal at fm=87 Hz, and better-than-normal at fm=4 and 16 Hz, and (ii) second-order AM detection thresholds were identical at all modulation rates in NH and HI listeners. Similar results were obtained when the audibility of the 2-kHz pure-tone carrier was equated for both groups, i.e. when listeners were tested at the same sensation level. These results demonstrate clearly that cochlear damage has no effect on the detection of complex temporal envelopes, and indicate that the distortion component must be generated by a more central non-linearity than cochlear compression, transduction, or short-term adaptation.

Temporal envelope perception in dyslexic children.

Speech intelligibility depends heavily on the accurate perception of auditory temporal envelope cues, that is the slower amplitude modulations present in the speech waveform. In a previous study, McAnally and Stein demonstrated that dyslexics may show impaired audibility (i.e. detectability) of these envelope cues. In the present psychophysical study, the ability to process temporal envelope cues was further investigated in dyslexic children by measuring detection thresholds of sinusoidal amplitude-modulation (SAM) and discrimination thresholds of SAM depth and SAM rate. Each threshold was measured at slow and fast SAM rates of 4 and 128 Hz, respectively. Overall, SAM thresholds were higher in dyslexics than in controls at both rates. The strongest deficit was observed at 4 Hz in the SAM detection task, but a deficit was also apparent at 128 Hz in the SAM discrimination tasks. Therefore, these results reveal that, in addition to reduced audibility of slow and fast envelope cues, some dyslexic children show poor encoding fidelity for these cues (as measured by the discrimination tasks). Overall, these findings are consistent with Tallal's hypothesis according to which the speech and reading deficits in some dyslexics may be caused by impaired temporal processes.