Christian Lorenzi

Speech perception problems of the hearing impaired reflect inability to use temporal fine structure

People with sensorineural hearing loss have difficulty understanding speech, especially when background sounds are present. A reduction in the ability to resolve the frequency components of complex sounds is one factor contributing to this difficulty. Here, we show that a reduced ability to process the temporal fine structure of sounds plays an important role. Speech sounds were processed by filtering them into 16 adjacent frequency bands. The signal in each band was processed by using the Hilbert transform so as to preserve either the envelope (E, the relatively slow variations in amplitude over time) or the temporal fine structure (TFS, the rapid oscillations with rate close to the center frequency of the band). The band signals were then recombined and the stimuli were presented to subjects for identification. After training, normal-hearing subjects scored perfectly with unprocessed speech, and were ≈90% correct with E and TFS speech. Both young and elderly subjects with moderate flat hearing loss performed almost as well as normal with unprocessed and E speech but performed very poorly with TFS speech, indicating a greatly reduced ability to use TFS. For the younger hearing-impaired group, TFS scores were highly correlated with the ability to take advantage of temporal dips in a background noise when identifying unprocessed speech. The results suggest that the ability to use TFS may be critical for “listening in the background dips.” TFS stimuli may be useful in evaluating impaired hearing and in guiding the design of hearing aids and cochlear implants.

Sound localization in noise in normal-hearing listeners

The ability to localize a click train in the frontal-horizontal plane was measured in quiet and in the presence of a white-noise masker. The experiment tested the effects of signal frequency, signal-to-noise ratio (S/N), and masker location. Clicks were low-pass filtered at 11 kHz in the broadband condition, low-pass filtered at 1.6 kHz in the low-pass condition, and bandpass filtered between 1.6 and 11 kHz in the high-pass condition. The masker was presented at either -90, 0, or +90 deg azimuth. Six signal-to-noise ratios were used, ranging from -9 to +18 dB. Results obtained with four normal-hearing listeners show that (1) for all masker locations and filtering conditions, localization accuracy remains unaffected by noise until 0-6 dB S/N and decreases at more adverse signal-to-noise ratios, (2) for all filtering conditions and at low signal-to-noise ratios, the effect of noise is greater when noise is presented at +/- 90 deg azimuth than at 0 deg azimuth, (3) the effect of noise is similar for all filtering conditions when noise is presented at 0 deg azimuth, and (4) when noise is presented at +/- 90 deg azimuth, the effect of noise is similar for the broadband and high-pass conditions, but greater for the low-pass condition. These results suggest that the low- and high-frequency cues used to localize sounds are equally affected when noise is presented at 0 deg azimuth. However, low-frequency cues are less resistant to noise than high-frequency cues when noise is presented at +/- 90 deg azimuth. When both low- and high-frequency cues are available, listeners base their decision on the cues providing the most accurate estimation of the direction of the sound source (high-frequency cues). Parallel measures of click detectability suggest that the poorer localization accuracy observed when noise is at +/- 90 deg azimuth may be caused by a reduction in the detectability of the signal at the ear ipsilateral to the noise.

Abnormal processing of temporal fine structure in speech for frequencies where absolute thresholds are normal.

The identification of nonsense syllables that were lowpass filtered at 1.5 kHz was compared for subjects with normal hearing and subjects with mild-to-severe hearing loss at high frequencies but with normal or near-normal hearing at low frequencies. Absolute thresholds were mostly within the normal range (<20 dB hearing level) for both groups for frequencies below 1.5 kHz. Performance was assessed with intact speech, speech that had been processed to preserve only temporal envelope cues in a few frequency bands (E speech), and speech that had been processed to remove envelope cues as far as possible while preserving temporal fine structure cues, again in a few frequency bands (TFS speech). For the intact speech and E speech, the hearing-impaired subjects performed slightly more poorly than the normal-hearing subjects, but this effect was significant only for the intact speech. For the TFS speech, the hearing-impaired subjects performed significantly more poorly than the normal-hearing subjects, with 12 out of 16 of the former performing at chance. The results indicate that, for people with hearing loss at medium to high frequencies, the processing of the TFS of speech can be degraded for frequencies where absolute thresholds are within the normal range.

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 Tallals hypothesis according to which the speech and reading deficits in some dyslexics may be caused by impaired temporal processes.

Effects of spectral smearing and temporal fine structure degradation on speech masking release.

This study assessed the effects of spectral smearing and temporal fine structure (TFS) degradation on masking release (MR) (the improvement in speech identification in amplitude-modulated compared to steady noise observed for normal-hearing listeners). Syllables and noise stimuli were processed using either a spectral-smearing algorithm or a tone-excited vocoder. The two processing schemes simulated broadening of the auditory filters by factors of 2 and 4. Simulations of the early stages of auditory processing showed that the two schemes produced comparable excitation patterns; however, fundamental frequency (F0) information conveyed by TFS was degraded more severely by the vocoder than by the spectral-smearing algorithm. Both schemes reduced MR but, for each amount of spectral smearing, the vocoder produced a greater reduction in MR than the spectral-smearing algorithm, consistent with the effects of each scheme on F0 representation. Moreover, the effects of spectral smearing on MR produced by the two schemes were different for manner and voicing. Finally, MR data for listeners with moderate hearing loss were well matched by MR data obtained for normal-hearing listeners with vocoded stimuli, suggesting that impaired frequency selectivity alone may not be sufficient to account for the reduced MR observed for hearing-impaired listeners.

Neuropsychological outcome in children with optic pathway tumours when first-line treatment is chemotherapy

Standard treatment of optic pathways gliomas consists of radiotherapy and surgery when feasible. Owing to the toxicity of irradiation, chemotherapy has emerged as an interesting therapeutic option, especially in young children. This study describes the neuropsychological profile of 27 children (aged between 1.5 and 15.7 years) with optic pathways gliomas treated with chemotherapy as first-line treatment. Eight of them also received radiotherapy as salvage treatment. Eight had neurofibromatosis type 1 (NF1). Intellectual outcome was preserved in children treated with chemotherapy only (mean=107±17) compared to children also receiving radiotherapy (mean IQ=88±24) or children having NF1 and treated with chemotherapy (mean IQ=80±13). Scores for abstract reasoning, mental arithmetic, chessboard/coding, perception, judgement of line orientation were lower in children irradiated than in those treated only by chemotherapy. Children with Nf1 showed subnormal IQ scores with marked impairment of short- and long-term memory. With respect to long-term neuropsychological outcome, our study shows that a chemotherapy-first strategy can preserve the intellectual outcome of these patients either by avoiding the need of radiotherapy or by delaying its use as much as possible.

Speech masking release in listeners with flat hearing loss: Effects of masker fluctuation rate on identification scores and phonetic feature reception

Consonant identification was measured for a stationary and amplitude-modulated noise masker in four listeners with flat cochlear hearing loss, and four age-matched normal-hearing listeners. The masker modulation rate was systematically varied between 2 and 128 Hz. Masking release (MR), that is better identification performance in fluctuating, than in stationary noise, was highest in a masker fluctuating at 8–16 Hz in all normal-hearing listeners. In comparison, MR was only observed in two out of the four impaired listeners. In these listeners, MR was poorer than normal, and peaked at lower rates, that is 2 or 8 Hz. MR corresponded to increased reception of information for voicing, place, and manner between 2 and 64 Hz in all normal-hearing listeners. In impaired listeners, increased reception of information was mainly observed for manner, and mainly reduced for place, but these differences were not significant. For all phonetic features, MR was observed at lower masker fluctuation rates (≤32 Hz) than in normal-hearing listeners. This study therefore shows that cochlear damage affects MR, both quantitatively and qualitatively. Sumario Se midió la identificación de consonantes con un ruido enmascarador estacionario y de amplitud modulada en cuatro sujetos con pérdidas auditivas cocleares planas, y en cuatro sujetos normo-oyentes de edades similares. La tasa de modulación del enmascarador se varió sistemáticamente entre 2 y 128 Hz. La sustracción del efecto de enmascaramiento (MR), que tiene un mejor desempeño de identificación con ruido fluctuante que con ruido estacionario, fue mayor con un enmascarador que fluctuó entre 8 y 16 Hz, en todos los sujetos normales. Comparativamente, el MR sólo se observó en dos de los cuatro sujetos hipoacúsicos. En estos sujetos, el MR fue menor al normal y su pico se obtuvo a tasas bajas, esto es, de 8 a 12 Hz. El MR en sujetos normo-oyentes correspondió a un incremento en la recepción de la información en sonoridad, punto y modo de articulación entre 2 y 64 Hz. En sujetos hipoacúsicos, se observó un aumento en la recepción de información en modo, y aunque se vio reducida principalmente en cuanto al punto de articulación, estas diferencias no fueron significativas. Para todas las características fonéticas, el MR se observó a tasas más bajas de fluctuación (≤32 Hz) que en los sujetos normo-oyentes. Este estudio, por consiguiente, muestra que el daño coclear afecta el MR tanto cualitativa como cuantitativamente.

Effects of periodic interruptions on the intelligibility of speech based on temporal fine-structure or envelope cues.

The intelligibility of speech signals processed to retain either temporal envelope (E) or fine structure (TFS) cues within 16 0.4-oct-wide frequency bands was evaluated when processed stimuli were periodically interrupted at different rates. The interrupted E- and TFS-coded stimuli were highly intelligible in all conditions. However, the different patterns of results obtained for E- and TFS-coded speech suggest that the two types of stimuli do not convey identical speech cues. When an effect of interruption rate was observed, the effect occurred at low interruption rates (<8 Hz) and was stronger for E- than TFS-coded speech, suggesting larger involvement of modulation masking with E-coded speech.

Neuronal correlates of perceptual amplitude-modulation detection

The goal of the present paper is to relate the coding of amplitude modulation (AM) in the auditory pathway to the behavioral detection performance. To address this issue, the detectability of AM was estimated by modelling a single neuron located in the central nucleus of the inferior colliculus (IC). The computational model is based on cochlear nucleus responses and a coincidence detection mechanism. The model replicated the main feature of the neuronal AM transfer function, namely a bandpass function. The IC-unit model was initially tuned to a 200-Hz modulation frequency. A single neurometric function for AM detection at this modulation frequency was generated using a 2-interval, 2-alternative forced-choice paradigm. On each trial of the experiments, AM was taken to be correctly detected by the model if the number of spikes in response to the modulated signal exceeded the number of spikes in an otherwise identical interval that contained an unmodulated signal. Psychometric functions for 4 human subjects were also measured under the same stimulus conditions. Comparison of the simulated neurometric and psychometric functions suggested that there was sufficient information in the rate response of an IC neuron well-tuned in the modulation-frequency domain to support behavioral detection performance.

Effects of age and hearing loss on the relationship between discrimination of stochastic frequency modulation and speech perception.

Objective: The frequency modulation (FM) of speech can convey linguistic information and also enhance speech-stream coherence and segmentation. The purpose of the present study was to use a clinically oriented approach to examine the effects of age and hearing loss on the ability to discriminate between stochastic patterns of low-rate FM and determine whether difficulties in speech perception experienced by older listeners relate to a deficit in this ability. Design: Data were collected from 18 normal-hearing young adults, and 18 participants who were at least 60 years old, nine of whom had normal hearing and the remaining nine who had a mild-to-moderate sensorineural hearing loss. Using stochastic frequency modulators derived from 5-Hz low-pass noise applied to a 1-kHz carrier, discrimination thresholds were measured in terms of frequency excursion (&Dgr;F) both in quiet and with a speech-babble masker present, stimulus duration, and signal-to-noise ratio (SNRFM) in the presence of a speech-babble masker. Speech-perception ability was evaluated using Quick Speech-in-Noise (QuickSIN) sentences in four-talker babble. Results: Results showed a significant effect of age but not of hearing loss among the older listeners, for FM discrimination conditions with masking present (&Dgr;F and SNRFM). The effect of age was not significant for the FM measures based on stimulus duration. &Dgr;F and SNRFM were also the two conditions for which performance was significantly correlated with listener age when controlling for effect of hearing loss as measured by pure-tone average. With respect to speech-in-noise ability, results from the SNRFM condition were significantly correlated with QuickSIN performance. Conclusions: Results indicate that aging is associated with reduced ability to discriminate moderate-duration patterns of low-rate stochastic FM. Furthermore, the relationship between QuickSIN performance and the SNRFM thresholds suggests that the difficulty experienced by older listeners with speech-in-noise processing may, in part, relate to diminished ability to process slower fine-structure modulation at low sensation levels. Results thus suggest that clinical consideration of stochastic FM discrimination measures may offer a fuller picture of auditory-processing abilities.