Informational masking of monaural target speech by a single contralateral formant

Recent research suggests that the ability of an extraneous formant to impair intelligibility depends on the variation of its frequency contour. This idea was explored using a method that ensures interference occurs only through informational masking. Three-formant analogues of sentences were synthesized using a monotonous periodic source (F0 = 140 Hz). Target formants were presented monaurally; the target ear was assigned randomly on each trial. A competitor for F2 (F2C) was presented contralaterally; listeners must reject F2C to optimize recognition. In experiment 1, F2Cs with various frequency and amplitude contours were used. F2Cs with time-varying frequency contours were effective competitors; constant-frequency F2Cs had far less impact. Amplitude contour also influenced competitor impact; this effect was additive. In experiment 2, F2Cs were created by inverting the F2 frequency contour about its geometric mean and varying its depth of variation over a range from constant to twice the original (0–200%). The impact on intelligibility was least for constant F2Cs and increased up to ~100% depth, but little thereafter. The effect of an extraneous formant depends primarily on its frequency contour; interference increases as the depth of variation is increased until the range exceeds that typical for F2 in natural speech

Spatial Release From Masking in Children: Effects of Simulated Unilateral Hearing Loss

The purpose of this study was twofold: 1) to determine the effect of an acute simulated unilateral hearing loss on children’s spatial release from masking in two-talker speech and speech-shaped noise, and 2) to develop a procedure to be used in future studies that will assess spatial release from masking in children who have permanent unilateral hearing loss. There were three main predictions. First, spatial release from masking was expected to be larger in two-talker speech than speech-shaped noise. Second, simulated unilateral hearing loss was expected to worsen performance in all listening conditions, but particularly in the spatially separated two-talker speech masker. Third, spatial release from masking was expected to be smaller for children than for adults in the two-talker masker

Informational masking of speech by acoustically similar intelligible and unintelligible interferers

Masking experienced when target speech is accompanied by a single interfering voice is often primarily informational masking (IM). IM is generally greater when the interferer is intelligible than when it is not (e.g., speech from an unfamiliar language), but the relative contributions of acoustic-phonetic and linguistic interference are often difficult to assess owing to acoustic differences between interferers (e.g., different talkers). Three-formant analogues (F1+F2+F3) of natural sentences were used as targets and interferers. Targets were presented monaurally either alone or accompanied contralaterally by interferers from another sentence (F0 = 4 semitones higher); a target-to-masker ratio (TMR) between ears of 0, 6, or 12 dB was used. Interferers were either intelligible or rendered unintelligible by delaying F2 and advancing F3 by 150 ms relative to F1, a manipulation designed to minimize spectro-temporal differences between corresponding interferers. Target-sentence intelligibility (keywords correct) was 67% when presented alone, but fell considerably when an unintelligible interferer was present (49%) and significantly further when the interferer was intelligible (41%). Changes in TMR produced neither a significant main effect nor an interaction with interferer type. Interference with acoustic-phonetic processing of the target can explain much of the impact on intelligibility, but linguistic factors—particularly interferer intrusions—also make an important contribution to IM

Dichotic integration of acoustic-phonetic information: Competition from extraneous formants increases the effect of second-formant attenuation on intelligibility

Differences in ear of presentation and level do not prevent effective integration of concurrent speech cues such as formant frequencies. For example, presenting the higher formants of a consonant-vowel syllable in the opposite ear to the first formant protects them from upward spread of masking, allowing them to remain effective speech cues even after substantial attenuation. This study used three-formant (F1+F2+F3) analogues of natural sentences and extended the approach to include competitive conditions. Target formants were presented dichotically (F1+F3; F2), either alone or accompanied by an extraneous competitor for F2 (i.e., F1±F2C+F3; F2) that listeners must reject to optimize recognition. F2C was created by inverting the F2 frequency contour and using the F2 amplitude contour without attenuation. In experiment 1, F2C was always absent and intelligibility was unaffected until F2 attenuation exceeded 30 dB; F2 still provided useful information at 48-dB attenuation. In experiment 2, attenuating F2 by 24 dB caused considerable loss of intelligibility when F2C was present, but had no effect in its absence. Factors likely to contribute to this interaction include informational masking from F2C acting to swamp the acoustic-phonetic information carried by F2, and interaural inhibition from F2C acting to reduce the effective level of F2

Across-formant integration and speech intelligibility:effects of acoustic source properties in the presence and absence of a contralateral interferer

The role of source properties in across-formant integration was explored using three-formant (F1+F2+F3) analogues of natural sentences (targets). In experiment 1, F1+F3 were harmonic analogues (H1+H3) generated using a monotonous buzz source and second-order resonators; in experiment 2, F1+F3 were tonal analogues (T1+T3). F2 could take either form (H2 or T2). Target formants were always presented monaurally; the receiving ear was assigned randomly on each trial. In some conditions, only the target was present; in others, a competitor for F2 (F2C) was presented contralaterally. Buzz-excited or tonal competitors were created using the time-reversed frequency and amplitude contours of F2. Listeners must reject F2C to optimize keyword recognition. Whether or not a competitor was present, there was no effect of source mismatch between F1+F3 and F2. The impact of adding F2C was modest when it was tonal but large when it was harmonic, irrespective of whether F2C matched F1+F3. This pattern was maintained when harmonic and tonal counterparts were loudness-matched (experiment 3). Source type and competition, rather than acoustic similarity, governed the phonetic contribution of a formant. Contrary to earlier research using dichotic targets, requiring across-ear integration to optimize intelligibility, H2C was an equally effective informational masker for H2 as for T2

The effect of an active transcutaneous bone conduction device on spatial release from masking

Objective: The aim was to quantify the effect of the experimental active transcutaneous Bone Conduction Implant (BCI) on spatial release from masking (SRM) in subjects with bilateral or unilateral conductive and mixed hearing loss. Design: Measurements were performed in a sound booth with five loudspeakers at 0\ub0, +/−30\ub0 and +/−150\ub0 azimuth. Target speech was presented frontally, and interfering speech from either the front (co-located) or surrounding (separated) loudspeakers. SRM was calculated as the difference between the separated and the co-located speech recognition threshold (SRT). Study Sample: Twelve patients (aged 22–76 years) unilaterally implanted with the BCI were included. Results: A positive SRM, reflecting a benefit of spatially separating interferers from target speech, existed for all subjects in unaided condition, and for nine subjects (75%) in aided condition. Aided SRM was lower compared to unaided in nine of the subjects. There was no difference in SRM between patients with bilateral and unilateral hearing loss. In aided situation, SRT improved only for patients with bilateral hearing loss. Conclusions: The BCI fitted unilaterally in patients with bilateral or unilateral conductive/mixed hearing loss seems to reduce SRM. However, data indicates that SRT is improved or maintained for patients with bilateral and unilateral hearing loss, respectively

Informational masking and the effects of differences in fundamental frequency and fundamental-frequency contour on phonetic integration in a formant ensemble

This study explored the effects on speech intelligibility of across-formant differences in fundamental frequency (ΔF0) and F0 contour. Sentence-length speech analogues were presented dichotically (left=F1+F3; right=F2), either alone or—because competition usually reveals grouping cues most clearly—accompanied in the left ear by a competitor for F2 (F2C) that listeners must reject to optimize recognition. F2C was created by inverting the F2 frequency contour. In experiment 1, all left-ear formants shared the same constant F0 and ΔF0F2 was 0 or ±4 semitones. In experiment 2, all left-ear formants shared the natural F0 contour and that for F2 was natural, constant, exaggerated, or inverted. Adding F2C lowered keyword scores, presumably because of informational masking. The results for experiment 1 were complicated by effects associated with the direction of ΔF0F2; this problem was avoided in experiment 2 because all four F0 contours had the same geometric mean frequency. When the target formants were presented alone, scores were relatively high and did not depend on the F0F2 contour. F2C impact was greater when F2 had a different F0 contour from the other formants. This effect was a direct consequence of the associated ΔF0; the F0F2 contour per se did not influence competitor impact

Role of The Cochlea and Efferent System in Children with Auditory Processing Disorder

Auditory processing disorder (APD) is characterized by difficulty listening in noisy environments despite normal hearing thresholds. APD was previously thought to be restricted to deficits in the central auditory system. The current work sought to investigate brainstem and peripheral mechanisms that may contribute to difficulties in speech understanding in noise in children with suspected APD (sAPD). Three mechanisms in particular were investigated: cochlear tuning, efferent function, and spatial hearing. Cochlear tuning was measured using stimulus frequency otoacoustic emission (SFOAE) group delay. Results indicate that children suspected with APD have atypically sharp cochlear tuning, and reduced medial olivocochlear (MOC) functioning. Sharper-than-typical cochlear tuning may lead to increased forward masking. On the contrary, binaural efferent function probed with a forward masked click evoked OAE (CEOAE) paradigm indicated that MOC function was not different in typically developing (TD) children and children suspected with APD. A third study with multiple OAE types sought to address this contradiction. Despite numerically smaller MOC inhibition in the sAPD group, MOC function was not significantly different between the two groups. Finally, spatial release from masking, localization-in-noise and interaural time difference thresholds were compared in TD and children with sAPD. Results indicate no significant difference in spatial hearing abilities between the two groups. Non-significant findings at group level in these studies may be related to the large heterogeneity in problems associated with APD. Fragmentation of APD into deficit specific disorders may facilitate research in identification of the specific anatomical underpinnings to listening problems in APD. Prior to conducting studies in children, three studies were conducted to optimize stimulus characteristics. Results of these studies indicate that the MOC may not be especially sensitive to 100 Hz amplitude modulation, as previously reported. Click stimulus presentation rates \u3e25 Hz activate the ipsilateral MOC reflex in typical MOC assays, contaminating contralateral MOC inhibition of CEOAEs. Finally, localization-in-noise abilities of TD children are on par with adults for a white noise masker, but not for speech-babble. This finding suggests that despite maturation of physiological mechanisms required to localize in noise, non-auditory factors may restrict the ability of children in processing complex signals

TEMPORAL CODING OF SPEECH IN HUMAN AUDITORY CORTEX

Human listeners can reliably recognize speech in complex listening environments. The underlying neural mechanisms, however, remain unclear and cannot yet be emulated by any artificial system. In this dissertation, we study how speech is represented in the human auditory cortex and how the neural representation contributes to reliable speech recognition. Cortical activity from normal hearing human subjects is noninvasively recorded using magnetoencephalography, during natural speech listening. It is first demonstrated that neural activity from auditory cortex is precisely synchronized to the slow temporal modulations of speech, when the speech signal is presented in a quiet listening environment. How this neural representation is affected by acoustic interference is then investigated. Acoustic interference degrades speech perception via two mechanisms, informational masking and energetic masking, which are addressed respectively by using a competing speech stream and a stationary noise as the interfering sound. When two speech streams are presented simultaneously, cortical activity is predominantly synchronized to the speech stream the listener attends to, even if the unattended, competing speech stream is 8 dB more intense. When speech is presented together with spectrally matched stationary noise, cortical activity remains precisely synchronized to the temporal modulations of speech until the noise is 9 dB more intense. Critically, the accuracy of neural synchronization to speech predicts how well individual listeners can understand speech in noise. Further analysis reveals that two neural sources contribute to speech synchronized cortical activity, one with a shorter response latency of about 50 ms and the other with a longer response latency of about 100 ms. The longer-latency component, but not the shorter-latency component, shows selectivity to the attended speech and invariance to background noise, indicating a transition from encoding the acoustic scene to encoding the behaviorally important auditory object, in auditory cortex. Taken together, we have demonstrated that during natural speech comprehension, neural activity in the human auditory cortex is precisely synchronized to the slow temporal modulations of speech. This neural synchronization is robust to acoustic interference, whether speech or noise, and therefore provides a strong candidate for the neural basis of acoustic background invariant speech recognition

SPATIAL HEARING AND FUNCTIONAL AUDITORY SKILLS OF CHILDREN WHO HAVE UNILATERAL HEARING LOSS

Children with unilateral hearing loss (UHL) are at increased risk for a range of developmental difficulties and delays, but there is substantial variability in developmental outcomes among this population. One explanation for the deficits observed among this population is that children with UHL have a reduced ability to compare sounds arriving at the two ears, which is critical for spatial hearing. Few studies have investigated the specific nature of the spatial hearing deficits that children with UHL experience. Defining this population’s spatial hearing deficits is critical for understanding and remediating the factors that contribute to the marked differences in developmental outcomes observed among children with UHL. The goal of this dissertation was to clarify the spatial hearing deficits experienced by children with UHL in natural listening environments. This goal was accomplished in two experiments under the following specific aims: (1) evaluate the effect of UHL on children’s ability to benefit from spatial separation of target and masker stimuli (spatial release from masking, SRM) for speech recognition in two-talker speech and speech-shaped noise, (2) assess localization on the azimuthal plane, and (3) determine the association between SRM and functional listening abilities as assessed by parent report for children with UHL. The first experiment addressed Aim 1 in a group of school-age children and adults with normal hearing who completed the experiment both with and without an acute simulated conductive UHL. The second experiment addressed Aims 1 through 3 in a group of children with longstanding sensorineural or mixed UHL, age-matched peers with normal hearing, and adults with normal hearing. Results from Aim 1 suggest that the real-world spatial hearing deficits resulting from UHL depend on the UHL type, degree, and/or duration, as well as characteristics of the listening environment. Findings from Aim 2 provide new insight into the spatial hearing cues used by children with UHL to localize sounds. Under Aim 3, results suggest that parent report of children’s functional hearing is associated with SRM for children with UHL. The findings from this dissertation provide a basis for our understanding of the deficits and individual differences observed in children with UHL.Doctor of Philosoph