Development of the auditory brainstem of birds: comparison between barn owls and chickens

Birds have proved to be extremely useful models for the study of hearing function. In particular, chickens and barn owls have been widely used by a number of researchers to study diverse aspects of auditory function. These studies have benefited from the advantages offered by each of these two species, including differences of auditory specialization. Direct comparisons between chickens and barn owls become complicated when the degree of auditory specialization and their modes of development are brought into consideration. In this article we review the available literature on the development of the auditory brainstem of chickens and barn owls in the context of such differences. In addition, we present a time line constructed on the basis of common stages of structural differentiation, rather than chronological time. We suggest that such a time line should be considered when discussing comparative data between these two species. Such an approach should facilitate the interpretation of similarities and differences observed in the developmental processes of the auditory system of chickens and barn owls.     

Acoustic cues to the voicing feature in tracheoesophageal speech.

Tracheoesophageal (TE) speakers often have difficulty producing the voiced/voiceless distinction. This limitation has been attributed to use of the pharyngoesophageal segment as the phonatory source. The nature of this tissue may preclude precise control of voicing onset, a contributing cue to a phoneme's voicing feature, at least in laryngeal speech. The purpose of this study was to determine whether voiced and voiceless consonants produced by TE speakers could be differentiated from those produced by laryngeal speakers using four acoustic measures associated with the voicing characteristic of consonants in laryngeal speech. Sixteen TE and ten laryngeal speakers produced five stop and fricative cognate pairs embedded in a carrier phrase. Three of the four acoustic measures contributed significantly to the discriminant models that differentiated accurately perceived TE and laryngeal samples. The three variables were consonant sound pressure level, consonant duration, and preceding vowel duration. In general, values for each measure were higher/longer for the TE group. The discriminant functions were interpreted as a reflection of TE speaker attempts at overarticulation.     

Acoustic trauma-induced auditory cortex enhancement and tinnitus

There is growing evidence suggests that noise-induced cochlear damage may lead to hyperexcitability in the central auditory system (CAS) which may give rise to tinnitus. However, the correlation between the onset of the neurophysiological changes in the CAS and the onset of tinnitus has not been well studied. To investigate this relationship, chronic electrodes were implanted into the auditory cortex (AC) and sound evoked activities were measured from awake rats before and after noise exposure. The auditory brainstem response (ABR) was used to assess the degree of noise-induced hearing loss. Tinnitus was evaluated by measuring gap-induced prepulse inhibition (gap-PPI). Rats were exposed monaurally to a high-intensity narrowband noise centered at 12 kHz at a level of 120 dB SPL for 1 h. After the noise exposure, all the rats developed either permanent (>2 weeks) or temporary (<3 days) hearing loss in the exposed ear(s). The AC amplitudes increased significantly 4 h after the noise exposure. Most of the exposed rats also showed decreased gap-PPI. The post-exposure AC enhancement showed a positive correlation with the amount of hearing loss. The onset of tinnitus-like behavior was happened after the onset of AC enhancement.     

Acoustic reflex dynamics and auditory brain stem responses in multiple sclerosis

Onset latency, rise time, and amplitude measures of the acoustic reflex and auditory brain stem responses (ABRs) to clicks were elicited from normal subjects and subjects with multiple sclerosis (MS). The results indicated that MS subjects exhibited response abnormalities including prolonged rise times of the acoustic reflex and prolonged absolute latencies, interwave latencies and interaural latencies, and poor response stability of the ABR. The results of this study indicate that measurement of acoustic reflex dynamics can add to the diagnostic value of the test beyond measurement of acoustic reflex threshold and decay. The combination of complete acoustic reflex and ABR tests is a valuable contribution to the diagnosis of patients with MS who have no auditory symptoms.     

Spectral contrasts underlying auditory stream segregation in goldfish (Carassius auratus)

This study investigates the effects of spectral separation of sounds on the ability of goldfish to acquire independent information about two simultaneous complex sources. Goldfish were conditioned to a complex sound made up of two sets of repeated acoustic pulses: a high-frequency pulse with a spectral envelope centered at 625 Hz, and a low-frequency pulse type centered at 240, 305, 390, or 500 Hz. The pulses were presented with each pulse type alternating with an overall pulse repetition rate of 40 pulses per second (pps), and a 20-pps rate between identical pulses. Two control groups were conditioned to the 625-Hz pulse alone, repeated at 40 and 20 pps, respectively. All groups were tested for generalization to the 625-Hz pulse repeated alone at several rates. If the two pulse types in the complex resulted in independent auditory streams, the animals were expected to generalize to the 625-Hz pulse trains as if they were repeated at 20 pps during conditioning. It was hypothesized that as the center frequency of the low-frequency pulse approached that of the 625-Hz pulse, the alternating trains would be perceived as a single auditory stream with a repetition rate of 40 pps. The group conditioned to alternating 625- and 240-Hz pulses generalized least, with maximum generalization at 20 Hz, suggesting that the animals formed at least one perceptual stream with a repetition rate of 20 pps. The other alternating pulse groups generalized to intermediate degrees. Goldfish can segregate at least one "auditory stream" from a complex mixture of sources. Segregation can be based on spectral envelope and grows more robust with growing spectral separation between the simultaneous sources. Auditory stream segregation and auditory scene analysis are shared among human listeners, European starlings, and goldfish, and may be primitive characteristics of the vertebrate sense of hearing.     

Mechanisms underlying azimuth selectivity in the auditory cortex of the pallid bat

A cochlear implant (CI) signal processing strategy named F0 modulation (F0mod) was compared with the advanced combination encoder (ACE) strategy in a group of four post-lingually deafened Mandarin Chinese speaking CI listeners. F0 provides an enhanced temporal pitch cue by amplitude modulating the multichannel electrical stimulation pattern at the fundamental frequency (F0) of the incoming speech signal. Word and sentence recognition tests were carried out in quiet and in noise. The responses for the word-recognition test were further segmented into phoneme and tone scores. Off-line implementations of ACE and F0mod were used, and electrical stimulation patterns were directly streamed to the CI subject's implant. To focus on the feasibility of enhanced temporal cues for tonal language perception, idealized F0 information that was extracted from speech tokens in quiet was used in the F0mod processing of speech-in-noise mixtures. The results indicated significantly better lexical tone perception with the F0mod strategy than with ACE for the male voice (p<0.05). No significant differences in sentence recognition were found between F0mod and ACE.     

Cortical centres underlying auditory temporal processing in humans: a PET study

We have used positron emission tomography (PET) to test a specific hypothesis of a neural system subserving auditory temporal processing (acoustical stimulus duration discrimination). Maps of the cerebral blood flow distribution during specific stimulations were obtained from five normally-hearing and otherwise healthy subjects. The auditory stimuli consisted of sounds of varying duration and of auditorily presented words in which the duration of the initial phoneme was manipulated. All stimuli alternated with conditions of silence in a subtraction paradigm. The blood flow distribution was mapped with O-15-labelled water. The results demonstrated that stimuli requiring recognizing, memorizing, or attending to specific target sounds during temporal processing generally resulted in significant activation of both frontal lobes and the parietal lobe in the right hemisphere. Based on these results, we hypothesise that a network consisting of anterior and posterior auditory attention and short-term memory sites subserves acoustical stimulus duration perception and analysis (auditory temporal processing).     

Spectral and temporal cues for speech recognition: implications for auditory prostheses

Features of stimulation important for speech recognition in people with normal hearing and in people using implanted auditory prostheses include spectral information represented by place of stimulation along the tonotopic axis and temporal information represented in low-frequency envelopes of the signal. The relative contributions of these features to speech recognition and their interactions have been studied using vocoder-like simulations of cochlear implant speech processors presented to listeners with normal hearing. In these studies, spectral/place information was manipulated by varying the number of channels and the temporal-envelope information was manipulated by varying the lowpass cutoffs of the envelope extractors. Consonant and vowel recognition in quiet reached plateau at 8 and 12 channels and lowpass cutoff frequencies of 16 Hz and 4 Hz, respectively. Phoneme (especially vowel) recognition in noise required larger numbers of channels. Lexical tone recognition required larger numbers of channels and higher lowpass cutoff frequencies. There was a tradeoff between spectral/place and temporal-envelope requirements. Most current auditory prostheses seem to deliver adequate temporal-envelope information, but the number of effective channels is suboptimal, particularly for speech recognition in noise, lexical tone recognition, and music perception.     

Effects of natural versus artificial spatial cues on electrophysiological correlates of auditory motion

The effect of the type of the auditory motion stimulus on neural correlates of motion processing were investigated using high-density electroencephalography. Sound motion was implemented by (a) gradual shifts in interaural time or (b) level difference; (c) motion of virtual 3D sound sources; or (d) successive activation of 45 loudspeakers along the horizontal plane. In a subset of trials, listeners (N = 20) performed a two-alternative forced-choice motion discrimination task. Each trial began with a stationary phase of the acoustic stimulus in a central position, immediately followed by a motion of the stimulus. The motion onset elicited a specific cortical response that was dominated by large negative and positive deflections, the so-called change-N1 and change-P2. The temporal dynamics of these components depended on the auditory motion cues presented: Free-field motion and virtual 3D motion were associated with earlier cortical responses and with shorter reaction times than shifts in interaural time or level. Also, free-field motion elicited much stronger onset responses than simulated motion. These findings suggest that natural-like stimulation using stimuli presented in the free sound field allows more reliable conclusions on neural processing of sound motion, whereas artificial motion stimuli, in particular gradual shifts in interaural time or level, seem to be less suited with respect to this aim.     

Acoustic and electrical pattern analysis of consonant perceptual cues used by cochlear implant users

It is hypothesized that for postlingually deafened adult cochlear implant (CI) users, a significant source of their perceptual performance variability is attributable to differences in their ability to discriminate the basic perceptual cues that are important in speech recognition. Previous research on 'electric hearing' has identified consistent perceptual cues for vowel recognition. However, the results on consonant perception by CI users are less clear. The primary purpose of this study is to present a quantitative method of evaluating potential 'electric cues' used by CI users in consonant identification. Since the actual input signals to the auditory periphery of CI users are electric in nature, we elected to measure the CI electric discharge patterns in addition to the original acoustic waveforms. The characteristics of the electric discharge patterns in response to intervocalic consonants were quantified and correlated with the dimensions of CI patients' perceptual spaces, which were computed from multidimensional scaling analyses of their consonant confusion matrices. The results agree with most, but not all, commonly accepted acoustic cues used by normal-hearing listeners. The correlation findings also suggest that CI users employ different sets of 'electric cues' in perceiving consonants that differ in their manner of articulation. Specifically, spectral and temporal cues associated with slowly changing formant structures and transitions, and features associated with frication and high-frequency noise, are all highly correlated with the perceptual dimensions of all CI users. However, rapidly changing formant transitions, such as those present in stop consonants, did not appear to play a significant role in consonant recognition by more poorly performing CI subjects. The perceptual results were consistent with our physical findings that the SPEAK coding strategy partially degraded the rapidly changing formant transitions.     

Effect of interaural level and phase cues on intervening interference in auditory working memory for loudness

The focus of this study was to gauge the influence of intervening interference on an intensity standard held within auditory working memory through measurement of the just noticeable difference (JND) for intensity. Additionally, the use of interaural phase differences and interaural level differences as spatial cues were employed to identify whether these indicators provided a means for release from interference. A series of tones, both with and without spatial cues, were presented to subjects and responses were obtained using the method of constant stimuli. The JND for intensity was measured in a control condition with a silent inter-comparison interval and three conditions containing intervening tones within the temporal gap between the standard and comparison stimuli. The presence of intervening interference produced a significant increase in the intensity difference needed for discrimination. Further, the provision of spatial cues did not result in a significant release from this interference. These results indicate that a release from interference is not obtained when listeners are required to rely entirely on information used for spatial location (i.e., overall intensity differences and interaural phase/intensity differences) without unique information identifying the sound source to aid in retention of relevant information within auditory working memory.     

Auditory processing disorders: Relationship to cognitive processes and underlying auditory neural integrity

Background

Auditory processing disorder (APD) in children has been reported and discussed in the clinical and research literature for many years yet there remains poor agreement on diagnostic criteria, the relationship between APD and cognitive skills, and the importance of assessing underlying neural integrity.

Purpose

The present study used a repeated measures design to examine the relationship between a clinical APD diagnosis achieved with behavioral tests used in many clinics, cognitive abilities measured with standardized tests of intelligence, academic achievement, language, phonology, memory and attention and measures of auditory neural integrity as measured with acoustic reflex thresholds and auditory brainstem responses.

Method

Participants were 63 children, 7–17 years of age, who reported listening difficulties in spite of normal hearing thresholds. Parents/guardians completed surveys about the child's auditory and attention behavior while children completed an audiologic examination that included 5 behavioral tests of auditory processing ability. Standardized tests that examined intelligence, academic achievement, language, phonology, memory and attention, and objective tests auditory function included crossed and uncrossed acoustic reflex thresholds and auditory brainstem responses (ABR) were also administered to each child.

Results

Forty of the children received an APD diagnosis based on the 5 behavioral tests and 23 did not. The groups of children performed similarly on intelligence measures but the children with an APD diagnosis tended to perform more poorly on other cognitive measures. Auditory brainstem responses and acoustic reflex thresholds were often abnormal in both groups of children.

Summary

Results of this study suggest that a purely behavioral test battery may be insufficient to accurately identify all children with auditory processing disorders. Physiologic test measures, including acoustic reflex and auditory brainstem response tests, are important indicators of auditory function and may be the only indication of a problem. The results also suggest that performance on behavioral APD tests may be strongly influenced by the child's language levels.     

Electrophysiological correlates of azimuth and elevation cues for sound localization in human middle latency auditory evoked potentials

OBJECTIVE: To study, in humans, the effects of sound source azimuth and elevation on primary auditory cortex binaural activity associated with sound localization. DESIGN: Middle Latency Auditory Evoked Potentials (MLAEPs) were recorded from three channels, in response to alternating polarity clicks, presented at a rate of 5/sec, at nine virtual spatial locations with different azimuths and elevations. Equivalent dipoles of Binaural Interaction Components (BICs) of MLAEPs were derived from 15 normally and symmetrically hearing adults by subtracting the response to binaural clicks at each spatial location from the algebraic sum of responses to stimulation of each ear alone. The amplified potentials were averaged over 4000 repetitions using a dwell time of 78 micro sec/address/channel. Variations in magnitudes, latencies and orientations of the dipole equivalents of cortical activity were noted in response to the nine spatial locations. RESULTS: Middle-latency BICs included six major components corresponding in latency to the vertex-neck recorded components of MLAEP. A significant decrease of equivalent dipole magnitude was observed for two of the components: Pa2 in response to clicks in the backward positions (medium and no elevation); and Nb in response to clicks in the back and front positions (medium and no elevation) in the midsagittal plane. In the coronal plane, Pa2 equivalent dipole magnitude significantly decreased in response to right-horizontal (no elevation) clicks. Significant effects on equivalent dipole latencies of Pa2 were found for backward positions (no elevation) in the midsagittal plane. No significant effects on Pa2 and Nb equivalent dipole orientations were found across stimulus conditions. CONCLUSIONS: The changes in equivalent dipole magnitudes and latencies of MLAEP BICs across stimulus conditions may reflect spectral tuning in binaural primary auditory cortex neurons processing the frequency cues for sound localization.     

Some neural mechanisms in the cat's auditory cortex underlying sensitivity to combined tone and wide-spectrum noise stimuli

In the auditory cortex of nitrous oxide-anesthetized, muscle-relaxed cats, single neurons were studied for their responsiveness to pure tones that were mixed acoustically with simultaneously gated wide-spectrum noise bursts presented using a calibrated sealed stimulating system. The intensities of both the tone and the noise were systematically varied, with a view to ascertaining the sensitivity of cortical cells to a characteristic frequency tone delivered in the presence of a noise mask. Neurons for which wide-spectrum noise provided a net excitatory influence typically displayed a 'strong-signal capture' effect; that is, the cell's responses were dominated by whichever of the two elements of the combined stimulus was the more effective when tested separately. These cells generally had monotonic tone rate intensity functions. Most of the cells that were suppressed by the noise displayed nonmonotonic pure tone rate intensity functions. When nonmonotonic cells were studied with the combined stimulus, the noise was found to produce an intensity-dependent suppression of their tone-evoked responses that could not be overcome by elevating the tone intensity. In contrast, for the minority of monotonic neurons whose tone-evoked responses were suppressed by noise, that suppression could be overcome by raising the tone intensity. None of the cells in the sample responded in a sustained fashion to continuous noise. In each of 11 cases examined, the effect of a continuous noise mask was to elevate tone thresholds and to prolong latent periods for tones; the magnitude of both of these effects depended on the intensity of the continuous noise mask.     

Acoustic schwannoma presenting as a tumor of the external auditory canal. Case report

An acoustic neurinoma involving the internal auditory canal, the vestibule, the cochlea, the middle ear, and extending into the cerebellopontine angle and the external auditory canal, is described in a 56-year-old woman. An initial episode of vertigo was followed by a 27-year history of progressive unilateral hearing loss leading to complete deafness and areflexia with central compensation. The tumor was removed by a two-step surgical procedure, and the histologic features were those of a schwannoma.