Representation of Waveform Periodicity in the Auditory Midbrain of the Bullfrog, Rana catesbeiana

The period of complex signals is encoded in the bullfrog’s eighth nerve by a synchrony code based on phase-locked responding. We examined how these arrays of phase-locked activity are represented in different subnuclei of the auditory midbrain, the torus semicircularis (TS). Recording sites in different areas of the TS differ in their ability to synchronize to the envelope of complex stimuli, and these differences in synchronous activity are related to response latency. Cells in the caudal principal nucleus (cell sparse zone) have longer latencies, and show little or no phase-locked activity, even in response to low modulation rates, while some cells in lateral areas of the TS (magnocellular nucleus, lateral part of principal nucleus) synchronize to rates as high as 90–100 Hz. At midlevels of the TS, there is a lateral-to-medial gradient of synchronization ability: cells located more laterally show better phase-locking than those located more medially. Pooled all-order interval histograms from short latency cells located in the lateral TS represent the waveform periodicity of a biologically relevant complex harmonic signal at different stimulus levels, and in a manner consistent with behavioral data from vocalizing male frogs. Long latency cells in the caudal parts of the TS (cell sparse zone, caudal magnocellular nucleus) code stimulus period by changes in spike rate, rather than by changes in synchronized activity. These data suggest that neural codes based on rate processing and time domain processing are represented in anatomically different areas of the TS. They further show that a population-based analysis can increase the precision with which temporal features are represented in the central auditory system.     

Binaural sensitivity to temporal fine structure and lateralization ability in children with suspected (central) auditory processing disorder

Objective

Previous studies have shown that a subgroup of children with suspected (central) auditory processing disorder (SusCAPD) have insufficient ability to use binaural cues to benefit from spatial processing. Thus, they experience considerable listening difficulties in challenging auditory environments, such as classrooms. Some researchers have also indicated the probable role of binaural temporal fine structure (TFS) in the perceptual segregation of target signal from noise and hence in speech perception in noise. Therefore, in the present study, in order to further investigate the underlying reason for listening problems against background noise in this group of children, their performance was measured using binaural TFS sensitivity test (TFS-LF) as well as behavioral auditory lateralization in noise test, both of which are based on binaural temporal cues processing.

Effects of age and background noise on processing a mistuned harmonic in an otherwise periodic complex sound

Older adults presented with short (i.e., 40 ms) harmonic complex tones show a reduced likelihood of hearing the mistuned harmonic as a separate sound. Here, we examined whether this age difference for the mistuned harmonic would generalize to a longer signal duration (i.e., 200 ms). We measured auditory evoked fields (AEFs) using magnetoencephalography while young and older adults were presented with harmonic complex tones that either had all partials of the tones in tune (single sound object) or contained a 4 or 16% mistuned harmonic (dual sound objects). The auditory stimuli were presented in isolation or embedded in low or moderate levels of continuous white noise. For each participant, we modeled the AEFs with a pair of dipoles in the superior temporal plane and examined the effects of age and noise on the amplitude and latency of the resulting source waveforms. The present study reveals similar noise-induced increases in N1m and object-related negativity in young and older adults which may be mediated via efferent feedback connections and/or changes in the temporal window of integration. We observed less age-related differences in concurrent sound segregation for stimuli that matched the duration of the temporal integration window of auditory perception (i.e., ～200 ms) than for short duration sounds (i.e., 40 ms). Possible explanations for this duration-dependent age-related decline in concurrent sound perception are a general slowing in auditory processing and/or lengthening of the temporal integration window.     

Temporal effects in simultaneous pure-tone masking: effects of signal frequency, masker/signal frequency ratio, and masker level

The effect of the temporal relationship between a pure-tone masker and a pure-tone signal in simultaneous masking was investigated in three experiments. The experiments extend previous work by: studying the temporal effect over a wide range of signal frequencies, studying the change in masking over time for several masker/signal frequency ratios, and studying the growth of masking for a brief signal at different temporal positions within a longer duration masker. In the first experiment, threshold was measured for a 20-ms signal temporally centered in a masker whose duration ranged from 20 ms to continuous. Signal frequency (fs) was 0.5, 1.0, 2.0, 4.0, or 8.0 kHz; masker frequency (fm) was 1.2 fs. For all signal frequencies, the amount of masking decreased as masker duration increased. In the second experiment, threshold was measured for a 20-ms, 1.0-kHz signal as a function of the signal's temporal position within a 400-ms masker whose frequency ranged from 1.0 to 1.25 kHz. For all but the 1.0-kHz masker, for which threshold was almost independent of the signal's temporal position, threshold decreased as signal onset was delayed relative to masker onset, but then increased slightly as the signal approached masker offset. In the final experiment, growth-of-masking functions were measured for a 20-ms, 1.0-kHz signal positioned at the beginning, at the temporal center, or at the end of a 400-ms masker whose frequency was 1.20 or 1.25 kHz. The masking functions generally were steepest for a signal at the onset of the masker and, for a given temporal position, steepest for the 1.20-kHz masker.     

The effects of age on temporal fine structure sensitivity in monaural and binaural conditions

Abstract

Objective: To extend the study of Hopkins and Moore (2011) by examining the effect of age in the medium age range on sensitivity to temporal fine structure (TFS), which is assumed to be represented in the patterns of phase locking in the auditory nerve. Design: Monaural TFS sensitivity was assessed using the TFS1 test (Moore & Sek, 2009) at centre frequencies of 850 and 2000 Hz, and binaural TFS sensitivity was assessed using the TFS-LF test (Hopkins & Moore, 2010a) at centre frequencies of 500 and 850 Hz, using a sensation level of 30 dB. Study sample: Thirty-five newly recruited normal-hearing subjects (thresholds better than 20 dB HL from 250 to 6000 Hz) were tested. Their ages ranged from 22 to 61 years. Results: There was a significant correlation between age and TFS sensitivity at all frequencies for both TFS tests. For the single centre frequency (850 Hz) that was used for both tests, scores for the two tests were modestly but significantly correlated. Conclusions: Sensitivity to TFS decreases with increasing age. The monaural and binaural TFS tests appear to reflect at least somewhat distinct auditory processes.     

Acoustic Temporal Modulation Detection in Normal-Hearing and Cochlear Implanted Listeners: Effects of Hearing Mechanism and Development

Temporal modulation detection ability matures over many years after birth and may be particularly sensitive to experience during this period. Profound hearing loss during early childhood might result in greater perceptual deficits than a similar loss beginning in adulthood. We tested this idea by measuring performance in temporal modulation detection in profoundly deaf children and adults fitted with cochlear implants (CIs). At least two independent variables could constrain temporal modulation detection performance in children with CIs: altered encoding of modulation information due to the CI-auditory nerve interface, and atypical development of central processing of sound information provided by CIs. The effect of altered encoding was investigated by testing subjects with one of two different hearing mechanisms (normal hearing vs. CI) and the effect of atypical development was studied by testing two different age groups. All subjects were tested for their ability to detect acoustic temporal modulations of sound amplitude. A comparison of the slope, or cutoff frequency, of the temporal modulation transfer functions (TMTFs) among the four subject groups revealed that temporal resolution was mainly constrained by hearing mechanism: normal-hearing listeners could detect smaller amplitude modulations at high modulation frequencies than CI users. In contrast, a comparison of the height of the TMTFs revealed a significant interaction between hearing mechanism and age group on overall sensitivity to temporal modulation: sensitivity was significantly poorer in children with CIs, relative to the other three groups. Results suggest that there is an age-specific vulnerability of intensity discrimination or non-sensory factors, which subsequently affects sensitivity to temporal modulation in prelingually deaf children who use CIs.     

The Role of Temporal Fine Structure Processing in Pitch Perception,Masking, and Speech Perception for Normal-Hearing and Hearing-Impaired People

Complex broadband sounds are decomposed by the auditory filters into a series of relatively narrowband signals, each of which can be considered as a slowly varying envelope (E) superimposed on a more rapid temporal fine structure (TFS). Both E and TFS information are represented in the timing of neural discharges, although TFS information as defined here depends on phase locking to individual cycles of the stimulus waveform. This paper reviews the role played by TFS in masking, pitch perception, and speech perception and concludes that cues derived from TFS play an important role for all three. TFS may be especially important for the ability to "listen in the dips" of fluctuating background sounds when detecting nonspeech and speech signals. Evidence is reviewed suggesting that cochlear hearing loss reduces the ability to use TFS cues. The perceptual consequences of this, and reasons why it may happen, are discussed.     

Response properties of the human frequency-following response (FFR) to speech and non-speech sounds: level dependence,adaptation and phase-locking limits

Abstract

Objective: The frequency-following response (FFR) is a neurophonic potential used to assess auditory neural encoding at subcortical stages. Despite the FFR’s empirical and clinical utility, basic response properties of this evoked potential remain undefined.

Design: We measured FFRs to speech and nonspeech (pure tone, chirp sweeps) stimuli to quantify three key properties of this potential: level-dependence (I/O functions), adaptation and the upper limit of neural phase-locking.

Study sample: n?=?13 normal-hearing listeners.

Results: I/O functions showed FFR amplitude increased with increasing stimulus presentation level between 25 and 80?dB SPL; FFR growth was steeper for tones than speech when measured at the same frequency. FFR latency decreased 4–5?ms with decreasing presentation level from 25 and 80?dB SPL but responses were ～2?ms earlier for speech than tones. FFR amplitudes showed a 50% reduction over 6?min of recording with the strongest adaptation in the first 60?s (250 trials). Estimates of neural synchronisation revealed FFRs contained measurable phase-locking up to ～1200–1300?Hz, slightly higher than the single neuron limit reported in animal models.

Conclusions: Findings detail fundamental response properties that will be important for using FFRs in clinical and empirical applications.     

Clinical assessment of balance: normative data, and gender and age effects

The purpose of this study was to provide age specific normative data of clinical gait and balance tests and to determine to what extent gender contributes to differences in postural control. Standing balance and walking performance was tested in 318 asymptomatic adults. The logistic regression, using both 10- and 30-second time limits as a dichotomization point, revealed a significant age effect for standing on foam with eyes closed, tandem Romberg with eyes closed (TR-EC), and one leg stance (eyes open and closed). The actual effect of decline was different for each test. Both tandem gait and dynamic gait index showed a ceiling effect up to 60 years of age, with a rapid decline of performance for subjects in their seventies. Linear regression equations indicated that for both men and women, timed up and go test (TUG) times increased with age, but even older subjects should perform the TUG in 10 seconds or less. Women performed significantly poorer on the TUG and TR-EC (30-second time limit).     

Effect of signal processing strategy and stimulation type on speech and auditory perception in adult cochlear implant users

Objective: The objective of this study was to investigate the effects of signal processing strategy on speech understanding and auditory function for adult cochlear implant (CI) users with a focus on the effects of sequential versus paired stimulation.

Design: Within-subjects, repeated measures design was utilised to compare performance between processing strategies and stimulation type on various measures of auditory function and subjective sound quality. Testing with subsequent strategies was completed after a total familiarisation time of two weeks.

Study Sample: Ten post-lingually deafened adult CI users were recruited from a clinical population. Participants had a minimum of 13?months CI experience. Ages ranged from 25–78?years. All participants had long-term experience with the optima strategy; eight with sequential stimulation and two with paired stimulation.

Results: We found no statistically significant effect of processing strategy. We observed an effect of stimulation type with sequential stimulation yielding significantly higher performance than paired stimulation for speech understanding in quiet and in noise, and subjective estimates of sound quality. No significant differences were noted across strategy or stimulation for music perception, spectral resolution or temporal resolution.

Conclusions: Many patients utilise paired stimulation – the default stimulation type in the clinical software; however, sequential stimulation yielded significantly higher outcomes on multiple measures.     

Neuronal populations in the gerbil PVCN: effects of age, hearing status and microcysts

This study was designed to test the hypothesis that microcysts in the gerbil auditory system are formed from neuronal somata. Six neuronal types (octopus, multipolar, bushy, elongate, miscellaneous and small) were distinguished, counted and measured along with the microcysts in the posteroventral cochlear nuclei (PVCNs) of 3, 12 and 36 month old gerbils. No decrease was observed in the numbers of neurons in any neuronal class, or in the neuronal population as a whole, in the PVCN of the gerbil as a function of age. Neither was any change observed in the PVCN area occupied by non-neuronal, non-microcyst elements. Neuronal sizes were unchanged between 3 and 12 months, but multipolar and bushy cells, as well as the total neuronal population decreased significantly in size between 12 and 36 months. The number and size of microcysts increased significantly between 3 and 12 months of age and accounts for increases in PVCN volume. The number and size of microcysts decreased significantly between 12 and 36 months. Thus, the appearance of microcysts can not result from the selective loss of any single class of neurons. Hearing was assessed in five 36 month old animals with auditory brainstem responses (ABR) and number and size of microcysts were found to correlate with hearing status, being largest and most numerous in animals with the best hearing, and smallest and fewest in the deaf animal. It is concluded that microcysts cannot represent a neurodegenerative disease of neuronal somata. Microcyst formation appears to be a dynamic process related to the degree of auditory stimulation.     

Rapid word-learning in normal-hearing and hearing-impaired children: effects of age, receptive vocabulary, and high-frequency amplification

OBJECTIVE: This study examined rapid word-learning in 5- to 14-year-old children with normal and impaired hearing. The effects of age and receptive vocabulary were examined as well as those of high-frequency amplification. Novel words were low-pass filtered at 4 kHz (typical of current amplification devices) and at 9 kHz. It was hypothesized that (1) the children with normal hearing would learn more words than the children with hearing loss, (2) word-learning would increase with age and receptive vocabulary for both groups, and (3) both groups would benefit from a broader frequency bandwidth. DESIGN: Sixty children with normal hearing and 37 children with moderate sensorineural hearing losses participated in this study. Each child viewed a 4-minute animated slideshow containing 8 nonsense words created using the 24 English consonant phonemes (3 consonants per word). Each word was repeated 3 times. Half of the 8 words were low-pass filtered at 4 kHz and half were filtered at 9 kHz. After viewing the story twice, each child was asked to identify the words from among pictures in the slide show. Before testing, a measure of current receptive vocabulary was obtained using the Peabody Picture Vocabulary Test (PPVT-III). RESULTS: The PPVT-III scores of the hearing-impaired children were consistently poorer than those of the normal-hearing children across the age range tested. A similar pattern of results was observed for word-learning in that the performance of the hearing-impaired children was significantly poorer than that of the normal-hearing children. Further analysis of the PPVT and word-learning scores suggested that although word-learning was reduced in the hearing-impaired children, their performance was consistent with their receptive vocabularies. Additionally, no correlation was found between overall performance and the age of identification, age of amplification, or years of amplification in the children with hearing loss. Results also revealed a small increase in performance for both groups in the extended bandwidth condition but the difference was not significant at the traditional p = 0.05 level. CONCLUSIONS: The ability to learn words rapidly appears to be poorer in children with hearing loss over a wide range of ages. These results coincide with the consistently poorer receptive vocabularies for these children. Neither the word-learning or receptive-vocabulary measures were related to the amplification histories of these children. Finally, providing an extended high-frequency bandwidth did not significantly improve rapid word-learning for either group with these stimuli.     

The cardiac component of the orienting response: effects of signal intensity, clinical interpretation, and procedural limitations.

The effects of stimulus intensity on the cardiac component of the orienting response were explored in male and female subjects. The acoustic stimuli were 10 recorded repetitions of an identical consonant vowel cluster spoken by an adult male. Heart rate data were obtained for both subject groups at three sensation levels across the baseline heart rate and at 10 stimulus presentation trials. A significant main effect was noted for trials. Significant linear and quadratic trend components were observed, reflecting habituation of the orienting response as a function of trials, and indicating the nonmonotonicity of heart rate change over trials. A lower mean heart rate was observed for Condition Two (20 dB SL) than for either of the other conditions, 10 dB and 30 dB SL respectively. Analysis of group variability and individual subjects' heart rate data suggested limitations in generalizing from group to individual data, as well as a need for further investigation if proper interpretation of single subject heart rate data are to be useful in the clinical situation.     

Learning to localize a broadband tonal complex signal with advanced hearing protectors and TCAPS: the effectiveness of training on open-ear vs. device-occluded performance

Abstract

(1) determine if untrained, normal-hearing listeners could learn, with regimented practice trials, to localise a dissonant tonal complex while wearing electronic hearing protectors and achieve equivalent performance to the unoccluded ear, (2) determine if different protector designs affect the localisation learning curve, and (3) determine if a sequential training regimen could be used to determine whether a device is not amenable to training effects. All subjects completed paired training and testing trials, in 12 Learning Units (LU), first with the open ear., then half with device A, a US Army TCAPS (INVISIO^® X50), and the other half with device B (a prototype). Both groups then finished with the opposite device. Ten subjects participated, with thresholds below 25 dBHL, bilateral symmetry below 15 dBHL, and naivety with electronic protectors and localisation testing. Subjects eventually approached their open ear capability with device A, but never reached equivalent open ear nor asymptotic performance with device B. Depending upon a protector’s sound transduction, frequency response, and other characteristics, different amounts of training are required to adapt. The training protocol has value in determining device acceptability based on the training burden required, especially when mission-related localisation performance is important.     

The Davis theory: A review,and implications of recent electrophysiological evidence

The Davis theory of mechano-electrical transduction asserts that the endocochlear potential and the hair cell resting potential summate to provide a driving force for current flow through the hair cell. However, while a variety of agents which depress the endocochlear potential simultaneously depress auditory nerve sensitivity and reduce the cochlear microphonic, recent reports suggest that the hair cells may be depolarised without such effects ensuing. The relevant literature is reviewed.     

Proximity of carotid canal wall to tympanic membrane: a human temporal bone study

OBJECTIVE: The objective was to examine the possible risk of injury to the internal carotid artery during procedures in the middle ear, including myringotomy. STUDY DESIGN: Histopathological morphometric study of human temporal bones. METHODS: One hundred forty-two human temporal bone specimens obtained from 92 individuals without any known ear disease were prepared for light microscopic study. Using 83 bones that were available for measurement, the thickness of the carotid canal wall (CCW), which is the medial wall of the bony portion of the eustachian tube, was measured. Using 15 bones selected for three-dimensional measurement, the closest distance from CCW to the anterior tympanic annulus was measured. Using all 142 temporal bone specimens, the CCW was examined to detect the presence of partial dehiscence. In one case, the images of CCW dehiscence and its surrounding structures were reconstructed by a personal computer. RESULTS: The thickness of the CCW was 0.00 to 0.73 mm (average thickness, 0.24 mm [+/-0.12 mm]). The distance from the CCW to the anterior tympanic annulus was 1.8 to 8.1 mm (average distance, 4.9 [+/-1.7 mm]). Dehiscence of CCW was observed in 7 (4.9%) of 142 temporal bone specimens. The reconstructed image showed that the posterior half of the dehiscence of CCW could be seen from the external ear canal. CONCLUSIONS: The CCW was found to be extremely thin or even dehiscent in some cases, rendering the internal carotid artery vulnerable during transtympanic procedures. The study's findings emphasized the need for judicious care when operating in the anterior mesotympanum.