Heath G. Jones

Biophysics of directional hearing in the American alligator (Alligator mississippiensis)

Physiological and anatomical studies have suggested that alligators have unique adaptations for spatial hearing. Sound localization cues are primarily generated by the filtering of sound waves by the head. Different vertebrate lineages have evolved external and/or internal anatomical adaptations to enhance these cues, such as pinnae and interaural canals. It has been hypothesized that in alligators, directionality may be enhanced via the acoustic coupling of middle ear cavities, resulting in a pressure difference receiver (PDR) mechanism. The experiments reported here support a role for a PDR mechanism in alligator sound localization by demonstrating that (1) acoustic space cues generated by the external morphology of the animal are not sufficient to generate location cues that match physiological sensitivity, (2) continuous pathways between the middle ears are present to provide an anatomical basis for coupling, (3) the auditory brainstem response shows some directionality, and (4) eardrum movement is directionally sensitive. Together, these data support the role of a PDR mechanism in crocodilians and further suggest this mechanism is a shared archosaur trait, most likely found also in the extinct dinosaurs.

Investigating long-term effects of cochlear implantation in single-sided deafness: a best practice model for longitudinal assessment of spatial hearing abilities and tinnitus handicap.

Objectives To evaluate methods for measuring long-term benefits of cochlear implantation in a patient with single-sided deafness (SSD) with respect to spatial hearing and to document improved quality of life because of reduced tinnitus. Patient A single adult male with profound right-sided sensorineural hearing loss and normal hearing in the left ear who underwent right-sided cochlear implantation. Methods The subject was evaluated at 6, 9, 12, and 18 months after implantation on speech intelligibility with specific target-masker configurations, sound localization accuracy, audiologic performance, and tinnitus handicap. Testing conditions involved the acoustic (NH) ear only, the cochlear implant (CI) ear (acoustic ear plugged), and the bilateral condition (CI+NH). Measures of spatial hearing included speech intelligibility improvement because of spatial release from masking (SRM) and sound localization. In addition, traditional measures known as “head shadow,” “binaural squelch,” and “binaural summation” were evaluated. Results The best indicator for improved speech intelligibility was SRM, in which both ears are activated, but the relative locations of target and masker(s) are manipulated. Measures that compare performance with a single ear to performance using bilateral auditory input indicated evidence of the ability to integrate inputs across the ears, possibly reflecting early binaural processing, with 12 months of bilateral input. Sound localization accuracy improved with addition of the implant, and a large improvement with respect to tinnitus handicap was observed. Conclusion Cochlear implantation resulted in improved sound localization accuracy when compared with performance using only the NH ear, and reduced tinnitus handicap was observed with use of the implant. The use of SRM addresses some of the current limitations of traditional measures of spatial and binaural hearing, as spatial cues related to target and maskers are manipulated, rather than the ear(s) tested. Sound testing methods and calculations described here are therefore recommended for assessing performance of a larger sample size of individuals with SSD who receive a CI.

Comparing sound localization deficits in bilateral cochlear-implant users and vocoder simulations with normal-hearing listeners.

Bilateral cochlear-implant (BiCI) users are less accurate at localizing free-field (FF) sound sources than normal-hearing (NH) listeners. This performance gap is not well understood but is likely due to a combination of compromises in acoustic signal representation by the two independent speech processors and neural degradation of auditory pathways associated with a patient’s hearing loss. To exclusively investigate the effect of CI speech encoding on horizontal-plane sound localization, the present study measured sound localization performance in NH subjects listening to vocoder processed and nonvocoded virtual acoustic space (VAS) stimuli. Various aspects of BiCI stimulation such as independently functioning devices, variable across-ear channel selection, and pulsatile stimulation were simulated using uncorrelated noise (Nu), correlated noise (N0), or Gaussian-enveloped tone (GET) carriers during vocoder processing. Additionally, FF sound localization in BiCI users was measured in the same testing environment for comparison. Distinct response patterns across azimuthal locations were evident for both listener groups and were analyzed using a multilevel regression analysis. Simulated implant speech encoding, regardless of carrier, was detrimental to NH localization and the GET vocoder best simulated BiCI FF performance in NH listeners. Overall, the detrimental effect of vocoder processing on NH performance suggests that sound localization deficits may persist even for BiCI patients who have minimal neural degradation associated with their hearing loss and indicates that CI speech encoding plays a significant role in the sound localization deficits experienced by BiCI users.

Effect of multi-electrode configuration on sensitivity to interaural timing differences in bilateral cochlear-implant users

Recent psychophysical studies in bilateral cochlear implant users have shown that interaural timing difference (ITD) sensitivity with electrical stimulation varies depending on the place of stimulation along the cochlear array. While these studies have measured ITD sensitivity at single electrode places separately, it is important to understand how ITD sensitivity is affected when multiple electrodes are stimulated together because multi-electrode stimulation is required for representation of complex sounds. Multi-electrode stimulation may lead to poorer overall performance due to interference from places with poor ITD sensitivity, or from channel interaction due to electrical current spread. Alternatively, multi-electrode stimulation might result in overall good sensitivity if listeners can extract the most reliable ITD cues available. ITD just noticeable differences (JNDs) were measured for different multi-electrode configurations. Results showed that multi-electrode ITD JNDs were poorer than ITD JNDs for the best single-electrode pair. However, presenting ITD information along the whole array appeared to produce better sensitivity compared with restricting stimulation to the ends of the array, where ITD JNDs were comparable to the poorest single-electrode pair. These findings suggest that presenting ITDs in one cochlear region only may not be optimal for maximizing ITD sensitivity in multi-electrode stimulation.

Evidence for a neural source of the precedence effect in sound localization

Normal-hearing human listeners and a variety of studied animal species localize sound sources accurately in reverberant environments by responding to the directional cues carried by the first-arriving sound rather than spurious cues carried by later-arriving reflections, which are not perceived discretely. This phenomenon is known as the precedence effect (PE) in sound localization. Despite decades of study, the biological basis of the PE remains unclear. Though the PE was once widely attributed to central processes such as synaptic inhibition in the auditory midbrain, a more recent hypothesis holds that the PE may arise essentially as a by-product of normal cochlear function. Here we evaluated the PE in a unique human patient population with demonstrated sensitivity to binaural information but without functional cochleae. Users of bilateral cochlear implants (CIs) were tested in a psychophysical task that assessed the number and location(s) of auditory images perceived for simulated source-echo (lead-lag) stimuli. A parallel experiment was conducted in a group of normal-hearing (NH) listeners. Key findings were as follows: 1) Subjects in both groups exhibited lead-lag fusion. 2) Fusion was marginally weaker in CI users than in NH listeners but could be augmented by systematically attenuating the amplitude of the lag stimulus to coarsely simulate adaptation observed in acoustically stimulated auditory nerve fibers. 3) Dominance of the lead in localization varied substantially among both NH and CI subjects but was evident in both groups. Taken together, data suggest that aspects of the PE can be elicited in CI users, who lack functional cochleae, thus suggesting that neural mechanisms are sufficient to produce the PE.

Postnatal development of cochlear microphonic and compound action potentials in a precocious species, Chinchilla lanigera

The development of sound-evoked responses in Chinchilla lanigera was studied from postnatal ages P0-1 (first 24 h) to adult. Cochlear microphonic (CMs) and compound action potentials (CAPs), representing ensemble sound-evoked activities of hair cells and auditory nerve fibers, respectively, were present as early as age P0-1. The data indicate that CM thresholds and sensitivities were generally adult-like (i.e., fall into adult ranges) at birth, but suprathreshold CM amplitudes remained below adult ranges through P28. CAP thresholds reached adult-like values between P7-P14, but the suprathreshold CAP amplitude continued to increase until ∼P28. The results confirm the auditory precociousness of the chinchilla.

The Effect of Microphone Placement on Interaural Level Differences and Sound Localization Across the Horizontal Plane in Bilateral Cochlear Implant Users.

Objective: This study examined the effect of microphone placement on the interaural level differences (ILDs) available to bilateral cochlear implant (BiCI) users, and the subsequent effects on horizontal-plane sound localization. Design: Virtual acoustic stimuli for sound localization testing were created individually for eight BiCI users by making acoustic transfer function measurements for microphones placed in the ear (ITE), behind the ear (BTE), and on the shoulders (SHD). The ILDs across source locations were calculated for each placement to analyze their effect on sound localization performance. Sound localization was tested using a repeated-measures, within-participant design for the three microphone placements. Results: The ITE microphone placement provided significantly larger ILDs compared to BTE and SHD placements, which correlated with overall localization errors. However, differences in localization errors across the microphone conditions were small. Conclusions: The BTE microphones worn by many BiCI users in everyday life do not capture the full range of acoustic ILDs available, and also reduce the change in cue magnitudes for sound sources across the horizontal plane. Acute testing with an ITE placement reduced sound localization errors along the horizontal plane compared to the other placements in some patients. Larger improvements may be observed if patients had more experience with the new ILD cues provided by an ITE placement.

Human middle-ear muscles rarely contract in anticipation of acoustic impulses

The Auditory Hazard Assessment Algorithm for Humans (AHAAH) is an electrical equivalence model of the human ear designed to predict the risk for auditory injury from a given impulse noise exposure. One concern with the model is that the middle-ear muscle contraction (MEMC) associated with the acoustic reflex is implemented as a protective mechanism for certain instances in which a person is “warned” prior to the impulse. The current study tested the assumption that the MEMC can be elicited by a conditioning stimulus prior to sound exposure (i.e., a “warned” response). In order to quantify the MEMC, we used laser-Doppler vibrometry to measure tympanic membrane motion in response to a reflex-eliciting acoustic impulse. After verifying the MEMC, we attempted to classically condition the response by pairing the reflex-eliciting acoustic impulse (unconditioned stimulus, UCS) with various preceding stimuli (conditioned stimulus, CS). Changes in the magnitude and/or time-course of the MEMC following repeated UCS...

Mixed stimulation rates to improve sensitivity of interaural timing differences in bilateral cochlear implant listeners

Normal hearing listeners extract small interaural time differences (ITDs) and interaural level differences (ILDs) to locate sounds and segregate targets from noise. Bilateral cochlear implant listeners show poor sensitivity to ITDs when using clinical processors. This is because common clinical stimulation approaches use high rates [∼1000 pulses per-second (pps)] for each electrode in order to provide good speech representation, but sensitivity to ITDs is best at low rates of stimulation (∼100-300 pps). Mixing rates of stimulation across the array is a potential solution. Here, ITD sensitivity for a number of mixed-rate configurations that were designed to preserve speech envelope cues using high-rate stimulation and spatial hearing using low rate stimulation was examined. Results showed that ITD sensitivity in mixed-rate configurations when only one low rate electrode was included generally yielded ITD thresholds comparable to a configuration with low rates only. Low rate stimulation at basal or middle regions on the electrode array yielded the best sensitivity to ITDs. This work provides critical evidence that supports the use of mixed-rate strategies for improving ITD sensitivity in bilateral cochlear implant users.

Anticipatory middle ear muscle contractions in damage-risk criteria

Anticipatory middle ear muscle contractions (MEMC) have been implemented as protective components of Damage-Risk Criteria (DRC) for impulsive noises. However, no studies have shown that anticipatory MEMC are pervasive among humans. This presentation describes a series of studies of the viability of assumed anticipatory MEMC obtained either through classical conditioning or while operating a model gun. Participants were adults with normal hearing, and the conditioning tasks varied on sensory modality and attention. Both between- and within-subjects designs were used. A conditioned response was defined as an MEMC occurring prior to the unconditioned stimulus and when only the conditioning stimulus was presented. These studies do not suggest that anticipatory MEMC should be included in DRC for impulsive noises.