Academic literature on the topic 'Cochlear implants. Hearing impaired Neural stimulation. Speech perception'

The modern cochlear implant (CI) is the most successful neural prosthesis developed to date. CIs provide hearing to the profoundly hearing impaired and allow the acquisition of spoken language in children born deaf. Results from studies enabled by the CI have provided new insights into ( a) minimal representations at the periphery for speech reception, ( b) brain mechanisms for decoding speech presented in quiet and in acoustically adverse conditions, ( c) the developmental neuroscience of language and hearing, and ( d) the mechanisms and time courses of intramodal and cross-modal plasticity. Additionally, the results have underscored the interconnectedness of brain functions and the importance of top-down processes in perception and learning. The findings are described in this review with emphasis on the developing brain and the acquisition of hearing and spoken language.

Background: Cochlear implants have shown vast improvements in speech understanding for those with severe to profound hearing loss; however, music perception remains a challenge for electric hearing. It is unclear whether the difficulties arise from limitations of sound processing, the nature of a damaged auditory system, or a combination of both. Purpose: To examine music perception performance with different acoustic and electric hearing configurations. Research Design: Chord discrimination and timbre perception were tested in subjects representing four daily-use listening configurations: unilateral cochlear implant (CI), contralateral bimodal (CIHA), bilateral hearing aid (HAHA) and normal-hearing (NH) listeners. A same-different task was used for discrimination of two chords played on piano. Timbre perception was assessed using a 10-instrument forced-choice identification task. Study Sample: Fourteen adults were included in each group, none of whom were professional musicians. Data Collection and Analysis: The number of correct responses was divided by the total number of presentations to calculate scores in percent correct. Data analyses were performed with Kruskal-Wallis one-way analysis of variance and linear regression. Results: Chord discrimination showed a narrow range of performance across groups, with mean scores ranging between 72.5% (CI) and 88.9% (NH). Significant differences were seen between the NH and all hearing-impaired groups. Both the HAHA and CIHA groups performed significantly better than the CI groups, and no significant differences were observed between the HAHA and CIHA groups. Timbre perception was significantly poorer for the hearing-impaired groups (mean scores ranged from 50.3–73.9%) compared to NH (95.2%). Significantly better performance was observed in the HAHA group as compared to both groups with electric hearing (CI and CIHA). There was no significant difference in performance between the CIHA and CI groups. Timbre perception was a significantly more difficult task than chord discrimination for both the CI and CIHA groups, yet the easier task for the NH group. A significant difference between the two tasks was not seen in the HAHA group. Conclusion: Having impaired hearing decreases performance compared to NH across both chord discrimination and timbre perception tasks. For chord discrimination, having acoustic hearing improved performance compared to electric hearing only. Timbre perception distinguished those with acoustic hearing from those with electric hearing. Those with bilateral acoustic hearing, even if damaged, performed significantly better on this task than those requiring electrical stimulation, which may indicate that CI sound processing fails to capture and deliver the necessary acoustic cues for timbre perception. Further analysis of timbre characteristics in electric hearing may contribute to advancements in programming strategies to obtain optimal hearing outcomes.

This report highlights research projects relevant to binaural and spatial hearing in adults and children. In the past decade we have made progress in understanding the impact of bilateral cochlear implants (BiCIs) on performance in adults and children. However, BiCI users typically do not perform as well as normal hearing (NH) listeners. In this article we describe the benefits from BiCIs compared with a single cochlear implant (CI), focusing on measures of spatial hearing and speech understanding in noise. We highlight the fact that in BiCI listening the devices in the two ears are not coordinated; thus binaural spatial cues that are available to NH listeners are not available to BiCI users. Through the use of research processors that carefully control the stimulus delivered to each electrode in each ear, we are able to preserve binaural cues and deliver them with fidelity to BiCI users. Results from those studies are discussed as well, with a focus on the effect of age at onset of deafness and plasticity of binaural sensitivity. Our work with children has expanded both in number of subjects tested and age range included. We have now tested dozens of children ranging in age from 2 to 14 yr. Our findings suggest that spatial hearing abilities emerge with bilateral experience. While we originally focused on studying performance in free field, where real world listening experiments are conducted, more recently we have begun to conduct studies under carefully controlled binaural stimulation conditions with children as well. We have also studied language acquisition and speech perception and production in young CI users. Finally, a running theme of this research program is the systematic investigation of the numerous factors that contribute to spatial and binaural hearing in BiCI users. By using CI simulations (with vocoders) and studying NH listeners under degraded listening conditions, we are able to tease apart limitations due to the hardware/software of the CI systems from limitations due to neural pathology.

Cochlear implants are now the treatment of choice for patients with severe to profound hearing loss. Inclusion criteria for cochlear implantation have expanded, and a whole array of implantable hearing devices have been introduced over the years. To date, more than 250 cochlear implantations have now been performed in the Philippines (Figure 1). In 2006, the first auditory brainstem implantation, and first vibroplasty or middle ear implantation in the country were done at the Philippine General Hospital (PGH). In 2008, the first electroacoustic stimulation or partial deafness cochlear implantation surgery in the country was performed at the Capitol Medical Center by Professor Joachim Müeller of the University of Würzburg and the author. This concept, that cochlear implantation can be performed for patients with residual hearing or only partial deafness, is quite novel. There are patients whose low frequency hearing below 1.5 kHz is still be quite good while high frequency hearing loss above 1.5 kHz is in the severe to profound range (Figure 2). For such patients speech discrimination scores will typically fall below 60% at 65 dB sound pressure level (SPL) in the best aided condition. This technological advancement, often called electroacoustic stimulation (EAS), was developed in 1999 after Christoph Von Ilberg demonstrated preserved residual low frequency hearing in a patient who underwent cochlear implantation such that the patient wore a hearing aid in the implanted ear.1 Currently, EAS devices are available from two manufacturers. Contraindications to the use of EAS are shown in Table 1. Candidates for EAS devices should have stable low frequency hearing. There should be no progressive or autoimmune sensorineural hearing loss. Also there should be no history of meningitis, otosclerosis, or any other malformation that might cause an obstruction. The patient’s air-bone gap should be < 15 dB. Finally, there should not be any external auditory canal problems that can impede placement of the ear mould for the acoustic component. There are two main components of the EAS system (Figure 3). The external component is made up of a microphone that picks up sounds and a processor that separately encodes low and high frequency energy. After processing, low frequency energy is converted into an acoustic signal via the loudspeaker located in the ear hook and delivered into the external auditory canal. This acoustic signal will vibrate the tympanic membrane and ossicles so that cochlear fluids as well as the relatively intact structures of the cochlea in the apical region are stimulated. In contrast, high frequency energy is coded into radio-wave-like signals which are transmitted transcutaneously to the internal receiver. There, electric signals are delivered to the electrode array that has been surgically implanted into the cochlea. Thus the auditory nerve receives information using two different pathways from low and high frequency sounds, and the auditory nerve signals are then transmitted to the brain. Our Experience: Of the more than 100 implantations done under the Philippine National Ear Institute “CHIP” or Cochlear and Hearing Implants Programme only one was a case of EAS implantation. This particular case demonstrates key principles and concepts that every otolaryngologist should consider. Among these are audiological evaluation, temporal bone imaging, surgical technique for hearing preservation and some quality of life issues. Audiological Evaluation A 33 year old man had been seen at the clinic for over 7 years, with serial audiograms (Figure 4-6) illustrating the presence of good and stable low frequency hearing while high frequency hearing loss increased somewhat. The patient had been continually advised to get the best hearing aids available. However, a series of high-end hearing aids did not solve his problem of poor hearing in noisy places nor his difficulty understanding words when watching television and movies. Figure 7A shows the speech perception scores of this patient obtained with a Word Intelligibility by Picture Identification (WIPI) test, a “closed-set test” using isolated words while Figure 7B represents speech scores when “open-set” Bamford-Kowal-Bench (BKB) Sentence Lists were presented to the listener in both quiet and noise prior to the implantation. Temporal bone imaging A combination of high resolution computerized tomography (HRCT) of the temporal bone with both coronal and axial cochlear views, and T2-weighted normal anatomic Fast Spin Echo (T2 FSE) or 3D Constructive Interference in Steady State (3D CISS) MRI sequences of the inner ear should be done. Results from both studies should ascertain whether the cochlear duct is patent, ruling out any cochlear fibrosis or obstructive pathology. This patient’s HRCT and 3-D CISS MRI studies showed no such cochlear obliteration that would have posed intraoperative difficulties and constituted contraindications to EAS surgery (Figure 8). Surgical Technique for Hearing Preservation A variety of techniques have evolved over the years into what is now commonly called minimally invasive cochlear implantation. Using minimally invasive techniques, residual hearing can indeed be preserved in over 80%-90% of patients 3,4 Initially, a “Soft Cochleostomy” technique was introduced. This entailed careful low-speed drilling of the promontory with a Skeeter® drill (Medtronic Xomed, Jacksonville FL, USA) followed by the use of a mini-lancet to make an opening in the membranous labyrinth. This method avoids direct suctioning and prevents ingress of blood and bone dust into the intracochlear compartment. Also, for this method, the endosteum is left intact after drilling a cochleostomy antero-inferior to the round window. This allows proper placement of the electrode into the scala tympani with less chance of injury to the basilar membrane. Later, a round window approach was introduced, and it also proved to be a reliable way to preserve residual hearing during cochlear implantation. For this method, a more direct round window approach is performed after careful drilling of the round window niche. A limited incision is made just large enough to allow the electrode to be inserted. For both methods, after the endosteal or round window membrane incision is made with a micro lancet, a very flexible electrode of 20 mm length is slowly inserted. During the insertion process, the cochleostomy or round window is kept under direct vision so that insertion forces are minimized. Topical antibiotics and steroids are applied at this time to reduce any inflammatory or apoptotic reactions related to the trauma of opening the cochlea and introducing an electrode. Finally, a soft tissue plug is placed tightly around the electrode entry point into the membranous labyrinth to prevent perilymph leakage. New electrode designs that are thinner and more flexible are important contributors to the preservation of hearing. Postoperative Outcomes and Quality of Life After about 4-6 weeks from the time of surgery the EAS implant is switched on. Based on our experience and that of others,3 speech perception performance improves with prolonged experience with the implant. Roughly 1 ½ years post-surgery this patient has achieved dramatic improvement in hearing both in quiet and in noise using the EAS compared to using only the hearing aid component or the CI component alone. Figure 9 shows this dramatic improvement in free-field pure tone thresholds. Figure 10 demonstrates the speech perception following EAS implantation compared to pre-EAS implantation. Audiologic evaluation done at the PGH Ear Unit using 20 phonetically balanced Filipino words familiar to the patient in quiet and with 55 dB masking noise in the side of the implanted ear clearly showed an advantage with the EAS configuration compared to either hearing aid or CI component alone. Even with noise, this patient actually performed better presumably because he may have concentrated more with the introduction of masking noise. Another factor of course is that the words have now become familiar to the patient with the previous testing done in quiet. Notably, he reported great subjective improvement after only 10 months post-surgery.5 Interestingly the patient’s only complaint during his last follow-up was that he had not been offered bilateral EAS implantation. It is always important for the otolaryngologist to consider the quality of hearing and quality of life of patients with hearing loss. Intervention should not end with a referral note to a hearing aid center or dispenser. It is important to request proof of improvement not only of hearing thresholds but of speech perception outcomes in quiet and in noise. That is, one should document actual performance with the device in place, regardless of the type of device (hearing aid, an EAS device, or a Cochlear implant). Minimal disturbance of the remaining intact structures of the cochlea of patients with low frequency residual hearing can be achieved by employing a meticulous surgical technique, by using the advanced and flexible electrodes developed by some manufacturers, and instilling intraoperative antibiotics and steroids. Thus when one is faced with a ski-slope type audiogram it is likely the patient with this audiogram will not benefit from hearing aids. Such patients should be offered the option of EAS implantation which combines good acoustic stimulation with electric stimulation using a shorter (than conventional cochlear implantation) but very flexible electrode system. Counseling must also be done with a special emphasis on the risk of losing residual hearing, and noting that post-operative rehabilitation may take a long period of time. This patient now has a better quality of life than was obtainable from the most expensive and advanced hearing aids in the market, and has demonstrated a new implantable solution to partial deafness. Truly, EAS technology has opened a new era in prosthetic rehabilitation for hearing impaired adults and children.5 Acknowledgement Dr. Maria Rina Reyes-Quintos is gratefully acknowledged for performing all the excellent audiological testing following the surgery while Susan Javier and Angie Tongko of Manila Hearing Aid Center performed all the audiological testing prior to the surgery. Ms. Celina Ann Tobias, Professional Education Manager of Med-El is also credited with thanks for preparing the figures, reviewing the manuscript and interviewing the patient regarding his hearing performance following the surgery.

More than 30,000 hearing-impaired human subjects have learned to use cochlear implants for speech perception and speech discrimination. To understand the basic mechanisms underlying the successful application of contemporary speech processing strategies, it is important to investigate how complex electrical stimuli delivered to the cochlea are processed and represented in the central auditory system. A deaf animal model has been developed that allows direct comparison of psychophysical thresholds with central auditory neuronal thresholds to temporally modulated intracochlear electrical signals in the same animals. Behavioral detection thresholds were estimated in neonatally deafened cats for unmodulated pulse trains (e.g., 30 pulses/s or pps) and sinusoidal amplitude-modulated (SAM) pulse trains (e.g., 300 pps, SAM at 30 Hz; 300/30 AM). Animals were trained subsequently in a discrimination task to respond to changes in the modulation frequency of successive SAM signals (e.g., 300/8 AM vs. 300/30 AM). During acute physiological experiments, neural thresholds to pulse trains were estimated in the inferior colliculus (IC) and the primary auditory cortex (A1) of the anesthetized animals. Psychophysical detection thresholds for unmodulated and SAM pulse trains were virtually identical. Single IC neuron thresholds for SAM pulse trains showed a small but significant increase in threshold (0.4 dB or 15.5 μA) when compared with thresholds for unmodulated pulse trains. The mean difference between psychophysical and minimum neural thresholds within animals was not significant (mean = 0.3 dB). Importantly, cats also successfully discriminated changes in the modulation frequencies of the SAM signals. Performance on the discrimination task was not affected by carrier rate (100, 300, 500, 1,000, or 1,500 pps). These findings indicate that 1) behavioral and neural response thresholds are based on detection of the peak pulse amplitudes of the modulated and unmodulated signals, and 2) discrimination of successive SAM pulse trains is based on temporal resolution of the envelope frequencies. Overall, our animal model provides a robust framework for future studies of behavioral discrimination and central neural temporal processing of electrical signals applied to the deaf cochlea by a cochlear implant.

Cochlear implants (CIs) have been remarkably successful at restoring speech perception for severely to profoundly deaf individuals. Despite their success, several limitations remain, particularly in CI users’ ability to understand speech in noisy environments, locate sound sources, and enjoy music. A new multimodal approach has been proposed that uses haptic stimulation to provide sound information that is poorly transmitted by the implant. This augmenting of the electrical CI signal with haptic stimulation (electro-haptic stimulation; EHS) has been shown to improve speech-in-noise performance and sound localization in CI users. There is also evidence that it could enhance music perception. We review the evidence of EHS enhancement of CI listening and discuss key areas where further research is required. These include understanding the neural basis of EHS enhancement, understanding the effectiveness of EHS across different clinical populations, and the optimization of signal-processing strategies. We also discuss the significant potential for a new generation of haptic neuroprosthetic devices to aid those who cannot access hearing-assistive technology, either because of biomedical or healthcare-access issues. While significant further research and development is required, we conclude that EHS represents a promising new approach that could, in the near future, offer a non-invasive, inexpensive means of substantially improving clinical outcomes for hearing-impaired individuals.

As elucidated by prior research, children with hearing loss have impaired vocal emotion recognition compared with their normal-hearing peers. Cochlear implants (CIs) have achieved significant success in facilitating hearing and speech abilities for people with severe-to-profound sensorineural hearing loss. However, due to the current limitations in neuroimaging tools, existing research has been unable to detail the neural processing for perception and the recognition of vocal emotions during early stage CI use in infant and toddler CI users (ITCI). In the present study, functional near-infrared spectroscopy (fNIRS) imaging was employed during preoperative and postoperative tests to describe the early neural processing of perception in prelingual deaf ITCIs and their recognition of four vocal emotions (fear, anger, happiness, and neutral). The results revealed that the cortical response elicited by vocal emotional stimulation on the left pre-motor and supplementary motor area (pre-SMA), right middle temporal gyrus (MTG), and right superior temporal gyrus (STG) were significantly different between preoperative and postoperative tests. These findings indicate differences between the preoperative and postoperative neural processing associated with vocal emotional stimulation. Further results revealed that the recognition of vocal emotional stimuli appeared in the right supramarginal gyrus (SMG) after CI implantation, and the response elicited by fear was significantly greater than the response elicited by anger, indicating a negative bias. These findings indicate that the development of emotional bias and the development of emotional perception and recognition capabilities in ITCIs occur on a different timeline and involve different neural processing from those in normal-hearing peers. To assess the speech perception and production abilities, the Infant-Toddler Meaningful Auditory Integration Scale (IT-MAIS) and Speech Intelligibility Rating (SIR) were used. The results revealed no significant differences between preoperative and postoperative tests. Finally, the correlates of the neurobehavioral results were investigated, and the results demonstrated that the preoperative response of the right SMG to anger stimuli was significantly and positively correlated with the evaluation of postoperative behavioral outcomes. And the postoperative response of the right SMG to anger stimuli was significantly and negatively correlated with the evaluation of postoperative behavioral outcomes.

Cochlear implants (CIs) have been remarkably successful at restoring hearing in severely-to-profoundly hearing-impaired individuals. However, users often struggle to deconstruct complex auditory scenes with multiple simultaneous sounds, which can result in reduced music enjoyment and impaired speech understanding in background noise. Hearing aid users often have similar issues, though these are typically less acute. Several recent studies have shown that haptic stimulation can enhance CI listening by giving access to sound features that are poorly transmitted through the electrical CI signal. This “electro-haptic stimulation” improves melody recognition and pitch discrimination, as well as speech-in-noise performance and sound localization. The success of this approach suggests it could also enhance auditory perception in hearing-aid users and other hearing-impaired listeners. This review focuses on the use of haptic stimulation to enhance music perception in hearing-impaired listeners. Music is prevalent throughout everyday life, being critical to media such as film and video games, and often being central to events such as weddings and funerals. It represents the biggest challenge for signal processing, as it is typically an extremely complex acoustic signal, containing multiple simultaneous harmonic and inharmonic sounds. Signal-processing approaches developed for enhancing music perception could therefore have significant utility for other key issues faced by hearing-impaired listeners, such as understanding speech in noisy environments. This review first discusses the limits of music perception in hearing-impaired listeners and the limits of the tactile system. It then discusses the evidence around integration of audio and haptic stimulation in the brain. Next, the features, suitability, and success of current haptic devices for enhancing music perception are reviewed, as well as the signal-processing approaches that could be deployed in future haptic devices. Finally, the cutting-edge technologies that could be exploited for enhancing music perception with haptics are discussed. These include the latest micro motor and driver technology, low-power wireless technology, machine learning, big data, and cloud computing. New approaches for enhancing music perception in hearing-impaired listeners could substantially improve quality of life. Furthermore, effective haptic techniques for providing complex sound information could offer a non-invasive, affordable means for enhancing listening more broadly in hearing-impaired individuals.

AbstractThe cochlear implant is one of the most successful medical prostheses, allowing deaf and severely hearing-impaired persons to hear again by electrically stimulating the auditory nerve. A trained audiologist adjusts the stimulation settings for good speech understanding, known as “fitting” the implant. This process is based on subjective feedback from the user, making it time-consuming and challenging, especially in paediatric or communication-impaired populations. Furthermore, fittings only happen during infrequent sessions at a clinic, and therefore cannot take into account variable factors that affect the user’s hearing, such as physiological changes and different listening environments. Objective audiometry, in which brain responses evoked by auditory stimulation are collected and analysed, removes the need for active patient participation. However, recording of brain responses still requires expensive equipment that is cumbersome to use. An elegant solution is to record the neural signals using the implant itself. We demonstrate for the first time the recording of continuous electroencephalographic (EEG) signals from the implanted intracochlear electrode array in human subjects, using auditory evoked potentials originating from different brain regions. This was done using a temporary recording set-up with a percutaneous connector used for research purposes. Furthermore, we show that the response morphologies and amplitudes depend crucially on the recording electrode configuration. The integration of an EEG system into cochlear implants paves the way towards chronic neuro-monitoring of hearing-impaired patients in their everyday environment, and neuro-steered hearing prostheses, which can autonomously adjust their output based on neural feedback.

Natural sounds convey information via frequency and amplitude modulations (FM and AM). Humans are acutely sensitive to the slow rates of FM that are crucial for speech and music. This sensitivity has long been thought to rely on precise stimulus-driven auditory-nerve spike timing (time code), whereas a coarser code, based on variations in the cochlear place of stimulation (place code), represents faster FM rates. We tested this theory in listeners with normal and impaired hearing, spanning a wide range of place-coding fidelity. Contrary to predictions, sensitivity to both slow and fast FM correlated with place-coding fidelity. We also used incoherent AM on two carriers to simulate place coding of FM and observed poorer sensitivity at high carrier frequencies and fast rates, two properties of FM detection previously ascribed to the limits of time coding. The results suggest a unitary place-based neural code for FM across all rates and carrier frequencies.