Localization of sound sources requires calculation of the difference in the sound arrival times between the two ears (interaural time difference). A critical process in this calculation is the coincidence detection of synaptic inputs from both ears in a brainstem circuit. In the avian species, a neural circuit composed of the nucleus magnocellularis (NM) and nucleus laminaris (NL) plays this role. NM neurons relay precisely-timed-spike signals from the auditory nerve to both sides of the NL, whereas the NL neurons work as coincidence detectors of the signals. Neurons in these nuclei show several differential features depending on their tuning frequency, thereby ensuring their signal processing against wide ranges of sound frequencies. For example, NM neurons differ in the morphology of the synaptic terminals and number of sodium channels in the axons, which enables the neurons to minimize the variations in spike jitter among frequencies. On the other hand, NL neurons differ in the location of the sodium channels in the axons, which allows the neurons to overcome the effects of input frequencies across sound frequencies. In this review, I shall summarize the recent findings on the cellular and molecular mechanisms involved in the calculation of the interaural time difference in the avian auditory brainstem.
We report a case of a 10-year-old girl with severe bilateral hearing loss. Her mother had suspected her daughter's hearing loss since she was 5 years old. The child had undergone several hearing tests over time, but her severe hearing loss was not discovered until we examined her at age 10. Even with a hearing aid, she could not hear well. We performed a cochlear implant 10 months after diagnosis. We believe that her hearing loss had not been diagnosed because it progressed slowly; she had mild hearing loss at low frequencies and had been diagnosed repeatedly as having normal-hearing. It is important to examine children's hearing loss carefully and to use an objective audiometry test.
The aim of the present study was to clarify the influence of social circumstances and medical factors on the communication modes in patients with prelingual hearing impairment having cochlear implants. Informed consent for participation in the study was obtained from 22 patients with cochlear implant who did not have multiple disabilities. These participants were divided into a hearing-dominant communication mode group (15 cases; hearing-dominant group) and a hearing plus visual communication mode group (7 cases; combined group). The differences in various medical factors and social factors were investigated between the two groups. Of the medical factors, the age at operation was lower and the hearing threshold level of cochlear implantation was lower in the hearing-dominant group than in the combined group. Of the social factors, the training time at home was significantly longer in the hearing-dominant group than in the combined group. There were no significant differences in relation to other factors, such as the education level of the parents, the working status, and the number of family members. These findings suggest that adequate selection of candidates for cochlear implant surgery and training of these patients at their own homes in addition to rehabilitation at the hospital are important for obtaining better results of cochlear implantation.
In general, patients with unilateral hearing loss may experience limitations in hearing in noisy situations and in recognizing the direction of the sound. For patients with unilateral hearing loss suffering from such hearing limitations, we recommend the use of a hearing aid for the affected ear. To select the optimal hearing aid, the baseline hearing level is determined by exposing the unaffected ear to the test sound; then, the unaffected ear is masked. However, the best method for masking and the extent of masking are still to be established. In an attempt to determine the optimal masking method, we masked both ears of 12 persons with normal hearing using earplugs and earmuffs, and checked the masking effect by examining the hearing levels. We found that masking of the ear canal with an earplug and covering the external ear with earmuffs was the most effective method for the masking; the masking effect obtained with this method was about 30-35-40-40-50dB (from 250Hz to 4000Hz). If the hearing level of the unaffected ear of the patients with unilateral hearing loss is better than the level of masking obtained in that ear, cross hearing may not occur with the use of this masking method; however, if it is worse, a difference of almost 50 dB may be observed in optimal setting of the hearing aid.
We evaluated speech intelligibility and sound impression with hearing aids in a noisy classroom between subjects with and without hearing-assistive devices. Test sounds were recorded through hearing aids worn on HATS (Head And Torso Stimulator), and presented to 30 normal hearing subjects. Monosyllables were used for measuring the intelligibility scores and short sentences for evaluating speech impression. These tests were conducted under both noisy and quiet conditions. Two types of hearing aids, one equipped with a noise suppression function and the other with no setting for noise suppression. The hearing-assistive devices used in the test included 2 FM systems, one, an FM transmitter equipped with a setting for noise suppression, and the other, without such a setting, and 1 induction loop system. The intelligibility score was analyzed by the percentage of corrected monosyllable answers, and the semantic differential scores of hearing impressions were analyzed by factor analysis. Subjects with hearing aids equipped with a noise suppression function showed significantly higher intelligibility scores and positive sound impressions, regardless of whether the environment was noisy or not. The intelligibility scores improved significantly and the differences in the intelligibility scores and speech impressions between subjects with the two types of hearing aids decreased after the introduction of hearing-assistive devices; higher intelligibility scores were obtained with the induction loop system. Factor analysis showed semantic differential scores with different features with each of the hearing-assistive devices.