Kumar Seluakumaran

Efferent Pathways Modulate Hyperactivity in Inferior Colliculus

Animal models have demonstrated that mild hearing loss caused by acoustic trauma results in spontaneous hyperactivity in the central auditory pathways. This hyperactivity has been hypothesized to be involved in the generation of tinnitus, a phantom auditory sensation. We have recently shown that such hyperactivity, recorded in the inferior colliculus, is still dependent on cochlear neural output for some time after recovery (up to 6 weeks). We have now studied the capacity of an intrinsic efferent system, i.e., the olivocochlear system, to alter hyperactivity. This system is known to modulate cochlear neural output. Anesthetized guinea pigs were exposed to a loud sound and after 2 or 3 weeks of recovery, single-neuron recordings in inferior colliculus were made to confirm hyperactivity. Olivocochlear axons were electrically stimulated and effects on cochlear neural output and on highly spontaneous neurons in inferior colliculus were assessed. Olivocochlear stimulation suppressed spontaneous hyperactivity in the inferior colliculus. This result is in agreement with our earlier finding that hyperactivity can be modulated by altering cochlear neural output. Interestingly, the central suppression was generally much larger and longer lasting than reported previously for primary afferents. Blockade of the intracochlear effects of olivocochlear system activation eliminated some but not all of the effects observed on spontaneous activity, suggesting also a central component to the effects of stimulation. More research is needed to investigate whether these central effects of olivocochlear efferent stimulation are due to central intrinsic circuitry or to coactivation of central efferent collaterals to the cochlear nucleus.

Unmasking effects of olivocochlear efferent activation on responses of inferior colliculus neurons

Behavioural studies suggest a role for the medial olivocochlear (MOC) system in improving detection and discrimination of signals in noise. Physiological studies in the cochlea support this notion, showing unmasking, an improvement of the dynamic range of primary auditory afferents in response to tones in noise after MOC system activation. However, little is known about unmasking effects in higher centres. We investigated the effects of MOC stimulation on the responses of neurons in the guinea pig inferior colliculus to masked tones. In 50% of neurons background noise increased the basal rate and decreased maximum rate, an effect similar to that reported in primary afferents. In 35% of neurons maximum rate was increased by the background noise. Activation of the MOC system caused an increased slope and output dynamic range in the majority of input-output functions. Statistical analysis showed that this increased the discriminability of the tones within the noise. Hence, antimasking effects of olivocochlear activation that have been described in the cochlea can still be observed in the midbrain. However, 27% of neurons showed a variety of effects that have not been described in primary afferents, suggesting involvement of central circuitry in modulating effects of MOC stimulation on higher centres.

Effects of centrifugal pathways on responses of cochlear nucleus neurons to signals in noise.

The medial olivocochlear (MOC) system, which originates in the brainstem and projects to the outer hair cells in the cochlea, is thought to be involved in improving signal detection in noisy backgrounds. This proposition arises from the observation that the input–output functions of auditory primary afferent fibres to pure tones recorded in a continuous background noise are unmasked by MOC activation, improving the dynamic range, and is supported by both animal and human behavioural experiments. However, it is not known how the unmasking effects observed in the cochlea are translated into higher auditory brain centres, such as the cochlear nucleus, where intrinsic circuitry can potentially modulate any effect. In this study we have investigated the effects of continuous background noise without and with MOC system activation, on responses of different neuron types in the ventral cochlear nucleus of the guinea pig. Results show that the unmasking effects of MOC system activation on tone responses in continuous background noise are present in the cochlear nucleus. These unmasking effects manifest themselves as decompression of input–output functions as well as an improved slope, which results in an improvement in intensity discrimination of the tones. The data show, however, that the strength of the unmasking effects of MOC system activation varies between the different neuronal types. Unmasking was not detected in onset chopper neurons despite its demonstrable presence in other neuronal types in the same animals. These observations may reflect the level of involvement of different neuronal types in intensity discrimination.

Effects of medial olivocochlear efferent stimulation on the activity of neurons in the auditory midbrain

Medial olivocochlear (MOC) efferents are known to suppress spontaneous activity and sound-evoked responses of primary afferents by their actions on outer hair cells in the cochlea. This study investigated the effects of MOC activation on the responses of single neurons in the central nucleus of the inferior colliculus (CNIC) of anaesthetized guinea pigs. Extracellular responses of CNIC neurons to contralateral tones were recorded with and without MOC stimulation in normal animals and in animals acutely treated with gentamicin to eliminate peripheral effects of MOC activation. In normal animals, input–output functions of CNIC neurons showed a variety of changes. Some effects resembled qualitatively those reported for primary afferents. However, other effects were also observed, including an increase of firing rates at medium- to high-tone levels and in a small number of neurons (10%), an increase in spontaneous activity. In addition, larger threshold shifts and larger reductions of spontaneous firing rates were observed as compared to effects seen in the periphery. In gentamicin-treated animals, activation of MOC efferents did not produce any changes in the input–output functions or spontaneous activity of CNIC neurons. This observation is consistent with the majority of MOC-induced changes in monaural responses in the CNIC being mediated by the actions of MOC terminals in the cochlea and resulting from the interplay between altered afferent input and central circuitry.

Hearing Risk among Young Personal Listening Device Users: Effects at High-Frequency and Extended High- Frequency Audiogram Thresholds

OBJECTIVE The usage of personal listening devices (PLDs) is associated with risks of hearing loss. The aim of this study is to evaluate the effects of music exposure from these devices on high-frequency hearing thresholds of PLD users. MATERIALS AND METHODS A total of 282 young adults were questioned regarding their listening habits and symptoms associated with PLD listening. Their audiogram thresholds were determined at high (3-8 kHz) frequencies and extended high frequencies (EHFs, 9-16 kHz). The preferred listening volumes of PLD users were used to compute their overall 8-h equivalent music exposure levels (LAeq8h). RESULTS Approximately 80% of the subjects were regular PLD users. Of these, 20.1% had LAeq8h of ≥75 dBA, while 4.4% of them had LAeq8h of ≥85 dBA, which carries a high risk of hearing damage. Compared with those exposed to LAeq8h of <75 dBA, subjects who had LAeq8h of ≥75 dBA reported a significantly higher incidence of tinnitus and difficulty in hearing others immediately after using PLDs. PLD users who were exposed to LAeq8h of ≥75 dBA and had been using their devices for ≥4 years also showed significantly higher mean audiogram thresholds compared with non-users at most EHFs tested. In addition, the thresholds of PLD users at EHFs showed a weak but significant positive correlation with their LAeq8h. CONCLUSION The present findings suggest that excessive exposure to music among PLD users may lead to initial effects on their hearing at very high frequencies.