Robustness of neuronal tuning to binaural sound localization cues against age-related loss of inhibitory synaptic inputs

Integrate-and-fire-type models of the lateral superior olive

Neurons of the lateral superior olive (LSO) in the auditory brainstem play a fundamental role in binaural sound localization. Previous theoretical studies developed various types of neuronal models to study the physiological functions of the LSO. These models were usually tuned to a small set of physiological data with specific aims in mind. Therefore, it is unclear whether and how they can be related to each other, how widely applicable they are, and which model is suitable for what purposes. In this study, we address these questions for six different single-compartment integrate-and-fire (IF) type LSO models. The models are divided into two groups depending on their subthreshold responses: passive (linear) models with only the leak conductance and active (nonlinear) models with an additional low-voltage-activated potassium conductance that is prevalent among the auditory system. Each of these two groups is further subdivided into three subtypes according to the spike generation mechanism: one with simple threshold-crossing detection and voltage reset, one with threshold-crossing detection plus a current to mimic spike shapes, and one with a depolarizing exponential current for spiking. In our simulations, all six models were driven by identical synaptic inputs and calibrated with common criteria for binaural tuning. The resulting spike rates of the passive models were higher for intensive inputs and lower for temporally structured inputs than those of the active models, confirming the active function of the potassium current. Within each passive or active group, the simulated responses resembled each other, regardless of the spike generation types. These results, in combination with the analysis of computational costs, indicate that an active IF model is more suitable than a passive model for accurately reproducing temporal coding of LSO. The simulation of realistic spike shapes with an extended spiking mechanism added relatively small computational costs.

The interference of tinnitus on sound localization was related to the type of stimulus

Spatial processing is a major cognitive function of hearing. Sound source localization is an intuitive evaluation of spatial hearing. Current evidence of the effect of tinnitus on sound source localization remains limited. The present study aimed to investigate whether tinnitus affects the ability to localize sound in participants with normal hearing and whether the effect is related to the type of stimulus. Overall, 40 participants with tinnitus and another 40 control participants without tinnitus were evaluated. The sound source discrimination tasks were performed on the horizontal plane. Pure tone (PT, with single frequency) and monosyllable (MS, with spectrum information) were used as stimuli. The root-mean-square error (RMSE) score was calculated as the mean target response difference. When the stimuli were PTs, the RMSE scores of the control and tinnitus group were 11.77 ± 2.57° and 13.97 ± 4.18°, respectively. The control group performed significantly better than did the tinnitus group (t = 2.841, p = 0.006). When the stimuli were MS, the RMSE scores of the control and tinnitus groups were 7.12 ± 2.29° and 7.90 ± 2.33°, respectively. There was no significant difference between the two groups (t = 1.501, p = 0.137). Neither the effect of unilateral or bilateral tinnitus (PT: t = 0.763, p = 0.450; MS: t = 1.760, p = 0.086) nor the effect of tinnitus side (left/right, PT: t = 0.389, p = 0.703; MS: t = 1.407, p = 0.179) on sound localization ability were determined. The sound source localization ability gradually deteriorated with an increase in age (PT: r² = 0.153, p < 0.001; MS: r² = 0.516, p = 0.043). In conclusion, tinnitus interfered with the ability to localize PTs, but the ability to localize MS was not affected. Therefore, the interference of tinnitus in localizing sound sources is related to the type of stimulus.

Age-Related Changes in Interaural-Level-Difference-Based Across-Frequency Binaural Interference

Low-frequency interaural time differences and high-frequency interaural level differences (ILDs) are used to localize sounds in the horizontal plane. Older listeners appear to be worse at horizontal-plane sound localization to compared younger listeners, but little is understood about age-related changes to across-frequency binaural processing. This study investigated if the frequency dependence of across-frequency ILD processing is altered for older compared to younger listeners, which was done by using an across-frequency binaural interference task (when the interaural difference sensitivity for a target sound is decreased by a spectrally remote interfering sound with zero interaural differences). It was hypothesized that as listeners experience advancing age and age-related high-frequency hearing loss (i.e., presbycusis), they will demonstrate worse binaural performance and experience more across-channel binaural interference (because of age-related temporal processing deficits), and will increasingly be affected by interferers at lower frequencies (because of age-related hearing loss) when compared to younger listeners. There were 11 older (>65 yrs) and 20 younger (<30 yrs) listeners with normal to near-normal audiometric thresholds up to 2 kHz. They were tested using a left-right ILD lateralization discrimination task. Single-tone ILD discrimination thresholds and across-frequency binaural interference were measured at 0.5, 1, 2, 4, and 8 kHz. ILD thresholds and interference were about twice as large for older compared to younger listeners. Interferers ≤1 kHz produced 2-3 times as much across-frequency binaural interference for older compared to younger listeners. Hearing thresholds were significant predictors of single-tone ILD thresholds; in addition, both target and interferer hearing thresholds were significant predictors of binaural interference. The results suggest a reweighting of binaural information that occurs with advancing age and age-related high-frequency hearing loss. This evidence of plasticity may help explain some of the age-related changes in spatial-hearing abilities.