High frequency sensitivity to interaural onset time differences in the bat inferior colliculus

Interaural time difference sensitivity under binaural cochlear implant stimulation persists at high pulse rates up to 900 pps

Spatial hearing remains one of the major challenges for bilateral cochlear implant (biCI) users, and early deaf patients in particular are often completely insensitive to interaural time differences (ITDs) delivered through biCIs. One popular hypothesis is that this may be due to a lack of early binaural experience. However, we have recently shown that neonatally deafened rats fitted with biCIs in adulthood quickly learn to discriminate ITDs as well as their normal hearing litter mates, and perform an order of magnitude better than human biCI users. Our unique behaving biCI rat model allows us to investigate other possible limiting factors of prosthetic binaural hearing, such as the effect of stimulus pulse rate and envelope shape. Previous work has indicated that ITD sensitivity may decline substantially at the high pulse rates often used in clinical practice. We therefore measured behavioral ITD thresholds in neonatally deafened, adult implanted biCI rats to pulse trains of 50, 300, 900 and 1800 pulses per second (pps), with either rectangular or Hanning window envelopes. Our rats exhibited very high sensitivity to ITDs at pulse rates up to 900 pps for both envelope shapes, similar to those in common clinical use. However, ITD sensitivity declined to near zero at 1800 pps, for both Hanning and rectangular windowed pulse trains. Current clinical cochlear implant (CI) processors are often set to pulse rates ≥ 900 pps, but ITD sensitivity in human CI listeners has been reported to decline sharply above ~ 300 pps. Our results suggest that the relatively poor ITD sensitivity seen at > 300 pps in human CI users may not reflect the hard upper limit of biCI ITD performance in the mammalian auditory pathway. Perhaps with training or better CI strategies good binaural hearing may be achievable at pulse rates high enough to allow good sampling of speech envelopes while delivering usable ITDs.

Effect of bat pinna on sensing using acoustic finite difference time domain simulation

The practicality of the finite-difference time-domain (FDTD) method was confirmed by comparing head-related transfer functions obtained from a three-dimensional (3D) digital model of a bat (Rhinolophus ferrumequinum nippon) head with acoustic experiments using a 3D printed physical model. Furthermore, we simulated the auditory directionality using a 3D digital model that was modified based on the pinna movement of a bat during echolocation and found that the alternating movements of the left and right pinna result in a binaural sound pressure difference for vertical sources. Using the FDTD method, suitable for simulating acoustics in large spaces, we could analyze in detail the binaural echoes that bats receive and the acoustic cues they use for echolocation.

Analysis of echolocation behavior of bats in "echo space" using acoustic simulation

BACKGROUND

Echolocating bats use echo information to perceive space, control their behavior, and adjust flight navigation strategies in various environments. However, the echolocation behavior of bats, including echo information, has not been thoroughly investigated as it is technically difficult to measure all the echoes that reach the bats during flight, even with the conventional telemetry microphones currently in use. Therefore, we attempted to reproduce the echoes received at the location of bats during flight by combining acoustic simulation and behavioral experiments with acoustic measurements. By using acoustic simulation, echoes can be reproduced as temporal waveforms (including diffracted waves and multiple reflections), and detailed echo analysis is possible even in complex obstacle environments.

RESULTS

We visualized the spatiotemporal changes in the echo incidence points detected by bats during flight, which enabled us to investigate the "echo space" revealed through echolocation for the first time. We then hypothesized that by observing the differences in the "echo space" before and after spatial learning, the bats' attentional position would change. To test this hypothesis, we examined how the distribution of visualized echoes concentrated at the obstacle edges after the bats became more familiar with their environment. The echo incidence points appeared near the edge even when the pulse direction was not toward the edge. Furthermore, it was found that the echo direction correlated with the turn rate of the bat's flight path, revealing for the first time the relationship between the echo direction and the bat's flight path.

CONCLUSIONS

We were able to clarify for the first time how echoes space affects echolocation behavior in bats by combining acoustic simulations and behavioral experiments.

Effects of place of stimulation on the interaural time difference sensitivity in bilateral electrical intracochlear stimulations: Neurophysiological study in a rat model

We examined the sensitivity of the neurons in the inferior colliculus (IC) in male and female rats to the interaural time differences (ITDs) conveyed in electrical pulse trains. Using bipolar pairs of electrodes that selectively activate the auditory nerve fibers at different intracochlear locations, we assessed whether the responses to electrical stimulation with ITDs in different frequency regions were processed differently. Most well-isolated single units responded to the electrical stimulation in only one of the apical or basal cochlear regions, and they were classified as either apical or basal units. Regardless of the cochlear stimulating location, more than 70% of both apical and basal units were sensitive to ITDs of electrical stimulation. However, the pulse rate dependence of neural ITD sensitivity differed significantly depending on the location of the stimulation. Moreover, ITD discrimination thresholds and the relative incidence of ITD tuning type markedly differed between units activated by apical and basal stimulations. With apical stimulation, IC neurons had a higher incidence of peak-type ITD function, which mostly exhibited the steepest position of the tuning curve within the rat's physiological ITD range of ±160 μs and, accordingly, had better ITD discrimination thresholds than those with basal stimulation. These results support the idea that ITD processing in the IC might be determined by functionally segregated frequency-specific pathways from the cochlea to the auditory midbrain.

Active head rolls enhance sonar-based auditory localization performance

Animals utilize a variety of active sensing mechanisms to perceive the world around them. Echolocating bats are an excellent model for the study of active auditory localization. The big brown bat (Eptesicus fuscus), for instance, employs active head roll movements during sonar prey tracking. The function of head rolls in sound source localization is not well understood. Here, we propose an echolocation model with multi-axis head rotation to investigate the effect of active head roll movements on sound localization performance. The model autonomously learns to align the bat's head direction towards the target. We show that a model with active head roll movements better localizes targets than a model without head rolls. Furthermore, we demonstrate that active head rolls also reduce the time required for localization in elevation. Finally, our model offers key insights to sound localization cues used by echolocating bats employing active head movements during echolocation.