Cochlear tuning estimates from level ratio functions of distortion product otoacoustic emissions.

Objective: Distortion product otoacoustic emission (DPOAE) levels plotted as a function of stimulus frequency ratio demonstrate a bandpass shape. This bandpass shape is narrower at higher frequencies compared to lower frequencies and thus has been thought to be related to cochlear mechanical tuning.Design: However, the frequency- and level-dependence of these functions above 8 kHz is largely unknown. Furthermore, how tuning estimates from these functions are related to behavioural tuning is not fully understood.Study Sample: From experiment 1, we report DPOAE level ratio functions (LRF) from seven normal-hearing, young-adults for f2 = 0.75-16 kHz and two stimulus levels of 62/52 and 52/37 dB FPL. We found that LRFs became narrower as a function of increasing frequency and decreasing level.Results: Tuning estimates from these functions increased as expected from 1-8 kHz. In experiment 2, we compared tuning estimates from DPOAE LRF to behavioural tuning in 24 normal-hearing, young adults for 1 and 4 kHz and found that behavioural tuning generally predicted DPOAE LRF estimated tuning.Conclusions: Our findings suggest that DPOAE LRFs generally reflect the tuning profile consistent with basilar membrane, neural, and behavioural tuning. However, further investigations are warranted to fully determine the use of DPOAE LRF as a clinical measure of cochlear tuning.

Relationship Between Behavioral and Stimulus Frequency Otoacoustic Emissions Delay-Based Tuning Estimates.

Purpose The phase delay of stimulus frequency otoacoustic emissions (SFOAEs) has been proposed as a noninvasive, objective, and fast source for estimating cochlear mechanical tuning. However, the implementation of SFOAEs clinically has been thwarted by the gaps in understanding of the stability of SFOAE delay-based tuning estimates and their relationship to behavioral measures of tuning. Therefore, the goals of this study were (a) to investigate the relationship between delay-based tuning estimates from SFOAEs and simultaneously masked psychophysical tuning curves (PTCs) and (b) to assess the across- and within-session repeatability of tuning estimates from behavioral and OAE measures. Method Three sets of behavioral and OAE measurements were collected in 24 normal-hearing, young adults for two probe frequencies, 1 and 4 kHz. For each participant, delay-based tuning estimates were derived from the phase gradient of SFOAEs. SFOAE-based and behavioral estimates of tuning obtained using the fast-swept PTC paradigm were compared within and across sessions. Results In general, tuning estimates were sharper at 4 kHz compared to 1 kHz for both PTCs and SFOAEs. Statistical analyses revealed a significant correlation between SFOAE delay-based tuning and PTCs at 4 kHz, but not 1 kHz. Lastly, SFOAE delay-based tuning estimates showed better intra- and intersession repeatability compared to PTCs. Conclusions SFOAE phase-gradient delays reflect aspects of cochlear mechanical tuning, in that a frequency dependence similar to that of basilar membrane tuning was observed. Furthermore, the significant correlation with PTCs at 4 kHz and the high repeatability of SFOAE-based tuning measures offer promise of an objective, nonbehavioral assay of tuning in human ears.

Optical stimulation of zebrafish hair cells expressing channelrhodopsin-2.

Vertebrate hair cells are responsible for the high fidelity encoding of mechanical stimuli into trains of action potentials (spikes) in afferent neurons. Here, we generated a transgenic zebrafish line expressing Channelrhodopsin-2 (ChR2) under the control of the hair-cell specific myo6b promoter, in order to examine the role of the mechanoelectrical transduction (MET) channel in sensory encoding in afferent neurons. We performed in vivo recordings from afferent neurons of the zebrafish lateral line while activating hair cells with either mechanical stimuli from a waterjet or optical stimuli from flashes of ∼470-nm light. Comparison of the patterns of encoded spikes during 100-ms stimuli revealed no difference in mean first spike latency between the two modes of activation. However, there was a significant increase in the variability of first spike latency during optical stimulation as well as an increase in the mean number of spikes per stimulus. Next, we compared encoding of spikes during hair-cell stimulation at 10, 20, and 40-Hz. Consistent with the increased variability of first spike latency, we saw a significant decrease in the vector strength of phase-locked spiking during optical stimulation. These in vivo results support a physiological role for the MET channel in the high fidelity of first spike latency seen during encoding of mechanical sensory stimuli. Finally, we examined whether remote activation of hair cells via ChR2 activation was sufficient to elicit escape responses in free-swimming larvae. In transgenic larvae, 100-ms flashes of ∼470-nm light resulted in escape responses that occurred concomitantly with field recordings indicating Mauthner cell activity. Altogether, the myo6b:ChR2 transgenic line provides a platform to investigate hair-cell function and sensory encoding, hair-cell sensory input to the Mauthner cell, and the ability to remotely evoke behavior in free-swimming zebrafish.