Comparison of Otoacoustic Emissions Within Gecko Subfamilies: Morphological Implications for Auditory Function in Lizards

Abstract

Otoacoustic emissions (OAEs) are sounds emitted by the ear and provide a non-invasive probe into mechanisms underlying peripheral auditory transduction. This study focuses upon a comparison of emission properties in two phylogenetically similar pairs of gecko: Gekko gecko and Hemidactylus turcicus and Eublepharis macularius and Coleonyx variegatus. Each pair consists of two closely related species within the same subfamily, with quantitatively known morphological properties at the level of the auditory sensory organ (basilar papilla) in the inner ear. Essentially, the comparison boils down to an issue of size: how does overall body size, as well as the innerear dimensions (e.g., papilla length and number of hair cells), affect peripheral auditory function as inferred from OAEs? Estimates of frequency selectivity derived from stimulus-frequency emissions (emissions evoked by a single low-level tone) indicate that tuning is broader in the species with fewer hair cells/shorter papilla. Furthermore, emissions extend outwards to higher frequencies (for similar body temperatures) in the species with the smaller body size/narrower interaural spacing. This observation suggests the smaller species have relatively improved high-frequency sensitivity, possibly related to vocalizations and/or aiding azimuthal sound localization. For one species (Eublepharis), emissions were also examined in both juveniles and adults. Qualitatively similar emission properties in both suggests that inner-ear function is adult like soon after hatching and that external body size (e.g., middle-ear dimensions and interaural spacing) has a relatively small impact upon emission properties within a species.

Interrelationships between spontaneous and low-level stimulus-frequency otoacoustic emissions in humans

“…When comparing SFOAEs across species (e.g., Bergevin, 2011;Joris et al, 2011), there may be systematic offsets as to what is the appropriate level for a given species. Or put another way, there may be differences across species where and how the low-level basilar membrane responses transitions from linear to compressive (e.g., w20e30 dB SPL for chinchilla, Ruggero et al, 1997).…”

“…All measurements reported here were obtained using similar methods as those reported previously (Shera and Guinan, 1999;Bergevin, 2011). A desktop computer housed a 24-bit soundcard (Lynx TWO-A, Lynx Studio Technology), whose synchronous root mean square (RMS) input/output was controlled using a custom dataacquisition system.…”

“…Such computations provided the corresponding 95% confidence intervals. Additional details regarding the computation of the loess fits can be found elsewhere (Shera and Guinan, 2003;Bergevin, 2011). Analysis required estimation of SOAE peak frequencies and the SFOAE phase at those frequencies.…”

Obtaining reliable phase-gradient delays from otoacoustic emission data

“…Among these are popular strategies such as applying data-exclusion criteria based on local SNR [e.g., ignoring delay values at frequencies where SFOAE level is less than a specified amount above the noise floor (e.g., Shera and Guinan, 2003;Bergevin, 2011)] and computing energy-weighted delays [in which delay values are weighted in proportion to a measure of the local emission "energy" and then averaged over some frequency band or group of subjects (e.g., Lineton and Wildgoose, 2009;Bentsen et al, 2011)]. For our tests of the SNR-based exclusion method, we set the criterion so that delays at frequencies with SNR < 15 dB-the mean SNR employed in the computation of P SFOAE -were excluded when computing the loess trend.…”

“…Although emission levels serve as convenient assays of cochlear function, important information is also carried by emission phase (e.g., Shera and Guinan, 1999). For example, measurements of reflectionsource OAE phase-or its time-domain counterpart, latency-have been used to probe mechanisms of emission generation (e.g., Zweig and Shera, 1995;Siegel et al, 2005;Shera et al, 2008;Bergevin and Shera, 2010;Meenderink and van der Heijden, 2010), to estimate the wavelength and delay of cochlear traveling waves (e.g., Neely et al, 1988;Shera and Guinan, 2003;Harte et al, 2009), to explore the effects of olivocochlear efferent feedback to the cochlea (e.g., Francis and Guinan, 2010), and to determine the characteristics of peripheral frequency selectivity in humans and other species (e.g., Shera et al, 2002Shera et al, , 2010Schairer et al, 2006;Bergevin et al, 2008;Lineton and Wildgoose, 2009;Bentsen et al, 2011;Bergevin, 2011;Joris et al, 2011).…”