Otoacoustic Interrelationships of the Barn Owl

Significant debate still exists about the biophysical mechanisms at work in otoacoustic emission (OAE) generation and how such may differ between mammals and non-mammals given gross morphological differences (e.g., existence of basilar membrane traveling waves, degree of tectorial membrane coupling). To further elucidate general principles at work, we examined the barn owl for interrelationships between spontaneous emissions (SOAEs) and those evoked using a single tone (SFOAEs). First, most ears exhibited SOAEs as a stable periodic ‘rippling’ whose peak-to-peak spacing was relatively constant (∼0.4 kHz). Some ears showed substantially larger narrowband peaks, although their statistical distributions were highly noisy. Second, significant interactions between a low-level tone and SOAE activity were observed via an interference pattern as the tone frequency was swept. Using a suppression paradigm to extract SFOAEs as the residual, the magnitude exhibited a stable pattern of peaks and valleys unique to each ear. Third, SFOAE phase exhibited significant accumulation as frequency was swept, with a phase-gradient delay of approximately 2 ms that was constant across frequency. The amount of SFOAE phase accumulation between adjacent SOAE peaks tended to cluster about an integral number of cycles, as previously observed for humans. Taken together, our data suggest that the principles underlying how active hair cells work together (e.g., entrainment, phase coherence) are shared between widely different inner ear morphologies, leading to the generation of OAEs with similar properties. INTRODUCTION Spontaneous otoacoustic emissions (SOAEs) are commonly regarded as evidence for an ‘active ear’, yet are relatively uncommon in mammals. Barn owls present an interesting opportunity for study as they have an inner ear morphology differing substantially to that of mammals [6], exhibit strong OAE activity [4, 17], and have well-characterized neurophysiology (e.g., known tonotopic map) [10, 11] and psychophysics [5]. Several theoretical studies have examined SOAE generation using a variety of biomechanical foundations (e.g., [14, 18]). Motivated by morphological differences in birds relative to mammals (see [12]), such as hair cell arrangement [6], tectorial membrane coupling [16], and the uncertain role of basilar membrane traveling waves [7], our present goal was to better elucidate general biophysical characteristics of the underlying active elements and their interactions. To this end, we systematically explored interrelationships within individual barn owl (Tyto alba) ears between SOAEs and stimulus frequency emissions (SFOAEs) evoked by a single, low-level tone. METHODS Measurements were made in 9 lightly anesthetized (ketamine/xylazine) owls with a vented middle ear. Animals were warmed by a heating blanket. An Etymotic ER-10C probe was coupled to the ear canal. SFOAEs were evoked by swept tones, generally between 1 and 11 kHz, and extracted using a suppression paradigm [9]. Probe levels varied from 0-50 dB SPL, with a suppressor level (Ls) 15 dB higher and 40 Hz higher in frequency. SOAE activity was analyzed by a variety of methods (see Results). Sample rate for all data acquisition was 44.1 kHz. Additional methodological details are similar to those described previously (e.g., [3, 15]). RESULTS SOAEs & Effects of an external tone Spontaneous emissions were observable in all ears examined. At a minimum, a rippling pattern atop the microphone noise floor was apparent. This rippling had a periodic structure to it, with a uniform spacing between maxima. In the 4 5 6 7 8 9 10 11 −30 −25 −20 −15 −10 −5 0 M ag n it u d e [d B S PL ] Frequency [kHz] TAG4eL_01 5 dB 1 2 3 4 5 6 7 8 9 10 11 −30 −25 −20 −15 −10 M ag n it u d e [d B S PL ] Frequency [kHz] 5 dB