Lauren A. Shaffer

Sources and mechanisms of DPOAE generation: Implications for the prediction of auditory sensitivity

Otoacoustic emissions (OAEs) have become a commonly used clinical tool for assessing cochlear health status, in particular, the integrity of the cochlear amplifier or motor component of cochlear function. Predicting hearing thresholds from OAEs, however, remains a research challenge. Models and experimental data suggest that there are two mechanisms involved in the generation of OAEs. For distortion product, transient, and high-level stimulus frequency emissions, the interaction of multiple sources of emissions in the cochlea leads to amplitude variation in the composite ear canal signal. Multiple sources of emissions complicate simple correlations between audiometric test frequencies and otoacoustic emission frequencies. Current research offers new methods for estimating the individual components of OAE generation. Input-output functions and DP-grams of the nonlinear component of the 2f2-f2 DPOAE may ultimately show better correlations with hearing thresholds. This paper reviews models of OAE generation and methods for estimating the contribution of source components to the composite emission that is recorded in the ear canal. The clinical implications of multiple source components are discussed.

Generation of DPOAEs in the guinea pig

In humans, distortion product otoacoustic emissions (DPOAEs) at frequencies lower than the f(2) stimulus frequency are a composite of two separate sources, these two sources involving two distinctly different mechanisms for their production: non-linear distortion and linear coherent reflection [Talmadge et al., J. Acoust. Soc. Am. 104 (1998) 1517-1543; Talmadge et al., J. Acoust. Soc. Am. 105 (1999) 275-292; Shera and Guinan, J. Acoust. Soc. Am. 105 (1999) 332-348; Kalluri and Shera, J. Acoust. Soc. Am. 109 (2001) 662-637]. In rodents, DPOAEs are larger, consistent with broader filters; however the evidence for two separate mechanisms of DPOAE production as seen in humans is limited. In this study, we report DPOAE amplitude and phase fine structure from the guinea pig with f(2)/f(1) held constant at 1.2 and f(2) swept over a range of frequencies. Inverse Fast Fourier Transform analysis and time-domain windowing were used to separate the two components. Both the 2f(1)-f(2) DPOAE and the 2f(2)-f(1) DPOAE were examined. It was found that, commensurate with human data, the guinea pig DPOAE is a composite of two components arising from different mechanisms. It would appear that the 2f(1)-f(2) emission measured in the ear canal is usually dominated by non-linear distortion, at least for a stimulus frequency ratio of 1.2. The 2f(2)-f(1) DPOAE exhibits amplitude fine structure that, for the animals examined, is predominantly due to the variation in amplitude of the place-fixed component. Cochlear delay times appear consistent with a linear coherent reflection mechanism from the distortion product place for both the 2f(1)-f(2) and 2f(2)-f(1) place-fixed components.

Effects of a suppressor tone on distortion product otoacoustic emissions fine structure: Why a universal suppressor level is not a practical solution to obtaining single-generator DP-grams

Objectives: The use of a suppressor tone has been proposed as the method of choice in obtaining single-generator distortion product (DP) grams, the speculation being that such DP grams will be more predictive of hearing thresholds. Current distortion product otoacoustic emissions (DPOAE) theory points to the ear canal DPOAE signal being a complex interaction between multiple components. The effectiveness of a suppressor tone is predicted to be dependent entirely on the relative levels of these components. We examine the validity of using a suppressor tone through a detailed examination of the effects of a suppressor on DPOAE fine structure in individual ears. Design: DPOAE fine structure, recorded in 10 normal-hearing individuals with a suppressor tone at 45, 55, and 65 dB SPL, was compared with recordings without a suppressor. Behavioral hearing thresholds were also measured in the same subjects, using 2-dB steps. Results: The effect of the suppressor tone on DPOAE fine structure varied between ears and was dependent on frequency within ears. Correlation between hearing thresholds and DPOAE level measured without a suppressor was similar to previous reports. The effects of the suppressor are explained in the theoretical framework of a model involving multiple DPOAE components. Conclusions: Our results suggest that a suppressor tone can have highly variable effects on fine structure across individuals or even across frequency within one ear, thereby making the use of a suppressor less viable as a clinical tool for obtaining single-generator DP grams.

What drives mechanical amplification in the mammalian cochlea

The recent report by Peter Dallos and colleagues of the gene and protein responsible for outer hair cell somatic motility (Zheng, Shen, He, Long, Madison, & Dallos, 2000), and the work of James Hudspeth and colleagues demonstrating that vestibular stereocilia are capable of providing power that may boost the vibration of structures within the inner ear (Martin & Hudspeth, 1999), presents the tantalizing possibility that we may not be far away from answering the question what drives mechanical amplification in the mammalian cochlea? This article reviews the evidence for and against each of somatic motility as the motor, and a motor in the hair cell bundle, producing cochlear mechanical amplification. We consider three models based on somatic motility as the motor and two based on a motor in the hair cell bundle. Available evidence supports a hair cell bundle motor in nonmammals but the upper frequency limit of mammalian hearing in general exceeds that of nonmammals, in many cases by an order of magnitude or more. Only time will tell whether an evolutionary dichotomy exists (Manley, Kirk, Köppl, & Yates, 2001).

Identifying, locating, and sequencing picture communication symbols: Contributions from developmental visuospatial and temporal memory

Abstract This study investigated visuospatial and temporal working memory through assessment of feature binding abilities of typically developing first-, third-, and fifth-grade school-age children. One hundred and twelve children were asked to identify, locate, and sequence various numbers and arrays of picture communication symbols during a picture span assessment task. Feature binding was assessed through analysis of object recall, location recall, object–location recall, object–sequence recall, and location–sequence recall. Significant increases in picture span occurred with increasing grade level. Objects were highly salient with most children having recall greater than 90%. Most children had difficulty with recall of location, object sequence, and location sequence. A significant increase in location–sequence binding ability was noted as grade level increased from first to third grade. When visuospatial and temporal working memory capacity is exceeded, limitations in the ability to bind multiple features (object, location, and sequence) may reduce children’s abilities to successfully use visual graphic displays for communication.

Cochlear fine structure in chinchillas

Spontaneous otoacoustic emissions (SOAEs) and distortion product otoacoustic emissions (DPOAEs) were evaluated in both ears of nine apparently normal‐hearing chinchillas selected because preliminary screening indicated that they had SOAEs in at least one ear. All chinchillas show DPOAE fine structure in both ears (even in ears without detectable SOAEs). The spacing of the SOAEs and the characteristics of DPOAE fine structure are compared with OAE measures (obtained using the same procedures) from two species of kangaroo rat (Dipodomys merriami and Dipodomys spectabilis) that have no SOAEs and no DPOAE fine structure, and human subjects who have fine structure. Similarities and differences between the species are used to evaluate this model of cochlear fine structures further [Talmadge et al., J. Acoust. Soc. Am. 98, 1517–1543 (1998)].

The effects of contralateral stimulation on synchronous evoked otoacoustic emissions

Synchronous evoked otoacoustic emissions (SEOAEs) provide evidence that the impedance of the human auditory system varies systematically with frequency. Frequency‐independent acoustic input to an intact human auditory system leads to regular patterns of maxima and minima in the acoustic stimulus measured in the ear canal. The pattern of SEOAEs is also seen in changes in the level of distortion product otoacoustic emissions (DPOAEs). Contralateral acoustic stimulation moves the entire pattern of SEOAEs up in frequency. Monitoring SEOAEs or DPOAEs at one frequency in the presence of contralateral stimulation would thus lead to a level increase at some frequencies and a reduction at others. This pattern was observed in a subject with elevated contralateral acoustic reflexes, making it probable that the effect is generated by efferent innervation. The implications for the use of oto‐ acoustic emissions to evaluate efferent function will be discussed together with the impact on cochlear models. [Work supported...

Spontaneous otoacoustic emission prevalence patterns in humans

In humans the prevalence of spontaneous otoacoustic emissions (SOAEs) depends on gender, pigmentation (race), and auditory status. Potential sources for variations in SOAE prevalence may include outer/middle ear characteristics and trauma to the auditory system. To explore these variables, auditory system structure, status, and history was obtained for each subject from whom SOAE recordings were obtained. Outer ear characteristics were determined from resonance peaks in a spectrum of white noise measured in the ear canal. Middle ear characteristics were obtained with multifrequency tympanometry. Auditory system trauma was determined by assessing hearing thresholds and from reports of noise exposure, medication history, and otologic surgery/illnesses. The results will be discussed in terms of their implications for clinical applications and models of the auditory system. [Work supported by the Showalter Foundation.]