The proposed research combines physiological measures of otoacoustic emissions (OAE) with psychophysical measures of intensity discrimination to study the perceptual consequences of variability of cochlear origin. OAEs are a measure of cochlear function. Recent studies have demonstrated that OAE response variability is similar at low levels for distortion product and stimulus frequency OAEs (DPOAE and SFOAE, respectively), but increases as a function of level for SFOAEs (Keefe et al., 2001; Schairer et al., 2002; Schairer and Keefe, 2002). This level-dependent variability limits detection of SFOAEs at higher stimulus levels, and it is not observed in DPOAEs or in measurements in a test cavity. The absence of variability in the test cavity suggests that its source is biological; its absence in DPOAEs suggests that the source cannot be the middle ear. The simplest explanation is that there is a level-dependent variability in the cochlear mechanics. Intensity discrimination is a behavioral measure of the listener's ability to detect a difference in level between two stimuli that are otherwise identical. Listener performance in intensity discrimination is limited entirely by internal noise, although the source of the noise is unknown. Stimuli in the SFOAE paradigm (i.e., a fixed-level primary condition) can be presented that are analogous to the stimuli in behavioral intensity discrimination. For a given SFOAEO function in a given fixed-level primary condition, SFOAE threshold can be identified and compared with the behavioral threshold for discrimination at the same pedestal level. It is predicted that for a given pedestal/primary level, there will be a positive, linear relationship between SFOAE threshold and intensity discrimination threshold when results are compared across a large group of subjects. If this is the case, it would suggest a common source of variability within the cochlea for the two measures. The main goals of the current application are to further investigate, in normal-hearing adults, the presence and repeatability of SFOAE variability with equal-frequency primaries in equal-level and fixed-level conditions, and with primary frequencies of various relationships, and to establish a relationship between SFOAE threshold and intensity discrimination threshold under comparable stimulus conditions. This research has implications for models of cochlear mechanics, which have not taken variability into account. Further, it suggests that cochlear variability limits intensity coding.
| Keefe, Douglas H; Schairer, Kim S; Ellison, John C et al. (2009) Use of stimulus-frequency otoacoustic emissions to investigate efferent and cochlear contributions to temporal overshoot. J Acoust Soc Am 125:1595-604 |
| Schairer, Kim S; Messersmith, Jessica; Jesteadt, Walt (2008) Use of psychometric-function slopes for forward-masked tones to investigate cochlear nonlinearity. J Acoust Soc Am 124:2196-215 |
| Schairer, Kim S; Ellison, John C; Fitzpatrick, Denis et al. (2006) Use of stimulus-frequency otoacoustic emission latency and level to investigate cochlear mechanics in human ears. J Acoust Soc Am 120:901-14 |
| Schairer, Kim S; Keefe, Douglas H (2005) Simultaneous recording of stimulus-frequency and distortion-product otoacoustic emission input-output functions in human ears. J Acoust Soc Am 117:818-32 |
