Volume 104, Issue 4, October 1998
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
Differentiation of cochlear pathophysiology in ears damaged by salicylate or a pure tone using a nonlinear systems identification technique104(1998); http://dx.doi.org/10.1121/1.423739View Description Hide Description
Mongolian gerbils were exposed to either α-ketoglutarate, salicylate, or an 8-kHz pure tone. Cochlear microphonic (CM) was recorded from the round window in response to 68 and 88 dB SPL Gaussian noise. A nonlinear systems identification technique provided the frequency-domain parameters of a third-order polynomial model characterizing cochlear mechano-electric transduction (MET). A series of physiologic indices were derived from further exploration of the model. Exposure to the 8-kHz pure tone and round window application of salicylate resulted in different changes in the polynomial parameters and physiologic indices even though the threshold shifts were similar. A general reduction of CM magnitude was found after the tone exposure, and an increase at low-mid frequencies was demonstrated in the salicylate group especially at the lower signal level. The slope of the MET curve was reduced by the acoustic overstimulation. The root or the operating point of the MET was shifted in opposite directions after the two treatments. Sound-pressure levels that saturate MET expanded in the tone exposure group and narrowed in the salicylate group. The signal level also had effects on these indices.
Temporal factors associated with cochlear nerve tuning to dual and single tones: A qualitative study104(1998); http://dx.doi.org/10.1121/1.423740View Description Hide Description
The simultaneous presentation of a 10- and 10.86-kHz tone produces an 860-Hz cochlear nerve difference tone (DT) response in the gerbil which persists for the duration of the stimulus. Forward masking shows this response is generated by neurons sharply tuned to the stimulus frequencies. When compared with the DT response, the cochlear nerve compound action potential (CAP) to a single tone is smaller in amplitude, has a higher nonmasked threshold, and produces a less sensitive tuning curve (TC). Forward maskers can also produce amplitude enhancement of the CAP, but this was not observed for the onset portion of the DT response. The CAP TC is as sharply tuned as the TC of either the DT onset response or the entire DT response. A comparison was made of tuning of the DT response to the onset, the first half and second half of the 23-ms duration probe stimulus, using either a 5- or 15-ms masker-probe interval. An increase of the tip threshold of the TC to all three portions of the stimulus occurred as the interval was increased between the end of the masker and the midpoint of the portion of the stimulus under question. The 15-ms masker-probe interval produced sharper TCs.
The effects of sensory hearing loss on cochlear filter times estimated from auditory brainstem response latencies104(1998); http://dx.doi.org/10.1121/1.423741View Description Hide Description
Derived-band auditory brainstem responses (ABRs) were obtained in 43 normal-hearing and 80 cochlear hearing-impaired individuals using clicks and high-pass noise masking. The response times across the cochlea [the latency difference between wave V’s of the 5.7- and 1.4-kHz center frequency (CF) derived bands] were calculated for five levels of click stimulation ranging from 53 to 93 dB p.-p.e. SPL (23 to 63 dB nHL) in 10-dB steps. Cochlear response times appeared to shorten significantly with hearing loss, especially when the average pure tone (1 to 8 kHz) hearing loss exceeded 30 dB. Examination of derived-band latencies indicates that this shortening is due to a dramatic decrease of wave V latency in the lower CF derived band. Estimates of cochlear filter times in terms of the number of periods to maximum response were calculated from derived-band latencies corrected for gender-dependent cochlear transport and neural conduction times. decreased as a function of hearing loss, especially for the low CF derived bands. The functions were similar for both males and females. These results are consistent with broader cochlear tuning due to peripheral hearing loss. Estimating filter response times from ABR latencies enhances objective noninvasive diagnosis and allows delineation of the differential effects of pathology on the underlying cochlear mechanisms involved in cochlear transport and filter build-up times.