Volume 107, Issue 5, May 2000
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
Frequency responses of two- and three-tone distortion product otoacoustic emissions in Mongolian gerbils107(2000); http://dx.doi.org/10.1121/1.428646View Description Hide Description
The frequency responses of distortion product otoacoustic emission (DPOAEs) were investigated in adult Mongolian gerbils. The main goal was to investigate in this species the extent to which DPOAE measurements might be useful in estimating cochlear frequency-tuning characteristics. Specifically, this study investigated the parameter space for generation of DPOAEs to determine those regions, if any, where the emission responses gave “simple” frequency responses, i.e., responses similar in form to typical neural responses. At the same time, it was desired to determine in this species the existence, extent, and nature of the more complex three-tone emission frequency responses as observed in some other species [e.g., Martin et al., Hearing Res. 136, 105–123 (1999)]. In the present work, two-tone frequency response curves ratio functions) were obtained by varying the lower frequency, while holding the frequency and both amplitudes constant. Only for frequencies, near 8 kHz did the response at the emission frequency, form a simple, relatively broad peak. At all lower frequencies, the two-tone frequency response curve was typically complex and composed of multiple peaks. In comparison, three-tone frequency responses were constructed by fixing the primary stimulus pair and varying a third tone widely in frequency and intensity Points in and which caused a criterion reduction in primary emission amplitude (at were used to construct emission suppression tuning curves (STCs). Only for primary frequencies, at 8 kHz and above were the emission STCs found to be simple, with shapes similar to neural frequency-tuning curves. At lower primary frequencies, particularly for relatively low primary frequency ratios (low three-tone responses were very complex. This complex response usually included a region of anomalous suppression in which very low suppression levels could result in significant decreases in the primary emission amplitude, often exceeding 12 dB. Regions of such anomalous suppression were typically observed under the following conditions: (1) for all frequencies from 0.5 to 4 kHz; (2) for frequencies between 1.4 and 8 kHz; (3) i.e., for frequencies 1–3 octaves above the primary frequency, (4) at levels often 10 dB lower or more than the usual “best frequency” threshold, i.e., even lower than the relative minimum threshold found near the primary stimulus frequencies; (5) exhibiting sharp amplitude decreases often accompanied by emission phase shifts of about 180 deg; (6) present in both cubic emissions and (7) to be less extreme at larger primary stimulus frequency ratios (larger and (8) less extreme at larger intensity ratios (larger Because of the anomalous behavior at frequencies below 8 kHz, “simple” emission STCs were typically only obtainable, if at all, near the extreme boundaries of the parameter space giving measurable emission amplitudes.
107(2000); http://dx.doi.org/10.1121/1.428647View Description Hide Description
Moderate acoustic trauma results in decreased cochlear sensitivity and frequency selectivity. This decrease is believed to be caused by damage to the cochlear amplifier that is associated with outer hair cells (OHCs) and their nonlinear electromechanical characteristics. A consequence of OHC nonlinearity is the acoustic enhancement effect, in which low-frequency electrically evoked otoacoustic emissions are enhanced by a simultaneous tone. The present study found that acoustic trauma reduced the acoustic enhancement effect and this reduction is correlated with the threshold at the electrode site. This result is consistent with the theory that trauma affects the mechanoelectric transduction process, thus affecting cochlear mechanical nonlinearity. Acoustic trauma also reduced the cochlear microphonic in a way that suggests that the number of functioning tension-gated channels and the stiffness of the gating springs were decreased. In some cases, the electromechanical transduction process was also found to be affected by acoustic trauma.
Auditory-nerve-fiber responses to high-level clicks: Interference patterns indicate that excitation is due to the combination of multiple drives107(2000); http://dx.doi.org/10.1121/1.428648View Description Hide Description
There has been no systematic study of auditory-nerve-fiber (ANF) responses to high-level clicks despite the advantages of clicks in revealing the natural resonances of a system. Cat single ANFs were studied using clicks up to 120 dB pSPL. Peri-stimulus-time (PST) histograms of responses were corrected for refractory effects, and compound PST (cPST) histograms were formed from rarefaction- and condensation-click PSTs. At low levels the responses followed the classic picture with each cPST appearing to be from a single resonant system followed by low-pass filtering that reduces high-frequency synchrony. In fibers across all characteristic frequencies, there were significantly different patterns at high click levels including several nonclassic features and “phase reversals,” i.e., a peak in the rarefaction-click PST at low levels was replaced at high levels by a peak at the same latency in the condensation-click PST. There were two separate regions of nonclassic features and phase reversals, which indicates that auditory-nerve fibers are excited by the combination at some stage in the cochlea of at least three excitation drives derived from the acoustic stimulus. These data support the interpretation that the cochlear partition vibrates in multiple resonant modes with each mode producing one excitation drive and that the mix of modes varies with sound level.