Index of content:
Volume 123, Issue 4, April 2008
- PHYSIOLOGICAL ACOUSTICS 
123(2008); http://dx.doi.org/10.1121/1.2871682View Description Hide Description
In this paper, two sets of experimental results to extract the two effective elastic moduli, the effective shear modulus, and the effective Poisson’s ratio for the gerbil cochlear partition are analyzed. In order to accomplish this, a geometrically nonlinear composite orthotropic plate model is employed. The model is used to predict both out-of-plane and in-plane motion of the partition under a static finite area distributed load. This loading condition models the small, but finite size, probe tips used in experiments. Both in-plane and out-of-plane motion are needed for comparison with recent experimental results. It is shown that the spatial decay rate (the space constant) for the in-plane deflection is different than for the out-of-plane deflection, which has a significant effect on the derived partition properties. The size of the probe tip is shown to have little influence on the results. Results are presented for two types of boundary conditions. Orthotropy ratios determined from the experimental data are found to vary with longitudinal position and choice of boundary conditions. Orthotropy ratios (the ratio of the two elastic moduli) are in the range of 65 close to the base to 10 in the upper middle turn of the cochlea.
123(2008); http://dx.doi.org/10.1121/1.2839138View Description Hide Description
Distortion product otoacoustic emission suppression (quantified as decrements) was measured for and , for a range of primary levels , suppressor frequencies , and suppressor levels in 19 normal-hearing subjects. Slopes of decrement-versus- functions were similar at both frequencies, and decreased as increased. Suppression tuning curves, constructed from decrement functions, were used to estimate (1) suppression for on- and low-frequency suppressors, (2) tip-to-tail differences, (3) , and (4) best frequency. Compression, estimated from the slope of functions relating suppression “threshold” to for off-frequency suppressors, was similar for 500 and . Tip-to-tail differences, , and best frequency decreased as increased for both frequencies. However, tip-to-tail difference (an estimate of cochlear-amplifier gain) was greater at , compared to . decreased to a greater extent with when , but, on an octave scale, best frequency shifted more with level when . These data indicate that, at both frequencies, cochlear processing is nonlinear. Response growth and compression are similar at the two frequencies, but gain is greater at and spread of excitation is greater at .
123(2008); http://dx.doi.org/10.1121/1.2888489View Description Hide Description
Previous non-invasive brain research has reported auditory cortical sensitivity to periodicity as reflected by larger and more anterior responses to periodic than to aperiodic vowels. The current study investigated whether there is a lower fundamental frequency (F0) limit for this effect. Auditory evoked fields (AEFs) elicited by natural-sounding periodic and aperiodic vowel stimuli were measured with magnetoencephalography.Vowel F0 ranged from normal male speech to exceptionally low values . Both the auditory N1m and sustained fields were larger in amplitude for periodic than for aperiodic vowels. The AEF sources for periodic vowels were also anterior to those for the aperiodic vowels. Importantly, the AEF amplitudes and locations were unaffected by the F0 decrement of the periodic vowels. However, the N1m latency increased monotonically as F0 was decreased down to , below which this trend broke down. Also, a cascade of transient N1m-like responses was observed in the lowest F0 condition. Thus, the auditory system seems capable of extracting the periodicity even from very low F0 vowels. The behavior of the N1m latency and the emergence of a response cascade at very low F0 values may reflect the lower limit of pitch perception.
Temporary hearing loss influences post-stimulus time histogram and single neuron action potential estimates from human compound action potentials123(2008); http://dx.doi.org/10.1121/1.2885748View Description Hide Description
An analytic compound action potential (CAP) obtained by convolving functional representations of the post-stimulus time histogram summed across auditory nerve neurons and a single neuron action potential was fit to human CAPs. The analytic CAP fit to pre- and postnoise-induced temporary hearing threshold shift (TTS) estimated in vivo and and the number of neurons contributing to the CAPs . The width of decreased with increasing signal level and was wider at the lowest signal level following noise exposure. latency decreased with increasing signal level and was shorter at all signal levels following noise exposure. The damping and oscillatory frequency of increased with signal level. For subjects with large amounts of TTS, had greater damping than before noise exposure particularly at low signal levels. Additionally, oscillation was lower in frequency at all click intensities following noise exposure. increased with signal level and was smaller after noise exposure at the lowest signal level. Collectively these findings indicate that neurons contributing to the CAP during TTS are fewer in number, shorter in latency, and poorer in synchrony than before noise exposure. Moreover, estimates of single neuron action potentials may decay more rapidly and have a lower oscillatory frequency during TTS.