Index of content:
Volume 127, Issue 2, February 2010
- PHYSIOLOGICAL ACOUSTICS 
Influence of inhibitory synaptic kinetics on the interaural time difference sensitivity in a linear model of binaural coincidence detection127(2010); http://dx.doi.org/10.1121/1.3282997View Description Hide Description
Temporal correlations between the sound waves arriving at the two ears are used to extract the azimuthal position of sound sources. Nerve cells in the mammalian medial superior olive (MSO) that extract these binaural correlations are sensitive to interaural time differences (ITDs) in the range of about . These neurons receive inputs from the two ears via four pathways, two excitatory and two inhibitory ones. In this paper, a simple linear model is fitted to the frequency dependence of ITD sensitivity of MSO neurons, which is quantified by the two parameters, characteristic phase and characteristic delay. The fit parameters are the relative delays and the relative strengths of the two inhibitory pathways and thus specify the underlying ITD-detecting circuit assuming a non-Jeffress-like situation, i.e., no excitatory delay lines but phase-locked inhibition. The fitting procedure finds the parameters of these inhibitory pathways such that they account for a desired frequency dependence of ITD sensitivity. It is found that positive characteristic delays require a finite amount of ipsilateral inhibition that arrives at roughly the same time as ipsilateral excitation as well as contralateral inhibition that lags contralateral excitation so much that it effectively leads excitation of the next cycle.
A computer model of auditory efferent suppression: Implications for the recognition of speech in noise127(2010); http://dx.doi.org/10.1121/1.3273893View Description Hide Description
The neural mechanisms underlying the ability of human listeners to recognize speech in the presence of background noise are still imperfectly understood. However, there is mounting evidence that the medial olivocochlear system plays an important role, via efferents that exert a suppressive effect on the response of the basilar membrane. The current paper presents a computer modeling study that investigates the possible role of this activity on speech intelligibility in noise. A model of auditory efferent processing [Ferry, R. T., and Meddis, R. (2007). J. Acoust. Soc. Am.122, 3519–3526] is used to provide acoustic features for a statistical automatic speech recognition system, thus allowing the effects of efferent activity on speech intelligibility to be quantified. Performance of the “basic” model (without efferent activity) on a connected digit recognition task is good when the speech is uncorrupted by noise but falls when noise is present. However, recognition performance is much improved when efferent activity is applied. Furthermore, optimal performance is obtained when the amount of efferent activity is proportional to the noise level. The results obtained are consistent with the suggestion that efferent suppression causes a “release from adaptation” in the auditory-nerve response to noisy speech, which enhances its intelligibility.
127(2010); http://dx.doi.org/10.1121/1.3279832View Description Hide Description
A procedure for extracting the nonlinear component of the stimulus-frequency otoacoustic emission (SFOAE) is described. This nSFOAE measures the amount by which the cochlear response deviates from linear additivity when the input stimulus is doubled in amplitude. When a tone was presented alone, the magnitude of the nSFOAE response remained essentially constant throughout the duration of the tone; response magnitude did increase monotonically with increasing tone level. When a wideband noise was presented alone, nSFOAE magnitude increased over the initial portion of the duration of the noise. When the tone and the wideband noise were presented simultaneously, nSFOAE magnitude decreased momentarily, then increased substantially for about the first and then remained strong for the remainder of the presentation. Manipulations of the noise bandwidth revealed that the low-frequency components were primarily responsible for this rising, dynamic response; no rising segment was seen with bandpass or highpass noise. The rising, dynamic nSFOAE response is likely attributable to activation of the medial olivocochlear efferent system. This perstimulatory emission appears to have the potential to provide information about the earliest stages of auditory processing for stimuli commonly used in psychoacoustical tasks.
Sensitization to masked tones following notched-noise correlates with estimates of cochlear function using distortion product otoacoustic emissions127(2010); http://dx.doi.org/10.1121/1.3277156View Description Hide Description
Neuronal gain adaptation has been proposed as the underlying mechanism leading to the perception of phantom sounds such as Zwicker tones and tinnitus. In this gain-adaptation theory, cochlear compression plays a significant role with weaker compression leading to stronger phantom percepts. The specific aim of this study was to find a link between the strength of neuronal gain adaptation and cochlear compression. Compression was assessed using distortion product otoacoustic emissions (DPOAEs). Gain adaptation is hypothesized to manifest itself in the sensitization observed for the detection of masked tones when preceded by notched noise. Perceptual thresholds for pure tones in notched noise were measured at multiple frequencies following various priming signals. The observed sensitization was larger than expected from the combined effect of the various maskers. However, there was no link between sensitization and compression. Instead, across subjects, stronger sensitization correlated with stronger DPOAEs evoked by low-level primaries. In addition, growth of DPOAEs correlated reliably with perceptual thresholds across frequencies within subjects. Together, the data suggest that short-term dynamic adaptation leading to perceptual sensitization is the result of an active process mediated by the outer hair cells, which are thought to modulate the gain of the cochlear amplifier via efferent feedback.