Volume 111, Issue 2, February 2002
Index of content:
- PSYCHOLOGICAL ACOUSTICS 
111(2002); http://dx.doi.org/10.1121/1.1436071View Description Hide Description
The relation between auditory filters estimated from psychophysical methods and peripheral tuning was evaluated using a computational auditory-nerve (AN) model that included many of the response properties associated with nonlinear cochlear tuning. The phenomenological AN model included the effects of dynamic level-dependent tuning, compression, and suppression on the responses of high-, medium-, and low-spontaneous-rate AN fibers. Signal detection theory was used to evaluate psychophysical performance limits imposed by the random nature of AN discharges and by random-noise stimuli. The power-spectrum model of masking was used to estimate psychophysical auditory filters from predicted AN-model detection thresholds for a tone signal in fixed-level notched-noise maskers. Results demonstrate that the role of suppression in broadening peripheral tuning in response to the noise masker has implications for the interpretation of psychophysical auditory-filter estimates. Specifically, the estimated psychophysical auditory-filter equivalent rectangular bandwidths (ERBs) that were derived from the nonlinear AN model with suppression always overestimated the ERBs of the low-level peripheral model filters. Further, this effect was larger for an 8-kHz signal than for a 2-kHz signal, suggesting a potential characteristic-frequency (CF) dependent bias in psychophysical estimates of auditory filters due to the increase in strength of cochlear nonlinearity with increases in CF.
111(2002); http://dx.doi.org/10.1121/1.1434944View Description Hide Description
Perturbation analyses have been applied in recent years to determine the relative contribution of individual stimulus components in detection and discrimination tasks. Responses to stimulus samples are compared to stimulus parameters to determine the details of the decision rule. Often, a linear model is assumed and it is of interest to determine the relative contribution of different stimulus elements to the decision. Here, biases in estimated relative weights are considered for the case where the decision variable is given by and the stimulus components, the are normally distributed, of equal variance, and mutually independent. The are the “true” combination weights, and n, k, and m are positive reals. The method used to estimate relative weights is the correlation coefficient between the and the observer’s responses. Estimates of relative do not depend on m but may depend on the mean values of the and the values of n and k (a dependence on the variance, holds even for linear transformations).
111(2002); http://dx.doi.org/10.1121/1.1430690View Description Hide Description
Masked thresholds are measured and simulated for bandpass-noise signals ranging in bandwidth from 4 to 256 Hz in the presence of a masking bandpass noise also ranging in bandwidth from 4 to 256 Hz. Signal and masker are centered at 2 kHz. To investigate the role of temporal processing in simultaneous masking, simulations were performed with the modulation-filterbank model by Dau et al. [J. Acoust. Soc Am. 102, 2906–2919 (1997)]. For a fixed masker bandwidth, thresholds are independent of the signal bandwidth as long as the signal bandwidth does not exceed the masker bandwidth and thresholds decrease with increasing masker bandwidth in those conditions. A simple modulation-low-pass filter (energy integrator) would be sufficient to describe the experimental results in those conditions. In contrast, the processing by a modulation filterbank is necessary to account for the conditions of “asymmetry of masking,” where thresholds for signals with bandwidths larger than the masker bandwidth are much lower than those for the reversed condition. In those conditions, the modulation-filterbank model is able to use the inherent higher modulation frequencies of the signal as an additional cue.
111(2002); http://dx.doi.org/10.1121/1.1436067View Description Hide Description
The ability of six human subjects to discriminate the velocity of moving sound sources was examined using broadband stimuli presented in virtual auditory space. Subjects were presented with two successive stimuli moving in the frontal horizontal plane level with the ears, and were required to judge which moved the fastest. Discrimination thresholds were calculated for reference velocities of 15, 30, and 60 degrees/s under three stimulus conditions. In one condition, stimuli were centered on 0° azimuth and their duration varied randomly to prevent subjects from using displacement as an indicator of velocity. Performance varied between subjects giving median thresholds of 5.5, 9.1, and 14.8 degrees/s for the three reference velocities, respectively. In a second condition, pairs of stimuli were presented for a constant duration and subjects would have been able to use displacement to assist their judgment as faster stimuli traveled further. It was found that thresholds decreased significantly for all velocities (3.8, 7.1, and 9.8 degrees/s), suggesting that the subjects were using the additional displacement cue. The third condition differed from the second in that the stimuli were “anchored” on the same starting location rather than centered on the midline, thus doubling the spatial offset between stimulus endpoints. Subjects showed the lowest thresholds in this condition (2.9, 4.0, and 7.0 degrees/s). The results suggested that the auditory system is sensitive to velocityper se, but velocity comparisons are greatly aided if displacement cues are present.
111(2002); http://dx.doi.org/10.1121/1.1436073View Description Hide Description
The hypothesis was investigated that selectively increasing the discrimination of low-frequency information (below 2600 Hz) by altering the frequency-to-electrode allocation would improve speech perception by cochlear implantees. Two experimental conditions were compared, both utilizing ten electrode positions selected based on maximal discrimination. A fixed frequency range (200–10 513 Hz) was allocated either relatively evenly across the ten electrodes, or so that nine of the ten positions were allocated to the frequencies up to 2600 Hz. Two additional conditions utilizing all available electrode positions (15–18 electrodes) were assessed: one with each subject’s usual frequency-to-electrode allocation; and the other using the same analysis filters as the other experimental conditions. Seven users of the Nucleus CI22 implant wore processors mapped with each experimental condition for 2-week periods away from the laboratory, followed by assessment of perception of words in quiet and sentences in noise. Performance with both ten-electrode maps was significantly poorer than with both full-electrode maps on at least one measure. Performance with the map allocating nine out of ten electrodes to low frequencies was equivalent to that with the full-electrode maps for vowelperception and sentences in noise, but was worse for consonant perception. Performance with the evenly allocated ten-electrode map was equivalent to that with the full-electrode maps for consonant perception, but worse for vowelperception and sentences in noise. Comparison of the two full-electrode maps showed that subjects could fully adapt to frequency shifts up to ratio changes of 1.3, given 2 weeks’ experience. Future research is needed to investigate whether speech perception may be improved by the manipulation of frequency-to-electrode allocation in maps which have a full complement of electrodes in Nucleus implants.