Volume 125, Issue 3, March 2009
Index of content:
- PSYCHOLOGICAL ACOUSTICS 
125(2009); http://dx.doi.org/10.1121/1.3075579View Description Hide Description
Detection of a pure tone signal in a narrowband noise masker can be improved by the introduction of coherently amplitude modulated masker bands in neighboring frequency regions, an effect called comodulation masking release (CMR). Experiment 1 tested the hypothesis that detection of a spectrally complex signal in a comodulated masker critically depends on the signal/masker interaction, with best sensitivity in conditions where the signal introduces across-frequency stimulus envelope differences. Consistent with this hypothesis, thresholds for a multi-frequency signal differed by approximately 10 dB depending on the relative patterns of signal/masker interaction across frequency. In comodulated maskers, there was no improvement in threshold relative to the single-frequency signal threshold even in cases where the multi-frequency signal introduced across-frequency envelope differences. Experiment 2 tested conditions that have previously been associated with large spectral integration in comodulated but not random maskers. Results depended on the masker configuration used as the reference condition, with comparable integration for random and comodulated noise in some cases. The results suggest that CMR obtained with a pure tone signal can differ greatly from that obtained with a complex signal, and that spectral integration is inversely related to the amount of CMR under some conditions.
125(2009); http://dx.doi.org/10.1121/1.3076045View Description Hide Description
Psychoacousticexperiments were conducted to investigate the role and interaction of fine-structure and envelope-based interaural temporal disparities. A computational model for the lateralization of binaural stimuli, motivated by recent physiological findings, is suggested and evaluated against the psychoacoustic data. The model is based on the independent extraction of the interaural phase difference (IPD) from the stimulus fine-structure and envelope. Sinusoidally amplitude-modulated 1-kHz tones were used in the experiments. The lateralization from either carrier (fine-structure) or modulator (envelope) IPD was matched with an interaural level difference, revealing a nearly linear dependence for both IPD types up to 135°, independent of the modulation frequency. However, if a carrier IPD was traded with an opposed modulator IPD to produce a centered soundimage, a carrier IPD of 45° required the largest opposed modulator IPD. The data could be modeled assuming a population of binaural neurons with a physiological distribution of the best IPDs clustered around 45°–50°. The model was also used to predict the perceived lateralization of previously published data. Subject-dependent differences in the perceptual salience of fine-structure and envelope cues, also reported previously, could be modeled by individual weighting coefficients for the two cues.
125(2009); http://dx.doi.org/10.1121/1.3050276View Description Hide Description
Informational masking of a target female talker by female distracters was measured with target and distracters presented from directly in front of the listener as a baseline condition. Next, it was found that if the distracters were also presented from directly in back of the listener, advanced or delayed by a few milliseconds with respect to the distracters in front, release from informational masking occurred. Release from informational masking was found for all delays within the Haas region of , with peak release of about . This peak occurred for a delay of and it was shown to be the result of delay-and-add filtering. Release from energetic masking was also found, but only for delays of or less.
125(2009); http://dx.doi.org/10.1121/1.3068457View Description Hide Description
A common finding in the cochlear implant literature is that the upper limit of rate discrimination on a single channel is about . The present study investigated rate discrimination using a procedure in which, in each block of two-interval trials, the standard could have one of the five baseline rates (100, 200, 300, 400, and ) and the signal rate was a given percentage higher than the standard. Eight Med-El subjects took part. The pattern of results was different than those reported previously: six Med-El subjects performed better at medium rates compared to both lower and higher rates. A similar pattern of results was obtained both with the method of constant stimuli and for 5000-pps pulse trains amplitude modulated at rates between 100 and . Compared to an unmatched group of eight Nucleus CI24 listeners tested using a similar paradigm and stimuli, Med-El subjects performed significantly better at and higher but slightly worse at . These results are discussed in relation to evidence on the limits of temporal pitch at low and high rates in normal-hearing listeners.
125(2009); http://dx.doi.org/10.1121/1.3068441View Description Hide Description
The addition of low-frequency acoustic information to real or simulated electric stimulation (so-called electric-acoustic stimulation or EAS) often results in large improvements in intelligibility, particularly in competing backgrounds. This may reflect the availability of fundamental frequency (F0) information in the acoustic region. The contributions of F0 and the amplitude envelope (as well as voicing) of speech to simulated EAS was examined by replacing the low-frequency speech with a tone that was modulated in frequency to track the F0 of the speech, in amplitude with the envelope of the low-frequency speech, or both. A four-channel vocoder simulated electric hearing. Significant benefit over vocoder alone was observed with the addition of a tone carrying F0 or envelope cues, and both cues combined typically provided significantly more benefit than either alone. The intelligibility improvement over vocoder was between 24 and 57 percentage points, and was unaffected by the presence of a tone carrying these cues from a background talker. These results confirm the importance of the F0 of target speech for EAS (in simulation). They indicate that significant benefit can be provided by a tone carrying F0 and amplitude envelope cues. The results support a glimpsing account of EAS and argue against segregation.