Phase effects on the perceived elevation of complex tones

Modeling sound-source localization in sagittal planes for human listeners

Monaural spectral features are important for human sound-source localization in sagittal planes, including front-back discrimination and elevation perception. These directional features result from the acoustic filtering of incoming sounds by the listener's morphology and are described by listener-specific head-related transfer functions (HRTFs). This article proposes a probabilistic, functional model of sagittal-plane localization that is based on human listeners' HRTFs. The model approximates spectral auditory processing, accounts for acoustic and non-acoustic listener specificity, allows for predictions beyond the median plane, and directly predicts psychoacoustic measures of localization performance. The predictive power of the listener-specific modeling approach was verified under various experimental conditions: The model predicted effects on localization performance of band limitation, spectral warping, non-individualized HRTFs, spectral resolution, spectral ripples, and high-frequency attenuation in speech. The functionalities of vital model components were evaluated and discussed in detail. Positive spectral gradient extraction, sensorimotor mapping, and binaural weighting of monaural spatial information were addressed in particular. Potential applications of the model include predictions of psychophysical effects, for instance, in the context of virtual acoustics or hearing assistive devices.

Localization of click trains and speech by cats: the negative level effect

Although localization of sound in elevation is believed to depend on spectral cues, it has been shown with human listeners that the temporal features of sound can also greatly affect localization performance. Of particular interest is a phenomenon known as the negative level effect, which describes the deterioration of localization ability in elevation with increasing sound level and is observed only with impulsive or short-duration sound. The present study uses the gaze positions of domestic cats as measures of perceived locations of sound targets varying in azimuth and elevation. The effects of sound level on localization in terms of accuracy, precision, and response latency were tested for sound with different temporal features, such as a click train, a single click, a continuous sound that had the same frequency spectrum of the click train, and speech segments. In agreement with previous human studies, negative level effects were only observed with click-like stimuli and only in elevation. In fact, localization of speech sounds in elevation benefited significantly when the sound level increased. Our findings indicate that the temporal continuity of a sound can affect the frequency analysis performed by the auditory system, and the variation in the frequency spectrum contained in speech sound does not interfere much with the spectral coding for its location in elevation.

Vertical-plane sound localization with distorted spectral cues

For human listeners, the primary cues for localization in the vertical plane are provided by the direction-dependent filtering of the pinnae, head, and upper body. Vertical-plane localization generally is accurate for broadband sounds, but when such sounds are presented at near-threshold levels or at high levels with short durations (<20 ms), the apparent location is biased toward the horizontal plane (i.e., elevation gain <1). We tested the hypothesis that these effects result in part from distorted peripheral representations of sound spectra. Human listeners indicated the apparent position of 100-ms, 50-60 dB SPL, wideband noise-burst targets by orienting their heads. The targets were synthesized in virtual auditory space and presented over headphones. Faithfully synthesized targets were interleaved with targets for which the directional transfer function spectral notches were filled in, peaks were leveled off, or the spectral contrast of the entire profile was reduced or expanded. As notches were filled in progressively or peaks leveled progressively, elevation gain decreased in a graded manner similar to that observed as sensation level is reduced below 30 dB or, for brief sounds, increased above 45 dB. As spectral contrast was reduced, gain dropped only at the most extreme reduction (25% of normal). Spectral contrast expansion had little effect. The results are consistent with the hypothesis that loss of representation of spectral features contributes to reduced elevation gain at low and high sound levels. The results also suggest that perceived location depends on a correlation-like spectral matching process that is sensitive to the relative, rather than absolute, across-frequency shape of the spectral profile.

Behavioral and modeling studies of sound localization in cats: effects of stimulus level and duration

Sound localization accuracy in elevation can be affected by sound spectrum alteration. Correspondingly, any stimulus manipulation that causes a change in the peripheral representation of the spectrum may degrade localization ability in elevation. The present study examined the influence of sound duration and level on localization performance in cats with the head unrestrained. Two cats were trained using operant conditioning to indicate the apparent location of a sound via gaze shift, which was measured with a search-coil technique. Overall, neither sound level nor duration had a notable effect on localization accuracy in azimuth, except at near-threshold levels. In contrast, localization accuracy in elevation improved as sound duration increased, and sound level also had a large effect on localization in elevation. For short-duration noise, the performance peaked at intermediate levels and deteriorated at low and high levels; for long-duration noise, this "negative level effect" at high levels was not observed. Simulations based on an auditory nerve model were used to explain the above observations and to test several hypotheses. Our results indicated that neither the flatness of sound spectrum (before the sound reaches the inner ear) nor the peripheral adaptation influences spectral coding at the periphery for localization in elevation, whereas neural computation that relies on "multiple looks" of the spectral analysis is critical in explaining the effect of sound duration, but not level. The release of negative level effect observed for long-duration sound could not be explained at the periphery and, therefore, is likely a result of processing at higher centers.