A recent development in human-computer interfaces is the virtual acoustic display, a device that synthesizes three-dimensional, spatial auditory information over headphones using digital filters constructed from head-related transfer functions (HRTFs). The utility of such a display depends on the accuracy with which listeners can localize virtual sound sources. A previous study [F. L. Wightman and D. J. Kistler, J. Acoust. Soc. Am. 85, 868-878 (1989)] observed accurate localization by listeners for free-field sources and for virtual sources generated from the subjects' own HRTFs. In practice, measurement of the HRTFs of each potential user of a spatial auditory display may not be feasible. Thus, a critical research question is whether listeners can obtain adequate localization cues from stimuli based on nonindividualized transforms. Here, inexperienced listeners judged the apparent direction (azimuth and elevation) of wideband noisebursts presented in the free-field or over headphones; headphone stimuli were synthesized using HRTFs from a representative subject of Wightman and Kistler. When confusions were resolved, localization of virtual sources was quite accurate and comparable to the free-field sources for 12 of the 16 subjects. Of the remaining subjects, 2 showed poor elevation accuracy in both stimulus conditions, and 2 showed degraded elevation accuracy with virtual sources. Many of the listeners also showed high rates of front-back and up-down confusions that increased significantly for virtual sources compared to the free-field stimuli. These data suggest that while the interaural cues to horizontal location are robust, the spectral cues considered important for resolving location along a particular cone-of-confusion are distorted by a synthesis process that uses nonindividualized HRTFs.

Two experiments are described in which listeners judge the apparent directions of virtual sound sources-headphone-presented sounds that are processed in order to simulate free-field sounds. Previous results suggest that when the cues to sound direction are preserved by the simulation, the apparent directions of virtual sources are nearly the same as the apparent directions of real free-field sources. In the experiments reported here, the interaural phase relations in the processing algorithms are manipulated in order to produce stimuli in which the interaural time difference cues signal one direction and interaural intensity and pinna cues signal another direction. The apparent directions of these conflicting cue stimuli almost always follow the interaural time cue, as long as the wideband stimuli include low frequencies. With low frequencies removed from the stimuli, the dominance of interaural time difference disappears, and apparent direction is determined primarily by interaural intensity difference and pinna cues.

A fundamental issue in human development concerns how the young infant's ability to recognize emotional signals is acquired through both biological programming and learning factors. This issue is extremely difficult to investigate because of the variety of sensory experiences to which humans are exposed immediately after birth. We examined the effects of emotional experience on emotion recognition by studying abused children, whose experiences violated cultural standards of care. We found that the aberrant social experience of abuse was associated with a change in children's perceptual preferences and also altered the discriminative abilities that influence how children categorize angry facial expressions. This study suggests that affective experiences can influence perceptual representations of basic emotions.

Listeners reported the apparent spatial positions of wideband noise bursts that were presented either by loudspeakers in free field or by headphones. The headphone stimuli were digitally processed with the aim of duplicating, at a listener's eardrums, the waveforms that were produced by the free-field stimuli. The processing algorithms were based on each subject's free-field-to-eardrum transfer functions that had been measured at 144 free-field source locations. The headphone stimuli were localized by eight subjects in virtually the same positions as the corresponding free-field stimuli. However, with headphone stimuli, there were more front-back confusions, and source elevation seemed slightly less well defined. One subject's difficulty with elevation judgments, which was observed both with free-field and with headphone stimuli, was traced to distorted features of the free-field-to-eardrum transfer function.

This article describes techniques used to synthesize headphone-presented stimuli that simulate the ear-canal waveforms produced by free-field sources. The stimulus synthesis techniques involve measurement of each subject's free-field-to-eardrum transfer functions for sources at a large number of locations in free field, and measurement of headphone-to-eardrum transfer functions with the subject wearing headphones. Digital filters are then constructed from the transfer function measurements, and stimuli are passed through these digital filters. Transfer function data from ten subjects and 144 source positions are described in this article, along with estimates of the various sources of error in the measurements. The free-field-to-eardrum transfer function data are consistent with comparable data reported elsewhere in the literature. A comparison of ear-canal waveforms produced by free-field sources with ear-canal waveforms produced by headphone-presented simulations shows that the simulations duplicate free-field waveforms within a few dB of magnitude and a few degrees of phase at frequencies up to 14 kHz.