(Fig. 8, right panels). However, the capacity of the auditory system to encode IPD declines sharply with increasing frequency [22]. We found that the average azimuthal tuning for the high frequency sound was actually less sharp than that for the low frequency sound (width at half-max: high = 42u614u, low = 32u612u; Fig. 4D,E), implying that the decline in the auditory system’s capacity to encode IPD at high frequencies is more severe than the increase in the rate of change in IPD with sound azimuth. Moreover, the interpretation of the high frequency IPD cue is ambiguous even in frontal space, since equivalent IPD values correspond to different azimuths separated by about 50u (Fig. 8, matching colors). We propose, therefore, that for sound localiza- tion in azimuth, low frequency cues dominate over high frequency cues, because of their superior spatial resolution and low spatial ambiguity. The amplitude of the sound that provides the cue is another factor that influences the contribution of a cue in the determina- tion of stimulus location. As the relative level of a frequency band increases, the neural representation of the sound’s location becomes progressively more influenced by the spatial information provided by that frequency band (Fig. 7). This neurophysiological effect could explain the shift in the distribution of behavioral responses that was observed when the amplitude of the subordinate (high frequency) sound was increased to well beyond that of the dominant sound (Fig. 2). In summary, the data indicate that when inferring the location of a sound source, the auditory system weights the information provided by different cues based on their relative spatial resolution, spatial ambiguity, and the relative amplitude of the sound that provided the cue. The representation of multiple sound sources In this study, we used multiple sound sources to create discrepant spatial cues. Previous studies have used multiple sounds for similar purposes. One group of studies employed the ‘‘precedence effect’’ whereby in response to slightly asynchronous sounds from different locations the auditory system attributes the location from the later sound to the location of the earlier sound. In this case, the auditory system groups the sounds, and differentially weights the spatial cues provided by the earlier sound [23]. Neurophysiological studies have revealed a strong suppression in the representation of the second sound in the auditory space map [24,25,26]. In other studies, paired, simultaneous sounds with identical waveforms were presented from different locations to produce a ‘‘phantom image’’ in the auditory space map that was located in between the locations of the two individual sounds [27]. This example of the ‘‘stereo effect’’ is due to acoustic interactions and not to neural processing. Studies most similar to ours involved presenting owls with simultaneous sounds from different locations, but with overlapping frequency spectra [28]. Unlike in the stereo experiments, the References 1. Blauert J (1997) Spatial Hearing: The Psychophysics of Human Sound Localization. Cambridge, MA: MIT Press. 2. Best V, Gallun FJ, Carlile S, Shinn-Cunningham BG (2007) Binaural interference and auditory grouping. J Acoust Soc Am 121: 1070–1076. 3. Heller LM, Trahiotis C (1996) Extents of laterality and binaural interference effects. J Acoust Soc Am 99: 3632–3637. 4. McFadden D, Pasanen EG (1976) Lateralization of high frequencies based on interaural time differences. J Acoust Soc Am 59: 634–639. 5. Heller LM, Trahiotis C (1995) Interference in detection of interaural delay in a sinusoidally amplitude-modulated tone produced by a second, spectrally remote sinusoidally amplitude-modulated tone. J Acoust Soc Am 97: 1808–1816. 6. Buell TN, Hafter ER (1991) Combination of binaural information across frequency bands. J Acoust Soc Am 90: 1894–1900. Hierarchy of Auditory Cues waveform microstructure differed between the two sounds in these experiments. Under these conditions, the owl’s auditory system represents the locations of both stimuli. This is because the auditory system evaluates IPD and ILD cues on a millisecond time-scale and, when there are two sources, the relative amplitudes of each frequency component for each source varies on this time- scale. As a result, for any given frequency at any moment in time, one source tends to be represented preferentially and, over time, both sounds are represented. This suggests that the flickering of the represented IPD value and ILD value between two sets of values within frequency channels on a millisecond time-scale is a reliable indicator of the presence of two sources at different locations. In our study, we used non-overlapping frequency bands, thereby eliminating this within-frequency indicator of multiple sources. In the absence of this indicator, spatial information from simultaneous sounds may be integrated according to the rules for cue dominance revealed in this study. Comparison with human psychophysics When humans are presented with simultaneous sounds with non-overlapping spectra from different azimuths, localization is biased towards the location of the low frequency sound, an effect that is reminiscent of the effect we observed in barn owls. In humans, the results indicate that sound localization cues are weighted differentially according to the spatial resolution provided by each cue: the discriminability index (d’) of each cue was sufficient to quantitatively predict the rules of integration. In addition, spatial ambiguity may also influence the relative weightings of cues in humans, although the contribution of this factor has not been tested. For humans, spatial ambiguity is not a factor for interpreting IPD because our auditory system does not measure IPDs for frequencies high enough to produce the same IPD value from different azimuths [1,7]. Spatial ambiguity is a factor, however, for interpreting ILD cues. ILD cues follow complex spatial patterns, and the complexity of the spatial pattern increases with frequency [29]. If spatial ambiguity contributes to the dominance hierarchy of cues for deriving sound source location in humans, as it appears to in owls, then lower frequencies should continue to dominate the localization of higher frequencies even above 4 kHz, for which ILD cues are most important for human localization. A similarity in the rules for the dominance hierarchy of sound localization cues in humans and owls suggests that, in both species, the auditory system infers the location of a sound by weighting differentially the highest resolution and least ambiguous cues. Author Contributions Conceived and designed the experiments: lW EK. Performed the experiments: lW PK. Analyzed the data: lW. Contributed reagents/ materials/analysis tools: lW. Wrote the paper: lW EK. 7. Zwislocki J, Feldman RS (1956) Just noticeable differences in dichotic phase. J Acoust Soc Am 28: 860–864. 8. Licklider JCR, Webster JC, Hedlum JM (1950) On the frequency limits of binaural beats. J Acoust Soc Am 22. 9. Wagner H, Takahashi T, Konishi M (1987) Representation of interaural time difference in the central nucleus of the barn owl’s inferior colliculus. J Neurosci 7: 3105–3116. 10. Payne RS (1971) Acoustic Location of Prey by Barn Owls (Tyto Alba). Journal of Experimental Biology 54: 535–573. 11. Keller CH, Hartung K, Takahashi TT (1998) Head-related transfer functions of the barn owl: measurement and neural responses. Hearing Research 118: 13–34. 12. Bregman AS (1990) Auditory scene analysis: the perceptual organization of sound. Cambridge, Mass: MIT Press xiii: 773 p. PLoS ONE | www.plosone.org 10 April 2010 | Volume 5 | Issue 4 | e10396