Hierarchy of Auditory Cues Figure 4. Summary of neural responses to individual sounds alone and paired sounds separated in azimuth. A. Population averaged responses as a function of stimulus azimuth and time post-stimulus onset (n = 46). Upper panel: responses to the low frequency sound (3–5 kHz).; middle panel: responses to the high frequency sound (7–9 kHz); lower panel: responses to both sounds together. Arrow on time axis demarcates the division between the early and late time period used in panels D and E. B. Normalized post-stimulus time histogram, averaged across all stimulus conditions plotted in A. C. Weighted average of responses to both sounds together as a function of time (from bottom panel of A). Solid circles: statistically shifted from 0 (p,.05; bootstrapped t-test); open circles: not significantly shifted from 0 (p..05; bootstrapped t-test). Leftward shifts: weighted average favors the high frequency location; Rightward shifts: weighted average favors the low frequency location. D. Same data as 4A, but displayed as tuning curves for the early time period (0–20 ms). E. Same data as 4A, but displayed as tuning curves for the late time period (20–50 ms). F. Histogram of the shift of the weighted average of the responses to both sounds together, relative to the weighted average of the additive prediction (the sum of the responses to each sound alone) for each recorded site for the early time period (0–20 ms). Positive values of shift represent a shift towards the low frequency location. G. Same as F, but for the late time period (20–50 ms). doi:10.1371/journal.pone.0010396.g004 Increasing the relative level of a sound increased the relative strength of the representation of its location when both sounds were presented together (Fig. 7). Nevertheless, the location of the low frequency sound continued to be represented differentially strongly, particularly during the late phase of the response, across the range of relative levels tested (Fig. 7, bottom row, black curve). Discussion When low (3–5 kHz) and high (7–9 kHz) frequency sounds of equal level occur simultaneously but originate from different locations, owls behave as though the sounds come from a single location. When the sound sources are separated in azimuth, owls Figure 5. Neural response as a function of average binaural level for the low and high frequency sound. Population averaged neural responses (n = 31) to the low (blue) or high (red) frequency sound for the late time period (20–50 ms). Responses at each site were aligned relative to the threshold for each sound and normalized by the maximum response to either sound. doi:10.1371/journal.pone.0010396.g005 tend to orient to the location of the low frequency sound; when they are separated in elevation, they tend to orient to the location of the high frequency sound. Thus, low frequency sounds dominate localization in azimuth, whereas high frequency sounds dominate localization in elevation. The pattern of neural activity in the OT space map can explain these remarkable behavioral results. In response to two simulta- neous low and high frequency sounds of approximately equal levels, the space map briefly represents the locations of both sounds, then shifts rapidly to a representation that heavily favors the dominant sound (the low frequency sound for sounds separated in azimuth and the high frequency sound for sounds separated in elevation) (Figs. 4,6). Thus, in response to spatially discrepant simultaneous sounds, the auditory space map rapidly and automatically suppresses the spatial representation of the subordinate stimulus and maintains the spatial representation of the dominant stimulus. The frequency-dependence of this effect indicates that the underlying mechanism must operate before the level of the OT, which receives inputs that are already broadly tuned to frequency. In the midbrain pathway that leads to the OT, the likely site is the ICX, where information converges across frequencies to create a map of space [18]. The dominance of the low frequency sound over the high frequency sound, observed for sounds separated in azimuth, was not due to low frequency masking of high frequency information. Low frequency spectral masking refers to the ability of a lower frequency sound to disrupt perception of a higher frequency sound. Our data cannot be explained by spectral masking because the frequency of the stimulus that dominated sound localization depended on whether the stimuli were separated in azimuth or elevation. Low frequency masking would predict low frequency dominance regardless of the direction of the spatial separation. Most of the data are based on recordings from multiple units. It is likely that some units in the multiunit recordings were PLoS ONE | www.plosone.org 7 April 2010 | Volume 5 | Issue 4 | e10396