responding more to the high frequency stimulus and others to the low frequency stimulus. This likelihood notwithstanding, unit responses to high frequency sounds were suppressed by the presence of a low frequency sound for azimuth separations, and unit responses to low frequency sounds were suppressed by the presence of a high frequency sound for elevation separations. Multiunit recordings increase confidence that this remarkable phenomenon is a general property of the entire population of tectal units. The stimulus location that owls orient toward behaviorally corresponds to the location represented in the space map .16 ms after sound onset (Figs. 4C and 6B). The transition from an initial representation of both stimuli to a differential representation of just one stimulus progresses during the first 8 ms of the neural response in the OT (Fig. 4C). This implies that the owl’s decision of where to orient, if based on OT activity, is determined by the pattern of neural activity more than 16 ms after sound onset, at least when the representation of stimulus location is shifting dynamically (Fig. 4C) due to conflicting spatial cues. When the level of the subordinate sound is much greater than that of the dominant sound, the owl’s orientation responses to the paired stimuli become variable and, in some cases, bimodally distributed (Fig. 2). Neural recordings from the OT space map exhibit a similar pattern. The bimodal distribution of late neural responses when the level of the high frequency sound is much greater than that of the low frequency sound (Fig. 7, lower right), indicates that when the relative level of the subordinate sound increases sufficiently, the representation of the location of the subordinate sound increases. A multiplicative rule can explain localization cue dominance A multiplicative rule for input integration would enhance responses when cues are mutually consistent and would suppress responses when cues are mutually contradictory. A multiplicative rule has been shown previously to operate in the ICX, the processing step before the OT [19,20]. A multiplicative rule, applied to the neural population data (Figs. 4 and 6), can account qualitatively for the dominance of low frequency cues in azimuth as well as for the dominance of high frequency cues in elevation. In azimuth, the low frequency sound drives no responses when the stimulus is more than 25u to the side of the RF center (Fig. 4D,E; L30u). According to a multiplicative rule, an absence of low frequency input would cancel the effect of the high frequency input, as observed in responses to both sounds together (Fig. 4D,E; black curve). In contrast, the high frequency sound continues to drive responses when the stimulus is located 25u or more to the side of the RF center (Fig. 4D,E; R30u). According to a multiplicative rule, the continued high frequency input would enhance responses to the low frequency sound, as observed. Similarly, the data from the individual site shown in Fig. 3 are consistent with a multiplicative rule operating on sub-threshold inputs [19,20] that exhibit the same spatial patterns as those of the population data. A multiplicative rule could also account for the dominance of high frequency cues when sounds are separated in elevation (Fig. 6D). The high frequency stimulus did not drive responses when the sound was located 30u or more above or below the RF center (Fig. 6D, +30u). According to a multiplicative rule, an absence of high frequency input would cancel responses to the low frequency input, as observed. In contrast, the low frequency sound continued to drive responses when the stimulus was more than 30u from the RF center (Fig. 6D, 230u), and this continued drive Hierarchy of Auditory Cues would enhance responses to the high frequency sound, as observed. Relative weighting of low and high frequency cues The information that is provided by a localization cue depends on its spatial resolution and on the spatial ambiguity in interpreting the cue. The spatial resolution of a cue depends both on the rate at which the cue’s value changes with source location and on the ability of the auditory system to discriminate those values. The spatial ambiguity in interpreting cue values arises because most cue values are produced by sounds from many different locations. We propose that, for localizing sounds in elevation, high frequency cues dominate over low frequency cues because of the superior spatial resolution of the high frequency cues and the higher neural gain afforded to high frequency channels. In elevation, the acoustic data show a 3-fold higher rate of change of the ILD cue (dB/deg) for the high frequency sound than for the low frequency sound (Fig. 8, left panels). In addition, in the brainstem nucleus that measures ILD, frequencies above 5 kHz are over-represented and the ILD sensitivity of neurons tuned to frequencies above 5 kHz is greater than that of neurons tuned to lower frequencies [21]. These factors are consistent with the sharper elevational tuning for the high frequency sound relative to the low frequency sound that we observed (Fig. 6). Another factor that favors the representation of the high frequency sound is that, on average, the high frequency sound drives nearly twice as many spikes as does the low frequency sound when each sound is presented alone (Fig. 5). The stronger response to the high frequency inputs may reflect the fact that only the higher frequencies contain high-resolution information about the eleva- tion of the source. Since barn owls are aerial predators, information about the elevation of an auditory stimulus is essential to targeting prey. In contrast, both high spatial resolution and low spatial ambiguity favor the low frequency cues when localizing in azimuth. The rate of change of the high frequency IPD cue (radians/deg) is twice as large as that for the low frequency cue Figure 8. Acoustic spatial cues generated by the low and high frequency sounds. ILD (left) and IPD (right), averaged across 8 owls, for the low frequency (3–5 kHz) and high frequency (7–9 kHz) sounds as a function of the elevation and azimuth of the speaker. IPD and ILD were averaged across the frequency range of each sound. doi:10.1371/journal.pone.0010396.g008 PLoS ONE | www.plosone.org 9 April 2010 | Volume 5 | Issue 4 | e10396