Hierarchy of Auditory Cues Figure 2. Effect of relative sound level on head orienting movements to simultaneous sounds separated in azimuth. Histograms of endpoints of head orienting movements for Owl D (left column) and Owl L (right column), representing response to all sound elevations. Either the sounds were equal level (1st row), or the level of the high frequency sound was greater than that of the low frequency sound by 10 dB (2nd row), 20 dB (3rd row), or 30 dB (4th row). In the plot, the low frequency sound location is represented by R15u (blue solid line), and the high frequency location is represented by L15u (red solid line), although in the experiments both relative positions were tested equally often. The dashed lines represent the average orientation to each sound presented alone (blue to the low frequency and red to the high frequency). Downward arrows indicate population averages. doi:10.1371/journal.pone.0010396.g002 The average response across the population of sampled sites (n = 46) is summarized in Fig. 4. Across the population, responses to the low and high frequency sounds presented alone were tuned for source azimuth. In response to the low frequency sound alone, units responded most strongly when the stimulus was centered in the RF (Fig. 4A, top panel) and did not respond when the stimulus was more than 25u to the side of the RF center. Similarly, in response to the high frequency sound alone, the units responded most strongly when the stimulus was centered in the RF (Fig. 4A, middle panel), but they also responded, though at a much lower level, when the source was as much as 45u to the side of the RF center (vertical spread of activity in Fig. 4A, middle panel), reflecting a broader average RF for the high than the low frequency stimulus (mean width at half-max: high = 42u614u, low = 32u612u; p,0.01, two-tailed t-test). In addition, the average response to the high frequency sound alone was 48% stronger than that to the low frequency sound alone. This difference in response strength reflected a greater response gain for the high than for the low frequency sound, as verified directly at a subset of sites (Fig. 5). When the low and high frequency sounds were presented simultaneously from different azimuths, the locations of both the low and the high frequency sounds were represented strongly at sound onset, but soon after, the location of the low frequency sound came to dominate the representation (Fig. 4A, bottom panel). The time-course of the population response (Fig. 4B) was used to define ‘‘early’’ (0–20 ms) and ‘‘late’’ response time periods (20–50 ms). The transition from the representation of both locations to a preferential representation of the location of the low frequency sound occurred during the early phasic response. The dynamics of the transition was analyzed by plotting the weighted average of the population response with 1 ms resolution (Fig. 4C). These data indicate that a rapid shift in the relative representations of the two locations occurred between 12 and 16 ms after sound onset, during which time the representation shifted from a representation centered at the location of the high frequency sound to a representation centered at the location of the low frequency sound. An analysis of the average activity patterns across the population of recording sites during the early and late phases of the response are shown in Fig. 4D,E. When both sounds were presented simultaneously (Fig. 4D, black curve), the strength of the early response to the two locations was not significantly different (two tailed t-test comparing responses to most effective stimulus position, p = 0.46), even though the response to the high frequency sound alone (Fig. 4D, red curve) was substantially greater than the response to the low frequency sound alone (Fig. 4D, blue curve; p,0.005, two tailed t-test comparing responses to most effective stimulus position). The response to the high frequency sound was suppressed by the presence of the low frequency sound (Fig. 4D; red versus black curve; p,1023, two-tailed t-test for most effective high frequency sound position), whereas the response to the low frequency sound was enhanced by the presence of the high frequency sound (Fig. 4D; blue versus black curve; p,0.05, two- tailed t-test for most effective low frequency sound position). The opposite effects of the low and high frequency sounds were even more pronounced during the sustained late phase of the response (Fig. 4E, right side). During the late phase, the location of the low frequency sound was more strongly represented than the location PLoS ONE | www.plosone.org 5 April 2010 | Volume 5 | Issue 4 | e10396