Hierarchy of Auditory Cues Figure 3. Neural responses at a sample site to individual sounds alone and to paired sounds separated in azimuth. The stimuli were positioned relative to the broadband RF center. A. Raster plots representing responses to the low frequency sound alone (top raster), the high frequency sound alone (middle raster), or both sounds presented together (bottom raster). When both sounds were presented together, the high and low frequency sounds were separated in virtual space by 30u. The position of the low frequency sound is written in blue type on the y-axis, and the position of the high frequency sound is in red type on the y-axis. The values of the low and the high stimuli differed by 30u, reflecting the spatial displacement between the two stimuli. Sound onset = 0 ms; upward arrow indicates 20 ms, the demarcation between early and late responses. Red arrowheads indicate high frequency center; blue arrowheads, low frequency center. B. Same data as in A, but displayed as tuning curves representing average response rates plotted separately for the early (top; 0–20 ms) and late (bottom; 20–50 ms) time period. Red curves represent responses to the high frequency sound alone, blue curves to the low frequency sound alone, and black curves to both sounds together. The position of the low frequency sound is written in blue type on the x-axis, and the position of the high frequency sound is in red type on the x-axis. doi:10.1371/journal.pone.0010396.g003 of the high frequency sound (Fig. 4E, right side, black curve; two- tailed t-test comparing responses to most effective stimulus position, p,0.01), even though the response gain to the low frequency sound alone was substantially less than that to the high frequency sound alone during this period (p,0.005, two tailed t-test comparing responses to most effective stimulus position). Additionally, for both the early and late phases of the population responses, the response to the low frequency sound was enhanced by the added presence of the high frequency sound (Fig. 4D,E, blue versus black curves; p,0.05, two-tailed t-test for most effective low frequency sound position). In contrast, the response to the high frequency sound was suppressed by the added presence of the low frequency sound (Fig. 4D,E, red versus black curves; p,1023, two-tailed t-test for most effective high frequency sound position). To quantify the shift towards the representation of the low frequency sound’s location, we compared for each site the center of mass of the responses to both sounds together with the predicted center of mass based on adding the responses to each sound alone (Fig. 4F,G). This analysis demonstrated a shift in the representa- tion of the two sounds that favored the representation of the low frequency location, and the magnitude of the shift was significantly greater during the late phase of the response (early shift: 561u; late shift: 1162u; p,1023; two-tailed t-test). Neural responses to sounds separated in elevation OT responses were tuned for stimulus elevation, as well as for azimuth (23 out of 23 sites displayed statistically significant tuning in elevation for the high frequency sound; 18 out of 23 displayed statistically significant tuning for the low frequency; 1-way ANOVA, p,.05). The population average activity revealed that tuning in elevation was much sharper for the high frequency sound alone than for the low frequency sound alone (Fig. 6C,D). When the low and high frequency sounds were presented simultaneously from different elevations, the locations of both sounds were represented in the early response immediately after sound onset, but the location of the high frequency sound dominated the representation in the late response (Fig. 6A,B). Responses to the high frequency sound alone were enhanced by the presence of the low frequency sound (Fig. 6D, left side; red versus black curve, p,1023 for two tailed t-test comparing late responses to most effective stimulus position). In contrast, responses to the low frequency sound alone were suppressed by the presence of the high frequency sound when the low frequency sound was located near the RF center (Fig. 6D, blue versus black curve, p,.01 for two tailed t-test comparing late responses for low frequency sound at 5u). Consequently, late responses to both sounds centered on the location of the high frequency sound. Effect of relative sound level on the neural representation of location The effect of changing the relative levels of the low and high frequency sounds was tested at a subset of sites (n = 31). To enable a comparison with the behavioral data (Fig. 2), this test was done with the sources separated in azimuth. PLoS ONE | www.plosone.org 6 April 2010 | Volume 5 | Issue 4 | e10396