Hierarchy of Auditory Cues Figure 1. Endpoints of head orienting movements in response to two spatially separated sounds. In A and C, acoustic stimuli were separated in azimuth; in B and D, stimuli were separated in elevation. A. Data from one owl (Owl L) when sounds were presented either alone or both sounds were presented together, separated in azimuth by 30u and at an elevation of +20u. Top: Azimuth of the high frequency sound was L15u; azimuth of the low frequency sound was R15u. Bottom: Azimuth of the high frequency sound was R15u; azimuth of the low frequency sound was L15u. Blue asterisks represent endpoints of head movements towards the low frequency sound alone; red crosses towards the high frequency sound alone; black circles towards both sounds together. The black cross represents the position of the zeroing visual stimulus; the colored crosses represent the position of the low (blue) and high (red) frequency sound. B. Data from Owl L when sounds were presented either alone or both sounds were presented together, separated in elevation by 30u and at an azimuth of 0u. Top: elevation of the high frequency sound was 215u; elevation of the low frequency sound was +15u. Bottom: elevation of the high frequency sound was +15u; elevation of the low frequency sound was 215u. C. Average endpoints of the head orienting movements for each of the 3 owls towards each sound alone, and towards both sounds together when the sounds were separated by 30u in azimuth. Sound elevation was either 220u, 0u, or +20u (all conditions randomly interleaved and all data are included in this plot). In the plot, the low frequency sound location is represented by R15u (blue dashed line), and the high frequency location by L15u (red dashed line), although in the experiments both relative positions were tested with equal frequency. Data from the different relative stimulus locations were combined because no statistical difference was observed in responses to sounds either to the right or left of the midline (two-tailed t- test, p..05). Error bars represent STD. 0u corresponds to the position of the visual target for the initial fixation. Each symbol represents a different owl: # is Owl B; % is Owl D; n is Owl L. D. Average endpoints for each of the 3 owls towards each sound alone, and towards both sounds together when the sounds were separated by 30u in elevation. Sound azimuth was either 220u, 0u, or +20u (all conditions randomly interleaved). The low frequency sound location is represented by +15u (blue dashed line), and the high frequency location by 215u (red dashed line), although in the experiments both relative positions were tested with equal frequency. Errorbars represent STD. Data from the different relative stimulus locations were combined because no statistical difference was observed in responses to sounds either above or below the visual plane (two-tailed t-test, p..05). Number of head movements reported for each owl: Owl B (200), Owl D (404), Owl L (480). We combined data from the relative orientations because no statistical difference was observed in orienting to sounds to the right and the left of the midline (two-tailed t-test, p..05). doi:10.1371/journal.pone.0010396.g001 (Fig. 3A, bottom raster; 3B, upper plot, black curve). Soon after stimulus onset, however, the pattern of the response changed: the site continued to respond only when the low frequency sound was in the RF (Fig. 3A, bottom, blue arrowhead; 3B, lower plot, black curve). Thus, when the low and high frequency sounds were presented together from different azimuths, the early response to the location of the high frequency sound was suppressed by the presence of the low frequency sound (p,1023, two-tailed t-test for most effective high frequency sound position). In contrast, the response to the low frequency sound was not significantly changed by the presence of the high frequency sound (p.0.05, two-tailed t-test for most effective low frequency sound position). PLoS ONE | www.plosone.org 4 April 2010 | Volume 5 | Issue 4 | e10396