Sound localization accuracy on vertical and horizontal planes.
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A bird needs to keep track not only of social interactions of conspecifics but also of their changing locations in space by determining their directions and distances. Current knowledge of accuracy in the computation of sound source location by birds is still insufficient, partly because physiological mechanisms of few species are studied in well defined laboratory settings, while field studies are performed in a variety of species and complex environments. Velocity gradients and reverberating surfaces may conceivably induce inaccuracy in sound source location (mainly elevation) by distorting the directional cues. However, most birds possess an inherently directional pressure difference receiver, which enhances the directional cues (mainly azimuth), and a computational mechanism in their auditory pathways to suppress echoes of redirected sound.
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Two potential sensory cues for sound location are interaural difference in response strength (firing rate and/or spike count) and in response latency of auditory receptor neurons. Previous experiments showed that these two cues are affected differently by intense prior stimulation; the difference in response strength declines and may even reverse in sign, but the difference in latency is unaffected. Here, I use an intense, constant tone to disrupt localization cues generated by a subsequent train of sound pulses. Recordings from the auditory nerve confirm that tone stimulation reduces, and sometimes reverses, the interaural difference in response strength to subsequent sound pulses, but that it enhances the interaural latency difference. If sound location is determined mainly from latency comparison, then behavioral responses to a pulse train following tone stimulation should be normal, but if the main cue for sound location is interaural difference in response strength, then post-tone behavioral responses should sometimes be misdirected. Initial phonotactic responses to the post-tone pulse train were frequently directed away from, rather than towards, the sound source, indicating that the dominant sensory cue for sound location is interaural difference in response strength.
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1. Binaurally time-shifted and intensity-unbalanced noise, delivered through earphones, induced owls to respond with a head-orienting behavior similar to that which occurs to free field auditory stimuli. 2. Owls derived the azimuthal and elevational coordinates of a sound from a combination of interaural time difference (ITD) and interaural intensity difference (IID). 3. IID and ITD each contained information about the azimuth and elevation of the signal. Thus, IID and ITD formed a coordinate system in which the axes were non-orthogonal. 4. ITD was a strong determinant of azimuth, and IID was a strong determinant of elevation, of elicited head turn.
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Listeners were tested under free head/torso movement conditions, using a brief pulsed broadband noise signal presented from any 1 of 20 loudspeakers in two intersecting hemicircumferential arrays: one in the horizontal plane, one in the vertical. The listeners' task was to judge the whereabouts of each signal while listening in normal, open-ears conditions or while wearing various types of standard or electronic earmuffs or compressible foam earplugs. Results showed that listeners could accomplish the task successfully in normal (open-ear) conditions. Results using earplugs, nonelectronic earmuffs, and two types of dichotic (dual microphone) electronic earmuffs showed decrements in performance similar to each other. These decrements took the general form of loss of appreciation of the vertical plane and of front-rear differences in the horizontal plane. Lateral discrimination was blurred but otherwise intact. Of two diotic (single microphone) electronic earmuffs, one completely disrupted localization function and the other came close to it.
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Current understanding gives predominant weight to stationary cues for auditory localization. Two experiments were conducted to investigate the possible existence of a dynamic cue. The first experiment involved localization of concealed sources in the upper median vertical plane (MVP) and showed, as expected, that elevation was not detectable with motionless listening when high-frequency energy was absent or when normal pinna function was distorted. Elevation under such conditions did become detectable with horizontal head rotations, provided low-frequency energy was present in the signal. This indicates that the basis of the dynamic cue is variation in the rate of transformation of low-frequency interaural time/phase differences. The second experiment involved localization of sources arrayed throughout upper and lower regions of the MVP and in the left lateral vertical plane (LVP); it showed that upper hemisphere sources can be distinguished somewhat from those in the lower hemisphere, even in motionless listening conditions, but more so with rotation. The greatest benefit for localization from rotation of the head appears to be gained for sources positioned in the front MVP.