Abstract
Despite their prevalence in nature, echoes are not perceived as events separate from the sounds arriving directly from an active source, until the echo's delay is long. We measured the head-saccades of barn owls and the responses of neurons in their auditory space-maps while presenting a long duration noise-burst and a simulated echo. Under this paradigm, there were two possible stimulus segments that could potentially signal the location of the echo. One was at the onset of the echo; the other, after the offset of the direct (leading) sound, when only the echo was present. By lengthening the echo's duration, independently of its delay, spikes and saccades were evoked by the source of the echo even at delays that normally evoked saccades to only the direct source. An echo's location thus appears to be signaled by the neural response evoked after the offset of the direct sound.
Highlights
In nature, sounds of interest are often followed by reflections from nearby objects
Echo-threshold is expected to correlate well with the length of the lag-alone segment. We investigated these alternative hypotheses in the barn owl, Tyto alba, an auditory predator whose sound localization is guided by activity on a topographic representation of auditory space in the external nucleus of its inferior colliculus [ICx, 18,19]
All cells had well-circumscribed spatial receptive fields (SRF; Fig. 1F) when assessed with 100 ms noise-bursts presented in virtual auditory space (VAS) [18,21,29]
Summary
Sounds of interest are often followed by reflections from nearby objects. Sounds arriving directly from the actively-emitting source dominate spatial perception. This phenomenon, known as localization dominance, is a major component of the precedence effect, a collection of auditory phenomena thought to allow for the segregation of direct sound from reflections [1,2,3,4]. As the delay between a direct sound and a reflection increases, subjects begin to report having heard the reflection. Localization dominance ends and echo-threshold is said to have been reached [5]. What causes the reflection to become perceptually salient?
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