In nature, sounds from an actively-emitting source arrives at the ears slightly before its echo, and this time advantage is thought to play a key role in our ability to perceive the location of the direct sound (localization dominance;LD) and be relatively unaware of the echoes and their points of origin (lag-discrimination suppression;LDS). LD and LDS, collectively known as the """"""""precedence effect"""""""", are thought to be the mechanisms by which we segregate acoustical signals of interest from background noise. This process declines with hearing loss and with age, and understanding the neural underpinnings is crucial to the design of assistive devices and rehabilitation strategies. While the fact that the direct sound arrives first is important in the precedence effect, experiments have also shown that signals without a arrival-time difference at the start of the stimulus can still evoke LD. The project proposed is to study LD and LDS in the absence of such onset cues in the barn owl, a nocturnal raptor that can hunt by hearing alone, guided by a map of frontal space in its midbrain. Pilot data suggests that owls, like humans, experience LD even without onset time cues. The project's hypothesis is that LD and LDS are due to consistent lead/lag relationships between the envelope features of the sound pair.
Aim 1 Localization Dominance. The hypothesis predicts that without the initial arrival-time difference, the strength of LD will depend on the similarity in the lead and lag envelope structures and the ability to sense the modulations of the envelope. The initial time-advantage is removed from the leading sounds by synchronously gating the sounds, and the envelope similarity and modulation frequencies of the sound pair are manipulated to determine the effect of these factors on LD. LD is assessed using the owl's natural propensity to turn its head in the perceived direction of a sound source.
Aim 2 Lag-Discrimination Suppression. Is LDS, the complementary phenomenon to LD, also influenced by the structure of the envelope? The owl's ability to discriminate the position of the lagging source will be investigated using the stimuli used in Aim 1. Discrimination is assessed with a newly developed, non-operant technique for probing the ability of subjects to detect small changes.
Aim 3 Neuronal Responses.
In Aim 3, we analyze the responses evoked by the synchronously gated stimuli in neurons of the owl's auditory space map. In first experiment, we test whether the suppression of responses to the lagging sound observed with conventional lead/lag stimuli (w/ initial transient disparity &ongoing envelope disparities) are also observable with synchronously gated stimuli. In a second experiment, we test a specific hypothesis about the nature of the suppression in the owl's space map that might allow for the resolution of temporal ambiguities that can arise when the envelope spectra are narrow.
In our daily environment, we are able to hear out sounds of interest and remain unaware of the echoes that accompany those sounds. Yet, the presence of echoes in a typical acoustical environment is easily witnessed by listening to a playback of a conversation recorded in, e.g., an office. Such soundtracks are often described as hollow or boomy, and these qualities are the result of echoes overlapping with the waves coming directly from the people conversing. Our project will determine how the brain segregates the sound coming directly from the source from the echoes that accompany it. We do so in the barn owl, a bird-of-prey that is especially adept at determining where sounds are coming from and can do so effectively in echo-ridden environments (e.g., barns). This ability to separate the acoustical signal from background noise declines with age and with hearing loss. The findings from our study will therefore help with the design of hearing aids that intelligently extracts of a signal from the background clutter.
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