People with normal hearing have the ability to listen to a desired target sound while filtering out unwanted sounds in the background. However, most patients with hearing impairment struggle in noisy environments, a perceptual deficit which current hearing aids and cochlear implants cannot resolve. Even though peripheral dysfunction of the ears undoubtedly contribute to this deficit, surmounting evidence has implicated central processing in the inability to detect sounds in background noise. Therefore, it is essential to better understand the underlying neural mechanisms by which target sounds are dissociated from competing maskers. This research focuses on two phenomena that help suppress background sounds: 1) dip-listening, and 2) directional hearing. When background noise fluctuates slowly over time, both humans and animals can listen in the dips of the noise envelope to detect target sound, a phenomenon referred to as dip-listening. Detection of target sound is facilitated by a central neuronal mechanism called envelope locking suppression. At both positive and negative signal-to-noise ratios (SNRs), the presence of target energy can suppress the strength by which neurons in auditory cortex track background sound, at least in anesthetized animals. However, in humans and animals, most of the perceptual advantage gained by listening in the dips of fluctuating noise emerges when a target is softer than the background sound. This raises the possibility that SNR shapes the reliance on different processing strategies, a hypothesis tested here in awake behaving animals. Neural activity of Mongolian gerbils is measured by chronic implantation of silicon probes in the core auditory cortex. Using appetitive conditioning, gerbils detect target tones in the presence of temporally fluctuating amplitude-modulated background noise, called masker. Using rate- vs. timing-based decoding strategies, analysis of single-unit activity show that both mechanisms can be used for detecting tones at positive SNR. However, only temporal decoding provides an SNR-invariant readout strategy that is viable at both positive and negative SNRs. In addition to dip-listening, spatial cues can facilitate the dissociation of target sounds from background noise. Specifically, an important cue for computing sound direction is the time difference in arrival of acoustic energy reaching each ear, called interaural time difference (ITD). ITDs allow localization of low frequency sounds from left to right inside the listener's head, also called sound lateralization. Models of sound localization commonly assume that sound lateralization from interaural time differences is level invariant. Here, two prevalent theories of sound localization are observed to make opposing predictions. The labelled-line model encodes location through tuned representations of spatial location and predicts that perceived direction is level invariant. In contrast, the hemispheric-difference model encodes location through spike-rate and predicts that perceived direction becomes medially biased at low sound levels. In this research, through behavioral experiments on sound lateralization, the computation of sound location with ITDs is tested. Four groups of normally hearing listeners lateralize sounds based on ITDs as a function of sound intensity, exposure hemisphere, and stimulus history. Stimuli consists of low-frequency band-limited white noise. Statistical analysis, which partial out overall differences between listeners, is inconsistent with the place-coding scheme of sound localization, and supports the hypothesis that human sound localization is instead encoded through a population rate-code.
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