Localization dominance, a phenomenon of the precedence effect, refers to the dominance of directional cues conveyed by sound arriving directly from the source over cues conveyed by reflected copies on the perception of sound source location. One theory of localization dominance is that leading sounds suppress neural responses to lagging sounds (Yin, 1994 Litovsky & Yin, 1998 a, b). Neurons in auditory nuclei respond best to a leading sound and have a reduced response to a lagging sound, supporting this hypothesis. It has been proposed that GABA-ergic or glycinergic inhibition suppresses neural responses to lagging sounds (Yin, 1994 Fitzpatrick et al. 1995 Pollack & Burger, 2002). An alternative hypothesis states that cochlear processing in low-frequency hearing animals alters directional cues conveyed by the leading and lagging sound, emphasising those present in the leading sound (Tollin, 1998 Hartung & Trahiotis 2001). Responses of single neurons in the inferior colliculus (IC) of anaesthetised guinea pigs were recorded to binaural click pair stimuli. Responses of some neurons were recorded before, during, and after iontophoresis of either the GABAa receptor antagonist gabazine, or the glycine receptor antagonist strychnine. Blocking glycine did not decrease neural suppression of the lagging click in 8/10 neurons. Blocking GAB A did not decrease neural suppression of the lagging click in 11/16 neurons. The neural representation of directional cues in the output of low-frequency neurons to the leading click of a binaural click pair differed from those actually conveyed by the stimulus in 20/20 neurons. Examination of the responses of several such neurons indicated responses to the leading click represented a direction between that conveyed by the leading and lagging click. The results supported the hypothesis that cochlear processing of binaural click pairs alters directional cues conveyed by the stimulus. Limited support was also found for the hypothesis that GABA-ergic and glycinergic suppress lagging click responses in some neurons.

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The auditory system must represent sounds with a wide range of statistical properties. One important property is the spectrotemporal contrast in the acoustic environment. The level of some sounds varies only a little over frequency and time, while the level of other sounds can vary a lot. This raises a contrast problem for neural coding: auditory neurons, with a limited dynamic range of firing rates, must be able to efficiently encode the sound level fluctuations in both low and high contrast sounds. In this thesis, I show how the contrast problem is solved in the primary areas of the ferret auditory cortex (AI/AAF). One hypothesis is that different neurons specialise for representing sounds of different contrasts. I find little evidence for such specialisation in the auditory cortex. Rather, the system adapts its coding to operate under different contrast conditions. I demonstrate that neurons in A1/ AAF rescale their gain to partially compensate for the spectrotemporal contrast of recent stimulation. When contrast is low, neurons increase their gain, becoming more sensitive to small changes in the stimulus, without changing their tuning. I quantify these gain changes and find that they resemble divisive normalisation, a phenomenon observed in many other neural systems. In a given stimulus context, auditory cortical neurons determine their gain predominantly on the basis of spectrotemporally local statistics. By comparing neural responses in All AAF with those in the inferior colliculus (IC), I show that this adaptive strategy of the auditory cortex is not simply inherited from the IC. When stimulus contrast changes, IC neurons undergo a variety of different changes in coding, which are on average weaker than in cortex. Together, these results suggest that the auditory cortex attempts to divide out contrast in its representation of an ongoing stimulus. This appears to be a novel property of the higher auditory pathway.