Agapiou, J.P.; (2007) Asymmetry in neural responses to interaurally time-delayed stimuli. Doctoral thesis , University of London.

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Abstract

The interaural time difference (ITD) between sounds at the two ears is an important cue in low-frequency sound localisation. Sensitivity to ITD is created in the medial superior olive (MSO), where neurons show an ITD-dependent response tuned to some preferred ITD. The traditional model of ITD sensitivity suggests that this best ITD arises from a difference in the length of projections from the two ears to the MSO. However, recent experiments in gerbil MSO instead indicate that glycinergic inhibitory mechanisms are responsible for establishing a neuron's best ITD. This study examined the role of mechanisms thought to underlie the establishment of the best ITD and their effects on shape of ITD tuning curves. The predictions of models of these mechanisms were then compared to responses to both pure-tone and broadband noise stimuli, recorded from binaural nuclei in the midbrain. Asymmetrical responses were observed, indicating some frequency-dependence in the mechanisms determining best delay. In addition, an envelope-sensitive component to the noise responses was observed at lower frequencies than previously reported. While the observed asymmetry was qualitatively consistent with the effect of carving inhibition, the traditional model of ITD sensitivity could not be distinguished from other mechanisms capable of introducing frequency-dependent delays. However, this study shows that mechanisms usually considered only to affect the best ITD, can have profound effects on ITD tuning curve shape. These effects can have a strong impact on the functional encoding of ITDs beyond the establishment of the best ITD.

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