Title

Neural encoding of interaural time differences in the midbrain of the unanesthetized rabbit

Date of Completion

January 2003

Keywords

Biology, Neuroscience

Degree

Ph.D.

Abstract

The auditory system has two primary functions: identification and localization of sound sources. A major cue for determining the location of low-frequency sounds is the interaural time difference (ITD) created by the unequal path lengths that a sound must travel to reach each ear. This dissertation examined how ITDs are coded in the inferior colliculus (IC) of the unanesthetized rabbit. ^ The basic anatomy and physiology of the IC have been well characterized. Counter to physiological and structural evidence for integration, a common assumption is that separate populations of neurons exist to code identification and localization cues. The coding of identification cues and localization cues are often studied separately. Here, amplitude modulation, a representative identification cue, was combined with ITDs. Neurons were able to simultaneously code modulation and ITDs. Modulation had a graded effect on firing rate from extreme enhancement to extreme suppression. Mechanisms to explain these effects were then studied. Physical differences in the modulated and unmodulated signals, including the interaural correlation, energy, and spectrum did not provide clear answers. Subsequently, the convergence of monaural and binaural inputs on single cells was tested by adding modulation to each ear separately. The results were consistent with adaptive mechanisms. The population of ITD-sensitive neurons differentially codes modulation by independently decreasing and increasing their firing rate to modulation while leaving ITD tuning unaffected. ^ Also addressed was the interaction of excitatory and inhibitory projections to the IC. The role of GABAergic inhibition in sharpening ITD tuning curves in the IC was studied using iontophoresis. The most salient effect GABA-antagonists was to substantially increase discharge rate. Blocking GABA caused the ITD tuning curves of many neurons to broaden. Therefore, GABA may keep a neuron at its optimal activity level, improving the signal-to-noise ratio and sharpening ITD tuning curves. Functionally, sharper ITD tuning curves may require fewer neurons to achieve a given level of acuity. ^ In summary, common neural mechanisms used to process a natural environment were explored. The structure of the IC and its place an obligatory stop in the auditory pathway make it an ideal nucleus for transforming the neural code. ^