The Synaptic Mechanisms Underlying Binaural Interactions in Rat Auditory Cortex
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The interaural level difference (ILD) is a sound localization cue first computed in the lateral superior olive (LSO) by comparing the loudness of sounds between the two ears. In the auditory cortex, one class of neurons is excited by contralateral but not ipsilateral monaural sounds. These "EO" neurons prefer ILDs where contralateral sounds are louder than ipsilateral sounds. Another class, the "PB" neurons, are unresponsive to monaural sounds but respond predominantly to binaural ILDs, when both ears receive simultaneous sounds of roughly equal loudness (0 ILD). Behavioral studies show that ILD sensitivity is invariant to increasing sound levels. However, in the LSO, ILD response functions shift towards the excitatory ear as sound level increases, indicating level-dependence. Thus, changes in firing rate can indicate either a change in sound location or sound level, or both. This suggests a transformation in level-sensitivity between the LSO and the perception of sound sources, yet the location of this transformation remains unknown. I performed recordings in the auditory cortex of the rat to test whether neurons were invariant to overall sound level. I found that with increasing sound levels, ILD responses were level-dependent, suggesting that level invariance of ILD sensitivity is not present in the rat auditory cortex. In general, neurons follow one of two processing strategies. The tuning of cortical cells typically follows the "inheritance strategy", such that the spiking output of the cell matches that of the excitatory synaptic input. However, cortical tuning can be modified by inhibition in the "local processing strategy". In this case, neurons are prevented from spiking at non-preferred stimuli by inhibition that overwhelms excitation. The tuning strategy of cortical neurons to ILD remains unknown. I performed whole-cell recordings in the anesthetized rat and compared the spiking output with synaptic inputs to ILDs within the same neurons. I found that the PB neurons showed evidence of the local processing strategy, which is a novel role for cortical inhibition, whereas the EO neurons utilized the inheritance strategy. This result suggests that an auditory cortical circuit computes sensitivity for midline ILDs. This dissertation includes previously published/unpublished co-authored material.