Ablation behavior experimentation on the hindbrain auditory system has led to the conclusion that the superior olivary complex (or SOC) in the medulla serves the auditory system as an "acoustic chiasm". The primary evidence is that unilateral lesions in the auditory pathway anywhere below this structure (that is, in the cochlea, cochlear nucleus, trapezoid body) result in sound localization deficits either in both hemifields of auditory space or in the hemifield ipsilateral to the lesion. In sharp contrast, unilateral lesions in the pathway anywhere above the SOC (that is, in the lateral lemniscus or beyond) result in sound localization deficits only in the hemifield contralateral to the lesion. Therefore, there is a clear functional parallel between the SOC and the optic chiasm even though there is no obvious anatomical parallel. The neuroanatomical and neurochemical substrate and the behavioral consequences of this chiasmatic process are the focus of the parent grant. The present FIRCA proposal will extend the parent grant's anatomical focus to the auditory forebrain, in particular to auditory cortex. In addition, it will extend the parent grant's behavioral focus (on azimuth discrimination mechanisms) to include moving sound-sources and possibly, more fundamental capacities of hearing (i.e., time-by-azimuth, frequency or intensity discriminations). Because the project entails rigorous psychophysical tests of hearing capacities before and after selective cortical ablations, the research should contribute to the understanding of auditory dysfunction in humans resulting from stroke or cortical injury.