This research program will evaluate the extent to which binaural information is processed hierarchically and represented topographically along the auditory neuraxis. Neural processing of binaural information begins with convergence at the superior olivary complex (SOC) of projections arising from the two ears, and it is commonly held that binaural interactions at this level establish a code that is relayed essentially unaltered along the ascending auditory pathway to the cortex. However, divergent monaural and binaural pathways converge at sites beyond SOC and thus provide structural substrates for the potential reprocessing of binaural information. This research program will ascertain the contribution of these higher level interactions of binaural information processing by comparing responses at SOC, inferior colliculus (IC) and primary auditory cortex (AI) to manipulation of average binaural level and interaural disparities in intensity and time. The responses of single neurons at these three levels of the primary auditory pathway in the gerbil (Meriones unguiculatus) will be assessed by extracellular recording and dichotic presentation of calibrated tones and noise. Preliminary data indicate several important differences in binaural responses at SOC and IC --- this program will determine the extent to which these differences represent emergent hierarchical processes. This detailed assessment of binaural processing will not only illuminate the emergence of new coding schemes in the ascending pathway, but will also permit examination in greater detail of the topographic organization of binaural responses. For example, this program will determine whether the gross binaural bands in AI mask a more specific organization based on sensitivities to interaural intensity differences and average binaural level. Ultimately, this examination of binaural processing at multiple levels of the auditory system will generate models of the synaptic mechanisms that shape binaural responses. In the final phase of this five year program, these mechanisms will be explored directly by intracellular recording. The knowledge gained from this research will extend our understanding of the principles of binaural information processing and, hence, our appreciation of the consequences of unilateral and bilateral hearing impairment.
