The overall objective of this project is to study the ability of humans to accurately localize sound in a reverberant environment despite the presence of many echoes. We will focus on the precedence effect (PE), an auditory phenomenon that is thought to result from active suppression of spatial information from echoes which reduces their potential for confusing our localization of the primary sound sources. The PE is experienced under experimental conditions when two similar sounds are presented from different locations with a brief delay, and the listener perceives only one sound whose apparent location is heavily dominated by the first source. The project has three aims: FIRST to determine what are the spatial properties of a sound that result in its being treated as an "echo" by the nervous system. We will measure how the PE changes when the relative locations of the leading and lagging (simulated echoes) sounds are varied in free field, by comparing the delays at which the lag is localized independently, and measuring its overall contribution to sound localization. This study will be a direct comparison with physiological studies I have conducted which point to a possible substrate of the PE in the inferior colliculus (IC), a nucleus which is thought to be important in spatial hearing. In addition, we will conduct studies under headphone conditions using "virtual space" stimuli to determine the role that each of the localization cues (interaural differences in timing, level and spectrum) play in the PE. SECOND to test the hypothesis that the PE is a binaural phenomenon. Although it is often assumed that the PE is binaural, there is evidence suggesting that there is a monaural component to the PE. This issue is critical for understanding the mechanisms that the auditory system might utilize to process complex arrays of multiple stimuli with varying time delays, as experienced in reverberant rooms. THIRD to study the PE in hearing-impaired listeners who are known to have severe problems comprehending speech in reverberant environments. The integrity of the PE in people with unilateral or bilateral hearing losses has never been studied. It is quite possible that some of the problems experienced by these listeners are due to their reduced ability to actively suppress echoes. The hypothesis will be tested that hearing impaired listeners manifest "weaker" PE. This issue has great potential for developing new diagnostic tests for the hearing-impaired using PE tasks, as well as improving hearing aids.