Physiological measures of the precedence effect and spatial release from masking in the cat inferior colliculus. R.Y. Litovsky 1,3 , C. C. Lane 1,2 , C. A. Atencio 1 and B. Delgutte 1,2 1 Massachusetts Eye and Ear Infirmary, Eaton Peabody Laboratory 243 Charles St. Boston MA 02114, USA 2 Harvard-MIT Speech and Hearing Sciences Program Cambridge, MA, USA 3 Boston University, Hearing Research Center 44 Cummington St., Boston MA 02215, USA litovsky@bu.edu 1. Introduction Human listeners localize and recognize auditory objects in complex acoustic envi- ronments. Little is known about the neural mechanisms underlying this ability, which is often degraded in the hearing impaired. We investigated responses of single-units in the anesthetized cat inferior colliculus (IC) for two stimulus situations that have some char- acteristics of complex environments and that depend on the location of the sound sources: (1) two brief stimuli that simulate a direct sound and a single reflection, and (2) a pair of simultaneous sounds, one of which may mask the other. The IC is an obvious target for these investigations because most IC neurons are sensitive to source direction and indi- vidual localization cues such as interaural time (ITD) and level (ILD) differences. (For a review, see Irvine, 1992). The first stimulus paradigm simulates the precedence effect (PE), which is the ob- servation that two sounds occurring in rapid succession are perceived as a single auditory object localized near the leading sound (Litovsky et al., 1999). Single-unit studies in the IC (Yin, 1994; Fitzpatrick et al., 1995; Litovsky & Yin, 1998a, b) have identified a pos- sible neural correlate of the PE in that the response to the lagging sound is suppressed for delays in which the PE occurs. A key question is whether these effects are due to a gen- eral suppressive mechanism akin to forward masking, or whether the neural suppression is specifically directional. To address this question, we characterize the relationship be- tween the directional neural responses to the leading sound and the lagging sound and how this relationship depends on individual localization cues. The second paradigm simulates a phenomenon called spatial release from masking (SRM), in which a signal is more easily detected when separated in space from a masker (e.g., Saberi et al., 1990). Single-unit studies of the IC have found that the neural detect- ability of a tone in noise can be improved if the signal ITD differs from the masker’s (Ji- ang et al., 1997). We extend these results to neural masking release in simulated free field, which depends on other localization cues (ILD and spectral features) besides ITD (Bronkhurst and Plomp, 1988).