Brown AD, Jones HG, Kan A, Thakkar T, Stecker GC, Goupell MJ, Litovsky RY. Evidence for a neural source of the precedence effect in sound localization. J Neurophysiol 114: 2991-3001, 2015. First published September 23, 2015 doi:10.1152/jn.00243.2015.-Normal-hearing human listeners and a variety of studied animal species localize sound sources accurately in reverberant environments by responding to the directional cues carried by the first-arriving sound rather than spurious cues carried by later-arriving reflections, which are not perceived discretely. This phenomenon is known as the precedence effect (PE) in sound localization. Despite decades of study, the biological basis of the PE remains unclear. Though the PE was once widely attributed to central processes such as synaptic inhibition in the auditory midbrain, a more recent hypothesis holds that the PE may arise essentially as a by-product of normal cochlear function. Here we evaluated the PE in a unique human patient population with demonstrated sensitivity to binaural information but without functional cochleae. Users of bilateral cochlear implants (CIs) were tested in a psychophysical task that assessed the number and location(s) of auditory images perceived for simulated source-echo (lead-lag) stimuli. A parallel experiment was conducted in a group of normal-hearing (NH) listeners. Key findings were as follows: 1) Subjects in both groups exhibited lead-lag fusion. 2) Fusion was marginally weaker in CI users than in NH listeners but could be augmented by systematically attenuating the amplitude of the lag stimulus to coarsely simulate adaptation observed in acoustically stimulated auditory nerve fibers. 3) Dominance of the lead in localization varied substantially among both NH and CI subjects but was evident in both groups. Taken together, data suggest that aspects of the PE can be elicited in CI users, who lack functional cochleae, thus suggesting that neural mechanisms are sufficient to produce the PE. sound localization; precedence effect; cochlear implants SOUND SOURCE LOCALIZATION subserves predator avoidance, prey capture, situational awareness, and, in humans and many other species, communication (for recent reviews see Grothe et al. 2010;Stecker and Gallun 2012). Although acoustic cues to sound location are degraded in many environments by reflections and reverberation, localization accuracy is generally robust across environments. This finding is often attributed to the "precedence effect" (PE), a well-known auditory phenomenon in which observers respond to the cues carried by the firstarriving sound, traveling directly from the source to the ears, rather than spurious cues carried by later-arriving signal reflections (Wallach et al. 1949; for review see Brown et al. 2015;Litovsky et al. 1999). The PE, conventionally studied with simulated echoes in a laboratory setting, consists of two primary phenomena termed fusion and localization dominance. Fusion refers to the perception of early-arriving (leading) and late-arriving (lagging) signals as ...