Sounds in a reverberant room can interfere with the direct sound source. The normal hearing (NH) auditory system has a mechanism by which the echoes, or reflections, can be minimized. This is called the precedence effect (PE). The precedence effect is a perceptual phenomenon where the cues of the preceding direct sound, the “lead,” dominate over later-arriving reflections, the “lag.” Varying the temporal delay between the lead and lag determines the PE. Furthermore, it has been primarily studied in NH listeners using pure tones or broadband stimuli. There are two key features of the PE: Fusion: At short lead-lag delays (LLDs), a single sound source can be perceived (Fig. 1c). The echo threshold (ET) is the delay at which two sound sources are perceived (Fig. 1b). NH ETs are in the range of 5-10 ms (Litovsky et al., 1999). Lateralization/Localization Dominance: Short LLDs allow the percept of the leading sound to dominate the overall perception of where the sound is located in space. The underlying mechanism that facilitates the PE is the binaural network in the brainstem that processes interaural timing differences (ITDs), or very short time delays between the ears, these are on the scale of microseconds (Brown and Stecker, 2013). In the current study, we manipulated lead-lag delays of narrowband noise bursts to assess the effect of frequency on the fusion and localization dominance in the PE over headphones. While all NH listeners exhibited PE similarly in the narrowband noise conditions according to two different frequencies (4 kHz and 10 kHz), these echo thresholds were slightly elevated in relation to previous PE literature (Litovsky et al., 1999; Brown and Stecker, 2013). The comparable effect seen across frequencies, in both the assessment of fusion and lateralization, shows that the PE has a strong presence across frequencies regarding a more complex signal; the use of narrowband stimuli may have contributed to these elevated echo thresholds. Furthermore, this data has implications for cochlear implant listeners because information is processed in these listeners through electrodes that have narrow frequency bands, therefore it may be possible to observe the PE in this population as well. Future studies will test cochlear implant listeners with this paradigm to explore if they are able to perceive sounds of a lead-lag pair at varying delays. Subjects: 6 normal hearing listeners; 3 male and 3 female ranging ages from years old. Stimuli presented over ER2 ear-insert headphones in a soundproof booth. Stimuli: “Lead-lag” pairs of either a 10 kHz or 4 kHz narrowband noise burst, presented in separate blocks; a low-pass noise masker was present in additional blocks. Stimuli were played at opposing timing differences of -500 µs and +500 µs with LLDs varied from 1-64 ms. Task: Simultaneous fusion (hearing one vs. two locations) and lateralization (indicating location of fused image in upper panel, or location of left-most of two images in lower panel) task. Participants were asked to touch on the screen to indicate where they heard the sound. Listeners completed 350 trials (50 trials at each LLD). METHODS Poster # Precedence Effect in Normal Hearing Listeners Frieda Powell, Tanvi Thakkar, and Ruth Litovsky University of Wisconsin-Madison, USA SRI Presentation Madison, WI 4 November, 2014 WAISMAN CENTER INTRODUCTION Figure 3: Average responses to whether one or two sounds were perceived (marked with points, with individual responses behind) depending on frequency and on presence of maskers LOCALIZATION DOMINANCE RESULTS Figure 4: Points show average lateralization responses for trials on which subjects reported one (black) or two (red) sounds. Participants performed as expected. CONCLUSIONS Brown AD, Stecker GC (2013). “The precedence effect: Fusion and lateralization measures for headphone stimuli lateralized by interaural time and level differences.” J Acoust Soc Am. 133(5): Brown AD et al. (2013). “The Precedence Effect: Insights from electric hearing.” Presented at the 16th Conference on Implantable Auditory Prosthesis, Lake Tahoe, CA Litovsky RY, Colburn HS, et al (1999). “The precedence effect.” J Acoust Soc Am. ;106(4 Pt 1): Wallach et al. (1949). “The precedence effect in sound localization” Am J Psychol. 62(3): REFERENCES We would like to thank all our participants and Cochlear Ltd for providing equipment and technical assistance. A special thank you to Shelly Godar for helpful comments and Andrew Brown for development of the software and program. NIH-NIDCD (R01 DC to RYL), and NIH-NICHD (P30 HD03352 to Waisman Center). ACKNOWLEDGEMENTS Binaural Hearing and Speech Laboratory FUSION RESULTS Listeners exhibited the PE in all four conditions tested: 4 kHz noise bursts with and without masker: average thresholds were comparable across these conditions (~16 ms). 10 kHz noise bursts with and without masker: average thresholds were comparable across these conditions (~16 ms). ETs for the different frequencies exhibited the same range of thresholds, illustrating that there was no effect of frequency. 4 kHz 4 kHz + Masker 10 kHz 10 kHz + Masker Lateralization/Localization Dominance: Localization dominance was generally robust in both the 4 and 10 kHz conditions, meaning that there was also no effect of frequency in the location of the lead vs. the lag when listeners were presented opposing locations of a lead-lag pair. Two locations were always reported when the LLD was above or near the ET, and one location was always reported at short LLDs below the ET. “One Location” “Two Locations” LLD = 1-64 ms Left Right Lead, +500 µs ITD Lag, -500 µs ITD Figure 2: a) and b) Image of screen where listeners indicated whether they heard one or two locations and where the sound was heard. Listeners were asked to judge the intracranial location and number of images heard on each trial. If two images were perceived, listeners were instructed to indicate the left-most image in the lower of 2 panels. c) Stimuli presented to listeners; stimuli were single click pairs (lead and lag), played at opposing ITDs and varying lead-lag delays (1-64 ms). Figure 1: Perception of a direct sound and a reflection according to the delay between them; a) Illustrates he location of the direct sound in relation to the listener, b) when the LLD is greater than the ET (on average when the LLD is greater than 5-10 ms), two sounds are perceived, c) when the LLD delay is below the ET (or less than 5-10 ms), only the leading/ direct sound is perceived. c. b. c. a.a. b. a. LLD > 5-10 ms LLD < 5-10 ms