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Understanding the impact of auditory object formation on lateralization in bilateral cochlear implant listeners Tanvi Thakkar, Alan Kan, and Ruth Y. Litovsky University of Wisconsin-Madison, Madison, WI e-mail: [email protected] WAISMAN CENTER INTRODUCTION METHODS Binaural Hearing and Speech Laboratory CIAP Lake Tahoe, CA 2017 Hypothesis: If stimulation rate is a major component of lateralization and auditory object formation… 1. Asymmetric across-ear rates will weaken object formation and yield poor lateralization. 2. Symmetric across-ear rates will strengthen object formation and yield good lateralization. RESULTS I: How many sounds were reported as a single auditory object? Components that may influence a listener's ability to successfully attend to a signal in noise include: As expected, BiCI and NH listeners have a high degree of AOF when information across the ears is the same. However, when information is different across the ears, range of AOF differs between BiCI and NH listeners suggesting that detection of rate differences with simultaneous presentation across the ear is more difficult with electrical stimulation (Fig. 3) . As expected, when AOF is poor, ITD lateralization is also poor. However, a high degree of AOF did not guarantee good ITD lateralization in both NH and BiCI listeners (Fig. 3 & 4). Overall, our results suggest that having AOF even when information is different across the ears is insufficient for good lateralization for both populations of listeners. 1. Steiger H. and Bregman, A. (1982b). Competition among auditory streaming, dichotic fusion, and diotic fusion. Perception & Psychophysics. 1982, 32(2), 153-162. 2. Kan A., and Litovsky RY (2015). Binaural hearing with electrical stimulation. Hear Res. 2015 Apr;322:127-37. 3. M. A. Stellmack and H. Dye, “The combination of interaural information across frequencies : The effects of number and spacing of components , onset and harmonicity,” vol. 93, no. May 1993, pp. 2933–2947, 1993. 4. Carlyon RP., Long CJ., and Deeks JM (2008). Pulse-rate discrimination by cochlear-implant and normal-hearing listeners with and without binaural cues. J Acoust Soc Am. 2008 Apr;123(4):2276-86. Figure 3: Proportion of ‘One’ sound responses (“One-Left”, “One-Center”, or “One-Right”) averaged across listeners. Error bars represent standard error. BiCI listeners (N=8): Presented with biphasic electrical pulse trains using synchronized research processors (Cochlear RF Generator) at a single pitch- matched pair of electrodes (Fig. 1). NH listeners (N=12): Presented with a Gaussian enveloped tone (GET) acoustic pulse train centered at 4 KHz. Experiment: single-interval task with 6 response options (Fig 2). “One-Left” “One- Center” “One-Right” “Left- dominant” “Center” “Right- dominant” Figure 2: Response options Bilateral cochlear implant (BiCI) listeners RESULTS II: How many responses were correctly lateralized (NH Listeners)? NH listeners show AOF for a narrow range of asymmetric rates across the ears. NH listeners never report hearing one sounds when rates differ more than -15% and +25%. BiCI listeners show AOF for a broader range of asymmetric rates across the ears. Normal hearing (NH) listeners DISCUSSION Response patterns across listeners appear to differ: 1. IBK, IBF, and IBK no longer lateralize the ITD of the perceived single fused image with the introduction of dichotic pulse rates 2. ICD, ICJ, ICP, IBQ, and IAU, lateralize by making more “left” and “right” decisions in the dichotic pulse-rate conditions. BiCI listeners with poorer ITD sensitivity appear to show poor lateralization and greater AOF regardless of rate symmetry. Binaural Sensitivity Having good sensitivity to binaural cues can help with locating the direction of a single coherent object Have good binaural sensitivity, leading to good lateralization of interaural time differences (ITDs) 1 . When provided with symmetric or identical across-ear information, listeners perceive a coherent auditory object 3 . Lack of auditory object formation (AOF) may lead to poor lateralization of ITDs. Binaural sensitivity is limited in clinical processors from poor representation of the ITD in the signal. However, listeners demonstrate ITD sensitivity when listening with research processors 2 . Sound processors operate independently and may encode the same signal differently between the two ears 4 . It is unknown whether poor ITD sensitivity with sound processors is due to poor AOF. RESULTS III: How many responses were correctly lateralized (BiCI Listeners)? Figure 6: Proportion of lateralized responses reported as “one” sound per target ITD for each individual BiCI listener; ITDs tested shown in top right of each panel; each BiCI listener’s sensitivity to ITDs of symmetric rates is shown in the top left. Responses similar to prediction Perceived Location “One-Left” “One-Center” “One-Right” Rate in right ear (pps) Proportion reported as “one ” sound in the correct direction 0 0.5 1 |ITD|=±500 IBF |ITD|=±500 IBK |ITD|=±1000 IBZ 0 0.5 1 |ITD|=±500 ICD |ITD|=±500 ICJ |ITD|=±500 ICP 0 0.5 1 |ITD|=±500 IBQ |ITD|=±800 IAU ~52 μs ~62 μs ~58 μs ~203 μs ~264 μs ~638 μs ~203 μs ~671 μs Responses deviating from prediction ITDs (in μs): randomized trial-by-trial. BiCI: 0 (all), ±500 (5 listeners), ±800 (2 listeners), ±1000 (1 listener). NH: 0, ±700. Rate conditions (pulses per second, pps): Left: always 100 pps. Right: 75, 85, 99, 100, 101, 125, 150, 175, 200, 250, 300 pps, interleaved. 20 repetitions for each ITD and rate combination. BiCI listeners NH listeners Auditory Object Formation “I hear two sounds…” Auditory object formation occurs when pieces of information within and across ears are bound together to perceive one coherent object ACKNOWLEDGEMENTS We would like to thank all our participants and Cochlear Ltd for providing equipment. This work is funded by NIH- NIDCD (R03DC015321 to AK and R01DC003083 to RYL), and NIH-NICHD (U54HD090256 to Waisman Center). Proportion reported as “one ” sound in the correct direction Rate in right ear (pps) If we assume that good AOF and good lateralization is characterized by a peak in the responses only when the rates are symmetric across the ears, then we observe that… BiCI listeners actually show inconsistent lateralization and AOF when rates are interaurally asymmetric. Figure 5: Proportion of correctly lateralized responses per target ITD presented, averaged across the group of BiCI listeners. Error bars represent standard error. Proportion reported as “one ” sound from total trials Perceived Location “One-Left” “One-Center” “One-Right” “Two-Left” “Two-Center” “Two-Right” Proportion reported as “two ” sounds in the correct direction REFERENCES 0 0.2 0.4 0.6 0.8 1 Prediction 0 1 0.5 “Two-Left” “Two-Center” “Two-Right” 3 NH listeners appear to show some, yet weaker lateralization when rates are symmetric even in the presence of AOF. However, NH listeners poorly lateralize when rates are completely asymmetric and AOF is lacking. Figure 4: Proportion of correctly lateralized responses per target ITD presented, averaged across the group of NH listeners. Predicted results shown in inset. Error bars represent standard error. Proportion reported as “one ” sound in the correct direction Rate in right ear (pps) Perceived Location Proportion reported as “two ” sounds in the correct direction “One-Left” “One-Center” “One-Right” 0 0.2 0.4 0.6 0.8 1 Rate in right ear (pps) c) Figure 1: Stimulus examples of BiCI (a,b) and NH (c) pulse-rate conditions. Left:100 pps & Right: 150 pps (example) Asymmetric electric pulse train (BiCI) b) Asymmetric GET pulse train (NH) Left:100 pps & Right: 150 pps (example) Left:100 pps & Right:100 pps ITD (μs) Symmetric electric pulse train (BiCI) a) Accurate lateralization ??? ??? Inaccurate lateralization Weak object formation Strong object formation “I hear one sound…” Listener ID Age Years of CI experience (Bilateral) Etiology Electrode Pair # L R IBF 64 8 Heredity 12 12 IBK 75 6 Heredity 14 13 IBQ 84 10 Meniere's 14 7 ICJ 66 6 Childhood illness 20 16 IAU 70 12 Heredity (from birth) 12 11 ICD 58 7 Moderate-to- severe loss at 3 years old 4 2 ICP 54 4 Deaf from childhood 12 14 IBZ 49 8 Gradual hearing loss; Heredity 12 12 Table 1: BiCI demographic data 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Poster #T38b
