NOT SIGNIFICANT Abstract Listening in noisy environments is difficult. Neural ”de-noising” mechanisms exist to improve the perceptual signal-to-noise ratio in such environments. This de-noising can be quantified using a “co- modulation masking release” (CMR) paradigm. People with hearing loss find noisy environments particularly troublesome, and have much weaker CMR. We tested the hypothesis that non-linear signal processing in the normal-hearing cochlea is the basis of CMR. Hypothesis: Frequency-dependent suppressive non-linearities in the cochlea result in a neural correlate of CMR in the auditory nerve. Introduction FREQUENCY SOUND LEVEL Unexpected improvement in masked signal-detection threshold when coherent (across-frequency) amplitude modulation is applied to a broadband masker [1]. Inner Hair Cells: Afferent Signal Transduction. Outer Hair Cells: “Cochlear Amplifier”. Adapted from Sachs & Kiang (1968) Frequency (kHz) Sound Pressure Level Adapted from Ruggero et al. (1992) Two-tone suppression: an (almost) instantaneous mechanical phenomenon – NOT neural inhibition! Co-Modulation Masking Release Begins in the Auditory Periphery Kareem R. Hussein 1 , Agudemu Borjigan 1 , Mark Sayles M.D., Ph.D. 1,2 1 Weldon School of Biomedical Engineering, Purdue University 2 Department of Speech, Language, & Hearing Sciences, Purdue University Methods Surgical preparation and recording • Anesthetized chinchilla (Chinchilla lanigera). • ketamine/xylazine/diazepam • Single-unit spike-time recording from ANFs • Dorsal fossa approach • Ipsi-lateral cerebellotomy • High impedance glass pipettes (30–90 MΩ) Acoustic stimuli • Presented signal tone, masked by a SAM tone at fiber’s center frequency (CF) and band-stop Gaussian “flanking band” noise. • Three acoustic signals used similar in past CMR studies [2,3] • RF: SAM + Signal. • CM: Signal + SAM + FB in phase with SAM. • CD: Signal + SAM + FB out of phase with SAM. Analyses: • Constructed tuning curve to find the unit’s CF and threshold, both used to design the CMR stimuli. • Vary sound pressure level of a band-stop Gaussian noise in the presence of a CF tone to find the noise level where maximum suppression of the CF-tone driven response occurs. • Present RF, CD, and CM stimuli in randomized order, for 20 repetitions. Construction of CMR acoustic stimuli Signal tone (a 50-ms long CF-tone pip) presented three times, centered on the last three amplitude envelope minima of the SAM tone on-frequency masker (OFM). Prediction Results REFERENCE CO-MODULATED CO-DEVIANT increasing signal level 0 5 10 15 20 25 30 35 40 45 50 0 1 2 3 4 5 6 7 Notched Noise Level (dB re. Threshold in quiet) Signal:Masker Synchronized−Rate Ratio experiment: 24−May−2017 track: 4 unit: 4 SR: 0 per sec. RF CM CD Histogram of LSR fiber showing firing rates effects from RF, CM and CD stimulus conditions with increasing signal level as you go down (top). Signal:Masker Sync- Rate Ratio as a function of increasing signal level (left). increasing signal level 0 5 10 15 20 25 30 35 40 45 50 0 0.5 1 1.5 2 2.5 3 Notched Noise Level (dB re. Threshold in quiet) Signal:Masker Synchronized−Rate Ratio experiment: 07−Jun−2017 track: 2 unit: 7 SR: 42 per sec. RF CM CD Histogram of HSR fiber showing firing rates effects from RF, CM and CD stimulus conditions with increasing signal level as you go down (top). Signal:Masker Sync- Rate Ratio as a function of increasing signal level (left). Signal:Masker Sync-Rate Ratio for different noise types. • Think of this measure as a signal-to-noise ratio, in the response dimension rather than the acoustic. • Low-pass noise maskers have most effect compared to notched- and high-pass noise maskers. Signal:Masker Sync-Rate Ratio differences between LSR and HSR fibers. Defines how much of the response is due to signal or masker. • In LSRs: CM, signal contributes more to response than masker, while for RF and CD, masker dominates the response. • In HSRs: the masker dominates for all conditions. Conclusions and Future Work • Proof of CMR in the nerve means relevant neural information is already present at the brain’s input. • Chinchilla ANF data support a role for cochlear non- linearities in mediating across-frequency co- modulation masking release. • Important to characterize the strength of this effect in the inputs to brainstem circuits. • This doesn’t mean there’s no role for brainstem (or cortical) processing in CMR. Acknowledgements Supported by start-up funds from the College of Engineering and College of Health & Human Sciences, Purdue University. KRH is supported by the SURF program. References [1] Hall J., et al. (1984). Detection in noise by spectro- temporal pattern analysis. The Journal of the Acoustical Society of America, 76, 50–56. [2] Pressnitzer D., et al. (2001). Physiological Correlates of Comodulation Masking Release in the Mammalian Ventral Cochlear Nucleus. The Journal of Neuroscience, 21, 6377– 6386. [3] Neuert V., et al. (2004). Responses of Dorsal Cochlear Nucleus Neurons to Signals in the Presence of Modulated Maskers. The Journal of Neuroscience, 25, 5789–5797. Basilar Membrane (top left), characteristic frequency of spontaneous rate fibers (top), and frequency tuning curves of several neurons (left). Description of constructed CMR acoustic stimulus conditions: Reference (RF), Co-modulated (CM), and Co-deviant (CD). Basilar membrane velocity responses to CF-tones (left) and frequency tuning curve showing suppressive regions represented in dashed blue and red lines (right). 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