# Physiological and Computational-Modeling Studies of Timbre Encoding in the Inferior Colliculus

> **NIH NIH F31** · UNIVERSITY OF ROCHESTER · 2022 · $46,752

## Abstract

Timbre, the quality that allows sounds to be distinguished when they are identical in pitch, level, and
duration, is a critical aspect of speech comprehension and music enjoyment. My proposal will fill a gap in neural
studies of timbre by testing the hypothesis that capture and off-CF inhibitory mechanisms lead to robust
representations of suprathreshold synthetic and natural-instrument timbre in the midbrain. To test my hypothesis,
I will record single-unit inferior colliculus (IC) responses from awake Dutch-belted rabbits. I will also develop a
new computational IC model based on these physiological responses.
 Spectral envelopes of harmonic sounds are correlated with the timbral perception of “brightness”. I
propose two mechanisms that contribute to spectral-envelope encoding: capture and off-characteristic frequency
(CF) inhibition. The first mechanism, capture, refers to the dominance of harmonics near spectral peaks over
auditory-nerve fibers tuned near the peaks. Capture is due to saturation of inner hair cells. Capture by a single
harmonic reduces the amplitude of low-frequency neural fluctuations in auditory-nerve fibers. Rates of IC
neurons are sensitive to low-frequency neural fluctuations as characterized by modulation transfer functions.
Ultimately, capture of auditory-nerve responses for fibers tuned near spectral peaks results in IC rate profiles
that encode spectral peaks. Preliminary results are partially consistent with spectral peaks of synthetic timbre
stimuli capturing peripheral responses, leading to a rate representation of salient spectral features in the
midbrain. However, another mechanism that could explain IC representations of timbre is off-CF inhibition, which
has been proposed to explain frequency-sweep sensitivity and psychophysical forward masking. A subcortical
computational model that features capture, but not off-CF inhibition, was able to predict preliminary responses
to synthetic timbre and narrowband tone-in-noise, but could not predict responses to wideband tone-in-noise or
natural timbre, indicating the need to update the model.
 I have developed experiments to test the hypothesis that timbre is robustly encoded in the midbrain via
capture and off-CF inhibition. Aim 1.1 will test the hypothesis that responses to wideband tone-in-noise are
strongly influenced by off-CF inhibition, and reducing the noise bandwidth increases the influence of capture. In
Aim 1.2 I will update a computational IC model by adding off-CF inhibition to test the hypothesis that capture and
off-CF inhibition are necessary to explain tone-in-noise stimuli. Aim 2.1 will test the hypothesis that the spectral
peak of a shaped harmonic complex, synthetic timbre, is robustly encoded in the IC over a range of
suprathreshold sound levels. Aim 2.2 bridges the gap between synthetic and natural timbre by recording
responses to real instrument sounds. Responses from Aim 2 will further test the new IC model. This project will
provide insight on t...

## Key facts

- **NIH application ID:** 10604855
- **Project number:** 1F31DC020630-01A1
- **Recipient organization:** UNIVERSITY OF ROCHESTER
- **Principal Investigator:** Johanna Fritzinger
- **Activity code:** F31 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2022
- **Award amount:** $46,752
- **Award type:** 1
- **Project period:** 2022-09-01 → 2025-08-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10604855

## Citation

> US National Institutes of Health, RePORTER application 10604855, Physiological and Computational-Modeling Studies of Timbre Encoding in the Inferior Colliculus (1F31DC020630-01A1). Retrieved via AI Analytics 2026-05-25 from https://api.ai-analytics.org/grant/nih/10604855. Licensed CC0.

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