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Robust Rate-Place Coding of Resolved Components in Harmonic and Inharmonic Complex Tones in Auditory Midbrain
Yaqing Su1,2, Bertrand Delgutte3,4
1Eaton-Peabody Labs, Massachusetts Eye & Ear, Boston, Massachusetts 02114.
Summary
Researchers found a robust "rate-place" code for harmonic complex tones in the auditory midbrain. This neural code is less sensitive to sound level and harmonic relationships, suggesting inhibition enhances pitch perception.
Area of Science:
- Auditory Neuroscience
- Computational Neuroscience
- Acoustic Perception
Background:
- Harmonic complex tones (HCTs) in speech and music create pitch based on fundamental frequency (F0), especially when harmonics are resolved.
- A rate-place code, where resolved harmonics map to neural firing rates along the tonotopic axis, is known in the auditory nerve and cortex.
- Understanding of this coding in intermediate auditory stages, like the inferior colliculus (IC), is limited.
Purpose of the Study:
- To investigate the neural coding of resolved harmonics in the inferior colliculus (IC).
- To characterize the properties of the rate-place code for harmonic complex tones (HCTs) and inharmonic complex tones (IHCTs) in the IC.
- To explore the role of inhibition in shaping neural responses within the IC.
Main Methods:
- Single-neuron recordings were conducted in the inferior colliculus (IC) of unanesthetized rabbits.
- Neurons were stimulated with HCTs and IHCTs varying in fundamental frequency (F0) and sound level.
- A spectral receptive-field model, including broadband inhibition, was used to predict neural responses.
Main Results:
- Many IC neurons exhibited rate-place coding, with firing rate peaks aligning resolved harmonics to their characteristic frequency.
- This IC rate-place code was most prominent for F0 > 800 Hz, moderately level-dependent (40 dB range), and insensitive to stimulus harmonicity.
- A model with broadband inhibition provided a better fit to neural responses than an excitatory-only model, suggesting inhibitory enhancement of the rate-place code.
Conclusions:
- The inferior colliculus (IC) employs a robust rate-place code for resolved harmonics, which is more stable across sound levels than peripheral codes.
- This midbrain code is insensitive to the harmonic relationships between frequency components, highlighting a transformation in auditory processing.
- Inhibition likely plays a crucial role in refining and enhancing the rate-place representation of pitch information in the auditory pathway.
Keywords:
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