Coding pitch and loudness, locating sound and the auditory cortex
- Mechanism
- For completeness
Pitch is read from where the cochlea vibrates most, loudness from how many fibres fire and how fast, and direction from tiny differences between the two ears.
Also called: Place theory, Decibel scale, Interaural time and level differences
See it in 3D: Cochlea · Vestibulocochlear nerve (VIII) · Auditory cortex · Geniculate bodies · Pons
What it is
The cochlea separates frequencies along its length, so each nerve fibre has a best frequency (place code); for low tones, fibres also fire in step with the wave (volley or phase-locking code). Loudness is coded by firing rate and by the number of fibres recruited, and is perceived on a compressed, logarithmic scale measured in decibels. Brainstem nuclei compare the two ears to locate sound, and the auditory cortex in the temporal lobe keeps the frequency map and analyses complex sounds such as speech.
How it works, step by step
- Each region of the basilar membrane resonates best to one frequency; inner hair cells there drive fibres tuned to it.
- Outer hair cells amplify the vibration of their own region (cochlear amplifier), sharpening tuning by tens of dB.
- Louder sound raises firing rate in tuned fibres and recruits neighbours along the membrane.
- Cochlear nuclei relay to both superior olives; the medial superior olive detects microsecond timing differences, the lateral olive compares levels.
- Signals ascend in the lateral lemniscus to the inferior colliculus, medial geniculate body and primary auditory cortex (Heschl's gyrus).
- Medial olivocochlear efferents return to the outer hair cells and reduce cochlear gain, protecting against noise and improving hearing in noise.
Equations
- I
- sound intensity
- I0
- reference intensity at the hearing threshold (10^{−12} W/m²)
+10 dB = ×10 intensity; +20 dB = ×100 intensity.
- P
- sound pressure
- P0
- 20 µPa
+20 dB = ×10 pressure.
Hearing reference values (young adult)
| Parameter | Value | Note |
|---|---|---|
| Frequency range | ≈ 20–20 000 Hz | narrows at high end with age |
| Most sensitive range | ≈ 1 000–4 000 Hz | |
| Normal conversation | ≈ 60 dB | |
| Risk of noise damage | ≥ 85 dBA with long or repeated exposure | |
| Maximum interaural time difference | ≈ 0.6–0.7 ms | sound from directly at one side |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- Noise-induced hearing loss first damages outer hair cells around 4 kHz, giving a typical audiogram notch.
- Presbycusis begins with high-frequency loss at the cochlear base, so consonants and speech in noise become hard to follow.
- Damage to auditory association cortex (dominant side) can impair understanding of speech despite normal hearing thresholds.
Test yourself
Think of the answer, then tap the question to see it.
A sound rises from 40 dB to 70 dB. By how much has its intensity increased?
1 000-fold: each 10 dB is ×10 intensity, so 30 dB is 10 × 10 × 10.
Which brainstem nucleus locates a low-frequency sound, and which cue does it use?
The medial superior olive, using the interaural time difference (arrival time between the two ears).
Why does a stroke in one auditory cortex not cause deafness in the opposite ear?
Auditory pathways cross at several levels from the cochlear nuclei upward, so each cortex receives input from both ears.
Where on the basilar membrane are high-frequency tones detected?
At the base, near the oval window, where the membrane is narrow and stiff; low tones peak near the apex.
Connections
See it in 3D5
The structures of Durgical's 3D anatomy where it happens.
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Inner Ear Anatomy and Cochlear Physiology (StatPearls (NCBI Bookshelf), 2024); Neuroanatomy, Temporal Lobe (StatPearls (NCBI Bookshelf), 2024); Noise-Induced Hearing Loss (National Institute on Deafness and Other Communication Disorders (NIH), 2024).
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Representative model, for educational purposes only.