MCAT content library / Psychology & sociology / Hearing and auditory coding
Follow sound from pressure vibration to cochlear mechanics and hair-cell signaling, then use frequency- and intensity-response data to localize an auditory deficit.
About 17 minutes, including cochlear-position and transduction experiments.
Useful foundations
On this page
- Sound is transmitted mechanically before it is coded neurally
- Concept: hair cells convert mechanical motion into neural output
- Worked application: frequency maps to cochlear position
- Pattern: intensity and frequency are different sound dimensions
- Rule: conductive and sensorineural deficits predict different measurements
- Check your reasoning
- Continue learning
- Sources
Sound is transmitted mechanically before it is coded neurally
Sound waves entering the external ear vibrate the tympanic membrane. The middle-ear ossicles transmit that mechanical vibration to the oval window, coupling air-borne pressure changes into fluid movement in the cochlea.
Within the cochlea, sound produces a traveling mechanical response along the basilar membrane. Different frequencies produce maximal responses at different positions. This mechanical frequency analysis occurs before auditory nerve firing represents the signal centrally.
A problem can therefore distinguish a transmission failure in the outer/middle ear from a receptor or neural-coding failure in the inner ear.
Concept: hair cells convert mechanical motion into neural output
Cochlear hair cells are mechanoreceptors. Movement of their stereocilia changes mechanically sensitive ion-channel activity, changing the hair cell’s membrane potential and neurotransmitter release onto auditory nerve fibers.
The hair cell does not simply “hear” a sound as a complete percept. It converts local mechanical motion into graded cellular signaling. Populations of auditory nerve fibers then carry frequency, timing, and intensity-related information into central auditory pathways.
Worked application: frequency maps to cochlear position
An original experiment records basilar-membrane displacement at three cochlear positions for three pure tones. Position 1 is near the base; Position 3 is nearer the apex. Values are normalized within the experiment.
| Tone | Position 1 | Position 2 | Position 3 |
|---|---|---|---|
| 8000 Hz | 1.00 | 0.25 | 0.05 |
| 2000 Hz | 0.22 | 1.00 | 0.28 |
| 400 Hz | 0.04 | 0.24 | 1.00 |
The position of maximal displacement changes systematically with frequency. If hair cells at Position 1 are selectively damaged, the strongest deficit should occur for high-frequency tones under this model, while lower-frequency responses centered farther away could remain relatively better preserved.
This is a place code for frequency. It does not imply that one hair cell alone determines the perceived pitch or that temporal coding is irrelevant at all frequencies.
Pattern: intensity and frequency are different sound dimensions
Increasing sound intensity increases the magnitude of the mechanical stimulus and can recruit stronger hair-cell and auditory-nerve responses. Changing frequency moves the pattern of maximal cochlear activation. A louder 400-Hz tone is not the same physical change as an 800-Hz tone.
An original hair-cell recording illustrates the distinction. At one best-frequency location, raising tone intensity from low to high increases receptor-response magnitude from 0.2 to 0.9. Changing the tone far away from that location’s best frequency reduces the response even if sound intensity is held constant.
Thus amplitude-related information and frequency-related information can be encoded simultaneously but should not be conflated.
Rule: conductive and sensorineural deficits predict different measurements
A conductive deficit reduces mechanical sound transmission before normal cochlear transduction. If a controlled experiment bypasses the impaired transmission route and directly drives cochlear vibration, receptor and neural responses can remain relatively intact.
A sensorineural deficit involving hair cells or auditory nerve pathways persists even when mechanical transmission to the cochlea is adequate. Measuring tympanic/ossicular transmission, cochlear mechanics, and neural output can therefore localize the affected stage.
Behavioral hearing loss alone does not identify the mechanism.
ACTIVE PRACTICE · EVIDENCE AND TRANSFER
Move the Tone Across the Cochlear Place Map
In the original basilar-membrane experiment, Position 1 lies near the cochlear base and Position 3 near the apex. Normalized displacement is measured at three sites for three equal-intensity pure tones.
Reported observations are from the lesson’s controlled teaching model. A preserved readout or group mean does not by itself establish a unique cellular or behavioral mechanism.
Check your reasoning
Choose an answer before opening its feedback.
Check 1
In the supplied tonotopic model, which cochlear region responds most strongly to the highest-frequency tone?
A. A region near the base
Correct. High-frequency displacement peaks nearer the base.
B. A region near the apex only
Review this choice. The apex is favored by lower frequencies in the model.
C. The tympanic membrane contains no frequency information
Review this choice. The question asks where cochlear displacement is maximal.
D. All cochlear positions must respond identically
Review this choice. The data show spatially selective responses.
Check 2
Mechanical vibration reaches the cochlea normally, but hair cells do not change membrane potential when their stereocilia move. Which stage is most directly defective?
A. Mechanotransduction in the hair cells
Correct. The mechanical stimulus arrives, but receptor conversion fails.
B. External-ear sound collection only
Review this choice. Cochlear vibration is already normal.
C. Lens accommodation
Review this choice. Accommodation is visual, not auditory.
D. Taste transduction
Review this choice. The measured failure is cochlear hair-cell signaling.
Check 3
A tone’s frequency remains 1000 Hz while its intensity increases. Which change is most directly expected?
A. Stronger mechanical and neural response magnitude without changing the stimulus frequency
Correct. Intensity and frequency are distinct dimensions.
B. The tone must become 2000 Hz
Review this choice. Greater intensity does not automatically double frequency.
C. The cochlea ceases to respond to all sound
Review this choice. Increased intensity generally strengthens the stimulus within safe modeled ranges.
D. Sound speed in the same medium must double
Review this choice. Intensity does not set sound speed.
Continue learning
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