A patient suffers noise-induced hearing loss after prolonged exposure to very loud high-frequency sounds. Which part of the cochlea is most likely damaged, and why?
AThe apex, because high frequencies cause peak displacement there
BThe base near the oval window, because high frequencies cause peak displacement there
CThe entire basilar membrane equally, because loud sounds affect all regions
DThe middle of the cochlea, because that region is most mechanically vulnerable
Tonotopy maps high frequencies to the base of the cochlea (near the oval window) and low frequencies to the apex. Because loud sounds damage the region of peak displacement, high-frequency noise destroys outer hair cells near the base first. This is why noise-induced hearing loss characteristically affects high-frequency perception before low-frequency perception — and why the apex (option A) is wrong despite being what many students incorrectly assume from 'base' meaning 'fundamental.'
Question 2 Multiple Choice
When a sound arrives slightly earlier at the right ear than the left, which brain structure primarily processes this cue, and for what purpose?
AThe lateral superior olive, to compute interaural level differences for high-frequency localization
BThe medial geniculate nucleus, to relay frequency information to cortex
CThe medial superior olive, to detect microsecond interaural timing differences for low-frequency localization
DThe auditory cortex, which reconstructs spatial position from spectral patterns
Interaural time differences (ITDs) — microsecond differences in sound arrival time between ears — are processed by the medial superior olive (MSO), which contains coincidence-detection neurons tuned to specific delays. ITDs dominate localization for low-frequency sounds. High-frequency localization relies instead on interaural level differences (ILDs) processed by the lateral superior olive (LSO). Confusing ITD/ILD and MSO/LSO is extremely common; the key link is: timing → MSO → low frequencies; level → LSO → high frequencies.
Question 3 True / False
Outer hair cells in the cochlea can actively contract in response to basilar membrane motion, amplifying vibration at their characteristic frequency.
TTrue
FFalse
Answer: True
Outer hair cells are active mechanical amplifiers — they contain the motor protein prestin and can rapidly change their length in response to membrane depolarization, boosting basilar membrane motion at their characteristic frequency by up to ~40 dB. This active mechanism sharpens frequency tuning and extends the dynamic range of hearing. It is also the reason ototoxic drugs and loud noise cause such severe hearing loss: destroying outer hair cells removes this amplification, dramatically reducing sensitivity and frequency resolution.
Question 4 True / False
The auditory cortex functions primarily as a passive relay station that simply decodes the frequency-sorted signals arriving from the medial geniculate nucleus.
TTrue
FFalse
Answer: False
The auditory cortex is an active, hierarchical processor analogous to visual cortex, not a passive decoder. It performs complex pattern analysis — extracting spectral and temporal features that distinguish vowels from consonants, familiar voices from unfamiliar ones, and music from noise. Cortical responses are not merely a readout of tonotopic input; they are shaped by attention, context, and learning. Single-axis tonotopic organization visible in primary auditory cortex gives way to increasingly complex feature selectivity in higher auditory areas.
Question 5 Short Answer
Why does the basilar membrane act as a mechanical Fourier transform, and why does this matter for how frequency information is encoded in the auditory nerve?
Think about your answer, then reveal below.
Model answer: The basilar membrane is narrow and stiff near the base (oval window) and wide and flexible near the apex, creating a continuous gradient of mechanical resonance. Different sound frequencies cause peak displacement at different positions — high frequencies at the base, low frequencies at the apex. Because hair cells sit at fixed positions along this membrane, each hair cell responds maximally to one frequency (its characteristic frequency). The auditory nerve therefore encodes frequency spatially (place coding): different nerve fibers carry information about different frequencies, and this tonotopic map is preserved through all auditory brainstem nuclei up to cortex.
The key insight is that the brain doesn't need to compute frequency — the mechanics of the cochlea perform this decomposition before any neural processing begins. This is why cochlear damage causes frequency-specific hearing loss (damage at the base → high-frequency loss; damage at the apex → low-frequency loss), and why cochlear implants with electrode arrays positioned along the cochlea can partially restore frequency discrimination by stimulating tonotopically appropriate positions.