Questions: Auditory System: Cochlea to Auditory Cortex

Question 1

A person suffers noise-induced hearing loss that permanently damages hair cells at the base of the cochlea. What pattern of hearing loss do you predict, and why?

• A. Loss of low-frequency hearing, because the base is closest to the incoming sound and processes all frequencies first
• B. Loss of all frequencies equally, since damage anywhere disrupts the entire basilar membrane's function
• C. Loss of high-frequency hearing, because high frequencies maximally displace the cochlear base — where the basilar membrane is narrow and stiff
• D. Loss of medium-frequency hearing, because the base processes a broad mid-range band

Answer: (C)

Tonotopy maps frequency to cochlear location: the base is narrow and stiff, maximally responsive to high frequencies; the apex is wide and flexible, maximally responsive to low frequencies. Damage to basal hair cells is therefore frequency-specific — it selectively eliminates high-frequency hearing. This is why noise-induced hearing loss (typically from high-intensity sounds with strong high-frequency components) is characterized by high-frequency deficits before low-frequency ones. The tonotopic map means cochlear damage is not just a reduction in overall sensitivity but a spatially organized pattern of deficit.

Question 2

Which sequence correctly describes how sound energy is converted into a neural signal in the cochlea?

• A. Sound wave → eardrum → ossicles → basilar membrane deflection → stereocilia bend → ion channels open → hair cell depolarizes → glutamate released → spiral ganglion neuron fires
• B. Sound wave → eardrum → direct nerve stimulation → cochlear nucleus → auditory cortex
• C. Sound wave → cochlear fluid pressure → direct spiral ganglion neuron activation → auditory cortex
• D. Sound wave → basilar membrane → ossicles → stereocilia → eardrum → neural signal

Answer: (A)

Mechanotransduction in the cochlea is a multi-step cascade: air pressure variations move the eardrum, which drives the ossicles (three small bones), which transmit vibrations into the cochlear fluid, which deflects the basilar membrane at the frequency-specific location. Hair cell stereocilia atop the basilar membrane bend with the deflection, mechanically opening tip-link ion channels. Potassium and calcium ions rush in, depolarizing the hair cell, which releases glutamate onto spiral ganglion neuron dendrites, generating action potentials in the auditory nerve. Each step is essential — mechanical, chemical, and electrical transduction are all required.

Question 3

The tonotopic organization established in the cochlea is progressively reorganized at each relay station (cochlear nucleus, inferior colliculus, auditory cortex) as processing becomes more complex.

• T. True
• F. False

Answer: False

Tonotopy is preserved — not reorganized — at every level of the auditory pathway. The cochlear nucleus, superior olivary complex, inferior colliculus, medial geniculate body, and primary auditory cortex all maintain frequency maps where neighboring neurons respond to neighboring frequencies. This preservation of the cochlear frequency map throughout the hierarchy is a fundamental organizing principle of the auditory system, analogous to the retinotopic maps preserved in the visual system. What changes at each level is not the frequency map but the sophistication of processing — from simple frequency detection to complex feature extraction like speech sounds and melodic contour.

Question 4

High-frequency sounds maximally deflect the base of the cochlear basilar membrane because the base is the point of entry for sound waves and therefore receives stimulation first.

• T. True
• F. False

Answer: False

The tonotopy of the basilar membrane is determined by mechanical properties, not by proximity to the sound source. The base is narrow and stiff, giving it a high resonant frequency — it vibrates maximally in response to high-frequency input. The apex is wide and flexible, giving it a low resonant frequency — it vibrates maximally in response to low-frequency input. This gradient of stiffness and width transforms the cochlea into a biological frequency analyzer, performing a spatial Fourier decomposition of the sound wave. The physical geography (stiffness gradient) is the cause; the tonotopic map is the result.

Question 5

Explain how the physical properties of the basilar membrane allow the cochlea to perform frequency analysis — separating a complex sound into its component frequencies.

Model answer: The basilar membrane varies continuously in width and stiffness from base to apex: it is narrow and stiff at the base (high resonant frequency) and wide and flexible at the apex (low resonant frequency). A sound wave entering the cochlea creates a traveling wave along the basilar membrane that grows in amplitude until it reaches the location whose resonant frequency matches the sound's frequency, then rapidly decays. A pure tone therefore produces maximal displacement at one specific point; a complex sound produces maximal displacement at multiple points simultaneously — one for each frequency component. Each location activates different hair cells, which activate different auditory nerve fibers, sending frequency-labeled signals to the brain. The basilar membrane thus performs a real-time spatial Fourier transform on the incoming sound.

This is analogous to a piano, where different strings (analogous to different basilar membrane locations) resonate at different frequencies. The cochlea achieves the same decomposition continuously and passively through mechanical tuning, not active computation. The tonotopic map that results from this physical decomposition is then preserved through all subsequent neural processing — it is the fundamental organizing principle of the entire auditory system.