What is the significance of the stiffness gradient in the basilar membrane?

• A. It makes all frequencies activate the same region.
• B. It allows different frequencies to activate specific regions of the membrane.
• C. It reduces sensitivity to high-frequency sounds.
• D. It has no impact on frequency encoding.

Answer: (B)

The stiffness gradient along the basilar membrane creates a mechanical frequency-to-place map. The base is stiff and narrow, so it responds best to high frequencies, while the apex is more compliant, preferring low frequencies. A sound’s traveling wave peaks at a location whose position depends on its frequency, causing hair cells at that region to transduce the vibration and send signals that reflect the frequency in a spatial pattern along the cochlea. This place coding underlies the tonotopic organization of auditory nerve fibers, enabling different frequencies to activate specific membrane regions. If all frequencies activated the same region, or if the gradient didn’t affect encoding, we wouldn’t be able to distinguish pitches as effectively.