basilar tuning

Meaning: Sound vibrations enter the cochlea at the oval window and set up a traveling wave along the basilar membrane. The membrane is narrow and stiff at the base (near the oval window) and wide and flexible at the apex, so the wave peaks at a position that depends on frequency. High frequencies peak near the base, low frequencies near the apex. Hair cells at the peak bend most, so the brain can infer pitch from which auditory nerve fibers are active. This location-to-pitch map is called tonotopic organization. The phrase "basilar tuning" is not a common textbook heading, but it points to this idea.

On the MCAT: Expect place theory (linked to Békésy) contrasted with frequency theory, in which firing rate matches the sound's frequency. Place coding accounts well for high pitches. Frequency coding helps with low pitches, and the volley principle extends it, with groups of neurons firing in turn. A typical question describes damage to hair cells at one end of the cochlea and asks which pitches are lost. Damage near the base impairs high-pitch hearing.

Common confusion: Students swap base and apex. The base is stiff, sits next to the oval window, and responds to high pitch. Also, pitch depends on frequency, whereas loudness depends on amplitude. The basilar membrane is in the inner ear (cochlea), not the middle ear.