How is loudness encoded in the auditory system?

• A. By the amplitude of basilar membrane vibration
• B. By the number of active hair cells and the firing rate of each active hair cell
• C. By the tympanic membrane displacement
• D. By the hair cell death rate

Answer: (B)

Loudness is encoded by the level of neural activity that reaches higher auditory centers—the brain relies on how many hair cells are activated and how fast the active cells fire. As sound intensity rises, more hair cells cross their thresholds and contribute to the signal, so more auditory nerve fibers are recruited. At the same time, the firing rate of each active hair cell’s afferent fiber tends to increase with intensity. The combination of increasing the number of active hair cells (more fibers firing) and increasing the firing rate of those cells provides a robust neural code for louder sounds. The mechanical motion of the basilar membrane and tympanic membrane are steps in the process, but loudness perception ultimately reflects the amount and pattern of neural activity produced by hair cells.