Summary: New research challenges long-standing ideas about how we perceive sound. The study shows that many sensory cells in the inner ear respond together to low-frequency sounds, providing the brain with simultaneous input that may make low-frequency perception more reliable and robust.
Source: Linköping University
Researchers at Linköping University in Sweden, together with collaborators at Oregon Health & Science University in the USA, report findings that revise our understanding of how low-frequency sound is processed in the inner ear.
Published in Science Advances, the study offers insights that could influence future cochlear implant design and improve outcomes for people with severe hearing loss.
Humans rely heavily on hearing to communicate and connect. Sound enters the outer ear, moves the eardrum, and travels into the spiral-shaped cochlea in the inner ear. There, specialized sensory cells called inner and outer hair cells convert mechanical vibrations into nerve signals the brain interprets as sound.
For about a century, the dominant view held that each sensory cell in the cochlea has a narrow “best” or optimal frequency at which it responds most strongly. Under that model, a cell tuned to 1000 Hz would respond far less to tones even slightly above or below that frequency, and the cochlea would be organized as a strict place code where different frequencies map to different locations.
This new study shows that the low-frequency region of the cochlea behaves differently. In the part of the cochlea that encodes sounds below roughly 1,000 Hz—the range that contains many vowel sounds of speech and many musical notes—many sensory cells respond at the same time to the same low-frequency tones. Instead of a sharp place-based frequency map, responses and temporal delays are similar across this low-frequency region.
“Many cells in the inner ear react simultaneously to low-frequency sound,” says Anders Fridberger, professor in the Department of Biomedical and Clinical Sciences at Linköping University. “We believe this collective response makes it easier to perceive low-frequency sounds, because the brain receives synchronous input from multiple sensory cells.”
That synchrony may also make low-frequency hearing more resilient. If some sensory cells are damaged, other nearby cells that respond to the same low-frequency signals can still supply information to auditory pathways, preserving perception to a greater extent than would be expected under a strict place-code model.
The finding has clear implications for human speech and music perception: vowel sounds and many musical tones—middle C on the piano, for example, is about 262 Hz—fall into the low-frequency range where this collective coding is observed. Understanding how the cochlea naturally encodes these frequencies can inform therapies and devices designed to restore hearing.
Cochlear implants are the leading intervention for severe sensorineural hearing loss. These devices use electrodes inserted into the cochlea to stimulate auditory nerves electrically. Current implant strategies are largely based on the century-old assumption of sharply localized frequency mapping. The authors suggest revising stimulation strategies for low frequencies so that implants more closely mimic the natural, distributed activation pattern found in the healthy cochlea.
“Changing stimulation methods to better reflect how low-frequency regions are naturally stimulated could improve the hearing experience for implant users,” says Anders Fridberger. The research team is now exploring practical ways to apply these findings, including new stimulation methods aimed at the low-frequency portions of the cochlea.
The experiments were performed on guinea pig cochleas, chosen because their low-frequency hearing shares similarities with human hearing. Results from such animal models offer foundational knowledge that can guide future human-focused research and implant development.
Funding: This work was supported by the U.S. National Institutes of Health and the Swedish Research Council.
About this auditory neuroscience research news
Author: Karin Söderlund Leifler
Source: Linköping University
Contact: Karin Söderlund Leifler – Linköping University
Image: The image is in the public domain
Original Research: Open access. “Best frequencies and temporal delays are similar across the low-frequency regions of the guinea pig cochlea” by Anders Fridberger et al., Science Advances.
Abstract
Best frequencies and temporal delays are similar across the low-frequency regions of the guinea pig cochlea
The cochlea typically maps tones of different frequencies to distinct anatomical locations: high-frequency tones produce strong responses at one location while producing little response elsewhere. This place code has informed auditory neuroscience and device design for many years.
However, frequency selectivity in cochlear regions that encode low-frequency sounds had not been systematically examined. The present study shows that low-frequency hearing follows a different principle: sound-evoked responses and temporal delays are similar across the low-frequency region, indicating a distributed coding strategy rather than a strict place code.
These findings challenge long-standing theories about cochlear organization and have broad implications for understanding how the brainstem and cortex process auditory information, as well as for optimizing stimulus delivery in auditory implants.