Feature Stories

Hearing with Precision

New work on auditory signal processing could lead to better hearing
  • close up of ear with graph overlay

“Our inner ear can process sensory signals with remarkable precision, but it comes with the cost of vulnerability, making it very easily damaged by noise and by aging."

-Geng-Lin Li

The inner ear is a remarkable sensing apparatus, says Geng-Lin Li, assistant professor of biology. “The temporal precision of the human auditory system is awe-inspiring, even by the standards of modern physics,” he notes. For example, humans can localize a sound by analyzing the time difference in arrival at our two ears, which can be as brief as 10 nanoseconds, or one hundredth of a millisecond. Many musicians, with training, can reach amazing precision, distinguishing the difference between tones at 1,000 Hz and 1,001 Hz.

Li, who was recently was awarded a five-year, $1.6 million grant by the National Institutes of Health to study auditory signal processing in the inner ear, says, “Our inner ear can process sensory signals with remarkable precision, but it comes with the cost of vulnerability, making it very easily damaged by noise and by aging. As we advance our basic understanding of hearing and satisfy our curiosity, new approaches could arise, allowing us to design better protection for people who work in a noisy environment.”

Comparing this auditory precision to that of the visual system, Li points out that the moving picture industry is built on the fact that our visual system cannot detect the time gap between pictures when the frame rate is about 20 frames per second, which translates to a 50-millisecond gap between pictures. In other words, we perceive smooth movements in film because our visual system has much less temporal acuity, that is, less ability to detect a time gap, compared to our auditory system, he says.

At the same time, the neurobiologist notes, our auditory system is very vulnerable to noise in the environment, and mowing the lawn for two hours without protection could cause permanent damage to our hearing.

Li is a member of the campus’s Institute for Applied Life Sciences (IALS) and one of the leaders in its Models to Medicines Center. IALS director Peter Reinhart says, “Professor Li’s research program focuses on neuron-to-neuron communication through synaptic transmission, a process that appears to go awry in a number of neurological diseases.” He adds that this work is enabled by extensive core facilities established through IALS, in particular two-photon uncaging microscopy, confocal imaging and electrophysiology.

Li says that because the mammalian cochlea is hidden behind bones, his experiments will instead be carried out in bullfrog hearing organs, which can be easily dissected even in the adult. The researchers use the patch-clamp recording technique, making a small hole in the cell membrane with very thin glass needles filled with solution. “With this powerful technique, you can not only record cell signals precisely,” Li notes, “but also stimulate cells precisely, which are apparently crucial in studying the signaling precision of the inner ear.”

He says hair cells in the inner ear are among the most difficult cells in the body to study because they involve extremely accurate communication of fast-changing signals. His experiments use two electrodes because he wants to target the communication between hair cells and auditory afferent fibers (AAFs) that connect the inner ear to the brain. “Each time, I use one electrode to stimulate a hair cell and another electrode to monitor responses on the connected AAF, and that tells me how the two communicate,” he explains.

Li’s student Fuu-Jiun Hwang, working on a Ph.D. in molecular and cellular biology, plus several undergraduates, including Anastasia Chobany, a communications disorders major, and Melisa Joseph, a biology major, are also gaining valuable lab experience working on this research project. The researchers hope that in five years they will be able to characterize calcium sensors in hair cells and neurotransmitter receptors in AAFs and propose new approaches in hearing protection.

UMass Amherst News Office