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"Adaptation" in Hearing: "We Have No Idea How It Works"


ENV reported last week on the slow advance that researchers are making toward understanding the fantastically sophisticated "biological acoustic system" through which we hear what's going on in the world. Not only hear, but -- crucially -- edit out what's of interest to us from the wall of sound that washes over us in a noisy environment, focusing in on it to the exclusion of everything else:

Through biomimetics, these researchers are advancing science, both by contributing to the understanding of the brain's operation, and by improving audio processing technology. It's one more example of how science progresses through design-based approaches.

No less crucial to hearing is the ability not only to edit what comes in but also to adapt to different levels of volume, instantaneously, to avoid injury while sacrificing nothing by way of sensitivity. Thus when the jackhammer right outside my window at Discovery Institute falls momentarily silent, I'm immediately able to hear very soft sounds like a colleague's footfall as he turns the corner in the carpeted hall on the way to come visit me.

With a view to helping patients with hearing loss, researchers at Stanford wanted to know how this gift of "adaptation" works. Well, they thought they know already. So did everyone else. But everyone was wrong (emphasis added):

"I would argue that adaptation is probably the most important step in the hearing process, and this study shows we have no idea how it works," [Anthony] Ricci said. "Hearing damage caused by noise and by aging can target this particular molecular process. We need to know how it works if we are going to be able to fix it."


Deep inside the ear, specialized cells called hair cells detect vibrations caused by air pressure differences and convert them into electrochemical signals that the brain interprets as sound. Adaptation is the part of this process that enables these sensory hair cells to regulate the decibel range over which they operate. The process helps protect the ear against sounds that are too loud by adjusting the ears' sensitivity to match the noise level of the environment.

The traditional explanation for how adaptation works, based on earlier research on frogs and turtles, is that it is controlled by at least two complex cellular mechanisms both requiring calcium entry through a specific, mechanically sensitive ion channel in auditory hair cells. The new study, however, finds that calcium is not required for adaptation in mammalian auditory hair cells and posits that one of the two previously described mechanisms is absent in auditory cochlear hair cells.

Experimenting mostly on rats, the Stanford scientists used ultrafast mechanical stimulation to elicit responses from hair cells as well as high-speed, high-resolution imaging to track calcium signals quickly before they had time to diffuse. After manipulating intracellular calcium in various ways, the scientists were surprised to find that calcium was not necessary for adaptation to occur, thus challenging the 30-year-old hypothesis and opening the door to new models of mechanotransduction (the conversion of mechanical signals into electrical signals) and adaptation.

I love the candor: "This study shows we have no idea how it works." While research giveth understanding, it sometimes also taketh it away -- revealing that what we thought we know ain't really so. The application of that recognition, that humility, should of course extend far behind the science of hearing.