‘Why don’t we all hear better?’

Rabinowitch frames his interest in this neural flip-flopping with a simple question: “When blind people, for instance, hear better, you can ask yourself, why don’t [the sighted] hear better in the first place? Why do we have to be blind in order to hear better?” he said. “It’s hard to understand without looking into the details.”

Studies in humans and other mammals can spot the different areas of the brain that switch on or off when senses are lost or boosted, but they often can’t go into more depth than that. Studying simpler animals like C. elegans allow researchers to pinpoint the exact cells — and even the molecules — involved.

To study how well the worms smell, Rabinowitch placed tiny amounts of different chemicals that smell like yummy worm treats near the animals and watched their behavior. (To Rabinowitch, they smell like rotten fruit, marzipan or butter.) Worms with super-smell start moving toward the chemical even when only tiny amounts of it are present; normal worms need more scent to respond.

Of the different scents, Rabinowitch said the marzipan odor is his favorite — and the worms love it too.

“I like to hang out with the worms and smell the marzipan,” he said.

Rabinowitch and his colleagues found that the neurons that respond to touch actively keep smell neurons in check in worms with normal senses. When those touch neurons become inactive, the smell neurons ramp up, uninhibited. So it could be that none of our senses are working to their full potential.

The team also pinpointed the molecular messenger that conveys signals from touch cells to smell cells, telling the latter to tone down. This type of molecule, known as a neuropeptide, could play a role in humans too, Rabinowitch said.

This type of cross-sensory mediation may be advantageous, the researchers speculated.

“It does make sense that there is a kind of preference for diversity of information than volume of information,” Rabinowitch said, but he emphasized that the brain’s predilection for many different, quieter signals over one strong input is as yet hypothetical.

Evolution may have selected for a sensory system with some space to flex, Bai said.

“You don’t want to always work at your extreme. The system will break that way,” he said. “You build stability by allowing room for change. When you lose one sense and the others compensate, that’s the stability built in.”

However, even in the simple worm, sensory plasticity is still a complex picture. The researchers also found that the worms lacking the sense of gentle touch have reduced abilities in some other senses, like feeling touch on their nose, a sense controlled by a different set of neurons than overall gentle touch. The scientists don’t yet understand why some senses are dampened while others are boosted.

The nitty gritty

There are parallels between microscopic worms that can’t feel their surroundings and blind humans, the researchers said. C. elegans worms do not have eyes, relying primarily on body touch to get around. They don’t need sight in their natural environment, living in dark, gritty soil or rotting fruit where they wouldn’t be able to see far anyway, Bai said.

“Humans are vision-dominant, but the worm is touch-dominant. That’s their eye,” he said. “They just have an eye all over their body.”

These findings make new predictions about how human senses could work, Rabinowitch said. Nobody’s yet looked at whether neurons for one sense affect other sensory circuits in people or other mammals. But based on their study, Rabinowitch thinks it could be happening.

In their worm study, the researchers found just a single touch neuron and single smell neuron involved in the sensory interaction. The smell cell that gets downregulated by the touch neuron is one of the first in the scent-sensing pathway. So it’s possible that, for example, vision neurons in sighted humans inhibit some of the very first neurons in the complex highway of nerves that conveys sound information from ear to brain.

“Even if it’s not the very first line of sensation … somewhere that’s very primary in the whole chain of processing could be a good place to look,” Rabinowitch said.

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