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Tuesday, February 26, 2008

Rat Whiskers Yields Insight Into Sensing Machinery

High-speed video of rats using their whiskers to explore different surfaces has given researchers significant insights into the subtle mechanics of their tactile sensory system. Such information is important because the rat tactile machinery is a widely used laboratory model for studying how energy from sound or touch is translated into neural activity. Thus, basic insights from studying the rat system could aid in understanding the senses of hearing and touch in all mammals, including humans.

Christopher Moore and colleagues published their findings in the Feb. 28, issue of the journal Neuron, published by Cell Press.

Previous studies of the mechanics of rat whiskers had analyzed the motions and neural signals generated by isolated rat whiskers, rather than the complex, subtle “micromotions” of the array of whiskers, called vibrissae, in a behaving animal.

“Research has proceeded without a thorough understanding of these signals because the inherent challenges in tracking high-speed, small-amplitude motion of thin vibrissae in a freely behaving animal precluded direct measurement of micromotions,” wrote the researchers.

In their experiments, the researchers trained rats to use their whiskers to discriminate between rough and smooth surfaces in a darkened chamber. Their reward for performing correctly was a sip of chocolate milk.

Using high-speed videography, the researchers recorded the subtle vibrations of the whiskers as the animals probed the surfaces with their characteristic “whisking” motion.

Detailed analysis of these vibrations revealed how the different surfaces produced different micromotions. The rough surfaces produced what the researchers termed “stick-slip-ring” events—a kind of twanging of the whiskers. In contrast, smooth surfaces generated a stream of infinitesimal “stick-slip” oscillations.

The analysis also revealed how the different-length whiskers on the animals’ snouts exhibited different resonance characteristics, contributing to the animals’ acute ability to “see” their environment with their whiskers.

“The present findings provide the first description of what is believed to be an essential surface cue, micromotions of the vibrissae,” conclude the researchers. “In so doing, they address fundamental questions that had until this point remained unanswered, such as whether intrinsic biomechanics would impact transduction meaningfully during active sensation and what range of velocities are produced during free behavior.”

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