High-speed imaging of whiskers
High-speed video has helped researchers uncover relationships between the biomechanical properties of rat whiskers and how rats move the whiskers when they are discriminating between smooth and rough surfaces. The way rats sense the environment through their whiskers commonly is used as a model for sensory processing in the mammal brain, and the novel approach is helping scientists understand the role of mechanical resonance in whisker sensing.
Rat whiskers grow in a pattern, with short whiskers near the nose and longer ones farther back. Resonance frequency is influenced strongly by whisker length, and the combination of length and growth patterning suggested to the researchers that different whiskers might convey information in different frequency bands, explained Jason T. Ritt of MIT in Cambridge, Mass. Some studies with anesthetized animals had indicated that the rats’ brains respond to the frequencies.
Ritt, along with Christopher I. Moore from MIT and Mark L. Andermann from Harvard Medical School in Boston, trained rats to use their whiskers to discriminate between rough and smooth surfaces. The experiment was designed to prevent scent clues, but it did not restrict the rats to using whiskers alone, which was a key part of the experiment, according to Ritt. “We thought that the divergent results between us and other labs using plucked whiskers might be due to differences in the way the whiskers were swept in those studies,” he said. So they let the animals choose how to use their whiskers, so that the motions observed were the most natural. As it turned out, the rats almost always used their whiskers. Only a few rarely touched the surfaces with their noses, and none used their paws.
For the surface-discrimination tests, the researchers used a pco.1200hs high-speed video camera from the Cooke Corp. of Romulus, Mich. A grid of high-power infrared LEDs from AOS Technologies AG of Baden, Switzerland, directed at a Mylar diffuser for backlighting operating at 880 nm produced dark whiskers on a bright background. For plucked whisker studies, they used a Redlake MotionScope with an incandescent illuminator designed to create pseudodark-field illumination so that the rats’ whiskers stood out bright against a dark background.
This single frame from a high-speed video shows a rat sweeping its whiskers laterally across the surface. The red lines show the tracked positions of an anterior whisker every third frame (1-ms period) prior to the underlying frame. Regions where tracks are more densely spaced indicate slower motion (sticking). The small white vertical bar demarcates the border between the rough and smooth surfaces, which were removed by intensity normalization. Reprinted with permission of Neuron.
The study showed that, indeed, smaller, anterior whiskers exhibited higher frequencies than the longer, posterior whiskers, and that these frequencies seem to play an important role in surface discrimination. The researchers also found that, on rough surfaces, the whiskers produced “stick slip ring” events, in which the whisker catches on the rough surface momentarily — until enough force builds up to free it, at which point it oscillates or twangs like a spring. Smoother surfaces have fewer ring events.
“Our results quantified the whisker motions at a scale that was previously unexplored and gave us much better insight into the kinds of inputs the nervous system uses in whisker-based touch,” Ritt said. “It provides a clear example of how the brain has to work with the physical properties of the body when sensing the world.”
Ritt said that the researchers now plan to combine high-speed videography with simultaneous recordings of brain activity to relate the observed micromotions directly to their encoding and processing in the brain.
Neuron, Feb. 28, 2008, pp. 599-613.
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