BIOMECHANICS 



By a Whisker 



Hairs (and steel wires) enable rats (and robots) to glean 

 information about their surroundings in remarkable detail. 



By Adam Summers 



When we were children, 

 my siblings and I spent 

 summers in the woods of 

 northern Canada. Sleeping in a rustic 

 cabin, and listening to mice scamper 



across the rafters just above us, my 

 fondest dream was that one of the ro- 

 dents would stumble and plop right 

 into my little sister's mouth. But for 

 reasons I didn't appreciate at the time, 

 she never had to contend with such 

 an assault, even on the darkest nights. 

 It seems that mice and rats have an 

 unusual sensory system that works in 

 total darkness: their whiskers. One 

 research team is trying to unravel the 

 workings of this hairy, touch-based 

 system, partly in a push for better ro- 

 botic sensors, and partly out of an 

 interest in basic neurobiology. 



ats and mice "whisk" 

 s they walk. Dozens 

 of/long hairs flank their 

 noseband the animals 

 pidly sweep them 

 forward and 



backward, exploring their environ- 

 ment the way a blind person might 

 probe with a flexible cane. [See illustra- 

 tion on this page\ Rats are known to 

 gather a remarkable amount of infor- 

 mation about objects they whisk — re- 

 markable because the hairs themselves 

 are dead keratin without any sensors, 

 just like the hairs on your head. Rats 

 usually whisk objects in the dark at a 

 frequency of about five to twelve 

 hertz; that is, they sweep their 

 whiskers back and forth between five 

 and twelve times a second. Sensitive 

 tissue at the base of each whisker re- 

 ceives the sensory information gath- 

 ered in these swipes and transmits it to 

 the brain. There, a kind of environ- 

 mental map is assembled. 



A group of investigators led by 

 Mitra J. Hartmann, a biomedical en- 

 gineer at Northwestern University in 

 Chicago, has been trying to build a 

 robot that "sees" its environment by 

 touch, much the same way a rat's 

 whiskers do. Hartmann's model fol- 

 lows the basic plan of a rat's muzzle, 

 but for simplicity she started with 

 only two rows of four whiskers, rep- 

 resented by miniature steel wires se- 

 cured to a frame. Attached to each 

 wire was a sensor that measured how 

 much it curved at the base. 



When the tip of a rat's whisker 

 slightly grazes a tasty hunk of cheese, 

 it bends into an arc with a distinct 

 curvature. If the cheese is closer, the 

 whisker bends even more, with a 

 different curvature. Happily for 

 Hartmann's analysis, the same me- 

 chanical principles apply whether 

 the whisker happens to be a straight 

 cylinder, as in her model, or one 

 that tapers to a fine tip, as in the real 

 rat whisker. By gathering the infor- 

 mation about curvature from Hart- 

 mann's small array of eight whiskers, 

 a computer was able to reconstruct 



Rat can explore the shape and texture of an object — here, a 

 crumpled soda can — in total darkness, by sweeping its 

 whiskers back and forth. Each whisker bends at some angle as 

 it brushes against the object and then breaks free. Tissue at 

 the base of each whisker senses the changing angles, which 

 the rat's brain combines into a map of the rat's environment. 



