Anyone who has watched a rat explore an object or environment knows that whiskers play a vital role in the inspection process. Unlike our hairs,whiskers are capable of discriminating between surfaces that differ ever so slightly in texture. With such finely tuned sensory abilities, it is clear that whiskers are more than overgrown hairs that respond to pressure changes. Instead, their acuity relies on a different mechanism. As recent work from two laboratories demonstrates, a whisker's mechanical resonance seems to hold the key to how a rat senses the world.
Some structures have the intriguing property of resonance: when vibrating at their resonant frequency, the amplitude of vibration is disproportionately large. Maria Neimark reasoned that if each whisker vibrates at unique resonance frequencies, a rat might be able to use its whiskers to perform a type of Fourier analysis, decoding the complex stimulus generated by brushing the whiskers over a textured surface. By deploying an array of differently tuned whiskers, vibrational patterns can be transformed into spatial patterns,simplifying the work the nervous system has to do to encode the stimulus.
Neimark and colleagues tested these ideas using whiskers on anesthetized rats and on isolated whiskers mounted on a metal beam. Stimuli were delivered using a piezoelectric device or by deflection with a rod, and the results measured using an optical switch. Whiskers were found to have sharp tuning curves and to resonate at frequencies between about 50 and 750 Hz. This resonance probably functions to amplify tactile signals: stimuli that vibrate a whisker near its resonance frequency increase its deflection as much as tenfold, which should deliver a larger stimulus to the neurons innervating the base of the whisker. Neimark and colleagues also demonstrated that, like a xylophone, the resonance frequencies of whiskers depends on their length, with longer whiskers having lower resonance frequencies. This, along with the fact that rat whiskers are organized into tidy rows, with longer ones towards the back, suggests that rats could use their array of differently sized whiskers to create a map of stimulus frequencies.
In a separate paper, Mitra Hartmann and colleagues mounted rat whiskers on a vibration table and delivered stimuli either at controlled frequencies or by brushing against the whisker with forceps, and recorded the whisker movements on high-speed video. At some frequencies, the tip of the whisker moved disproportionately more than the base, demonstrating the whisker's resonant properties. In addition, the authors examined whisker movements using videotape of a rat exploring a chamber in which a vertical bar had been mounted. This experiment revealed that as a rat passes a nearby object,whiskers deflected by the object oscillate after passing the object,suggesting that whiskers can be used to detect edges of objects and perhaps even to determine the distance between the animal's snout and the object. One of the most intriguing points raised by Hartmann's study is the suggestion that rats may be able to actively modulate the damping on individual whiskers by altering blood flow and muscle activity around the whisker base. If so, the resonance of whiskers can be actively altered, suggesting that the simple spatial map proposed by Neimark and her colleagues may not be so simple after all.