Artificial whiskers sense shape & texture of objects

Researchers in the Department of Mechanical Engineering at Northwestern University in Evanston, Illinois have developed  artificial mechanical whiskers that can sense the shape and texture of objects. (Read more at New Scientist.) 

This film clip shows an array of four mechanical whiskers brushing against a sculpted face:

Whiskers in the animal kingdom

A variety of organisms use whiskers to explore their environment and to produce three-dimensional representations of their surroundings, and it is in rats that most of the research into the neural encoding of sensory information received from whiskers has been carried out.

For rats and other rodents, which lead a largely nocturnal existence and therefore have poor eyesight, the whiskers – or vibrissae – are a major source of sensory information which aids in navigation and support. Rats have whiskers arranged in rows of arcs on both sides of the face. A pad on each side of the face contains about 35 large whiskers arranged in 5 rows and about 7 arcs. Each whisker in the same arc scans a different trajectory in the immediate surroundings, while whiskers in the same row scan roughly the same trajectory. A comparison of the activity of different whiskers in the same arc provides spatial information in the vertical axis, while the location of an object along the horizontal axis is encoded in the time interval between activity different whiskers in the same row.

The rat’s whiskers are very mobile – they can be projected as much as 5 cm in front of the animal’s face – and extremely sensitive. The whiskers are constantly moved back and forth (whisked) in a rhythmic motion with a frequency of 4-10 Hz; high-speed cameras show that this can be done 10 times a second.

Each whisker is surrounded by spindle-like mechanoreceptors, and innervated by neurons whose cell bodies are located in the trigeminal ganglion. Information from the whiskers is conveyed to the somatosensory cortex (also called the barrel cortex) of the brain, via the brain stem and thalamus, along two parallel pathways, called the lemniscal and paralemniscal pathways. Neurons in the former pathway have small receptive fields and large diameter axons, and respond to stimuli very quickly, whereas neurons in the latter pathway have larger receptive fields and slower responses;

Each axon in the trigeminal nerve (N.V. in the figure on the left) innervates a single whisker follicle. When these axons reach the central nervous system, they form whisker-related bundles which terminate in four brainstem nuclei(the principal sensory nucleus, spinal trigeminal complex subnucleus oralis, interpolaris and caudalis). These bundles are referred to as barrelettes, and are present in all but the subnucleus oralis; the bundled organization is also seen in the thalamus and somatosensory cortex.

In the cortex, the sensory afferents are arranged somatotopically, with the arrangement of neurons in the contralateral somatosensory cortex corresponding exactly to the arrangement of whiskers on the side of the face. Highly sensitive whiskers that are innervated by high densities of sensory neurons are represented by larger areas of the barrel cortex than less sensitive ones, just as the face and hands are grossly over-represented in the human somatosensory cortex. The characteristic three dimensional shape of the neuron clusters in the somatosensory cortex led to their being called ‘barrels’.

It is now well established that each whisker on the rat’s face is represented by a barrel in the contralateral somatosensory cortex; this is know called the one barrel-one vibrissa hypothesis. This can be demonstrated by the removal of a whisker in a newborn rat, which results in the complete absence of the corresponding barrel in the somatosenosry cortex, and, as a consequence, the enlargement of adjacent barrels. Electrophysiological studies of the properties of cells in the barrel cortex also confirm the one barrel-one vibrissa hypothesis.

The whiskers of aquatic mammals are generally far more sensitive than those of rats. For example, whereas individual whiskers on the face of the Norwegian rat are innervated by 100-150 axons, those of the sea lion are innervated by up to 1,500. Aquatic mammals use their whiskers to detect tiny changes in water turbulence while catching prey. Toothless whales, such as dolphins, have whiskers surrounding their blowholes, which may be used to detect the density of plankton in their surroundings. But it is the manatees which have the most sophisticated whiskers in the animal kingdom. They have whiskers all over the surface of their bodies, and their fleshy lips are covered with about 600 large whiskers, which they use to feel and grasp objects.

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