The Cutting Edge of Haptics

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With haptics, the illusion is created with applied forces. Typically, when someone moves his or her finger over a sharp or pointed edge, both vertical and lateral forces are applied to the skin, says Robles-De-La-Torre. But he found that the brain could be fooled into thinking it's being poked simply by applying lateral forces.

To create this illusion of sharpness, Robles-De-La-Torre, working with Carlo Alberto Avizzano and colleagues at the Scuola Superiore Sant'Anna in Pisa, Italy, used a haptic interface called GRAB, which consists of a thimble connected to the end of a motorized, extendable arm. A user is able to move the thimble freely when placing their forefinger in it. Then carefully controlled motors provide force feedback, so the thimble's movement is impeded in ways that create "virtual" surfaces.

By setting up the system so subjects can move only their finger along one axis, from left to right, the researchers were able make people feel like they were running a finger over a range of different sharp and pointed edges, just by applying lateral resistance to their movement. The sensation was so convincing that the subjects were even able to match the shapes of the edges to images of them, such as a saw tooth or a hump with a pointed peak.

Robles-De-La-Torre believes this illusion occurs because of a tradeoff that exists between different types of sensory information. Specifically, when we explore objects that contain small details, the force information is more important than proprioception, the sensory information that comes from our muscles and skin to tell our bodies how far a finger or limb has moved. As a result, we may feel like our finger is actually moving as it rides up one of these sharp edges, if they're not too big, he says.

Queen's O'Modhrain says that a lot of work has been done in rendering either large geometric shapes using force feedback, or fine-grained surfaces using vibrating skin actuators. But she says this recent work shows that it should be possible to render surfaces that lie between these two extremes without having to apply pressure to the skin.

It may even be possible to simulate the feelings of pain that can accompany a sharp object, says Robles-De-La-Torre, because the short pulse of lateral force that stimulates the sharpness sensation may also stimulate skin receptors normally associated with pain. But to prove this hypothesis, more tests are needed. "Pain is a complex phenomenon," he says.