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Haptics: Pressure

Haptics is the study of the sense of touch. Now, touch has many parts to it: It covers our body, and senses changes in the air, heat and cold, it provides our pain response, and tells us the texture of anything we touch, or that touches us.

Below all that, at it's most basic level, haptics detects pressure.

Any contact with the skin exerts pressure. If we have a funny tummy, the gas creation exerts pressure on the stomach. A pounding migraine pushes out, feeling like it will burst your skull. Wherever you look, the fundamental sensation of touch, is pressure.

So what tells us when we are under pressure?

The central nervous system descends via the brainstem from the back of the brain, and moves out to cover the entire body. Major nerve tendrils run across the face, along the shoulders, down the arms and legs, and cover the midsection. They reach into the most intimate areas, and they head to the outer extremities, continually branching out into smaller and smaller divisions as they go. Some paths head into the body, deep into the organs themselves, sending commands what to do, and relaying data back to the brain.

At the very end of the branching, nerve fibres are down to single-cell widths. The axions are only large enough to transmit one signal stream, serially, each. At the far ends of these threads are sensory devices. Each one different, for a different purpose. One detects texture, one detects acids, one detects heat, one detects cold. The last, the pacinian corpuscle, detects pressure.


A Pacinian Corpuscle

The pacinian corpuscle consists of a single nerve fibre, it's sheath stripped, in the centre of what could best be described as a gas bag onion. Concentric layers of webbed sheathing surround the nerve, radiating outwards like the skins of an onion. Bearing a strong resemblance to cargo netting in their basic function, these layers serve the opposite purpose. They are there to detect and amplify any change in the pressure around them. This change is then detected by the specific way the corpuscle deforms, and this is in turn, transmitted down the nerve as an electrical signal pattern.

Interestingly, the pacinian corpuscle cannot differentiate between increased pressure, and lessened pressure. This is because increased pressure and lessened pressure both deform the corpuscle, and do so in opposite ways - oner pushing it in, the other pulling it out, but the fibres of the corpuscle itself, deform the same way. It does not matter whether a part of it is compressed or stretched, all the corpuscle can detect, is that part has changed, and registers such. Thus, it is actually possible to simulate intense pressure on the skin, by vacuum sucking it.

Pressure sensors exist in different concentrations in different parts of the body. The greatest concentrations, are in the fingertips, with the fingers, sexual organs, and face running closely behind. For the rest of the body, pressure sensors, and, in fact, touch sensors in general, are more widely spaced.

There is a simple test you can do, to illustrate this point. Take two pencils, place them vertically, points down on a desk, and tape them together like that - so that both points will press into you at the same time. Now, take your new pressure tool, and press it firmly into your palm. You will feel two discrete points pushing in. Now try the same with your back; the sole of your foot; your thigh. Sometimes it will feel like two separate pencils, sometimes it will feel like just one. The reason it feels like just one, is because the pressure sensors are so far apart in those areas, that they cannot pick up and differentiate two separate points a little less than a centimetre apart.

In practice, all the touch sensors group around the hair folicules. So, wherever there is the possibility of hair, or goose bump growing/forming, there is a single pacinian corpuscle under the skin. This amounts to tens of thousands of corpuscles along the arms, along the legs, and across the face. Head hair has even more, and a few hundred across the back. All in all, a hundred thousand, or more pacinian corpuscles make up the pressure system.

The brain deals with this massive number of sensors in a quite clever way - they only transmit when pressuure changes. Once the corpuscle is deformed, it stays silent until its shape is changed once again. In this way, the brain is only made aware of pressure when it changes - hence why you stop feeling your clothing after a while, unless you're moving.

Haptic garments

VR tends to use garments which fit over parts of the body, to create artificial pressure sensations and trigger the corpuscles.

Haptic Vests

The haptic vest is one of the older, more primitive haptic devices. It gets away with being so primitive as it only targets the torso, where pressure sensors are few and far between. They typically work via an actuator array, or a shape memory alloy single actuator, which expands and presses against the skin, to create pressure. Alternatively, a rumble vest may be used, which vibrates very quickly, confusing the corpuscles into believing they are under more pressure than they are.


A basic haptic vest

Haptic Datagloves

Haptic data gloves fit over the hand like a normal glove. They are riddled with sensors at the joints, and actuators on the palm and finger pads. They respond to the movement of the user's hand, replicating those movements into the VR environement. When the virtual representation of a hand grasps something, the gloves send force feedback information to the hand - they exert pressure over the areas that correspond to the areas of the virtual hand in contact with the object.

Direct Neural Stimulation

There is another way to deal with full-body haptics. Rather than try and deal with ha hundred thousand corpuscles, an alternative is to jack into the brainstem directly, into the nerve bundle all these signals pass through, on their way to the brain. Then feed artificial signals into it, as if the body is being touched, but without the actual touching taking place.

Limited versions of this are being used in prosthetic devices.


Ex-Marine Claudia Mitchell and her thought-controlled arm

The human arm is connected to the central nervous system by four main nerves, which betweeen them, control all joint movements and positioning. Instead of communicating with a missing arm, the stubs of these nerves are re-routed from the arm stub and into the chest, where they can be assured a plentiful blood supply. The ends of these nevers and then fixed to sensors, which pick up the most delicate electrical activity, and pass that to a computer, which works out what the signals are asking for. There is of course, nothing to stop signals from passing back.

The arm above, does not, as of this writing, pass signals back, outside the lab.

Bibliography

An Actuator for the Tactile Vest ? A Torso-Based Haptic Device
http://csdl2.computer.org/persagen/DLAbsToc.jsp?resourcePath=/dl/proceedings/&toc=comp/
proceedings/haptics/2003/1890/00/1890toc.xml&DOI=10.1109/HAPTIC.2003.1191305

First woman fitted with bionic arm
http://www.virtualworldlets.net/Archive/IndividualNews.php?News=1659

VWN Haptics Database
http://www.virtualworldlets.net/Resources/Menu.php?Category=75

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