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Haptics: Texture
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Haptics: Texture

Haptics is the study of the sense of touch. There are many parts to touch, and five different types of touch nerve in the nervous system. The sense of touch covers our body like a sheath: Every square centimetre of skin has haptic nerves. They tell us about the world around us: pressure, texture, heat, cold, damage.

Here, we are looking at texture.

So what detects texture?

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 axons 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 pressure, one detects damage, one detects heat, one detects cold. The last, Meissner's corpuscle, detects texture.


A Meissner Corpuscle

A Meissner's corpuscle is a strange structure. The nerves that enter them, shed their myelin sheath, the electrically-insulating layer that typically protects nerves. The lack of the sheath, typically slows nerve impulses down, and allows them to interfere with one another if too close together. After dropping the sheath, the nerves enter an enclosed structure, made mostly of a support structure of separare cells. Within this, the nerve splits into many filaments, each of which takes a different path through the three dimensional cell structure, forming something that looks a bit like a system of plant roots that have just been pulled out of a flowerpot too small for them.

This tightly wound structure is enclosed in its own metaphorical flowerpot: The same cells that form the lattice structure, wrap around the outside of the corpuscle, preventing any tendrils from escaping, and forming a bulb-like structure.

This then forms up in layers, in the same manner as happens in the pressure sensor, the pacinian corpuscle.

This onion structure is not coincidence; the detection of texture, is actually a detection of pressure. By detecting extremely fine changes in pressure, continually, Meissner corpuscles build up an exact picture of the grain of any surface, by detecting these minute pressure changes.

The corpuscles are in the highest part of the epidermis of the skin, just below the dermis. Any slight physical vibration will cause the soft bulb to deform, crushing one or more filaments of the nerve, which send activation signals back up the trunk. The unsheathed nerve segments interfere with one another's signals, building up a much higher signal than a single touch would normally generate. This is what allows them to detect the finest movement. As the corpuscle deforms, the signals intensify; the onion layering around it workingl like amplifiers - focussing signals inwards. Even when fully deformed, the signal does not cease, as the very act of returning to its original shape disturbs the corpuscle's nerve fibers, which causes them to transmit again.

However, the corpuscles are also phasic, and if a particular deformation occurs with regularity, they will eventually tune it out. This is the reason you stop feeling the texture of your clothes if they are in contact with your skin continually.

They are distributed throughout the skin, but concentrated in areas especially sensitive to light touch, such as the fingertips, palms, soles, lips, tongue, face and the skin of the male and female genitala.

Texture-Based VR Interfaces

At this time, most touch-based VR interfaces can handle pressure, but because texture is an extremely fine version of pressure, they do not handle texture that well. Those that do, tend to approach it by using vibration to simulate light touches of different materials.

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 At the moment, very, very few handle texture, as the fidelity required is not really yet possible.

CyberTouch Tactile Feedback

Immersion's CyberTouch gloves are one of the first texture-replicating interfaces. Small vibrotactile stimulators sit under the tip of each finger, and on the palm. Each stimulator individually vibrates to a programmed limit, to match any of a small range of surface textures.

Specifications

* Vibro-tactile actuators: 6; one on each finger, one on the palm
* Vibrational Frequency: 0 – 125 Hz
* Vibrational Amplitude: 1.2 N peak-to-peak at 125 Hz (max)

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.

At the moment, this is possible with pressure. There has not yet been research on the signals used in texture, sufficient to recreate. However, there are no technological barriers to recreating it via this method, as it is only te electrical signals generated that are eplicated, same as for pressure.

Further Reading

Feeling bumps and holes without a haptic interface: the perception of pseudo-haptic textures (Registration necessary for full text)

Vibratory Tactile Display of Image-Based Textures (Registration necessary for full text)

CyberTouch Tactile Feedback Glove

Staff Comments

 


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