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CUTANEOUS SENSES

La everyday life people depend on their cutaneous (somesthetic, or skin) senses much

more than they realize. There is however, an ongoing question as to how many

cutaneous sense there are : this confusion exists in part because of the bases on which

the senses can be classified. As Geldard (1972. Pp 258, 259) points out, we can classify

them qualitatively (on the basis of their observer similarity-that is, the sensations

generated), in terms of the stimulus (i.e .. the form of energy that triggers the sensation,

as thermal, mechanical, chemical, or electrical).

Or automically (in accordance) with the nature of the sense organs or tissuesspe

involved). With respect to the anatomic structures involved, it is still not clear how

many distinct types of nerve endings there are, but for convenience Geldard considers

the skin as housing there more or less separate systems of sensitivity, one for pressure

reception, one for pain, and one responsive to temperature changes. Although it has

been suggested that any particular sensation is triggered by the stimulation of a specific

corresponding type of nerve ending or receptor, this notion seems not to be warranted,

Rather , it is now, generally believed that the various receptors are specialized in their

functions through the operation of what geldard refers to as the priciple of patterned

response.

Some of the cutancous reseptors are responsive to more than one form of energy.

Through complex interaction among the various types of nerve endings, being

stimulated by various forms and amounts of energy, we experience a wide variety of

specific sensations that we endow with such labels as touch, contact, tickle, and

preassure (Geldard, 1972).

For the most part, tactual displays have utilized the hand and finger as the priacipal

receptors of information. Craft workers, for centuries, have use their sense of touch to

detect irregularities and surface roughness in their work. Interestingly Lederman (1978)

reports that surface irregularities can be detected more accurately whwn the person

moves an intermediate piece of paper or thin cloth over the surface than when the bare

fingers are used alone. Inspection of the coachwork of Aston Martin sport cars, for

example is done by rubbing over the surface with a cotton glove on the hand.

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Not all parts of the hand are equally sensitive to touch. One common meassure of touch

sensitivity is the two point threshold, the smallest distance between two pressure points

at which the point are perceived as separate. Figure 6-12 shows the median two-point

threshold for fingertip, finger, and palm (vallbo and Johansson 1978). The sensitivity

increases (i,e., the two-point threshold becomes smallers) from the palm to the

fingertips. Thus, tactual displays should be used with extreme caution in low

temperature environtments. Stevens (1982), however, found that cooling or warming

the object being felt caused a marked improvement (averaging 41 percent) in the skins

acuity relative to thermally neutral stimulation. With this bit of background

information, let us look at or more appropriately, feel our way through some specific

types of tactual displays.

TACTUAL DISPLAY

Although we depend very much upon our cutaneous senses in everyday living, these

senses have been used only to alimited degree as the basis for the intentional

transmission of information to people by the use of tactual displays. The primary uses

of tactual displays to date have been as subtitutes for hearing. Especially as aids to the

deaf, and as subtitutes for seeing. Especially as aids to the blind.

The most frequent types of stimuli used for tactual displays have been mechanical

vibration or electric impluses. Information can be transmitted by mechanical vibration

based on such physical parameters as location of vibrators, frequensy, Intensity

.