4 Haptics as a Substitute for Vision
Learning Objectives
The haptic perceptual system uses a combination of tactile and kinaesthetic information about the environment. Traditionally, this sense has been called touch, but it is nowadays often called haptics to emphasize that not only the sensors in the skin are involved, but also the sensors in muscles, tendons and joints. A related name is active touch. Assistive technology based on haptics has been historically important for the visually impaired and blind person, particularly in the fields of obstacle avoidance for mobility and for accessing print resources. For example, the haptic low technology aid of the long cane made obstacle avoidance when travelling a reality and for printed information, the advent of the haptic medium of Braille made the knowledge and information of printed books accessible.
Despite significant developments with aural interfaces particularly for print, computer systems and mobile telecommunications, haptics will always retain an essential role in the exploration, understanding and use of the physical environment. For this reason it is important to have a basic understanding of the fundamentals and applications of the haptic perceptual system.
The learning objectives for the chapter include:
•Gaining an understanding of the principles and fundamentals of haptics.
•Obtaining an appreciation of the interrelationships between visual and haptic perceptual systems.
•Learning about the different ways in which haptics are used in assistive technology systems.
•Understanding the advantages and also the limitations of haptics in assistive technology systems for the visually impaired or blind person.
4.1 Introduction
The hands are remarkable organs that have had an enormous importance for both the biological and cultural development of human beings. In fact, the hand was in an advanced state long before humans appeared on the scene. Hands have an
136 4 Haptics as a Substitute for Vision
impressive ability to adapt to different kinds of manipulation tasks, from working with miniature detail to loading heavy objects. It is not as often realised that not only is the hand a marvellous performer, but also it is a highly competent sense organ. Good performance presupposes efficient on-line information about the relationship between the hands and the environment in which they act, a fact that robot developers have had to be quite aware of. There is not yet (ever?) a robot hand that can do all the tasks a human hand can do.
Less well known is the fact that the feet are important as sense organs, too. They can detect information about the ground being walked upon, its slant, material and hardness. This is essential knowledge for safe locomotion. Sighted people are to a large extent informed about these features via their eyes, but blind people have mainly to rely on their feet as sensors (Cratty 1971). Other parts of the body surface can also provide information about the environment, for instance, the back and the abdomen, as will be discussed below. The skin is the largest sense organ and very important in many respects.
In textbooks the sensory capacities of the hands are often subsumed under the skin senses. This includes the perception obtained via cutaneous sensors, thus not only touch, but also perception of heat, cold and pain. It is certainly true that the tactual information obtained via the skin has a basic importance, but this is not the complete story. When the hands are in function, there is a close cooperation between the sensors in the skin and sensors in the muscles, tendons and joints (the kinaesthetic sense), as well as muscles performing the exploration, with all these factors coordinated by the neural system (Cholewiak and Collins 1991; Johnson 2002; Wing et al. 1996).
4.1.1 Physiological Basis
Haptic perception is supported by a large variety of receptor and fibre types in the hairless skin and deeper tissues, in all 13 different types of sensory afferents, including four mechanoreceptive types providing information about skin deformation and four proprioceptive types informative about muscle length, muscle force and joint angle. These two groups of types are the most important for haptic perception, the former type for information via the skin, the latter type for information via muscles, tendons and joints. Each of the receptor types is specialized for specific kinds of information functions, for example, one type for form and texture perception by being sensitive to edges, corners, points and curvature, another type for perception of hand conformation and of forces parallel to the skin surface. Concerning the proprioceptive information the most important contribution seems to come from the muscle length afferents with some possible contribution from joint afferents. Most of the receptor types are engaged in a majority of the activities of the hand. A thorough analysis of the complex physiological basis and its relation to perception was presented by Johnson (2002).
4.1 Introduction |
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4.1.2 Passive Touch, Active Touch and Haptics
When touch is described as a skin sense it is often considered as a passive receiver of stimulation from the environment. In opposition to this view, pioneers as Katz (1989) in his seminal work, first published in 1925, and Gibson (1962) emphasized the observers as active explorers of the environment with their hands. The hand is regarded as a perceptual system based on active exploration to collect information, and active touch is considered to be superior to passive touch. To stress the importance of activity the sense is often called “active touch” or “haptics” (after a Greek word for touching). The latter term will most often be used here, as well as the adjective “haptic”, sometimes alternating with “touch” and “tactile” or “tactual” in contexts where these terms are traditional. The use of the latter two terms is not consistent, but “tactile” is often used for the physical stimulus (for instance in tactile pictures) and “tactual” for perceptual aspects.
Even if it is has been shown in some contexts that the kinaesthetic component is very useful (see, for instance, the work due to Austin and Sleight 1952, Cronin 1977 and Loo et al. 1983), it has sometimes been doubted that activity is a necessary condition for touch to function well. There is experimental evidence that active and passive touch give equivalent results for the perception of texture (Lederman 1981) and small patterns (Vega-Bermudez et al. 1991). Magee and Kennedy (1980) found even better results for passive than for active touch in identifying objects in raised line drawings, interpreting the result to depend on favourable conditions for attention to the perceptual task in the passive case. Further experimental analysis is reported by Richardson et al. (2004) and a theoretical discussion can be found in Hughes and Jansson (1994). The outcome of this discussion was that active exploration is favourable for the efficiency of haptics for most tasks, but there may be tasks where it is not necessary. Symmons et al. (2004) provided an overview of studies concerning the active-passive problem and found the results to a large extent to be task-dependent. Johnson (2002) suggested that passive touch requires more concentration, and that the difference between the two kinds of touch is similar to the difference between situations with dim and bright light in vision.
4.1.3 Exploratory Procedures
The movements performed to collect information via the hands are usually not random, but goal-directed. There are specific movements to get specific kinds of information. Lederman and Klatzky (1987) suggested a number of basic exploratory procedures: among others, lateral motion for perceiving texture, pressure for perceiving hardness, static contact for perceiving temperature, unsupported holding for perceiving weight, enclosure (enclosing the object in a hand or both hands) for perceiving global shape and volume, and contour following for perceiving global shape and exact shape. Other exploratory procedures suggested are wielding to get information about several properties of an object (Turvey and Carello 1995) and shaking, for example, a container with liquid, to be informed about the amount it contains (Jansson et al., 2006). One of the problems with haptic displays, to be dis-