Haptic sensing is fundamental to our experience of the physical world, and in its absence we are profoundly limited in our abilities to function—as illustrated vividly by the case of the individual IW, described in chapter 2. Nevertheless we are often unaware of its importance until we are in some way incapacitated by injury or disability. Even then we may comment more on the loss of motor function—that is, that our hands are clumsy—rather than our impaired tactile abilities. Most of the time we are unaware of the continuous tactile stimulation we get through contact with the clothes we wear or the chair supporting us, and it is only when something unexpected occurs that our attention is drawn to our skin. This highlights one of the affordances of tactile and haptic sensing: that we can respond to incoming information rapidly.
Human perception is inherently multisensory; multiple senses are engaged simultaneously as we perceive the world. In our daily lives, we automatically combine information from our senses of touch and kinesthesia with that arising from our other senses. In deciding whether a strawberry we see (visual sensing) at the market is ripe, for example, we may initially squeeze it (haptic sensing), then smell it (olfactory sensing), and finally bite into it (gustatory sensing) to confirm that our sense of taste is consistent with our visual, haptic, and olfactory impressions. The information we get from our various senses may be redundant or complementary. There are situations where we defer to the haptic sense as the superior sense in making a judgment, such as evaluating the feel of a fabric, the finish on a piece of furniture, or the weight of a fruit. In other circumstances, we may simply want to confirm haptically what we have seen or heard. The bidirectional nature of haptic sensing, in which we not only perceive the world but act directly on it to obtain those sensory impressions, makes it unique among our senses. When we look at how people use their hands to discover the properties of objects they encounter, we find remarkable consistency in the types of movements used. Moreover, when a specific property is of interest, for example whether a piece of fruit is ripe, the movement is inevitably optimal for perceiving that property (i.e., compliance).
Tactile display technology has a long history, dating back to the development of braille and other sensory substitution systems that were designed to compensate for sensory loss. More recently, vibrotactile displays have become a ubiquitous feature in smartphones, tablets, and other small electronic devices. The fact that such communication is private and unobtrusive has obvious appeal. For most of these devices, tactile communication has been limited to providing the user with alerts and notifications. Although some of these systems allow a much greater range of tactile communication capabilities by customizing the inputs delivered, such functionality has not been widely adopted. These features may be considered less useful because of the limits of our haptic information processing capacity, compared to vision and audition, and the requirement that users focus their attention on the tactile signal to determine its meaning. With the advent of surface haptic displays based on variable friction technologies, we see this limitation overcome, in that such displays involve both the visual and haptic senses. As surface haptic display technologies mature, high-fidelity textures will be rendered, markedly enhancing our interactions with a broad range of devices.
The use of the sense of touch as a navigation aid seems to be one of the most promising general applications of tactile displays. The reports of people walking into lampposts or into traffic while looking at a cell phone for directions would become less frequent if the directions were provided by smart shoes or clothing equipped with vibrating motors and sensors that communicated with a phone’s navigation app. The use of tactile feedback to convey information about posture seems particularly promising for athletes and fitness trainers. In these applications, directional tactile feedback can be used in conjunction with an accompanying app to indicate which joint to move or how to correctly align the position of the body for a specific activity. In this sphere of wearables and interactive clothing, there is potential for considerable growth.
Haptic interfaces in which the user makes contact with an object in a real or virtual environment, and feels its properties such as weight or stiffness, have been evaluated in a diverse range of applications. They have been used in areas such as medical and dental training scenarios, prototyping, gaming, education, and assistive technology, but as yet have not been widely adopted. This is partly due to the cost of such systems, the perceived value associated with incorporating haptic interfaces in larger systems, and the absence of any multipurpose display that can readily be adapted for different applications. Future haptic displays will require highly realistic haptic rendering so that the user experiences sensations similar to those afforded by direct manual interactions with the environment.
In an era when so much communication takes place via the Internet and without any direct person-to-person contact, the significance of the loss of haptic communication, whether a handshake, a pat on the shoulder, or a gentle caress, cannot be overestimated. Interpersonal touch has been shown to affect people’s attitudes toward others who are delivering services and can facilitate bonding between people. To address this shortcoming of modern communication systems, a number of simple devices have been developed over the past decade to enable long-distance tactile communication. These include a ring that squeezes the finger and a jacket that “hugs” the wearer when activated. Such devices have yet to meet commercial success, and so it is not clear whether the absence of any rigorous empirical testing prior to device development has impeded their adoption. In developing such devices it will be critical to identify the type of tactile experience that people desire for long-distance interpersonal communication, making sure to rely as much on our knowledge of human haptic perception as on the availability of new technologies.
Many fundamental questions remain to be answered about haptic perception, about the underlying sensory mechanisms involved in processing information from the skin and the muscles, and about how different sensory modalities interact. Still, we have seen remarkable advances over the past 20 years in our understanding of the human haptic system and in the development of technologies enabling us to study it more effectively. The importance of haptic perception to our daily interactions must not be underestimated, and its absence in many spheres of activity presents a challenge that will have to be surmounted.