Chapter 9. The Mechanics of Feeling

From textures to temperature and vibrations to slippage, our sense of touch serves as the mechanical interface between our bodies and the physical world. This same capability can also be harnessed to provide an added sense of realism to computer-based simulations. To understand the application of tactile sensorial feedback capabilities within virtual and augmented reality simulations, it is important to first understand how we actually feel the real world. In this chapter we explore the mechanics of our sense of touch, its extraordinary range of capabilities, and how tactile and proprioceptive cues supplement sight and hearing.

The Science of Feeling

Consider the touch of a friend, the texture of the different fabrics in your clothing, or the forces you feel as you swing a golf club or baseball bat.

These and other sensations are possible because humans are equipped with a marvelously complex system of sensors, neural pathways, and areas of the brain that together detect, relay, and process thousands of pieces of information each second about the physical environment we are within, the external forces acting on us, as well as the position, orientation, and movement of our body. Collectively referred to as the somatic sensory system, there are two subsystems that hold particular relevance to the overall subject matter of this book. The first is responsible for our ability to detect and perceive mechanical stimuli, including those representing external pressure, vibration, flutter, textures, skin stretching, and so on. This is referred to as our tactile sense. The second, known as our kinesthetic sense, enables us to detect and perceive joint angles, as well as tension and other forces exerted in our muscles and tendons. The combination of the two senses generally falls under the term haptics.


Note

Use of the word haptics has been the source of tremendous confusion and more than a few epic arguments within and between the virtual reality, robotics, and medical communities. Due in part to the “us too” phenomenon, significant disagreement has evolved on proper use of the term. Does haptics refer to our collective tactile and kinesthetic senses, or does it refer to a class of man-machine interfaces providing mechanical stimuli to address these senses, edging out tried and true phrases such as tactile/force feedback devices? For clarity, this book will use the term sparingly and in most instances in relation to matters concerning physiology.


In this chapter we will explore both systems. They are crucial to understanding the challenges of implementing tactile and force feedback technologies within virtual and augmented reality systems.

We begin this exploration with the part of our physical body where most of these sensations originate: our skin.

Anatomy and Composition of the Skin

Our skin is the sensory organ responsible for our sense of touch, the first of the five primary senses to develop and respond to stimulation during gestation (Huss, 1977). In fact, a human fetus will actually begin to respond to touch at about the eighth week of development (Rantala, 2013). A medium-sized organ, our skin spans an area covering approximately 1.6 to 1.9 sq. meters (Rinzler, 2009) and accounts for approximately 15% of our overall body weight (Kanitakis, 2001).

We have two general types of skin—hairy and glabrous (naturally hairless)—both of which are capable of a remarkably complex set of functions falling into three main categories: protection, regulation, and sensation.

Protection—Located at the interface between the external environment and our internal physiology, skin serves as a protective barrier against a variety of threats, including mechanical impact and pressure (Sembulingam and Sembulingam, 2012), dangerous ultraviolet (UV) radiation (Brenner and Hearing, 2008), and biological pathogens (Nestle et al., 2009).

Regulation—Our skin serves the vital role of regulating and stabilizing body temperature through constriction and dilation of blood vessels (Charkoudian, 2003) and secretion and evaporation of water.

Sensation—Also serving as the primary interface with our physical surroundings, our skin contains an extensive network of nerve cells specifically designed to detect changes in the environment as well as generate sensations we feel related to the properties of our physical world with which we have contact. It is with these nerve cells that we are able to detect the tactile and force feedback cues provided by such devices as a vibrating controller used for navigating in, and manipulation of, augmented and virtual reality simulations.

Skin Layers

As detailed in Figure 9.1, human skin is composed of two primary layers, each of which varies in thickness and function. The top, outermost layer is referred to as the epidermis (from the Greek word epi, meaning “on top,” and derma, meaning “the skin”) (Oxford, 2015). This layer serves as a physical and chemical barrier separating interior physiology and the external environment (Madison, 2003; Denda, 2000). A dynamic structure, the epidermis is made of tightly packed, scale-like cells that regenerate approximately every 45 days. The epidermis is thinnest on the eyelids (.05 mm) and thickest on the palms and the soles of the feet (1.5 mm) (NIH, 2006).

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Credit: Image by guniita © 123RF.com

Figure 9.1 Cross-section of human skin detailing the two primary layers and underlying supportive structure of cells specialized in accumulating and storing fats that acts as an energy reserve.

The second layer, known as the dermis, provides support for the epidermis and accounts for approximately 80 percent of the thickness of the skin. The dermis is principally composed of connective tissue (collagen) containing hair follicles, glands, blood vessels, nerves, and sensory receptors. It gives the skin its elasticity and serves the structural purpose of cushioning the body (Boundless, 2015). Also varying in thickness depending on location (1.5 to 4 mm), the dermis layer is highly vascular and contains more than 11 miles of blood vessels (Jablonski, 2013). This venous network keeps the skin oxygenated and supplied with nutrients and plays a vital role in the skin’s healing and heat regulation processes.

A third, innermost layer (although technically not considered part of the skin) is known as the hypodermis or subcutis. This layer is predominantly composed of cells specialized in accumulating and storing fats. The hypodermis also serves to attach the dermis to the muscles and bones and supplies nerves and blood vessels to the dermis. Several sensory receptors are also located in this layer.

Tactile Perception

Tactile sensing is based on sensory information produced by specialized cutaneous receptors (those located in the skin) that are triggered through physical contact with the stimuli such as is experienced through active touching (Gibson, 1962) or through an external force acting on our body, such as the touch of a friend, a thumping low-frequency tone from a bass kicker used in flight simulators, or heat from an open flame.

Similar to receptors found in our eyes and ears, cutaneous receptors can also be considered transducers because they effectively convert energy, such as mechanical stimulus or thermal energy, into electrical signals (nerve impulses). The receptor types involved in our sense of touch are primarily divided into three categories: nociceptors (for detection of stimuli producing pain), thermoreceptors (for detection of stimuli related to temperature), and those with the most relevance to the subject matter of this book, mechanoreceptors (for detection of mechanical stimuli and physical interaction).

Mechanoreceptors

As detailed in Figure 9.2, human skin contains four major types of receptors that respond specifically to different types of mechanical stimulation resulting from physical interaction with our surroundings: Meissner corpuscles, Merkel disks, Pacinian corpuscles, and Ruffini endings. Known as mechanoreceptors, these sensors send information to our central nervous system regarding touch, pressure, vibrations, and cutaneous tension (Purves et al., 2001). All four are also categorized as low-threshold (or highly sensitive) receptors because each responds to weak mechanical stimulation.

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Credit: Illustration by BruceBlaus via Wikimedia under a CC 3.0 license

Figure 9.2 Cross-section of human skin detailing cutaneous structures and sensory mechanisms.

All mechanoreceptors function in the same basic manner:

1. An external stimulus or force acts on the surface of the skin.

2. The force is transferred deeper into the skin where it stimulates a mechanoreceptor.

3. The mechanoreceptor generates an action potential (electrical impulse).

4. The action potential is sent along afferent nerves to the central nervous system resulting in a conscious perception, behavioral response, or both (Geffeney and Goodman, 2012).

Mechanoreceptor Classification

The four types of mechanoreceptors we will be exploring in this section are classified in two different ways: by their rate of adaptation and by the size of their receptive fields.

Rate of Adaptation

Mechanoreceptors, like other sensory receptors in the human body, have an interesting performance characteristic known as adaptation. When a stimulus acts on our skin, the appropriate mechanoreceptors respond by firing off an initial series of impulses along afferent nerves (nerves carrying impulses from receptors or sense organs toward the central nervous system) signaling the presence of an external stimuli or change in the environment. The stronger the stimulus or displacement of tissue, the greater the frequency of the neural response (Knibestöl, 1973). This response has been shown to logarithmically increase with pressure (Muniak et al., 2007). How quickly the receptors adapt, or return to a passive state, depends on the type. The basic concept behind slow and rapidly adapting mechanoreceptors is illustrated within Figure 9.3.

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Credit: Illustration by S. Aukstakalnis

Figure 9.3 This figure illustrates the basic concept of sensory adaptation and the change over time in the responsiveness of a mechanoreceptor to continual stimulation. Slow-adapting receptors continue firing as long as the stimulus is present. Rapidly adapting receptors respond quickly at the initial onset but stop firing if the stimulus remains constant, firing again to signal the removal of the stimuli.1

1 There are a number of variations to the slow and rapidly adapting firing patters of different mechanoreceptors based on stimulus, the detailing of which goes far beyond the general scope and intent of this chapter. For more information on this topic, see Johansson and Vallbo (1979, 1983, 2014); Vallbo and Johansson (1984); Burgess (2012); Martini et al. (2013); and Hao et al. (2015).

Slow-adapting receptors (Merkel disks, Ruffini endings), designated using the initials (SA), optimally respond to contact forces including initial and continuous pressure, edges and intensity, as well as cutaneous tension (skin stretch) (Purves et al., 2001). Your ability to hold this book at your side without dropping it is a direct result of the response from slow-adapting receptors. You are, quite literally, constantly aware of the mass, weight, shape, and edges of the book while it is in your hands. This awareness comes from slow-adapting mechanoreceptors sending a constant stream of sensory information to the central nervous system. Rapidly adapting receptors (Meissner corpuscles, Pacinian corpuscles), designated using the initials (RA), optimally respond to rapidly changing stimuli such as vibrations and changes in textures. Rapidly adapting receptors also respond to initial contact and motion, but not to steady pressure (Talbot et al., 1968). Your ability to discern between the feeling of a fuzzy peach and the surface of concrete is a direct result of rapidly adapting receptors. Vibrotactile displays found in gaming controllers and smartphones also rely upon their capability.

Receptive Fields and Mechanoreceptor Distribution

As depicted in Figure 9.4, a mechanoreceptor’s receptive field is the region or surface area of skin on the hand and elsewhere that, adequately stimulated, will trigger a response from an individual receptor. Receptor field sizes range from 1 to 2 mm2 all the way up to larger areas, including entire fingers and sizeable portions of the palm, depending on receptor type and location on the body (Kortum, 2008; Johansson and Vallbo, 1979). Those receptors with the smallest receptive fields (Merkel disks and Meissner corpuscles) are found in the highest densities and are located in the epidermis layer closest to the surface of the skin. In contrast, those receptors with the largest receptive fields (Ruffini endings and Pacinian corpuscles) are fewer in number and located deeper in the skin within the dermis and subcutaneous layers.

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Credit: Hand image by julenochek © 123RF.com

Figure 9.4 Cutaneous receptors form overlapping receptive fields that, when appropriately stimulated, produce action potentials in afferent nerves that carry the signals to the central nervous system.

This layering, or overlap, of receptive fields plays a crucial role in our ability to carry out all forms of manual tasks, ranging from grasping and lifting a bulky cantaloupe off the counter in your kitchen to the delicate task of threading a sewing needle or placing a contact lens in your eye.

Receptor Distribution and Spatial Resolution

It is estimated that there are approximately 17,000 individual mechanoreceptors in the glabrous skin of each hand (Vallbo and Johansson, 1984). Interestingly, tactile perception improves with decreasing finger size, which explains why women have a more finely tuned and delicate sense of touch (Peters et al,. 2009). High-density receptors with the smallest receptive fields (Merkel disks and Meissner corpuscles) have the highest spatial resolutions, enabling us to identify with considerable accuracy where external stimuli are acting on the skin, such as when a mosquito has decided to stop by for a meal. In contrast, low-density receptors with the larger receptive fields (Ruffini endings and Pacinian corpuscles) have a significantly lower spatial resolution. Although they are far less precise in signaling “where” a force is acting on the skin, these receptors are far more sensitive to light touch.

Mechanoreceptor Details

All four main mechanoreceptors differ in physical size, geometry, range of sensitivity to static and dynamic stimuli, location in the skin, population density, specific manner of function, and more. With all of these features, it is important to remember that despite the specificity of the descriptions, there is considerable overlap in the response of different mechanoreceptors to a single stimulus event. As an example, the controller buzzing in your hand as you manipulated an object within a virtual environment or the thump felt on your chest coming from a gaming vest will elicit a response from several different receptors, including those detecting deep pressure, cutaneous stretching, and vibration (which translates into perception of texture).

Merkel Disks

A Merkel disk is a specialized sensory nerve ending characterized by a disk-shaped epithelial cell attached to an afferent nerve fiber. Typically this nerve fiber is a branch of medium-to-large diameter nerves serving a cluster of disks (Netter, 2013). As shown in Figure 9.6, Merkel disks are generally located in skin layers at the boundary between the epidermis and the dermis and are found in the greatest numbers within glabrous skin in such locations as beneath the ridges forming our fingerprints (Bear et al., 2007, p 389). When the appropriate stimulus is present, deformation of the disk wall opens ion channels in the nerve fiber. The influx of sodium (Na+) ions results in action potential firing (Maksimovic et al., 2014).

Image

Credit: Illustration by BruceBlaus via Wikimedia under a CC 3.0 license

Figure 9.6 Merkel disks are slow adapting, sensitive to light touch and vibration, and respond maximally to low frequencies in the 5–15 Hz range.

Response Characteristics

Merkel disks are slow-adapting (SA) receptors that are highly sensitive to detailed surface patterns and sustained light mechanical stimulus, and they play a key role in such tasks as reading Braille due to their small receptive field (Noback et al., 2005; Gentaz, 2003). Merkel disk receptors respond maximally to low frequencies (5–15 Hz) (Gilman, 2002).

Meissner Corpuscles

A Meissner corpuscle (also referred to as a tactile corpuscle) is characterized by an elongated, capsule-like geometry. It contains flattened, horizontally stacked laminar cells within which are coiled, meandering, and afferent nerve fibers (Cauna and Ross, 1960). When pressure deforms the corpuscle, the nerve fibers are stimulated and action potentials are produced in the nerve (Dahiya et al., 2010). When the stimulus is removed, the corpuscle regains its shape, producing another series of action potentials (Johnson, 2001).

As shown in Figure 9.7, the boundary between the dermis and epidermis is not uniform, but contains small undulations and protuberances known as dermal papillae extending from the dermis layer up into the overlying epidermis. Meissner’s corpuscles are located within about every fourth papilla near the surface of the skin (Freinkel and Woodley, 2001). The highest densities of Meissner’s corpuscles are found in glabrous skin most sensitive to touch—notably that of the fingers, palms, and soles of our feet (Johansson and Vallbo, 1979; McCarthy et al., 1995; Dillon et al., 2001; Kelly et al., 2005).

Image

Credit: Illustration by BruceBlaus via Wikimedia under a CC 3.0 license

Figure 9.7 Meissner corpuscles are rapidly adapting receptors found close to the surface of the skin, are sensitive to fine textures and slippage, and respond maximally to mid-range frequencies (20–50 Hz).

Response Characteristics

Meissner corpuscles are rapidly adapting (RA) and respond to the onset and removal of the mechanical stimulus. They are particularly efficient in transducing information when textured objects are moved across the skin (Purves et al., 2001; Burgess, 2012), the slippage of objects (Barker and Cicchetti, 2012), as well as shape changes in exploratory and discriminatory touch (Mancall and Brock, 2011). Meissner corpuscles respond maximally to mid-range frequencies (20–50 Hz) (Gilman, 2002) such as might be produced from a smooth cotton shirt (Klein and Thorne, 2006).

Pacinian Corpuscles

A Pacinian corpuscle (alternatively known as a Lamellar corpuscle) is characterized by an oval-shaped geometry and is composed of several dozen concentric lamellae (thin layers) made of fibrous connective tissue separated by layers of fluid. The entire corpuscle is encased on the outside by collagen. At the center of the corpuscle is a cavity containing one or more afferent nerve fibers (Purves et al., 2001).

When physical pressure from an external force acts on the corpuscle and causes a deformation of the structure, the nerve fiber at the center is also bent or stretched, opening ion channels (chemical gates) into the outer membrane and allowing the influx of sodium ions. The greater the external force, the greater the deformation of the corpuscle and the larger the influx of sodium ions, resulting in the generation of an action potential within the afferent nerve fiber that is forwarded to the central nervous system.

The largest of the four major types of mechanoreceptors, Pacinian corpuscles are also the fewest in number (Kandel, 2000). As shown in Figure 9.8, Pacinian corpuscles are located deep in the dermis layer of the skin and even within the subcutaneous fat.

Image

Credit: Illustration by BruceBlaus via Wikimedia under a CC 3.0 license

Figure 9.8 Pacinian corpuscles are rapidly adapting and found deep within the skin; they are sensitive to deep, unsustained pressure and high-frequency vibrations (200–300 Hz range).

Response Characteristics

Pacinian corpuscles, like Meissner corpuscles, are rapidly adapting (RA) and respond to the onset and removal of the mechanical stimulus. They are most sensitive to deep pressure—such as from a poke—but not sustained pressure. They also respond well to high-frequency vibrations applied to the skin in the 200–300 Hz range (Bear et al., 2007, p 391; Gilman, 2002), although the source is poorly localized due to the size of the receptive field (Noback et al., 2005). Pacinian corpuscles are so sensitive that they have been shown to actually be able to detect sound waves when water is the coupling agent (Ide et al., 1987).

Ruffini Ending

A Ruffini ending (alternatively known as a Ruffini corpuscle) is characterized by a spindle-shaped geometry with complex, tree-like ends (dendritic, from the Greek déndron). The structure contains a dense entanglement of nerve endings encapsulated in collagen connective tissue.

As shown in Figure 9.9, Ruffini endings are located deep in the dermis layer of the skin. Found at their highest densities in the folds of the palm, over the joints, and along the edges of fingernails, when the skin is stretched, collagen fibers enclosed in the spindle-shaped capsule compress the nerve endings, resulting in the release of action potentials (Gardner, 2010).

Image

Credit: Illustration by BruceBlaus via Wikimedia under a CC 3.0 license

Figure 9.9 Ruffini endings are very slow adapting and sensitive to sustained pressure, skin stretch, and slippage, and they respond to high frequencies (300–400 Hz).

Response Characteristics

Ruffini ending are slow adapting (SA), sensitive to sustained pressure, skin stretch, and slippage, and contribute to the control of finger position and movement (Barrett and Ganong, 2012; Mountcastle, 2005). Ruffini endings respond maximally to high frequencies (300–400 Hz) and show very little adaptation (Guyton and Hall, 2001).

Mechanoreceptor Performance Summary

Based upon the descriptions provided, Table 9.1 and Figure 9.10 summarize the performance characteristics of the four major mechanoreceptors.

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Table 9.1 Summary of Primary Mechanoreceptor Characteristics

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Credit: Receptor illustrations by BruceBlaus via Wikimedia under a CC 3.0 license., Hand image by ratru © 123RF.com, Derivative work—S. Aukstakalnis

Figure 9.10 Based on type, mechanoreceptors in the skin of the palm and fingers vary by distribution and receptive field size.

Thus far we have explored the physiology and basic function of the sensors responsible for generating cutaneous tactile sensations. As our fingers explore an object, this information helps our brain understand subtle details about the physical features in our surroundings.

In this next section we will briefly explore the physiology and basic function of an equally important set of sensors responsible for our kinesthetic sense, which helps us perceive joint angles, as well as tension and other forces experienced in our muscles and tendons. An example of the relevance to the subject matter of this book can be found in the resistance and rumbling (known as force feedback) experienced using a quality gaming steering wheel.

Kinesthetic Sensing

Kinesthesia is the sense that permits us to detect bodily position, weight, and movement in our muscles, tendons, and joints. In short, it is the sense of body awareness. A part of the somatic sensory system described at the beginning of this chapter, kinesthesia is a sense we use nonstop without conscious thought. It helps us decide if we will turn sideways (and by how much) when passing someone in a doorway and whether we will fit in the middle seat on a plane flight. It will cue our brain if we are walking off the edge of a step (due to angles in our ankle, knee, and hip) and even help to plant a layup during a game of basketball.

An understanding of this sensory channel is useful because, in addition to the increasing research and development efforts in the area of tactile and force feedback devices discussed in Chapter 10, “Tactile and Force Feedback Devices” (primarily output devices), there is experimentation with systems that measure a user’s physical exertion as a form of input (Ponto et al., 2012).

Proprioceptors

A key aspect of our kinesthetic sense (the adjective form of kinesthesia) is what is known as proprioception, which is the unconscious sense of movement, the related strength of effort, and relative positions of neighboring parts of the body (Mosby’s, 1994). Considerably different from mechanoreception, where external stimuli trigger a neural response, with proprioception the body itself is acting as a stimulus to its own receptors (Sherrington, 1906). As a simple example, even with your eyes closed, you know if your arms are at your sides or extended over your head, if your fingers are spread apart, or if you knees are bent. There is no need to see them or think about them. You just “know.”

When you move your body, be it shifting in your chair, crossing your legs, walking, or reaching for an object, everything begins with nerve impulses from the central nervous system triggering the process. During these movements, tissues in the joints change shape, including skin, muscles, tendons, and ligaments (Adrian, 1929; Grigg, 1994). In turn, various nerves and receptors within these tissues begin firing.

Research suggests that the signals provided by two unique sensory nerves—muscle spindles and Golgi tendon organs—are key to our proprioceptive sense (Proske and Gandevia, 2009, 2012).

Muscle Spindles

Muscle spindles are small, elongated sensory organs enclosed within a capsule and are found in nearly all skeletal muscles of the human body (Purves et al., 2001). As is obvious from Figure 9.11, a muscle spindle derives its name from actual geometry and structure. The concentration of spindles varies depending on specific muscle function, with higher counts found in muscles involved with delicate movements (Taylor, 2006).

Image

Credit: Image courtesy of N. Stifani, distributed under a CC 4.0 license, Derivative work—S. Aukstakalnis

Figure 9.11 A muscle spindle is a proprioceptor located deep within and oriented parallel to extrafusal muscle fibers.

Similar to Meissner and Pacinian corpuscles, the main body of a muscle spindle is composed of a capsule of collagen. Within this capsule and oriented parallel to the regular muscle fibers are specialized intrafusal muscle fibers, around which is wound an afferent nerve ending (dendrite) known as an annulospiral ending.

When regular muscle fibers stretch, tension in the intrafusal fibers increases, opening ion gates and stimulating the annulospiral nerve ending. This results in the generation of an action potential. The greater the tension, the greater the frequency of impulses fired off. These impulses quickly reach the central nervous system, and a return signal is sent to control the extent to which the muscle is allowed to stretch and, in doing so, prevent damage (Prochazka, 1980; Sherwood, 2015).

Golgi Tendon Organs

Unlike muscle spindles, which measure stretch, another sensory receptor, the Golgi tendon organ, measures tension in tendons (the tough, fibrous material connecting muscles to bone). When you lift an object, be it a baseball or heavy weight, or a limb is acted on by an external force, Golgi tendon organs tell you how much tension is being exerted by the muscle. From the viewpoint of a force feedback device such as those discussed in Chapter 10, Golgi tendon organs are the sensors that tell you how much force is being imparted to your hand or other limbs.

Muscle spindles are located within actual muscle fibers, as shown in Figure 9.12, but Golgi receptors are located in the tendons that attach a muscle to a bone. The core of a Golgi receptor consists of collagen fibers running parallel to the direction of normal muscle fibers. Interwoven into the collagen fibers are afferent nerve fibers. When a muscle stretches, tension is produced within the tendon. In turn, this tension results in the collagen fibers being pulled tight, stimulating the afferent nerve. Variation in the tension of the collagen fibers results in a variable firing rate of the nerve. These signals, in turn, enable physical output to be modulated (Grey et al., 2007; Mileusnic and Loeb, 2006).

Image

Credit: Derivative work based on public domain image courtesy of Neuromechanics via Wikimedia

Figure 9.12 Golgi tendon organs consist of sensory nerve endings interwoven among collagen fibers. When tension in the tendon increases due to muscle stretching, the collagen fibers stimulate the nerves.

Conclusion

In this chapter we have explored the primary mechanisms enabling our sense of touch, from the sensors responsible for generating cutaneous tactile sensations that help our brain understand subtle details about the physical features of objects and our surroundings, to those enabling us to perceive stronger mechanical forces acting on our body. In Chapter 10 we will explore a range of technologies and devices leveraging these processes and mechanisms enabling more intuitive man-machine interfaces.

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