The Power of Touch

Handling Objects in Museum and Heritage Context

Elizabeth Pye

• Publisher: Routledge
• Year: 2016
• eBook ISBN:
  9781315417431
• Topic: Art
• Subtopic: Art General

Despite the fact that we have a range of senses with which to perceive the world around us, museums and other cultural institutions have traditionally used sight as the main way to convey information. In everyday life, though, we use touch constantly in conjunction with sight. Why, then, does it play so small a role in the study and enjoyment of museum objects? Contributors to this volume explore how the sense of touch can be utilized in cultural institutions to facilitate understanding and learning.

PART 1

SCIENCE OF TOUCH

1

WEIGHING UP THE VALUE OF TOUCH

Alan Wing, Christos Giachritsis and Roberta Roberts

Introduction

Vision often appears to determine the way we perceive the world. However, touch is the sensory modality that verifies the reality of what we see by allowing us to confirm the physical presence of objects and people around us. Vision may prompt us to make contact with an object or person but, by touching, we reinforce the subjective impact of that object or person. For example, we may decide to touch an object on the basis of its appearance (looks interesting) but the final sense of the object will also be based on its feel (feels good).

Touch provides us with knowledge about the location, geometry, and weight of an object by integrating information from the cutaneous receptors in the skin and proprioceptive receptors in the muscles. These receptors convert the mechanical effects of contact forces into electrical impulses in the nerves and are termed mechanoreceptors. The muscle mechanoreceptors provide cues to large- and medium-scale geometric features of an object, which allow us to perceive its size and shape as well as its weight and hardness. The skin mechanoreceptors provide us with information about small-scale geometry, which allows us to perceive texture. The skin also has receptors that allow us to detect thermal conductivity, or temperature, relative to the body.

Temperature is a very useful sensory cue, for example in discriminating metal (high thermal conductivity, so it feels cold) from wood (low thermal conductivity, so it feels warm), and also provides an important protective function. However, the sensing principles and information processing by the nervous system for temperature are quite different from those for the mechanoreceptors and it is the latter that we focus on in this chapter.

Size and shape inform us about the type of object we are handling (and thus its use, given we are familiar with the object, say, a garment). However, texture and weight indicate its quality (cotton, wool, fur). In a cultural heritage context, perceiving the geometric and material properties of an ancient artefact through touching and handling may be expected to provide us with clues about the way people used to live; for example, we might better appreciate the technologies they used.

Later in this chapter, we discuss human sensitivity to weight and how this sensitivity may vary depending on the way we grasp, handle, and manipulate an object. However, we begin by reviewing the neuro-physiology of touch. We describe the nature of sensory coding in general, focus on the neural receptors that respond to mechanical events underlying touch, and outline the processes in the brain that take the incoming sensory signals and interpret them or use them for controlling actions. Finally, we provide a brief overview of the motor system of the brain, which is responsible for the control of movement and is critically dependent on touch sensory input.

Sensory Coding of Environmental Events

How do our senses tell us about the environment? Information from the environment is available in various forms of energy including mechanical, thermal, and electromagnetic. Using specialised receptors, our senses respond to such sources of energy and transform them into electrical signals, a series of millivolt electrical pulses, whose timing is related to the strength of the source. Different properties of an object may be uniquely signalled by different senses, for example, temperature by touch and wavelength of reflected light (colour) by vision. However, generally a given attribute of an object may serve as input to several senses because a common energy source leads to correlated physical effects on receptors in different sensory systems. Thus, as the water in a kettle starts to boil, the turbulence produces a pattern of stimulation that affects hearing, touch (through vibration of the kettle handle), and vision.

Once the sensory receptors have encoded the stimulus into a stream of electrical pulses, the pulses are transmitted via the nerves to different regions of the brain (Figure 1.1). Since different sensory inputs are all transmitted electrically, with nothing in the signal to indicate its origin, the target brain areas have to be specialised in order to utilise the incoming sensory information according to its particular sensory source (e.g. eyes, hands, ears, nose). For instance, processing of the sensory input in brain areas responsible for touch differs from processing by those areas underlying vision. Moreover, for each sense, the relevant brain area reflects the distribution of effects across the sensory receptors. Thus, in the case of touch, different body regions are represented in separate but neighbouring areas of the brain, effectively creating a map representing the body surface.

Figure 1.1: Cortical processing of touch. The somatosensory cortex receives sensory input via the thalamus from mechanoreceptors in the skin and muscles. Somatosensory inputs may affect motor actions directly via the motor cortex or indirectly via the posterior parietal cortex, which is important in the integration of touch and visual information.

What Is Touch?

Information about an object’s size, surface texture, shape, or orientation is gained by making contact with its surfaces. When contact is made, there is a deformation of the mechanoreceptors in the epidermal layer just below the outer surface of the skin. Although these tactile sensors are small (a fraction of a millimetre across), depending on how deep they lie some of them signal deformation of the overlying skin over a distance of several millimetres. Contact with any part of the body generally provides us with at least a vague sensation, that is, we know we have been touched and roughly where the touch occurred. However, our ability to make fine discriminations about the information varies depending on which part of the body comes into contact with the surfaces. This is because the tactile mechanoreceptors are not evenly distributed over all parts of the body but are concentrated in certain areas with special functional significance, such as the hands, and particularly the tips of the fingers (Johansson & Vallbo 1983). We are therefore better able to localise where we have been touched, or, for example, to discriminate between one and two closely spaced contacts on our hands, than if the touch contact is made, say, on the back of the arm.

The mechanoreceptors in the skin respond in two different ways when subject to deformation, either maintaining a steady series of electrical pulses while the deformation continues or responding only to changes in deformation due to pressure increases or decreases. Tactile receptors that respond preferentially to change are very sensitive to stimuli vibrating around 200 times per second, or to brief events that set up such vibrations in the skin. The functional significance is most likely to be in detecting initial contact when grasping an object and in sudden changes in skin stresses when a grasped object starts to slip or someone taps you on the shoulder to attract your attention. For example, researchers examined the responses of mechanoreceptors in the skin (using fine-wire electrodes inserted into volunteers’ arms) when lifting an object from a table (Macefield et al. 1996). They showed that the sudden change in skin stresses associated with successful lift was marked by a burst of activity from the skin mechanoreceptors. They argued that, since lifting is normally associated with such sensory input, the absence of such activity from the mechanoreceptors could form the basis for triggering feedback correction to the lifting action if the object fails to lift, perhaps because it is heavier than expected.

The combination of static and dynamic sensing exhibited by the skin mechanoreceptors has parallels in the mechanoreceptors embedded within the muscles. Termed muscle spindles, these receptors convey information about muscle length (static) and change in length (dynamic) associated with movement of the joint about which the muscle acts. Muscles act in pairs about joints, and opposing pairs flex, stabilise, or extend joints depending on the balance of the tension exerted by the muscles. In signalling muscle length, the spindles provide the brain with an indication of joint angle. Such information may relate to the consequences of muscle action or to forces imposed on the limb as a result of contact with external objects. For example, the degree of flexion of the fingers in holding an object in the hand indicates whether the object is small or large. Combining information from shoulder, elbow, and wrist joints with information about the length of the upper arm and forearm provides a basis for determining hand position relative to the body. This class of sensory information is termed proprioceptive to reflect the body sensing its own internal state.

The Sensory Brain: Processing of Touch Information

Cutaneous and proprioceptive information from the skin and muscle mechanoreceptors is transmitted to the brain via the spinal cord and the thalamus, the latter being an important relay structure deep in the brain (see Figure 1.1). From the thalamus the information is transmitted to the primary somatosensory receiving area, (S-I), in the parietal lobe of the cerebral cortex, on the opposite side (contralateral) to the site of stimulation. (For a detailed review of these and other neuroanatomical topics, see Kandel et al. 2000). Thus, touch inputs from the left side of the body terminate in S-I in the right hemisphere of the brain and those from the right terminate in left S-I.

An individual neuron in S-I is responsive to stimulation over a limited region of the body surface, called the receptive field of the neuron. If a stimulus moves across the skin, it may cross the receptive fields of several neurons in S-I. Neurons with adjacent receptive fields tend to lie adjacent to one another in S-I and spatial relations between receptive fields correspond approximately to the spatial arrangement of neurons in the cortex. However, as already noted, the number of somatosensory neurons representing an area of skin is determined by the density of sensory receptors in that area. It is as though the scale of the S-I map is adjusted to the function of the body part, with a larger scale for those parts of the body whose sensory function is more important (Figure 1.2). Thus, for example, the cortical sensory map has relatively more space devoted to representing the face than the torso. Note also how much space is devoted to the hand and fingers. Thus the two areas with the most complex sensory and motor capabilities of the body are supported by the greatest amount of cortical tissue.

Figure 1.2 Somatosensory homunculus showing the distribution of the tactile inputs to the brain. (adapted from Penfield & Rasmussen 1950, taking account of Servos et al. 1999)

A number of distinct regions has been identified within S-I (Brodman’s areas 3a, 3b, 1, 2). If different patterns of stimulation are applied to the skin, recordings of the activity of single neurons in these different areas show that processing of touch information undergoes a series of progressive transformations. In area 3b, which is early in the progression, the neurons respond to simple skin contact. Neurons in areas 1 and 2 signal more complex relational properties, which depend on integration of information across a number of neurons that subserve simple contact in area 3b. In this way, successive stages in cortical processing allow progressive elaboration of the nature of the touch stimulus. Neurons in area 3a (adjacent to area 3b) receive proprioceptive input from muscle receptors. They have important connections with neurons in area 2. Thus the latter area affords an opportunity for integration of cutaneous and proprioceptive information. This supports, for example, interpretation of cutaneous input from adjacent fingers in relation to information about the relative position of the fingers, a key component in recognising objects from their three-dimensional shape (stereognosis). An important target for touch information from S-I is posterior parietal cortex (PPC) (see Figure 1.1). In addition to receiving information from S-I, PPC also receives inputs from the visual and auditory systems. This brain area is important for integrating information from the different sensory modalities to build up a single integrated perception of an object.

The Motor Brain: Controlling the Muscles

Movements result from muscle action driven by activity in primary motor cortex (M-I) which lies in the frontal region of the brain just forward of S-I. Just as S-I provides a map-like representation of touch input, so adjacent areas of M-I drive activity in groups of muscles occupying adjacent areas of the body. Moreover, the amount of cortical tissue in M-I devoted to different areas of the body reflects the complexity of movement that can be achieved. This means that the face and hand have much larger representation (relative to their surface area on the body) than, for example, the torso.

Functioning of the neurons in the brain depends on the blood supply to support the metabolic needs of the cells. Loss of blood supply can lead to cell death, and the resulting impairment provides an index of the function of the part of the brain involved. Loss of blood supply to M-I results in weakness or complete loss of muscle control on the contralateral side of the body (the motor pathways cross from one side of the body to the other on the way from M-I to the spinal cord and on to the muscles). Although it is quite common to have weakness of movement affecting all of one side of the body (hemiparesis), sometimes the impairment of function can be localised, for example just affecting the hand and arm.

The ability to move our limbs plays a key role in allowing us to gauge the weight of objects. Skilled weight judgements are an important part of everyday life. An obvious application is in making purchasing decisions about quantity. However, sensing weight not only shapes what we think about an object but also how we handle it. A heavier object, for instance, requires more force to move than a light object and requires greater care to avoid an uncontrolled collision when setting it down. The weight of an object placed on the palm of the hand can be inferred from the pressure it exerts when the hand rests palm up on a table. But for many years it has been known that weight judgements are more accurate if muscle action is involved (Flanagan 1996). In active lifting, a ‘sense of effort’ appears to operate that is attributed to monitoring the level of muscle commands used to support the object. In lifting an object, the level of muscle activity changes with posture; for example, less shoulder muscle activity is required to lift an object close to the body than one held away from the body. This is because muscles have to overcome torque due to the weight of the object multiplied by its distance from the shoulder (you expend less energy if you li...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• List of Illustrations
• Preface
• Introduction: The Power of Touch
• Part 1 Science of Touch
• Part 2 History of Touch
• Part 3 Professional Touch
• Part 4 Touch and Memory
• Part 5 Touch and Discovery
• Part 6 Virtual Touch
• Index
• About the Contributors