Try this. Close your eyes and ask a friend to hand you an everyday object. What can you learn about the object without even looking at it (or listening to or smelling it)? A lot! You’ll quickly acquire a wealth of information: its size … its shape … what it’s made of. You might even be able to identify exactly what the object is.

This information comes through your haptic system, the perceptual system through which people acquire information about objects by touching them (Lederman & Klatzky, 2009). (The word haptic comes from the Greek word for “touch.”)

Not all types of touch are the same. As James J. Gibson (1962) pointed out, there are two types:

The two types of touch differ psychologically. Through active touching, you perceive physical properties of objects in the environment—for example, whether they are smooth, rough, warm, or cold (Turvey & Carello, 2011). When being touched, your psychological experience centers on a point of your body; for example, if the object is a bowling ball that touches you by landing on your foot, you feel pain in your foot.

In this section of the chapter, we’ll first look at the types of information you acquire by using your haptic system. After reviewing these psychological processes, we’ll move down to a biological level of analysis, to learn about the nervous-system mechanisms that make haptic information gathering possible.

Let’s first look at the precision, or acuity, of the haptic system. We’ll then see how this system reveals information about (1) the surfaces of objects (their texture, temperature, and hardness), and (2) objects’ size and shape.

ACUITY. If somebody taps you, you can locate the tap: It was felt on your arm, or shoulder, or head, or back. How precisely can you locate it, though? This is a question about the acuity of the haptic system, that is, the accuracy with which the system can identify whether it has been stimulated, where the stimulation has occurred, and the number of stimulations that occurred.

Researchers have developed psychophysical procedures to measure haptic system acuity. A classic method is the two-point procedure, developed by the nineteenth-century German scientist Ernst Weber. In the two-point procedure, researchers touch the skin of a research participant with two stimuli (e.g., two pencil points). The space between the stimuli varies across experimental trials. Sometimes they are spaced well apart, but other times they are so close that people can’t even tell there are two stimuli; it feels as if there is just one. On each trial, then, participants are asked whether they experience one point touching their skin or two. The smallest distance at which they can detect two different stimuli touching their skin is called the two-point threshold. This distance indicates the acuity of the haptic system; a smaller distance (i.e., a lower threshold) means greater acuity (a greater ability to distinguish between stimuli).

A key finding is that the two-point threshold distance varies across different parts of the body. At some regions of the body, the distance is quite small; the haptic system has excellent acuity. At other parts, acuity is relatively poor; that is, the two-point threshold is large. The differences between regions of the body are considerable (Figure 5.44); some two-point thresholds are more than 10 times larger than others. The greatest acuity is found at the fingers. The lowest is at the shoulder, back, and various areas of the leg.

SURFACES: TEXTURE, TEMPERATURE, AND HARDNESS. The haptic system delivers information about the surfaces of objects. The type of information received depends on how you explore the surface. Different types of hand and finger movements deliver different types of information (Figure 5.45).

When you move your fingertips back and forth across the surface of an object, you acquire information about texture—whether the surface is rough or smooth. The physical property that has the greatest influence on your perception of roughness is the distance between raised areas on the surface. Whether you move your finger across the surface quickly or slowly, a surface with gaps between raised areas will feel rough (Lederman & Klatzky, 2009).

Touch also provides information about temperature. When holding your fingers still on an object’s surface, you usually find that the object feels slightly cool. The surface temperature of your skin is similar to the inner temperature of your body, which is warmer than most indoor and outdoor environmental temperatures, so the object in the environment feels cool.

The temperature of objects can affect other judgments. When people touch two objects and try to identify whether they are made of different materials, the objects’ temperatures affect people’s identification. It is harder to identify differences between materials if they are heated to the same temperature (Lederman & Klatzky, 2009).

A third surface property you perceive haptically is hardness. Pressing against an object’s surface—such as a melon to judge its ripeness or a bicycle tire to judge whether it needs air—tells you how hard it is. Research reveals that people are very good at perceiving the stiffness of materials. When presented with objects made from materials that varied systematically in physical stiffness, participants could judge hardness accurately based on haptic perception of the degree to which the surface of the object deformed (changed shape) under the pressure of their fingers (Tiest & Kappers, 2009).

SIZE AND SHAPE. The haptic system also delivers information about an object’s size and shape. Exploring an object by touch with your eyes closed, you can identify its approximate length and width. By holding the object and moving it around, you can estimate its mass (weight).

A principle you learned earlier, in the section on the visual system and brightness, applies also to the haptic system and perception of mass. You can apply that principle here to answer the following question: How big a difference in physical mass is required for the haptic system to detect that the mass of an object has changed? The principle of just noticeable differences is that there is no single, fixed answer to that question. If somebody hands you two envelopes, one weighing 1 ounce and the other 2 ounces, you’ll notice the difference. If somebody hands you two dumbbells, one weighing 24 pounds, 15 ounces, and the other 25 pounds, you won’t notice. The smallest detectable difference increases as the mass of objects increases.

Like other perceptions, the perception of mass is subject to illusions. For example, when judging the weight of two objects that weigh the same but are of different sizes, size influences weight judgments: Larger objects are perceived as lighter. Even with practice lifting the objects, people judge that smaller objects are heavier than larger ones when the objects are, in reality, the same weight (Flanagan & Beltzner, 2000).

Let’s now look under the skin—literally—to examine the biological bases of haptic perception.

The body contains, underneath the skin, cutaneous receptors, which are receptor cells that convert physical stimulation into nervous-system impulses (Iggo & Andres, 1982). For example, you feel a tap on your leg because the tap activates cutaneous receptors, which send impulses that travel to your spinal cord and then to your brain. There are different types of cutaneous receptors, and their response to stimuli differs in two main ways:

These differences produce different types of sensatory experience. Pricking your finger with a pin feels different from pressing on the side of your hand because the two types of stimulation activate different types of receptors.

One distinct type of cutaneous receptor responds to changes in temperature. It transduces physical temperature into neural signals that lead to the experience of hot and cold.

Imagine you’re playing the game Twister, all twisted up after a few moves. Now imagine that the only lamp in the room falls over and hits you on the head, plunging you into darkness and causing you temporarily to lose memory of the fact that you’re playing Twister. Even though you can’t see your arms and legs, and don’t remember moving your hands and feet to the various colored circles, you still know you’re twisted up. You can feel it.

This feeling comes from your kinesthetic system, the perceptual system that delivers information about the orientation of your body and its various parts. A key source of these signals is nervous system inputs coming from your muscles. Even when signals from the skin and bones are blocked (e.g., through anesthesia), people are aware of the position of their limbs thanks to information coming from the muscular system (Matthews, 1982). This indicates that kinesthetic feedback from muscles is a unique perceptual system.

Brain-imaging studies also indicate that the kinesthetic system is a distinct perception system (Guillot et al., 2009). Research participants were asked to imagine (1) visual imagery: what it would look like if they performed various muscle movements; and (2) kinesthetic imagery: what it would feel like to perform the movements. Visual and kinesthetic imagery activated different regions of participants’ brains. This further shows that the kinesthetic system is a distinct, sixth system of perception.

In addition to nervous system input from muscles, another biological mechanism that contributes to kinesthetic abilities is the vestibular system, found within the inner ear. The vestibular system contains small fluid-filled structures. When you tilt or rotate your head, the fluid moves. The movement triggers receptor cells that provide information to the brain—information needed for proper balance. If the vestibular system is not functioning properly, people experience vertigo, a sense of dizziness and spinning when their head changes position (Bhattacharyya et al., 2008).