Glance around you. Notice the incredible variety of colors, shades, shadows, and images. Listen carefully to the diversity of sounds, loud and soft, near and far. Focus on everything that’s touching you—your clothes, your shoes, the chair you’re sitting on. Now, inhale deeply through your nose and identify the aromas in the air.

With these simple observations, you have exercised four of your senses: vision, hearing, touch, and smell. As we saw in Chapter 2, the primary function of the nervous system is communication—the transmission of information from one part of the body to the other. Where does that information come from? Put simply, your senses are the gateway through which your brain receives all its information about the environment. It’s a process that is so natural and automatic that we typically take it for granted until it is disrupted by illness or injury. Nevertheless, as Mike’s story demonstrates, people with one nonfunctional sense are amazingly adaptive. Often, they learn to compensate for the missing environmental information by relying on their other senses.

In this chapter, we will explore the overlapping processes of sensation and perception. Sensation refers to the detection and basic sensory experience of environmental stimuli, such as sounds, images, and odors. Perception occurs when we integrate, organize, and interpret sensory information in a way that is meaningful. Here’s a simple example to contrast the two terms. Your eyes’ physical response to light, splotches of color, and lines reflects sensation. Integrating and organizing those sensations so that you interpret the light, splotches of color, and lines as a painting, a flag, or some other object reflects perception. Mike’s visual world reflects this distinction. Although his eye was accurately transmitting visual information from his environment (sensation), his brain was unable to make sense out of the information (perception).

Where does the process of sensation leave off and the process of perception begin? There is no clear boundary line between the two processes as we actually experience them. In fact, many researchers in this area of psychology regard sensation and perception as a single process.

Although the two processes overlap, we will present sensation and perception as separate discussions. In the first half of the chapter, we’ll discuss the basics of sensation—how our sensory receptors respond to stimulation and transmit that information in usable form to the brain. In the second half of the chapter, we’ll explore perception—how the brain actively organizes and interprets the signals sent from our sensory receptors.

KEY THEME

Sensation is the result of neural impulses transmitted to the brain from sensory receptors that have been stimulated by physical energy from the external environment.

KEY QUESTIONS

We’re accustomed to thinking of the senses as being quite different from one another. However, all our senses involve some common processes. All sensation is a result of the stimulation of specialized cells, called sensory receptors, by some form of energy.

Imagine biting into a juicy strawberry. Your experience of hearing the strawberry crunch is a response to the physical energy of vibrations in the air, or sound waves. The sweet, slightly tart taste of the strawberry is a response to the physical energy of dissolvable chemicals in your mouth, just as the distinctive sharp aroma of the strawberry is a response to airborne chemical molecules that you inhale through your nose. The prickly feel of the strawberry’s skin is a response to the pressure of the strawberry against your hand. And the bright red color of the strawberry is a response to the physical energy of light waves reflecting from the irregularly shaped sphere.

Sensory receptors convert these different forms of physical energy into electrical impulses that are transmitted via neurons to the brain. The process by which a form of physical energy is converted into a coded neural signal that can be processed by the nervous system is called transduction. These neural signals are sent to the brain, where the perceptual processes of organizing and interpreting the coded messages occur. FIGURE 3.1 illustrates the basic steps involved in sensation and perception.

We are constantly being bombarded by many different forms of energy. For instance, at this very moment radio and television waves are bouncing around the atmosphere and passing through your body. However, sensory receptors are so highly specialized that they are sensitive only to very specific types of energy (which is lucky, or you might be seeing Friends reruns in your brain right now). So, for any type of stimulation to be sensed, the stimulus energy must first be in a form that can be detected by our sensory receptor cells. Otherwise, transduction cannot occur.

SENSORY THRESHOLDS

Along with being specialized as to the types of energy that can be detected, our senses are specialized in other ways as well. We do not have an infinite capacity to detect all levels of energy. To be sensed, a stimulus must first be strong enough to be detected—loud enough to be heard, concentrated enough to be smelled, bright enough to be seen. The point at which a stimulus is strong enough to be detected because it activates a sensory receptor cell is called a threshold. There are two general kinds of sensory thresholds for each sense—the absolute threshold and the difference threshold.

The absolute threshold refers to the smallest possible strength of a stimulus that can be detected half the time. Why just half the time? It turns out that the minimum level of stimulation that can be detected varies from person to person and from trial to trial. Because of this human variability, researchers have arbitrarily set the limit as the minimum level of stimulation that can be detected half the time. Under ideal conditions (which rarely occur in normal daily life), our sensory abilities are far more sensitive than you might think (see TABLE 3.1). Can stimuli that are below the absolute threshold affect us? We discuss this question in the Science Versus Pseudoscience box, “Subliminal Perception.”

The other important threshold involves detecting the difference between two stimuli. The difference threshold is the smallest possible difference between two stimuli that can be detected half the time. Another term for the difference threshold is just noticeable difference, which is abbreviated jnd.

The just noticeable difference will vary depending on its relation to the original stimulus. This principle of sensation is called Weber’s law, after the German physiologist Ernst Weber (1795–1878). Imagine holding a pebble (the original stimulus). If a second pebble is placed in your hand, you will notice an increase in weight. But if you start off holding a very heavy rock (the original stimulus), you probably won’t detect an increase in weight when the same pebble is balanced on it. Weber's law holds that for each sense, the size of a just noticeable difference is a constant proportion of the size of the initial stimulus. So, whether we can detect a change in the strength of a stimulus depends on the intensity of the original stimulus.

What Weber’s law underscores is that our psychological experience of sensation is relative. That is, there is no simple, one-to-one correspondence between the objective characteristics of a physical stimulus, such as the weight of a pebble, and our psychological experience of it.

SENSORY ADAPTATION

Suppose your best friend has invited you over for a spaghetti dinner. As you walk in the front door, you’re almost overwhelmed by the odor of onions and garlic cooking on the stove. However, after just a few moments, you no longer notice the smell. Why? Because your sensory receptor cells become less responsive to a constant stimulus. This gradual decline in sensitivity to a constant stimulus is called sensory adaptation. Once again, we see that our experience of sensation is relative—in this case, relative to the duration of exposure.

Because of sensory adaptation, we become accustomed to constant stimuli, which allows us to quickly notice new or changing stimuli. This makes sense. If we were continually aware of all incoming stimuli, we’d be so overwhelmed with sensory information that we wouldn’t be able to focus our attention. So, for example, once you manage to land your posterior on the sofa, you don’t need to be constantly reminded that the sofa is beneath you.