Chapter 7 Introduction

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What would your mental life be like if you had no senses? What if, from birth, you could not see, hear, touch, taste, smell, or in any other way sense the world around you? You would not be able to react to anything because reaction requires sensory input. You would not be able to learn anything because learning begins with sensory input. Would you be able to think? What could you think about with no knowledge gained from the senses? Philosophers, from Aristotle on, have pondered these questions and have usually concluded that without sensation there would be no mental life. It is no wonder that the study of the senses has always been a fundamental part of the science of psychology.

Sensory systems have evolved in all animals for the purpose of guiding their behavior. To survive and reproduce, animals must respond to objects and events in the world in which they live. They must move toward food and mates, for example, and away from predators and precipices. Sensory systems did not evolve to provide full, objective accounts of the world’s physical properties. Rather, they evolved to provide the specific kinds of information that the animal needs in order to survive and reproduce. To understand an animal’s sensory systems is to understand its way of life. For instance, frogs’ eyes contain “bug detectors,” neurons that respond only to small, moving dark spots and that trigger tongue movements in the spot’s direction; they also contain color detectors that are ideally tuned to distinguish the hue of the pond from that of the grass and lily pads (Muntz, 1964). Many species of migrating birds have a magnetic sense, sensitive to the earth’s magnetic field, that enables them to fly in the correct direction even on cloudy nights, when visual cues are not available (Hughes, 1999). Bats, which fly at night, have tiny, almost useless eyes but huge ears. They emit ultrasonic (beyond the range of human hearing) beeps and hear the echoes, which they use to detect objects (such as edible insects) and barriers as they navigate in complete darkness (Griffin, 1986).

This chapter and the next one are about sensation and perception. Roughly speaking, sensation refers to the basic processes by which sensory organs and the nervous system respond to stimuli in the environment and to the elementary psychological experiences that result from those processes (such as our experience of the bitterness of a taste, loudness of a sound, or redness of a sight). Perception, in contrast, refers to the more complex organizing of sensory information within the brain and to the meaningful interpretations extracted from it (such as “This is strong coffee,” “My alarm clock is ringing,” or “That object is an apple”). Thus, the study of perception is more closely tied to the study of the brain, thought, and memory than is the study of sensation. The distinction is fuzzy, however, because the organizing of stimulus information in ways useful for extracting meaning actually begins during the earliest steps of taking that information in.

This chapter begins with a brief overview of basic processes involved in sensation. We then provide a discussion of psychophysics, which is an approach to finding and describing reliable relationships between physical stimuli and sensory experiences of those stimuli. The chapter continues with sections on smell, taste, pain, and hearing. The next chapter deals exclusively with vision, which is by far the most thoroughly studied of the senses.