OUR SENSES ENLIGHTEN AND EMPOWER US. Through sensation, we form an immediate and relevant picture of the world and our place in it, informed by our past experience and preparing us for probable futures. Sensation provides immediate answers to three ongoing and essential questions: Is something there? What is it? and What’s changed? To answer these questions, all sensory systems perform two fundamental functions: detection and discrimination. Because our world and our needed responses to it change with time, sensory systems can both preferentially respond and adapt to changing stimuli in the short term, and also learn to modify our responses to stimuli as our needs and circumstances change.
Since ancient times, humans have been fascinated by the nature of sensory experience. Aristotle defined five senses—vision, hearing, touch, taste, and smell—each linked to specific sense organs in the body: eyes, ears, skin, tongue, and nose. Pain was not considered to be a specific sensory modality but rather an affliction of the soul. Intuition, often referred to colloquially as a “sixth sense,” was not yet understood to depend upon the experience of the classic sensory systems. Today, neurobiologists recognize intuition as inferences derived from previous experience and thus the result of cognitive as well as sensory processes.
In this chapter, we consider the organizational principles and coding mechanisms that are universal to all sensory systems. Sensory information is defined as neural activity originating from stimulation of receptor cells in specific parts of the body. Our senses include the classic five senses plus a variety of modalities not recognized by the ancients but essential to bodily function: the somatic sensations of pain, itch, temperature, and proprioception (posture and movement of our own body); visceral sensations (both conscious and unconscious) necessary for homeostasis; and the vestibular senses of balance (the position of the body in the gravitational field) and head movement.
Sensation informs and enriches all life, and the fundamentals of sensory processing have been conserved throughout vertebrate evolution. Specialized receptors in each of the sensory systems provide the first neural representation of the external and internal world, transforming a specific type of stimulus energy into electrical signals (Figure 17–1). All sensory information is then transmitted to the central nervous system by trains of action potentials that represent particular aspects of the stimulus. This information flows centrally to regions of the brain involved in the processing of individual senses, multisensory integration, and cognition.
The major sensory modalities in humans are mediated by distinct classes of receptor neurons located in specific sense organs. Each class of receptor cell transforms one type of stimulus energy into electrical signals that are encoded as trains of action potentials (see Figure 17–4). The principal receptor cells include photoreceptors (vision), chemoreceptors (smell, taste, and pain), thermal receptors, and mechanoreceptors (touch, hearing, balance, and proprioception). The classic five senses—vision, smell, taste, touch, and ...