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  • Psychophysics Relates the Physical Properties of Stimuli to Sensations

    • Psychophysical Laws Govern the Perception of Stimulus Intensity

    • Psychophysical Measurements of Sensation Magnitude Employ Standardized Protocols

    • Sensations Are Quantified Using Probabilistic Statistics

    • Decision Times Are Correlated with Cognitive Processes

  • Physical Stimuli Are Represented in the Nervous System by Means of the Sensory Code

    • Sensory Receptors Are Responsive to a Single Type of Stimulus Energy

    • Multiple Subclasses of Sensory Receptors Are Found in Each Sense Organ

    • Neural Firing Patterns Transmit Sensory Information to the Brain

    • The Receptive Field of a Sensory Neuron Conveys Spatial Information

  • Modality-Specific Pathways Extend to the Central Nervous System

    • The Receptor Surface Is Represented Topographically in Central Nuclei

    • Feedback Regulates Sensory Coding

    • Top-Down Learning Mechanisms Influence Sensory Processing

  • An Overall View

Since ancient times humans have been fascinated by the nature of sensory experience. The Greek philosopher Aristotle defined five senses—vision, hearing, touch, taste, and smell—each linked to specific sense organs in the body: the eyes, ears, skin, tongue, and nose (Figure 21–1). 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 something beyond the experience of classic sensory systems. Today neurobiologists are more likely to describe intuition as inferences derived from previous experience and thus the result of cognitive rather than sensory processes.

Figure 21–1
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. 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 hearing—and the sense of balance are mediated by receptors in the eye, nose, mouth, skin, and inner ear, respectively. The other somatosensory modalities—thermal senses, pain, and proprioception— are mediated by receptors distributed throughout the body.

In this chapter we consider the organizational principles and coding mechanisms universal to all sensory systems. We define sensory information as neural activity originating from stimulation of receptor cells in specific parts of the body. These 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 proprioception (posture and movement of our own body), pain, itch, and temperature; 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.

The extent to which features of sensory processing have been conserved in the course of human evolution seems nothing short of astonishing. In each of the sensory systems receptors provide the first neural representation of ...

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