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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
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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.
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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.
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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 ...