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Chapter 4

Chapter 4: Sensation

Chapter Review

THE ORIGINS OF KNOWLEDGE

  • The study of sensory processes grew out of questions about the origin of human knowledge. The empiricists argued that all knowledge comes through stimuli that excite the senses. However, the only way to get information about distal stimuli (the objects or events in the world) is through the proximal stimuli (the energies that impinge on a sensory surface). The empiricists therefore argued that much of perception is built up through learning by association.

PSYCHOPHYSICS

  • Research in psychophysics seeks to relate the characteristics of the physical stimulus to both the quality and intensity of the sensory experience. One psychophysical measurement is the absolute threshold. Another measurement is the difference threshold, producing a just-noticeable difference ( jnd). According to Weber’s law, the jnd is a constant fraction of the intensity of the comparison stimulus. Building on this principle, Fechner’s law states that the strength of a sensation grows as the logarithm of stimulus intensity.
  • Data in psychophysical procedures are influenced by a perceiver’s sensory sensitivity as well as her decision criteria. These two factors can be assessed separately, though, via a signal-detection procedure.

A SURVEY OF THE SENSES

  • Sensory codes are the rules by which the nervous system translates the properties of the proximal stimulus into neural impulses. Psychological intensity is usually coded by the rates of firing by the neurons and by the sheer number of neurons triggered by the stimulus.
  • Other codes are for sensory quality. In some cases, qualitative differences within a sensory modality are best described by specificity theory—that different sensory qualities (e.g., red versus green) are signaled by different neurons, just as the different sense modalities are signaled by different nerves. More commonly, sensory coding is best described by pattern theory, which holds that certain sensory qualities arise because of different patterns of activation across a whole set of neurons.
  • Certain properties can be observed in all of the sensory systems—including the phenomenon of adaptation—the tendency to respond less to a stimulus that has been around and unchanging for some time.
  • The vestibular sense signals movements of the head, and helps us know which way is “up” and which is “down.” The receptors for this sense are in the semicircular canals in the inner ear.
  • The skin senses include several distinct subsystems, and lead to the separate sensations of pressure, temperature and pain. Even within these systems, we must distinguish different types of receptors—for example, one type that fires when the temperature rises, and another that fires in response to a drop in skin temperature.
  • The sense of pain depends on specialized receptors that respond to various forms of tissue damage and temperature extremes. However, the experience of pain is also influenced by other mechanisms, including the endorphins, and by neural circuits that provide a “gateway” blocking the transmission of some signals from the nociceptors.
  • The sense of smell is triggered by receptors in the olfactory epithelium, which then send their neural signals to glomeruli in the olfactory bulb. The experience of a specific smell is coded by a pattern of activity across the glomeruli. Smell has many functions—helping animals to find food and avoid predators, and, in many circumstances, providing a means of communicating within a species. The chemicals used for these communications are called pheromones.
  • The receptors for taste are located on the papillae found primarily on the tongue. There are five types of receptors, and each type is sensitive to a wide range of inputs. Once again, therefore, the qualities of taste (sweet vs. salty, sour vs. bitter) are coded by a pattern of responding across the five receptor types.

HEARING

  • Sound waves can vary in amplitude and frequency, and set up vibrations in the eardrum that are then transmitted by the auditory ossicles to the oval window, whose movements create waves in the cochlea. Within the cochlea is the basilar membrane, which contains the auditory receptors that are stimulated by the membrane’s deformation. According to the place theory, the experience of pitch is based on the place of the membrane that is most stimulated; each place is especially responsive to a particular frequency and generates a particular pitch sensation. According to the frequency theory, the experience of pitch depends on the firing frequency of the auditory nerve. Evidence suggests that both theories are correct—the perception of higher frequencies depends on the place stimulated on the basilar membrane, and the perception of lower frequencies depends on firing frequency.

VISION

  • Vision is our primary distance sense. Its stimulus is light, which can vary in intensity and wavelength. Some structures of the eye, such as the iris and the lens, control the amount of light entering the eye and form a proper proximal stimulus—the retinal image. Once on the retina, the light stimulus is transduced by the rods and cones. Acuity is greatest in the fovea, where the density of cones is greatest.
  • Rods and cones differ markedly in function. The rods operate at low light intensities and are insensitive to differences in hue. The cones function at much higher illumination levels and are responsible for sensations of color.
  • The various components of the visual system interact constantly, and these interactions actively shape and transform the stimulus input. One kind of interaction involves contrast effects, including brightness contrast.These effects serve to accentuate edges—as in the case of Mach bands.The physiological mechanism underlying this effect is lateral inhibition, a clear example of how the visual system refines the stimulus information by emphasizing some aspects of the input and understating others.
  • Visual sensations vary in color; and color sensations can be ordered on the basis of their hue, brightness, and saturation.Normal human color vision is trichromatic, depending on three cone types. However, some facts do not fit with this trichromatic conception, because colors come in pairs—as shown by the phenomena of complementary colors, color contrast, and negative afterimages.
  • Opponent-process theory proposes that the output of the cones serves as input for a further layer of mechanisms that recode the signal into three opponent-process pairs: red-green, blue-yellow, and black-white.
  • Shape perception depends on specialized detector cells that respond to certain characteristics of the stimulus, such as curves and straight edges. The optimal input for each cell—that is, a stimulus of a certain shape and size at a certain position—defines the cell’s receptive field. In cats and monkeys, feature detectors seem to respond maximally when a line or edge of a specific orientation is in view. Other cells, deeper within the visual system, assemble these elements in order to detect larger configurations and more complex patterns.
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