Developmental psychology.pdf

Sensory Processes 139
Semicircular canals
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Figure 5.14
Nonauditory Labyrinth. There are
two parts. The vestibular system,
which includes the saclike structures
called the utricle and saccule,
indicates body position, balance, arid
linear motion. The other part, the
semicircular canals, mediate rotary
motion
Even when this system is functioning properly, an individual may show poor
balance, sometimes because the vestibular branch of the auditory nerve has been damaged
and does not carry the vestibular impulse to the brain. Some people who have
poor hearing also have poor balance, a problem that may originate in the receptor
areas.
Research and Demonstrations In lobsters, the weighted particles on the hair cells
must be replaced periodically, as the animal acquires a new shell, and the substances
used for this purpose are generally stones and sand obtained from the ocean floor. In
some early experiments, conditions were arranged so that no materials other than iron
filings were available for replacing the sloughed-off stones, and the animals used them.
Later, when a magnet was placed above these aquatic creatures, they rolled over on
their backs and remained in this position. This action returned the normal gravitational
pull, which comes from the animal's underside rather than its back (Prentiss, 1901).
When one vestibular system is damaged in the frog, it immediately tilts its
head toward that side. The disturbance is temporary, however, for after a while the
symptom disappears (Schaefer & Meyer, 1978). Apparently the frog learns to compensate
for this loss through the use of other senses, especially vision.
Passive Motion
William James, in his early demonstration laboratory, had an apparatus for testing the
sense of motion in frogs. Placing an amphibian on a flat, rotating disk, he found that
it first turned its head in the direction opposite to the rotation, and later the frog "waltzed
around" with its entire body in this same direction (James, 1908). Apparently the frog,
like the human being, attempts to compensate for any disruption of its normal state
of equilibrium.
James was studying the sense of passive motion, which occurs whenever an
individual is traveling from one place to another but not actively moving. This experience
occurs in elevators, roller coasters, and automobile rides.
Structures and Functions Generally, two types of passive motion are specified, the
first of which is mediated by the vestibular system. In the experience of linear motion,
one's rate of straight-line movement is changed, an