CHARACTERISTICS OF SENSATIONS
Certain
characteristics of sensations will help you understand how the sensory areas
work with informa-tion from the receptors.
1.
Projection—the sensation seems to come from
the area where the receptors were
stimulated. If you touch this book, the sensation of touch seems to be in your
hand but is actually being felt by your cere-bral cortex. That it is indeed the
brain that feels sensations is demonstrated by patients who feel phantom pain after amputation of a
limb. After loss of a hand, for
example, the person may still feel that the hand is really there. Why does this
hap-pen? The receptors in the hand are no longer pres-ent, but the severed nerve
endings continue to generate impulses. These impulses arrive in the parietal
lobe area for the hand, and the brain does what it has always done and creates
the projection, the feeling that the hand is still there. For most amputees,
phantom pain diminishes as the severed nerves heal, but the person often
experiences a phantom “presence” of the missing part. This may be helpful when
learning to use an artificial limb.
2. Intensity—some
sensations are felt more distinctly and
to a greater degree than are others. A weak stimulus such as dim light will
affect a small num-ber of receptors, but a stronger stimulus, such as bright
sunlight, will stimulate many more recep-tors. When more receptors are
stimulated, more impulses will arrive in the sensory area of the brain. The
brain “counts” the impulses and projects a more intense sensation.
3. Contrast—the effect of
a previous or simultaneous sensation
on a current sensation, which may then be exaggerated or diminished. Again,
this is a func-tion of the brain, which constantly compares sensa-tions. If, on
a very hot day, you jump into a swimming pool, the water may feel quite cold at
first. The brain compares the new sensation to the previous one, and since
there is a significant differ-ence between the two, the water will seem colder
than it actually is.
4. Adaptation—becoming
unaware of a continuing stimulus.
Receptors detect changes, but if the stim-ulus continues it may not be much of
a change, and the receptors will generate fewer impulses. The water in the
swimming pool that seemed cold at first seems to “warm up” after a few minutes.
The water has not changed temperature, and the recep-tors for cold have no
changes to detect, therefore they generate fewer impulses. The sensation of
cold lessens, and we interpret or feel that as increasing warmth. For another
example, look at your left wrist (or perhaps the right one). Many of us wear a
watch and are probably unaware of its presence on the arm most of the time. The
cuta-neous receptors for touch or pressure adapt very quickly to a continuing
stimulus, and if there is no change, there is nothing for the receptors to
detect.
5. After-image—the sensation
remains in the con-sciousness even after the stimulus has stopped. A familiar
example is the bright after-image seen after watching a flashbulb go off. The
very bright light strongly stimulates receptors in the retina, which generate
many impulses that are perceived as an intense sensation that lasts longer than
the actual stimulus.
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