Sensory Physiology
201
Axons from neurons of the parietal and temporal lobes go
to association areas in the frontal lobes that are part of the lim-
bic system. Through these connections, sensory information
can be invested with emotional and motivational signifi cance.
Further perceptual processing involves not only arousal,
attention, learning, memory, language, and emotions, but also
comparison of the information presented via one type of sensa-
tion with that presented through another. For example, we may
hear a growling dog, but our perception of the event and our
emotional response vary markedly, depending upon whether
our visual system detects the sound source to be an angry ani-
mal or a tape recording.
Factors That Affect Perception
We put great trust in our sensory-perceptual processes despite
the inevitable modifi cations we know the nervous system
makes. Factors known to affect our perceptions of the real
world include:
1.
Sensory receptor mechanisms (e.g., adaptation) and
processing of the information along afferent pathways
can infl uence afferent information.
2.
Factors such as emotions, personality, experience, and
social background can infl uence perceptions so that
two people can be exposed to the same stimuli and yet
perceive them differently.
3.
Not all information entering the central nervous
system gives rise to conscious sensation. Actually,
this is a very good thing because many unwanted
signals are generated by the extreme sensitivity of our
sensory receptors. For example, under ideal conditions,
the rods of the eye can detect the fl ame of a candle
17 miles away. The hair cells of the ear can detect
vibrations of an amplitude much lower than those
caused by blood fl ow through the ears’ blood vessels
and can even detect molecules in random motion
bumping against the ear drum. It is possible to detect
one action potential generated by a certain type of
mechanoreceptor. Although these receptors are capable
of giving rise to sensations, much of their information
is canceled out by receptor or central mechanisms to be
discussed later. In other afferent pathways, information
is not canceled out—it simply does not feed into parts
of the brain that give rise to a conscious sensation.
To use an example cited earlier, stretch receptors in
the walls of some of the largest blood vessels monitor
blood pressure as part of refl ex regulation of this
pressure, but people have no conscious awareness of
their blood pressure.
4.
We lack suitable receptors for many energy forms. For
example, we cannot directly detect ionizing radiation
and radio or television waves.
5.
Damaged neural networks may give faulty perceptions
as in the bizarre phenomenon known as
phantom
limb,
in which a limb lost by accident or amputation is
experienced as though it were still in place. The missing
limb is perceived to be the site of tingling, touch,
pressure, warmth, itch, wetness, pain, and even fatigue.
It seems that the sensory neural networks in the
central nervous system that are normally triggered by
receptor activation are, instead, activated independent
of peripheral input. The activated neural networks
continue to generate the usual sensations, which the
brain perceives as arising from the missing receptors.
6.
Some drugs alter perceptions. In fact, the most
dramatic examples of a clear difference between the real
world and our perceptual world can be found in drug-
induced hallucinations.
In summary, for perception to occur, there can be no
separation of the three processes involved—transducing stim-
ulus energy into action potentials by the receptor, transmit-
ting data through the nervous system, and interpreting data.
Sensory information is processed at each synapse along the
afferent pathways and at many levels of the central nervous
system, with the more complex stages receiving input only
after the more elementary systems have processed the infor-
mation. This hierarchical processing of afferent information
along individual pathways is an important organizational prin-
ciple of sensory systems. As we will see, a second important
principle is that information is processed by
parallel
pathways,
each of which handles a limited aspect of the neural signals
generated by the sensory transducers. A third principle is that
information at each stage along the pathway is modifi ed by
“top-down” infl uences serving the emotions, attention, mem-
ory, and language. Every synapse along the afferent pathway
adds an element of organization and contributes to the sen-
sory experience so that what we perceive is not a simple—or
even an absolutely accurate—image of the stimulus that origi-
nally activated our receptors.
We conclude our general introduction to sensory system
pathways and coding with a summary of the general principles
of the organization of the sensory systems (
Table 7–1
).
Table 7–1
Principles of Sensory System
Organization
1. Specifi c sensory receptor types are sensitive to certain
modalities and submodalities.
2. A specifi c sensory pathway codes for a particular modality or
submodality.
3. The specifi c ascending pathways are crossed so that sensory
information is generally processed by the side of the brain
opposite the stimulated side of the body.
4. In addition to other synaptic relay points, most specifi c
ascending pathways synapse in the thalamus on their way to
the cortex.
5. Information is organized such that initial cortical processing
of the various modalities occurs in different parts of the
brain.
6. Specifi c ascending pathways are subject to descending
controls.
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