Download Sensory organs and perception

Sensation and Perception.
Importance of sensory systems
of the organism and their role in
behavior
Sensitivity training
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The techniques employed by Kurt Lewin at the
National Training Laboratories in Maine and his
colleagues, collectively known as sensitivity training,
were widely adopted for use in a variety of settings.
Initially, they were used to train individuals in business,
industry, the military, the ministry, education, and other
professions. In the 1960s and 1970s, sensitivity training
was adopted by the human potential movement,
which introduced the “encounter group.”
Although encounter groups apply the basic T-group
techniques, they emphasize personal growth, stressing
such factors as self-expression and intense emotional
experience.
Vision is the process of transforming light
energy into neural impulses
that can then be interpreted by the brain.
The eye composition
The rods an cones
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The retina, lining the back of the eye, consists of ten layers of
cells containing photoreceptors (rods and cones) that convert
the light waves to neural impulses through a photochemical
reaction. Aside from the differences in shape suggested by their
names, rod and cone cells contain different light-processing
chemicals (photopigments), perform different functions, and are
distributed differently within the retina.
Cone cells, which provide color vision and enable us to
distinguish details, adapt quickly to light and are most useful in
adequate lighting. Rod cells, which can pick up very small
amounts of light but are not color-sensitive, are best suited for
situations in which lighting is minimal. Because the rod cells are
active at night or in dim lighting, it is difficult to distinguish
colors under these circumstances. Cones are concentrated in the
fovea, an area at the center of the retina, whereas rods are
found only outside this area and become more numerous the
farther they are from it. Thus, it is more difficult to distinguish
colors when viewing objects at the periphery of one’s visual
field.
Processing the visual information
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Branches of the optic nerve cross at a junction in the
brain in front of the pituitary gland and underneath
the frontal lobes called the optic chiasm and ascend
into the brain itself. The nerve fibers extend to a part
of the thalamus called the lateral geniculate nucleus
(LGN), and neurons from the LGN relay their visual
input to the primary visual cortex of both the left and
right hemispheres of the brain, where the impulses
are transformed into simple visual sensations.
Objects in the left visual field are viewed only
through the right brain hemisphere, and vice versa.
The primary visual cortex then sends the impulses to
neighboring association areas which add meaning or
“associations”to them.
The field of vision
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A human has a field of vision that covers almost 180°,
although binocular vision is limited to the approximately
120° common to both eyes. The field extends upward
about 60° and down about 75°
Because the nerve fibers from the left half of the retina
of the left eye go to the left side of the brain and fibers
from the left half of the right eye cross the optic chiasm
and go to the left side of the brain as well, all the
information from the two left half-retinas ends up in the
left half of the brain.
And because the lens of the eye reverses the image it
sees, it is information from the right half of the visual
field that is going to the left visual cortex. Likewise,
information from the left half of the visual field goes to
the right visual cortex.
Binocular vision
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Binocular depth cues are based on the simple
fact that a person’s eyes are located in
different places.
One cue, binocular disparity, refers to the fact
that different optical images are produced on
the retinas of both eyes when viewing an
object.
By processing information about the degree
of disparity between the images it receives,
the brain produces the impression of a single
object that has depth in addition to height
and width.
Examining of eyes
Hearing
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Hearing is the ability to perceive sound. The ear, the
receptive organ for hearing, has three major parts:
the outer, middle, and inner ear. The pinna or outer
ear—the part of the ear attached to the head, funnels
sound waves through the outer ear.
The sound waves pass down the auditory canal to
the middle ear, where they strike the tympanic
membrane, or eardrum, causing it to vibrate. These
vibrations are picked up by three small bones
(ossicles) in the middle ear named for their shapes:
the malleus (hammer), incus (anvil), and stapes
(stirrup).
The stirrup is attached to a thin membrane called the
oval window, which is much smaller than the
eardrum and consequently receives more pressure.
Hearing
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As the oval window vibrates from the
increased pressure, the fluid in the coiled,
tubular cochlea (inner ear) begins to
vibrate the membrane of the cochlea
(basilar membrane) which, in turn, bends
fine, hairlike cells on its surface.
These auditory receptors generate
miniature electrical forces which trigger
nerve impulses that then travel via the
auditory nerve, first to the thalamus and
then to the primary auditory cortex in the
temporal lobe of the brain.
Sound perception thresholds
Ear inspection
Touch sensation
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Touch is the skin sense that allows us to
perceive pressure and related sensations,
including temperature and pain.
The sense of touch is located in the skin,
which is composed of three layers: the
epidermis, dermis, and hypodermis.
Different types of sensory receptors, varying
in size, shape, number, and distribution within
the skin, are responsible for relaying
information about pressure, temperature, and
pain.
Sensory receptors encode various types of information about objects
with which the skin comes in contact. We can tell how heavy an object
is by both the firing rate of individual neurons and by the number of
neurons stimulated. (Both the firing rate and the number of neurons
are higher with a heavier object.) Changes in the firing rate of neurons
tell us whether an object is stationary
or vibrating, and the spatial organization of the neurons gives us
information about its location.
Pain
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Pain is physical suffering resulting from some sort of
injury or disease, experienced through the central
nervous system.
Pain is a complex phenomenon that scientists are still
struggling to understand. Its purpose is to alert the
body of damage or danger to its system, yet
scientists do not fully understand the level and
intensity of pain sometimes experienced by people.
Long-lasting, severe pain does not serve the same
purpose as acute pain, which triggers an immediate
physical response.
Pain that persists without diminishing over long
periods of time is known as chronic pain. It is
estimated that almost one-third of all Americans
suffer from some form of chronic pain.
Spreading of pain information
in the body
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Pain signals travel through the body along
billions of special nerve cells reserved
specifically for transmitting pain messages.
These cells are known as nociceptors.
The chemical neurotransmitters carrying the
message include prostaglandins, bradykinin—
the most painful substance known to
humans—and a chemical known as P, which
stands for pain. Prostaglandins are
manufactured from fatty acids in nearly every
tissue in the body. Analgesic pain relievers,
such as aspirin and ibuprofen, work by
inhibiting prostaglandin production.
Spreading of pain information
in the body
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As they travel, the pain messages are sorted
according to severity. Recent research has discovered
that the body has two distinct pathways for
transmitting pain messages.
The epicritic system is used to transmit messages of
sudden, intense pain, such as that caused by cuts or
burns. The neurons that transmit such messages are
called A fibers, and they are built to transmit
messages quickly.
The protopathic system is used to transmit less
severe messages of pain, such as the kind one might
experience from over-strenuous exercise. The C
fibers of the protopathic system do not send
messages as quickly as A fibers.
The smell and the taste
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Olfaction is one of the two chemical senses: smell
and taste. Both arise from interaction between
chemical and receptor cells.
In olfaction, the chemical is volatile, or airborne.
Breathed in through the nostrils or taken in via the
throat by chewing and swallowing, it passes through
either the nose or an opening in the palate at the
back of the mouth, and moves toward receptor cells
located in the lining of the nasal passage. As the
chemical moves past the receptor cells, part of it is
absorbed into the uppermost surface of the nasal
passages called the olfactory epithelium, located at
the top of the nasal cavity.
The smell and the taste
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When a person eats, chemical stimuli taken in
through chewing and swallowing pass
through an opening in the palate at the back
of the mouth and move toward receptor cells
located at the top of the nasal cavity, where
they are converted to olfactory nerve
impulses that travel to the brain, just as the
impulses from olfactory stimuli taken in
through the nose.
The olfactory and gustatory pathways are
known to converge in various parts of the
brain, although it is not known exactly how
the two systems work together.
Sensory deprivation
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Sensory deprivation experiments of the 1950s have
shown that human beings need environmental timulation
to function normally. In a classic early experiment,
college students lay on a cot in a small, empty cubicle
nearly 24 hours a day, leaving only to eat and use the
bathroom.
They wore translucent goggles that let in light but
prevented them from seeing any shapes or patterns, and
they were fitted with cotton gloves and cardboard cuffs
to restrict the sense of touch.
The continuous hum of an air conditioner and U-shaped
pillows placed around their heads blocked out auditory
stimulation.
Sensory deprivation
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Initially, the subjects slept, but eventually they became
bored, restless, and moody. They became disoriented
and had difficulty concentrating, and their performance
on problem-solving tests progressively deteriorated the
longer they were isolated in the cubicle.
Some experienced auditory or visual hallucinations.
Although they were paid a generous sum for each day
they participated in the experiment, most subjects
refused to continue past the second or third day.
After they left the isolation chamber, the perceptions of
many were temporarily distorted, and their brain-wave
patterns, which had slowed down during the experiment,
took several hours to return to normal.
Sensory deprivation
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The deterioration in both physical and psychological
functioning that occurs with sensory deprivation has
been linked to the need of human beings for an optimal
level of arousal.
Too much or too little arousal can produce stress and
impair a person’s mental and physical abilities. Thus,
appropriate degrees of sensory deprivation may actually
have a therapeutic effect when arousal levels are too
high.
A form of sensory deprivation known as REST (restricted
environmental stimulation), which consists of floating for
several hours in a dark, soundproof tank of water heated
to body temperature, has been used to treat drug and
smoking addictions, lower back pain, and other
conditions associated with excessive stress.
Summary of senses inspection