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Sensation vs Perception The text describes
sensation as 'activation of the sensory receptors' but, as I said in class, it
is far more than simply the sensory neurons' activation.
1. Sensation is how data about the body and
outside world are translated by your sensory systems into data that
is received by the brain.
Sensory systems are an extension of the nervous system. Sensory receptor cells are
neurons which are highly excitable in response not to neurotransmitters, but
to specific kinds of stimulation from the environment.
Sensations include all the forms of information available about the environment
outside and feedback from the body itself is translated ('transduced')
into the code of neural impulses by highly reactive specialized neurons (sensory
receptors) which are excited into firing off action potentials, starting nerve impulses
that are carried to the brain by special nerve pathways.
2. Perception is what your brain does with the sensory data
in order to make sense
of what is going on outside the brain. The brain organizes
sensory input into meaningful patterns that are learned over time from repeated exposure
to and interaction with that outside environment. Specific networks of neurons
in the brain are organized to deal with their particular type of inputs,
transforming them into sensations specific to that system..
Perception occurs in the brain.
A person who is hallucinating is seeing things that are not present in the
environment because the visual system in his brain is being activated. Pressing
firmly on your (closed) eyes causes you to see spots or areas of color in
motion, not because they are present in the environment, but because you are
stimulating, through the optic nerve, the visual part of your brain.
Sensory systems in general:
A. Each sensory system consists of several steps in transforming environmental information into our perceptions of the world:
- The sensory organs which are designed to receive specific kinds of
environmental data
- The specialized sensory neurons within the sensory organs which transduce (change or convert) the
particular kinds of original information (light, sound, etc) into coded nerve
impulses (sensory coding),
- The nerves that carry that information to a specific area of the brain that
does the primary organization of incoming data, and then
- The secondary areas and networks in the brain that assemble and
transform that
information into perceptions, patterns that make sense to us.
Localization of function;
The brain-based functions of reception and integration of coded sensory
information are located in specific areas of the cortex dedicated to that
particular sense. This is called the localization of function; if
electrically stimulated, each specific area will produce only the sensations it
specializes in.. The receptor cells in the brain for that system can only respond as if
the information is 'correct' for that system (as if it is light in the visual system, or
sound in the auditory system, etc.); if an auditory nerve were to be stimulated by an
electrode, it would produce the sensation of sound, even in a silent room.
Once the sensation passes through the initial portion of the cortex dedicated to
that sense, the 'messages' are then sent on to other parts of the cortex for further
processing, integration with other related sensations, and storage in
the memory circuits of the brain.
(In synaesthesia, the subjective experience of a sensation other than the
one being directly stimulated by the environment outside the brain: ie, touch translated
into the sensation of color or specific words having a 'taste'. This is not yet well
understood, but presumably, one sensory system can then stimulate another within the
brain itself, sort of like having crossed wires....)
B. The mechanics of transmission of the
senses is basic/common to all sensory systems, in that, whatever the original
stimulus (a chemical or physical stimulation), each sensory receptor's neurons convert
(transduces) that 'information' into the basic neural impulse (Neurons
either fire or don't fire, and, as with all neurons,
information is relayed as electrically processes within neurons and then
chemically transmitted between neurons).
However, the type of neurotransmitter varies and may be inhibitory or excitatory, and the
rate of firing or pattern of nerve impulses can convey complex information (just as the
Morse code, though simple, can spell out any thought you can put into words).
C. Sensory neurons and networks of neurons have specialized
functions. They are feature detectors, data reduction mechanisms, change
detectors, and their sensitivity is affected by a number of other neuronal
processes.
- feature detectors Sensory systems are feature
detectors; they attuned to specific aspects or features or all the data
available to each system. For instance, humans have taste receptors for only five
flavors (includinghe newly accepted 'umami', or brothy, tastes) although it may be that
other animals have a wider range of taste receptors. (The complexity of most
'flavors' is due to the sense of smell which is activated by the food as well asthe sense
of taste.)
- data reduction mechanisms: Sensory systems not only gather
and transmit environmantal data, they also serve as data reduction
mechanisms. Sensory systems can only take in a limited range or spectrum of all
available environmental information. This range varies between species and also for
individuals over time, depending on age, illness, disability, mood, alertness, etc. The
bottom limits of what the brain can sense in any one sensory system is called the 'absolute'
threshold. Also, each system varies in how much of an increase or decrease in
actual stimulation is required to make the brain aware of a difference (the difference
threshold or Just Noticeable Difference or JND).
Weber's Law states that the JND is a constant proportion of the original level of
stimulation, and that that proportion varies with the particular sensory system. For
instance, we are very acutely aware of differences in pitch, noticing a 1/3 of 1% change,
but quite insensitive in taste, needing a 20% change in the sweetness or sourness of
something before we can detect a difference.
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Psychophysics is the science of measuring the senses in terms of
their ranges (thresholds) and limitations under varying conditions.
Subliminal sensations: ("Sub"= below, "limen"= threshold)
Sensations which are not below the threshold needed for neural transmission, but
which are not consciously registered in the brain. |
- Change detectors: Sensory systems are most sensitive to changes
in levels of environmental stimulation. With exposure to constant levels, even if very
powerful, the activated cells fire more slowly over time; in order to experience the
sensation more strongly, it needs to be increased, and decreases will be more noticeable
as well. This is called sensory adaptation, and it explains
our tendency to keep turning our radios, CD players, TV, up louder and louder. (Did you
ever turn on your car in the morning only to be loudly blasted by your radio? It
didn't seem that loud when you were driving home last night!)
- The strength of a sensory input can also be altered by selective
attention: Various brain structures can block, divert, or facilitate the
sending of sensory messages to the cortex, depending on the situation. Some messages have
to take priority in supporting survival: if you are choking, your brain's priority is to
get air into your lungs, not to attend to the horrified expression on your dinner
partner's face! Also, even in less demanding situations, you can only pay attention to a
limited number of things, but attention increases your sensitivity to a stimulus due to the
meaning you associate with that input. (For instance, when in a conversation with one
person at a party, you hear someone behind you mention your name in a different
conversation, and 'tune in' to that conversation instead of the original one!) And stimuli
that indicate a threat or a problem to be dealt with will be more strongly
perceived (as
when an infant's fussing alerts the same sleeping parent who stayed in a deep sleep
when a much louder car alarm went off on the street).
- Last, sensory gating may take place in the spinal cord in
directing pain messages to be sent to the brain. Emergencies, indicated by sharp shooting
pains, take precedence over dull aches, which serve as reminders of past injuries or
indicate chronic conditions.
- Sensory input to the brain can also be reduced by perceptual
defenses (as when the sensory inputs produce an uncomfortable emotion).
For example, subconscious perceptual
defenses can keep us unaware of psychologically disturbing or embarrassing
stimuli. (If you really want the student sitting next to you to hear a 'dirty' word, you
may have to say it louder!) (Better not!)
- Selective attention can also increase sensitivity to signals from
the environment that are alarming or demand the person's attention.
- Other factors
that increase sensory input are repetition (when it has slight variations, such as when a
faucet drips), intensity, (if the stimulus is very strong, loud, bright, etc.) or
saturation (clear or unmixed with other sensations), or contrast (as in 'visual pop-out' phenomena).
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The visual system. We went over the structure of the eye,
noting how the amount of light, the stimulus for the visual system has to be
controlled by the iris, a circular set of muscles that
expand or contract the pupil, of opening into the eye's interior.
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(The 'red eyes' that are sometimes seen in flash photography
are the result of the pupil being dilated (wide open) in dark conditions
and not having time to shut down when the flash goes off. The red color is
the light of the flash hitting the blood vessels on the back surface of
the eye. The 'red-eye reduction' feature of modern cameras prevents this
by having a brief flash go off before the picture is taken so that the
pupil can close down.) |
The light passes first through the clear, curved, sensitive outer surface
called the cornea. The cornea redirects the angle of the light
rays and begins the process of focusing the image in the eye. The cornea is
filled with a clear viscous fluid that holds the shape of its curve. The cornea
also functions as a very sensitive protective covering for the delicate
structures of the eye, which causes you to blink and your eyes to water when
touched by anything foreign.
Next, the light passes through the iris and then into the lens, a
clear jelly-like flexible structure shaped somewhat like a 'One-a-Day' type
vitamin pill. When pulled at the edges by the contracting ciliary
muscles, this lens further bends or redirects the light rays to fine-tune the
focus on the back of the eye.
The interior of the eye is filled with a clear jelly-like fluid that allows
the light to shine onto the retina, the lining of the orb of
the eye. The retina has a layer of light sensitive cells which
convert, or transduce, the light into neural impulses that get
forwarded to the brain. The retina wraps around most of the interior
surface of the eye.
The retinal has two kinds of light-reactive cells: the rods,
which are sensitive to motion, light and dark, and peripheral vision, and the cones,
which are sensitive to fine detail and to color but need bright light in which
to operate. The cones are concentrated in a small spot called
the fovea, so the area of sharp focus is very small. (If you
stare at one word in the middle of the page in your book, the words on either
side quickly become blurry as the field of vision widens. Thus, we have to move
our eyes along a line of print to be able to read it.)
The rods and cones are actually behind a system of connecting cells that
collect the nerve impulses from the light-sensitive cells and pass them out of
the eye to the brain by way of the optic nerve. Both the blood
vessels and the nerve fibers all leave the eye at the same spot which is the
only place the eye is anchored to the skull, enabling it to pivot freely.
However, because do many nerve fibers and blood vessels have to go through all in
one space, there is no room for rods or cones, and this results in a 'blind
spot'.
We tested the ability to see this 'blind spot'. Not everyone can, as your eye
is also constantly in motion*, shifting the area 'hidden' by this spot to other
places in the field of vision. Also, because the blind spot doesn't fall on the
same part of the field of vision for both eyes, what one eye can't see, the
other can, 'filling in the blank' for that area.
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* These tiny rapid movements of the eye prevent sensory
adaptation, so that the rods and cones are constantly being stimulated by
new information and do not slow down in their rate of firing. Thus, vision
stays clear rather than fading |
Theories of color vision: There are two main theories, both of which seem
to explain aspects of how we perceive color.
- The trichromatic theory is based on the fact
that cones appear to be differentially sensitive to different
light frequencies. Some respond more to red frequencies, some to
blue and some to green. (It's not that a red-sensitive
cone does not fire in response to blue or green, but that it
fires at a different rate. The brain then translates the various
rates of firing into the 'colors' that we 'see'.)
- The opponent-process theory of color
perception states that receptor cells in both the inner layer of
retinal cells and in the brain are arranged in pairs that can
sense either red or green, blue or yellow (and
black or white). According to this process, continual
viewing of one color of a pair weakens the ability to inhibit
the opposite sensation; thus, when you stare at red and then
look at a white surface, you 'see' the opposite color of the
pair, green.
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| Total color blindness, a sex-linked recessive trait,
is very rare. It is more common in men because it is inherited when
a male gets the defective X chromosome from his mother (and has no
healthy X chromosome to counteract it) or when a female inherits two
defective X chromosomes, one from her mother and one from her
father. More commonly, a person may experience partial color
blindness, in which one of the three types of cones is missing
or the lens is becoming covered with a yellow clouding which screens
out shorter wave lengths, and so the mixed messages from the cones
to the brain, representing the full range of colors, is distorted.
Red/green color weakness one of the more common, causing problems
for people in telling medications apart or seeing traffic lights
turn red. |
One more fact about color and vision: although the
rods cannot 'see' color, they are more sensitive to light in the
frequency of blue-green than any others. Thus the lights on an
airport runway at night are blue/green in color.
The cones, however, are more sensitive to yellow-green frequencies,
seeing them as brighter than the other frequencies. |
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The basics of hearing:
Hearing is based on waves of alternating compressions and rarefactions of
molecules in the air: (For a visual demonstration, go to sound
waves.) Humans hear between 20 to 20,000 vibrations (or hertz)
per minutes. (Other animals can hear below and above these thresholds.)
Sound waves transduced by eardrum (tympanic membrane) into other kinds of
vibrations by the membranes( tympanic and the one covering the oval
window on the cochlea) and the three tiny bones (or ossicles)
and then back into waves in the fluid of the cochlea. The actual
transduction into neural impulses is done by the hair cells of the
cochlea.
Hair cells by themselves cannot discern different tones; they all react
alike in response to bending when they touch the membranes that are
above them. Frequency (or tone) is transmitted by several different means:
1) up to a certain level, the hair cells vibrate at the same rate as the
sound waves. Sound waves of 100 hertz make the hair cells fire 100 times a
second. This is the frequency theory. 2) The book says that then at
4,000 hertz, the location of the hair cells that are firing
indicates to the brain the higher pitches /tones. Hair cells that are
stimulated nearer the oval window are higher in sound, while those more
distant from the oval window are lower frequency tones. This is the place
theory of hearing. The problem with this is that neurons cannot fire
faster than 100 times a second. The book does not explain this, but the
way we hear sounds between 100 and 1,000 Hz is based on the volley
principle, in which the hair cells do not all fire art once, but in
volleys or waves at 100 Hz intervals; thus 3 volleys of cells firing tells
the brain that you have heard 300 Hz.
| What sensory systems have in common: |
Optical system |
Auditory system |
| Type/nature of environmental information |
Visible spectrum of all the waves of electromagnetic
radiation . Frequency of waves creates colors, brightness by
amplitude. |
Waves of compression and rarefaction (less dense)
molecules: sound vibrations Frequency of waves creates tone (pitch),
and loudness by amplitude |
| Structures/mechanisms that gather/carry/prepare info
for correct reception by sensory neurons |
Outer surface, cornea, protects and gather
light rays to pass into interior of eye. Iris muscles control
size of pupil (opening into interior of eye) to control how much
light enters (also protects the retina from damage by too much
light). Lens further focuses light rays onto retina, at the back of
eye, where rods and cones are located. |
Outer structure of ear (pinna) gathers sound waves
into auditory canal which carries them to tympanic membrane. Length
of auditory canal protects eard drum from injury, (but, unlike ear,
no structure prevents damage to transducers due to too much sound).
Vibrations of air carried by eardrum and ossicles to oval window of
cochlea where they are converted into waves of fluid. |
| Detecting/transducing/coding mechanisms |
Rods and cones respond differently to specific
features of incoming information (light's bright ness and hue/color,
motion, details) and transduce or translate the information into
coded neural impulses. |
Hair cells in the cochlea bent by waving membrane and
transduce information into neural impulses. Unlike rods/cones, only
one kind of hair cell; frequency and loudness indicated by where
(and how many) hair cells are firing, as well as how frequently they
fire. |
| Transmission structures |
Optical information carried by optic nerve from retina
to brain. Information from nasal side of visual field of each
eye goes to the opposite hemisphere while stimuli from outer side of
each retina goes to same side hemisphere |
Auditory information carried by auditory nerve from
retina to brain. Information from each ear passes to opposite
hemisphere of brain. |
| Reception in 'CPU'*, the brain Specific area, due to
localization of function. |
Occipital lobes receive incoming optical information
and further encode features such as lines, angles, patterns of light
and dark. Distribution of this information then connected to other
parts of brain that influence what the brain then makes of this
information based on innate and learned perceptions. |
Temporal lobes receive and process auditory
information and pass it on to other parts of brain: 'hearing"
takes place in the brain, which makes sense, based on past
experience, of the patterns of neural impulses. |
* In some ways, the system is like a computer, with the brain serving
as the 'Central Processing Unit' (CPU) which takes the information coded
by the keyboard and 'processes' it. Inputs from one
sensory system are correlated with simultaneous stimuli to build up,
through repeated paired exposures, a 'gestalt' that consists of all the
senses involved in 'knowing' what this particular combination of stimuli
indicates. Example: After repeated exposure to simultaneous sensory inputs
every time you experience an apple as the source of sensory stimuli, your
brain combines/processes the sight, smell, taste and feel of an apple into
the full 'gestalt' of an apple
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Motion sickness:
The theory of sensory conflict explains the dizziness and nausea we
sometimes feel when confronted by missmatched sensory inputs. Did you ever
attend an Imax Film and get a bit dizzy when surrounded by the
motion on the huge screen? That's because your
sensory systems, other than vision, were telling you that you were sitting
still, which conflicted with the visual information you were processing.
In a car, the interior of the car and your body are moving at the same
time, making it seem, visually, that you are holding still, but your
vestibular systems are telling you that you are in motion.
Result, car sickness! That's why they say you should sit in the
front seat: more of your visual inputs will be about a world in motion,
confirming what your other sensory systems are telling your brain.
In space, without gravity to hold the fluids in a stable position in
the semicircular canals, your vestibular system gets an overload of mixed
messages, yet the interior of the space capsule appears, visually, to be
stable. About 80% of all potential astronauts get space sickness as a
result. (Some get used to it, but not all.) Virtual reality can also
cause illness, a sort of 'lack of motion sickness': what you see is
moving, but your body and head sense that you are holding still. |
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