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=                               Sense                                =
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                             Introduction
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A sense is a physiological capacity of organisms that provides data
for perception. The senses and their operation, classification, and
theory are overlapping topics studied by a variety of fields, most
notably neuroscience, cognitive psychology (or cognitive science), and
philosophy of perception. The nervous system has a specific sensory
nervous system, and a sense organ, or sensor, dedicated to each sense.

Humans have a multitude of sensors. Sight (vision, visual sense),
hearing (audition, auditory sense), taste (gustation, gustatory
sense), smell (olfaction, olfactory sense), and touch
(somatosensation, somatosensory sense) are the five traditionally
recognized senses. The ability to detect other stimuli beyond those
governed by these most broadly recognized senses also exists, and
these sensory modalities include temperature (thermoception),
kinesthetic sense (proprioception), pain (nociception), balance
(equilibrioception), vibration (mechanoreception), and various
internal stimuli (e.g. the different chemoreceptors for detecting salt
and carbon dioxide concentrations in the blood, or sense of hunger and
sense of thirst). However, what constitutes a sense is a matter of
some debate, leading to difficulties in defining what exactly a
distinct sense is, and where the borders lie between responses to
related stimuli.

Other animals also have receptors to sense the world around them, with
degrees of capability varying greatly between species. Humans have a
comparatively weak sense of smell and a stronger sense of sight
relative to many other mammals while some animals may lack one or more
of the traditional five senses. Some animals may also intake and
interpret sensory stimuli in very different ways. Some species of
animals are able to sense the world in a way that humans cannot, with
some species able to sense electrical and magnetic fields, and detect
water pressure and currents.


                              Definition
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A broadly acceptable definition of a sense would be "A system that
consists of a group of sensory cell types that responds to a specific
physical phenomenon, and that corresponds to a particular group of
regions within the brain where the signals are received and
interpreted." There is no firm agreement as to the number of senses
because of differing definitions of what constitutes a sense.

The senses are frequently divided into exteroceptive and
interoceptive:
* 'Exteroceptive senses' are senses that perceive the body's own
position, motion, and state, known as 'proprioceptive senses'.
External senses include the traditional five: sight, hearing, touch,
smell and taste, as well as thermoception (temperature differences)
and possibly an additional weak magnetoception (direction).
Proprioceptive senses include nociception (pain); equilibrioception
(balance); proprioception (a sense of the position and movement of the
parts of one's own body).
* 'Interoceptive senses' are senses that perceive sensations in
internal organs.

Non-human animals may possess senses that are absent in humans, such
as electroreception and detection of polarized light.

In Buddhist philosophy, Ayatana or "sense-base" includes the mind as a
sense organ, in addition to the traditional five. This addition to the
commonly acknowledged senses may arise from the psychological
orientation involved in Buddhist thought and practice. The mind
considered by itself is seen as the principal gateway to a different
spectrum of phenomena that differ from the physical sense data. This
way of viewing the human sense system indicates the importance of
internal sources of sensation and perception that complements our
experience of the external world.


 Sight
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Sight or vision (adjectival form: visual/optical) is the capability of
the eye(s) to focus and detect images of visible light on
photoreceptors in the retina of each eye that generates electrical
nerve impulses for varying colors, hues, and brightness.  There are
two types of photoreceptors: rods and cones. Rods are very sensitive
to light but do not distinguish colors.  Cones distinguish colors but
are less sensitive to dim light. There is some disagreement as to
whether this constitutes one, two or three senses. Neuroanatomists
generally regard it as two senses, given that different receptors are
responsible for the perception of color and brightness. Some argue
that stereopsis, the perception of depth using both eyes, also
constitutes a sense, but it is generally regarded as a cognitive (that
is, post-sensory) function of the visual cortex of the brain where
patterns and objects in images are recognized and interpreted based on
previously learned information. This is called visual memory.

The inability to see is called blindness. Blindness may result from
damage to the eyeball, especially to the retina, damage to the optic
nerve that connects each eye to the brain, and/or from stroke
(infarcts in the brain).  Temporary or permanent blindness can be
caused by poisons or medications.

People who are blind from degradation or damage to the visual cortex,
but still have functional eyes, are actually capable of some level of
vision and reaction to visual stimuli but not a conscious perception;
this is known as blindsight. People with blindsight are usually not
aware that they are reacting to visual sources, and instead just
unconsciously adapt their behavior to the stimulus.

On February 14, 2013 researchers developed a neural implant that gives
rats the ability to sense infrared light which for the first time
provides living creatures with new abilities, instead of simply
replacing or augmenting existing abilities.


 Hearing
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Hearing or audition (adjectival form: auditory) is the sense of sound
perception. Hearing is all about vibration. Mechanoreceptors turn
motion into electrical nerve pulses, which are located in the inner
ear. Since sound is vibration, propagating through a medium such as
air, the detection of these vibrations, that is the sense of the
hearing, is a mechanical sense because these vibrations are
mechanically conducted from the eardrum through a series of tiny bones
to hair-like fibers in the inner ear, which detect mechanical motion
of the fibers within a range of about 20 to 20,000 hertz, with
substantial variation between individuals. Hearing at high frequencies
declines with an increase in age. Inability to hear is called deafness
or hearing impairment. Sound can also be detected as vibrations
conducted through the body by tactition. Lower frequencies that can be
heard are detected this way. Some deaf people are able to determine
the direction and location of vibrations picked up through the feet.


 Taste
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Taste or gustation (adjectival form: gustatory) is one of the
traditional five senses. It refers to the capability to detect the
taste of substances such as food, certain minerals, and poisons, etc.
The sense of taste is often confused with the "sense" of flavor, which
is a combination of taste and smell perception. Philippe Mercier - The
Sense of Taste - Google Art Project Flavor depends on odor, texture,
and temperature as well as on taste. Humans receive tastes through
sensory organs called taste buds, or gustatory calyculi, concentrated
on the upper surface of the tongue. There are five basic tastes:
sweet, bitter, sour, salty and umami. Other tastes such as calcium and
free fatty acids may also be basic tastes but have yet to receive
widespread acceptance. The inability to taste is called ageusia.

Flies and butterflies have taste organs on their feet, allowing them
to taste anything they land on. Catfish have taste organs across their
entire bodies, and can taste anything they touch, including chemicals
in the water.


 Smell
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Smell or olfaction (adjectival form: olfactory) is the other
"chemical" sense. Unlike taste, there are hundreds of olfactory
receptors (388 according to one source), each binding to a particular
molecular feature. Odor molecules possess a variety of features and,
thus, excite specific receptors more or less strongly. This
combination of excitatory signals from different receptors makes up
what we perceive as the molecule's smell. The sense of smell Bequest
of [[Mrs E.G. Elgar, 1945 Museum of New Zealand Te Papa Tongarewa.]]
In the brain, olfaction is processed by the olfactory system.
Olfactory receptor neurons in the nose differ from most other neurons
in that they die and regenerate on a regular basis. The inability to
smell is called anosmia. Some neurons in the nose are specialized to
detect pheromones.

An example of smell in non-mammals is that of sharks, which combine
their keen sense of smell with timing to determine the direction of a
smell. They follow the nostril that first detected the smell. Insects
have olfactory receptors on their antennae. Although it is unknown to
the degree and magnitude which non-human animals can smell better than
humans.


 Touch
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Touch or somatosensation (adjectival form: somatic), also called
tactition  (adjectival form: tactile) or mechanoreception, is a
perception resulting from activation of neural receptors, generally in
the skin including hair follicles, but also in the tongue, throat, and
mucosa. A variety of pressure receptors respond to variations in
pressure (firm, brushing, sustained, etc.). The touch sense of itching
caused by insect bites or allergies involves special itch-specific
neurons in the skin and spinal cord. The loss or impairment of the
ability to feel anything touched is called tactile anesthesia.
Paresthesia is a sensation of tingling, pricking, or numbness of the
skin that may result from nerve damage and may be permanent or
temporary.


 Balance and acceleration
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Balance, equilibrioception, or vestibular sense is the sense that
allows an organism to sense body movement, direction, and
acceleration, and to attain and maintain postural equilibrium and
balance. The organ of equilibrioception is the vestibular labyrinthine
system found in both of the inner ears. In technical terms, this organ
is responsible for two senses of angular momentum acceleration and
linear acceleration (which also senses gravity), but they are known
together as equilibrioception.

The vestibular nerve conducts information from sensory receptors in
three ampulla that sense motion of fluid in three semicircular canals
caused by three-dimensional rotation of the head.  The vestibular
nerve also conducts information from the utricle and the saccule,
which contain hair-like sensory receptors that bend under the weight
of otoliths (which are small crystals of calcium carbonate) that
provide the inertia needed to detect head rotation, linear
acceleration, and the direction of gravitational force.


 Temperature
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Thermoception is the sense of heat and the absence of heat (cold) by
the skin and internal skin passages, or, rather, the heat flux (the
rate of heat flow) in these areas. There are specialized receptors for
cold (declining temperature) and for heat (increasing temperature).
The cold receptors play an important part in the animal's sense of
smell, telling wind direction.  The heat receptors are sensitive to
infrared radiation and can occur in specialized organs, for instance
in pit vipers. The thermoceptors in the skin are quite different from
the homeostatic thermoceptors in the brain (hypothalamus), which
provide feedback on internal body temperature.


 Proprioception
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Proprioception, the kinesthetic sense, provides the parietal cortex of
the brain with information on the movement and relative positions of
the parts of the body. Neurologists test this sense by telling
patients to close their eyes and touch their own nose with the tip of
a finger. Assuming proper proprioceptive function, at no time will the
person lose awareness of where the hand actually is, even though it is
not being detected by any of the other senses. Proprioception and
touch are related in subtle ways, and their impairment results in
surprising and deep deficits in perception and action.


 Pain
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Nociception (physiological pain) signals nerve-damage or damage to
tissue. The three types of pain receptors are cutaneous (skin),
somatic (joints and bones), and visceral (body organs). It was
previously believed that pain was simply the overloading of pressure
receptors, but research in the first half of the 20th century
indicated that pain is a distinct phenomenon that intertwines with all
of the other senses, including touch. Pain was once considered an
entirely subjective experience, but recent studies show that pain is
registered in the anterior cingulate gyrus of the brain. The main
function of pain is to attract our attention to dangers and motivate
us to avoid them. For example, humans avoid touching a sharp needle,
or hot object, or extending an arm beyond a safe limit because it is
dangerous, and thus hurts. Without pain, people could do many
dangerous things without being aware of the dangers.


 Magnetoception
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Magnetoception (or magnetoreception) is the ability to detect the
direction one is facing based on the Earth's magnetic field.
Directional awareness is most commonly observed in birds, which rely
on their magnetic sense to navigate during migration. It has also been
observed in insects such as bees. Cattle make use of magnetoception to
align themselves in a north-south direction. Magnetotactic bacteria
build miniature magnets inside themselves and use them to determine
their orientation relative to the Earth's magnetic field. There has
been some recent (tentative) research suggesting that the Rhodopsin in
the human eye, which responds particularly well to blue light, can
facilitate magnetoception in humans.


 Sexual stimulation
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Sexual stimulation is any stimulus (including bodily contact) that
leads to, enhances and maintains sexual arousal, and may lead to
orgasm. Distinct from the general sense of touch, sexual stimulation
is strongly tied to hormonal activity and chemical triggers in the
body. Although sexual arousal may arise without physical stimulation,
achieving orgasm usually requires physical sexual stimulation
(stimulation of the Krause-Finger corpuscles found in erogenous zones
of the body).


 Other internal senses{{anchor|Interoception}}
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An internal sense also known as interoception is "any sense that is
normally stimulated from within the body".  These involve numerous
sensory receptors in internal organs. Interoception is thought to be
atypical in clinical conditions such as alexithymia.
Some examples of specific receptors are:
* Hunger is a sensation that is governed by a set of brain structures
(e.g., the hypothalamus) that are responsible for energy homeostasis.
* Pulmonary stretch receptors are found in the lungs and control the
respiratory rate.
* Peripheral chemoreceptors in the brain monitor the carbon dioxide
and oxygen levels in the brain to give a feeling of suffocation if
carbon dioxide levels get too high.
* The chemoreceptor trigger zone is an area of the medulla in the
brain that receives inputs from blood-borne drugs or hormones, and
communicates with the vomiting center.
* Chemoreceptors in the circulatory system also measure salt levels
and prompt thirst if they get too high; they can also respond to high
blood sugar levels in diabetics.
* Cutaneous receptors in the skin not only respond to touch, pressure,
temperature and vibration, but also respond to vasodilation in the
skin such as blushing.
* Stretch receptors in the gastrointestinal tract sense gas distension
that may result in colic pain.
* Stimulation of sensory receptors in the esophagus result in
sensations felt in the throat when swallowing, vomiting, or during
acid reflux.
* Sensory receptors in pharynx mucosa, similar to touch receptors in
the skin, sense foreign objects such as mucous and food that may
result in a gag reflex and corresponding gagging sensation.
* Stimulation of sensory receptors in the urinary bladder and rectum
may result in sensations of fullness.
* Stimulation of stretch sensors that sense dilation of various blood
vessels may result in pain, for example headache caused by
vasodilation of brain arteries.
* Cardioception refers to the perception of the activity of the heart.
* Opsins and direct DNA damage in melanocytes and keratinocytes can
sense ultraviolet radiation, which plays a role in pigmentation and
sunburn.
*Baroreceptors relay blood pressure information to the brain and
maintain proper homeostatic blood pressure.


 Time
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Chronoception refers to how the passage of time is perceived and
experienced. Although the sense of time is not associated with a
specific sensory system, the work of psychologists and neuroscientists
indicates that human brains do have a system governing the perception
of time, composed of a highly distributed system involving the
cerebral cortex, cerebellum and basal ganglia. One particular
component, the suprachiasmatic nucleus, is responsible for the
circadian (or daily) rhythm, while other cell clusters appear to be
capable of shorter-range (ultradian) timekeeping.

One or more dopaminergic pathways in the central nervous system appear
to have a strong modulatory influence on mental chronometry,
particularly interval timing.


 Agency
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The sense of agency refers to the subjective feeling of having chosen
a particular action. Some conditions, such as schizophrenia, can lead
to a loss of this sense, causing a person to feel like a machine or
even leading to delusions of being controlled from some outside
source. The opposite extreme occurs too, with some people experiencing
everything in their environment as if they had decided that it would
happen.

Even in non-pathological cases, there is a measurable difference
between making a decision and the feeling of agency. Through methods
such as the Libet experiment, a gap of half a second or more can be
detected from the time when there are detectable neurological signs of
a decision having been made to the time when the subject actually
becomes conscious of the decision.

There are also experiments in which an illusion of agency is induced
in psychologically normal subjects. In Wegner and Wheatley 1999,
subjects were given instructions to move a mouse around a scene and
point to an image about once every thirty seconds. However, a second