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Senses are the physiological methods of perception. The
senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, but most notably
neuroscience, cognitive psychology (or cognitive
science), and philosophy of perception.
Definition of "sense"
There is no firm agreement amongst neurologists as to exactly how many senses there are. The disagreements stem from a lack of
consensus as to what the definition of a sense should be. Although schoolchildren are still routinely taught that there are five
senses (sight, hearing, touch, smell, taste; a classification first devised by Aristotle), it is generally agreed that there are at least nine different senses in humans, and a minimum of two
more observed in other organisms.
A broadly acceptable definition of a sense would be "a system that consists of a sensory cell type (or group of cell types)
that respond to a specific kind of physical energy, and that correspond to a defined region (or group of regions) within the
brain where the signals are received and interpreted". Where disputes arise is with regard to the exact classification of the
various cell types and their mapping to regions of the brain.
List of senses
Using this definition several senses can be identified. This list begins with those five senses defined by Aristotle and hence
probably most familiar to the reader.
Seeing or vision describes
the ability to detect light by the eye and the
brain to interpret the image as "sight". There is disagreement as to whether or not this
constitutes one, two or even three distinct senses. Neuroanatomists generally regard it as two senses, given that different
receptors are responsible for the perception of colour (the frequency of light) and brightness (the energy of light). Some argue
that the perception of depth also constitutes a sense, but it is generally regarded that this is really a cognitive (that is,
post-sensory) function derived from having stereoscopic vision (two eyes) and is not a sensory perception as such.
Hearing or audition is the
sense of sound perception and results from tiny hair fibres in the inner ear detecting the motion of atmospheric particles within (at best) a range of 20 to 20000 Hz. Sound
can also be detected as vibration by tactition. Lower and higher frequencies than can be heard are detected this way only.
Taste or gustation is one of the two "chemical"
senses. It is well-known that there are at least four types of taste "bud" (receptor) on the tongue and hence, as should now be expected, there are anatomists who argue that these in fact constitute four or
more different senses, given that each receptor conveys information to a slightly different region of the brain.
The four well-known receptors detect sweet, salt, sour, and bitter, although the receptors for sweet and bitter have not been
conclusively identified. A fifth receptor, for a sensation called "umami", was first
theorised in 1908 and its existence confirmed in 2000 (see [1] ). The umami receptor detects the amino acid glutamate, a flavor commonly found in meat, and in artificial flavourings
such as monosodium glutamate.
Smell or olfaction is the other "chemical"
sense. Unlike taste, there are hundreds of olfactory receptors, each binding to a particular molecular feature, according to
current theory. The combination of features of the odor molecule makes up what we perceive as the molecule's smell. 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.
Note that in aquatic organisms there is essentially no difference between smell and taste.
The remaining senses can be considered types of touch or physical feeling.
Tactition is the sense of pressure
perception, generally in the skin.
Thermoception is the sense of heat
and the absence of heat (cold), also by the skin and including internal
skin passages. It is also the first of the group of senses not identified explicitly by Aristotle. Again there is some
disagreement about how many senses this actually represents--the thermoceptors in the skin are quite different from the homeostatic thermoceptors which provide feedback on internal body temperature. How
warm or cold something feels does not only depend on temperature, but also on
specific heat capacity and heat conductance; e.g., warm metal feels warmer than
warm wood, and cold metal feels colder than cold wood, because metal has a higher thermal
conductivity than wood. Wind feels cold because of the heat withdrawn for evaporation of sweat or other moisture, and
because an isolating layer of warm air around the body blows away; however, in the case of hot air, wind makes it feel hotter,
for a similar reason as the latter.
Nociception is the perception of pain.
It can be classified as from one to three senses, depending on the classification method. The three types of pain receptors are
cutaneous (skin), somatic (joints and bones) and visceral (body organs).
Equilibrioception is the perception of
balance and is related to cavities containing fluid in the inner ear. There is some disagreement as to whether
or not this also includes the sense of "direction" or orientation. However, as with depth perception earlier, it is generally
regarded that "direction" is a post-sensory cognitive awareness.
Proprioception is the perception of body
awareness and is a sense that people rely on enormously, yet are frequently not aware of. More easily demonstrated than
explained, proprioception is the "unconscious" awareness of where the various regions of the body are located at any one time.
(This can be demonstrated by anyone closing their eyes and waving their hand around. 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).
Based on this outline and depending on the chosen method of classification, somewhere between 9 and 21 human senses have been
identified. Additionally, there are some other candidate physiological experiences which may or may not fall within the above
classification (for example the sensory awareness of hunger and thirst).
Non-human senses
Electroception (or "electroreception"), the most
significant of the non-human senses, is the ability to detect electric
fields. Several species of fish, sharks and rays have evolved the capacity to sense
changes in electric fields in their immediate vicinity. Some fish passively sense changing nearby electric fields, some generate
their own weak, electric fields and sense the pattern of field potentials over their body surface, and some use these generating
and sensing capacities for social communication. The mechanisms by which electroceptive fishes construct a spatial representation
from very small differences in field potentials involve comparisons of spike latencies from different parts of the fish's
body.
The only mammal which is known to demonstrate electroception is the platypus
(see [2] ).
Humans (and probably other mammals) can detect electric fields indirectly by detecting the effect they have on hairs. An
electrically charged balloon, for instance, will exert a force on human arm hairs, which can be felt through tactition and
identified as coming from a static charge (and not from wind or the like).
Magnetoception (or
"magnetoreception") is the ability to detect fluctuations in magnetic fields and is most commonly observed in birds. Although there
is no dispute that this sense exists in many avians (it is essential to the navigational abilities of migratory birds) it is not
a well understood phenomenon (see [3] ).
Magnetotactic
bacteria build miniature magnets inside themselves and use them to determine their orientation relative to the Earth's
magnetic field.
Echolocation is the ability to determine
orientation to other objects through interpretation of reflected sound (like sonar).
Bats and dolphins are noted for this ability,
though some other mammals and birds do as well. It is most often used to navigate through poor lighting conditions or to identify
and track prey. There is presently an uncertainty as to whether this is simply an extremely developed post-sensory interpretation
of auditory perceptions, or actually constitutes a separate sense. Resolution of the issue will require brain scans of animals
while they actually perform echolocation, a task which has proved difficult in practice.
Pit vipers and some boas have organs that
allow them to detect infrared light, such that these snakes are able to sense the
body heat of their prey. This sense is in addition to and distinct from normal vision.
See also
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