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Sense

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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, most notably neuroscience, cognitive psychology (or cognitive science), and philosophy of perception. The nervous system has a sensory system dedicated to each sense.

Definition of sense

There is no firm agreement among neurologists as to the number of senses because of differing definitions of what constitutes a sense. One definition states that a sense is a faculty by which outside stimuli are perceived. The conventional five senses are sight, hearing, touch, smell, taste: a classification traditionally attributed to Aristotle. However, humans have at least nine different senses, and there are at least two other senses in other organisms.

A broadly acceptable definition of a sense would be "a system that consists of a group 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." Disputes about the number of senses arise typically regarding the classification of the various cell types and their mapping to regions of the brain.

Senses

Sight

Sight or vision is the ability of the brain and eye to detect electromagnetic waves within the visible range (light) interpreting the image as "sight." There is 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 colour (the frequency of photons of light) and brightness (amplitude/intensity - number of photons of light). Some argue that stereopsis, the perception of depth, also constitutes a sense, but it is generally regarded as a cognitive (that is, post-sensory) function of brain to interpret sensory input and to derive new information. The inability to see is called blindness.

Hearing

Hearing or audition is the sense of sound perception. In humans, it results from tiny hair fibres in the inner ear detecting the motion of a membrane which vibrates in response to changes in the pressure exerted by atmospheric particles within a range of 20 to 22000 Hz, with substantial variation between individuals. Sound can also be detected as vibrations conducted through the body by tactition. Lower and higher frequencies than that can be heard are detected this way only. The inability to hear is called deafness.

Taste

Taste or gustation is one of the two main "chemical" senses. It is well-known that there are at least four types of taste "bud" (receptor) on the tongue and hence there are anatomists who argue that these constitute four or more different senses, given that each receptor conveys information to a slightly different region of the brain. The inability to taste is called ageusia.

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. The umami receptor detects the amino acid glutamate, a flavor commonly found in meat and in artificial flavourings such as monosodium glutamate.

Note that taste is not the same as flavor; flavor includes the smell of a food as well as its taste.

Smell

Smell or olfaction is the other "chemical" sense. Unlike taste, there are hundreds of olfactory receptors, 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. 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.

Touch

Touch, also called tactition or mechanoreception, is the sense of pressure perception, generally in the skin. There are a variety of pressure receptors that respond to variations in pressure (e.g., firm, brushing, and sustained). The inability to feel anything or almost anything is called anesthesia. Paresthesia is a sensation of tingling, pricking, or numbness of a person's skin with no apparent long term physical effect.

Non-human senses

Analogous to human senses

Other living organisms have receptors to sense the world around them, including many of the senses listed above for humans. However, the mechanisms and capabilities vary widely.

Smell

Among non-human species, dogs have a much keener sense of smell than humans, although the mechanism is similar. Insects have olfactory receptors on their antennae.

Vision

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. The common vampire bat may also have an infrared sensor on its nose. Infrared senses are, however, just sight in a different light frequency range. It has been found that birds and some other animals are tetrachromats and have the ability to see in the ultraviolet down to 300 nanometers. Bees are also able to see in the ultraviolet.

Balance

Ctenophores have a balance receptor (a statocyst) that works very differently from the mammalian semi-circular canals.

Not analogous to human senses

In addition, some animals have senses that humans do not, including the following:

  • 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 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 electric field 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 order of mammals that is known to demonstrate electroception is the monotreme order. Among these mammals, the platypus has the most acute sense of electroception.

Body modification enthusiasts have experimented with magnetic implants to attempt to replicate this sense, however in general humans (and probably other mammals) can detect electric fields only 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). This is however not electroception as it is a post-sensory cognitive action.

  • Echolocation is the ability to determine orientation to other objects through interpretation of reflected sound (like sonar). Bats and cetaceans are noted for this ability, though some other animals use it, as well. It is most often used to navigate through poor lighting conditions or to identify and track prey. There is currently an uncertainty whether this is simply an extremely developed post-sensory interpretation of auditory perceptions or it actually constitutes a separate sense. Resolution of the issue will require brain scans of animals while they actually perform echolocation, a task that has proven difficult in practice. Blind people report they are able to navigate by interpreting reflected sounds (esp. their own footsteps), a phenomenon which is known as Human echolocation.

  • Magnetoception (or "magnetoreception") is the ability to detect fluctuations in magnetic fields and is most commonly observed in birds, though it has also been observed in insects such as bees. 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. There is experimental and physical evidence to suggest this sense exists in a weak form in humans.

Magnetotactic bacteria build miniature magnets inside themselves and use them to determine their orientation relative to the Earth's magnetic field.

  • Pressure detection uses the lateral line, which is a pressure-sensing system of hairs found in fish and some aquatic amphibians. It is used primarily for navigation, hunting, and schooling. Humans have a basic relative-pressure detection ability when eustachian tube(s) are blocked, as demonstrated in the ear's response to changes in altitude.

  • Polarized light direction / detection is used by bees to orient themselves, especially on cloudy days. Cuttlefish can also perceive the polarization of light.

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